Essential idea: The structure of the wall of the small intestine allows it to move, digest and absorb food.
6.1 Digestion and absorption
UNDERSTANDINGS:
U6.1.1 The contraction of circular and longitudinal muscle of the small intestine mixes the food with enzymes and moves it along the gut.
U6.1.2 The pancreas secretes enzymes into the lumen of the small intestine.
Pancreas contains two types of gland tissue:
- Small groups of cells secrete insulin glucagon into the blood
- Remainder synthesises and secretes digestive enzymes into the gut in response to eating a meal - mediated by hormones synthesised in stomach and by enteric nervous system.
Digestive enzymes are synthesised in pancreatic gland cells on ribosomes on RER, processed in Golgi apparatus and secreted by exocytosis. Ducts within the pancreas merge into larger ducts, finally forming one pancreatic duct, through which about a litre of pancreatic juice is secreted per day into the lumen of the small intestine.
Contents of pancreatic juice:
- Amylase to digest starch
- Lipases to digest triglycerides and phospholipids
- Proteases to digest protein and peptides
U6.1.3 Enzymes digest most macromolecules in food into monomers in the small intestine.
The enzymes secreted by the pancreas into the lumen of the small intestine carry out these hydrolysis reactions:
- starch is digested to maltose by amylase
- triglycerides are digested to fatty acids and glycerol or fatty acids and monoglycerides by lipase
- phospholipids are digested to fatty acids, glycerol and phosphate by phospholipase
- proteins and polypeptides are digested to shorter peptides by protease
NOTE:
- nucleases digest DNA and RNA into nucleotides
- maltase digests maltose into glucose
- lactase digests lactose into glucose and galactose
- sucrase digests sucrose into glucose and fructose
- exopeptidases are proteases that digest peptides by removing single amino acids either from the carboxy or amino terminal of the chain until only a dipeptide is remaining
- dipeptidases digest dipeptides into amino acids
Since the small intestine is so large in length, food takes hours to pass through, allowing time for digestion of most macromolecules to be completed. Some subtances remain largely undigested because human cannot synthesise the necessary enzymes. Cellulose for example is not digested and passes on to the large intestine as one of the main components of dietary fibre.
U6.1.4 Villi increase the surface area of epithelium over which absorption is carried out.
U6.1.5 Villi absorb monomers formed by digestion as well as mineral ions and vitamins.
The epithelium that covers the villi must form a barrier to harmful substances, which at the same time being permeable enough to allow useful nutrients to pass through.
Villi cells absorb these products of digestion of macromolecules in food:
- glucose, fructose, galactose and other monosaccharides
- any of the 20 amino acids used to make proteins
- fatty acids, monoglycerides and glycerol
- bases from digestion of nucleotides.
They also absorb substances required by the body and present in foods but not needing digestion:
- mineral ions such as calcium, potassium and sodium
- vitamins such as ascorbic acid (vitamin C).
Some harmful substances can pass through the epithelium and are subsequently removed from the blood and detoxified by the liver.
Some harmless but unwanted substances are also absorbed, including many of those that give food its artificial colour and flavour - these pass out in urine. Small numbers of bacteria also pass through the epithelium but are quickly removed from the blood by phagocytic cells in the liver.
U6.1.6 Different methods of membrane transport are required to absorb different nutrients.
UNDERSTANDINGS:
U6.1.1 The contraction of circular and longitudinal muscle of the small intestine mixes the food with enzymes and moves it along the gut.
- Circular and longitudinal muscle in the wall of gut are smooth muscles consisting of relatively short cells which exert continuous moderate force with some more vigorous contractions.
- Waves of muscle contraction = peristalsis contract circular muscles behind food preventing it being pushed back to the mouth and the longitudinal contractions move food along. Contractions are controlled unconsciously by enteric nervous system.
- Peristalsis occurs in one direction, when vomiting occurs, abdominal muscles are used.
- Food is moved a few centimetres at a time hence progression through intestine is much slower allowing time for digestion.
- Main function of peristalsis = churning with semi digested food to mix it with enzymes and speed up digestion.
U6.1.2 The pancreas secretes enzymes into the lumen of the small intestine.
Pancreas contains two types of gland tissue:
- Small groups of cells secrete insulin glucagon into the blood
- Remainder synthesises and secretes digestive enzymes into the gut in response to eating a meal - mediated by hormones synthesised in stomach and by enteric nervous system.
Digestive enzymes are synthesised in pancreatic gland cells on ribosomes on RER, processed in Golgi apparatus and secreted by exocytosis. Ducts within the pancreas merge into larger ducts, finally forming one pancreatic duct, through which about a litre of pancreatic juice is secreted per day into the lumen of the small intestine.
Contents of pancreatic juice:
- Amylase to digest starch
- Lipases to digest triglycerides and phospholipids
- Proteases to digest protein and peptides
U6.1.3 Enzymes digest most macromolecules in food into monomers in the small intestine.
The enzymes secreted by the pancreas into the lumen of the small intestine carry out these hydrolysis reactions:
- starch is digested to maltose by amylase
- triglycerides are digested to fatty acids and glycerol or fatty acids and monoglycerides by lipase
- phospholipids are digested to fatty acids, glycerol and phosphate by phospholipase
- proteins and polypeptides are digested to shorter peptides by protease
- This does not complete the process of digestion into molecules small enough to be absorbed
- The wall of the small intestine produces a variety of other enzymes, which digest more substnaces
- Some enzymes produced by gland cells in the intestine wall may be secreted in intestinal juice but most remain immobilised in the plasma membrane of epithelium cells lining the intestine
- They are active there and continue to be active when the epithelium cells are abraded off the lining and mixed with the semi-digested food.
NOTE:
- nucleases digest DNA and RNA into nucleotides
- maltase digests maltose into glucose
- lactase digests lactose into glucose and galactose
- sucrase digests sucrose into glucose and fructose
- exopeptidases are proteases that digest peptides by removing single amino acids either from the carboxy or amino terminal of the chain until only a dipeptide is remaining
- dipeptidases digest dipeptides into amino acids
Since the small intestine is so large in length, food takes hours to pass through, allowing time for digestion of most macromolecules to be completed. Some subtances remain largely undigested because human cannot synthesise the necessary enzymes. Cellulose for example is not digested and passes on to the large intestine as one of the main components of dietary fibre.
U6.1.4 Villi increase the surface area of epithelium over which absorption is carried out.
- In the human digestive systm nutrients are absorbed principally in the small intestine
- The rate of absorption depends on the surface area of the epithelium that carries out the process
- The small intestine in adults is approximately 7 metres long and 25-30 millimetres wide
- There are folds on its inner surface called plicae circulares, from which project many tiny finger-like structures of tissue called villi
- The individual epithelial cells also have finger-like projections, called microvilli
- These projections give the intestise a large surface area to increase the rate of absorption by a factor of about 10
- Villi are present on the mucosa of the small intestine
- A single villus is between 0.5 and 1.5mm long and there can be as many as 40 of them per square millimetre of small intestine wall
U6.1.5 Villi absorb monomers formed by digestion as well as mineral ions and vitamins.
The epithelium that covers the villi must form a barrier to harmful substances, which at the same time being permeable enough to allow useful nutrients to pass through.
Villi cells absorb these products of digestion of macromolecules in food:
- glucose, fructose, galactose and other monosaccharides
- any of the 20 amino acids used to make proteins
- fatty acids, monoglycerides and glycerol
- bases from digestion of nucleotides.
They also absorb substances required by the body and present in foods but not needing digestion:
- mineral ions such as calcium, potassium and sodium
- vitamins such as ascorbic acid (vitamin C).
Some harmful substances can pass through the epithelium and are subsequently removed from the blood and detoxified by the liver.
Some harmless but unwanted substances are also absorbed, including many of those that give food its artificial colour and flavour - these pass out in urine. Small numbers of bacteria also pass through the epithelium but are quickly removed from the blood by phagocytic cells in the liver.
U6.1.6 Different methods of membrane transport are required to absorb different nutrients.
Passive diffusion - The natural movement of a substance usually from area of highest to lowest concentration down a concentration gradient. simple diffusion is the most basic form of transport and is considered passive transport because unlike active transport, simple diffusion does not require the use of ATP (energy).
Since membranes are held together by weak forces, certain molecules can slip between the lipids in the bilayer and cross from one side to the other. This spontaneous process is termed diffusion. This process allows molecules that are small and lipophilic (lipid-soluble), including most drugs, to easily enter and exit cells.
Facilitated diffusion - In order to cross the hydrophobic interior of the bilayer, water-soluble molecules (those that are either charged or have polar groups) and large molecules require the action of membrane transport proteins. These integral membrane proteins provide a continuous protein-lined pathway through the bilayer. There are two classes of membrane transport proteins: carrier proteins, which literally carry specific molecules across and channel proteins, which form a narrow pore through which ions can pass. Channel proteins carry out passive transport (aka facilitated diffusion), in which ions travel spontaneously down their gradients.
Primary and Secondary Active Transport - The active transport of molecules across cell membranes is one of the major factors on molecular level for keeping homeostasis within the body. This kind of transport requires energy as they transport molecules against their concentration gradient. It is divided into two types according to the source of energy used, called primary active transport and secondary active transport. In primary active transport, the energy is derived directly from the breakdown of ATP. In the secondary active transport, the energy is derived secondarily from energy that has been stored in the form of ionic concentration differences between the two sides of a membrane.
Exocytosis - In general, large macromolecules (e.g. proteins, viruses, lipoprotein particles) require more complex mechanisms to traverse membranes and are transported into and out of cells selectively via endocytosis and exocytosis (secretion). Interestingly, endocytosis and exocytosis are not only important for the import/export of large molecules. Often, essential small molecules that are hydrophobic or toxic (e.g., iron) travel through the bloodstream bound to proteins, which enter and exit cells via these mechanisms.
EXAMPLES:
Triglycerides:
1. digested into fatty acids and monoglycerides
2. absorbed by simple diffusion into villus epithelium cells as they can pass between phospholipids in the plasma membrane
3. fatty acids are also absorbed by facilitated diffusion as there are fatty acid transporters, which are proteins embedded into the membrane of the microvilli
4. once inside the epithelium cells, fatty acids are combined with monoglycerides to produce triglycerides, which cannot diffuse back into the lumen
5. triglycerides coalesce with cholesterol to form droplets with a diameter of about 0.2 micrometres, which become coated in phospholipids and protein
6. these lipoprotein particles are released by exocytosis through the plasma membrane on the inner side of the villus epithelium cells
7. they then either enter the lacteal and are carried away in the lymph, or enter the blood capillaries in the villi.
Glucose:
1. cannot pass through the plasma membrane by simple diffusion - it is a polar molecule and thus hydrophobic
2. sodium-potassium pumps in the inwards-facing part of the plasma membrane pump sodium ions by active transport from the cytoplasm to the interstitial spaces inside the villus and potassium ions in the opposide direction
3. this creates a low concentration of sodium ions inside villus epithelium cells
4. sodium-glucose co-transporter proteins in the microvilli transfer a sodium ion and a glucose molecule together from the intestinal lumen to the cytoplasm of the epithelium cells
5. this type of facilitated diffusion is passive but it depends on the concentration gradient of sodium ions created by active transport
6. glucose channels allow glucose to move by facilitated diffusion from the cytoplasm to the interstitial spaces inside the villus and on into blood capillaries in the villus.
Since membranes are held together by weak forces, certain molecules can slip between the lipids in the bilayer and cross from one side to the other. This spontaneous process is termed diffusion. This process allows molecules that are small and lipophilic (lipid-soluble), including most drugs, to easily enter and exit cells.
Facilitated diffusion - In order to cross the hydrophobic interior of the bilayer, water-soluble molecules (those that are either charged or have polar groups) and large molecules require the action of membrane transport proteins. These integral membrane proteins provide a continuous protein-lined pathway through the bilayer. There are two classes of membrane transport proteins: carrier proteins, which literally carry specific molecules across and channel proteins, which form a narrow pore through which ions can pass. Channel proteins carry out passive transport (aka facilitated diffusion), in which ions travel spontaneously down their gradients.
Primary and Secondary Active Transport - The active transport of molecules across cell membranes is one of the major factors on molecular level for keeping homeostasis within the body. This kind of transport requires energy as they transport molecules against their concentration gradient. It is divided into two types according to the source of energy used, called primary active transport and secondary active transport. In primary active transport, the energy is derived directly from the breakdown of ATP. In the secondary active transport, the energy is derived secondarily from energy that has been stored in the form of ionic concentration differences between the two sides of a membrane.
Exocytosis - In general, large macromolecules (e.g. proteins, viruses, lipoprotein particles) require more complex mechanisms to traverse membranes and are transported into and out of cells selectively via endocytosis and exocytosis (secretion). Interestingly, endocytosis and exocytosis are not only important for the import/export of large molecules. Often, essential small molecules that are hydrophobic or toxic (e.g., iron) travel through the bloodstream bound to proteins, which enter and exit cells via these mechanisms.
EXAMPLES:
Triglycerides:
1. digested into fatty acids and monoglycerides
2. absorbed by simple diffusion into villus epithelium cells as they can pass between phospholipids in the plasma membrane
3. fatty acids are also absorbed by facilitated diffusion as there are fatty acid transporters, which are proteins embedded into the membrane of the microvilli
4. once inside the epithelium cells, fatty acids are combined with monoglycerides to produce triglycerides, which cannot diffuse back into the lumen
5. triglycerides coalesce with cholesterol to form droplets with a diameter of about 0.2 micrometres, which become coated in phospholipids and protein
6. these lipoprotein particles are released by exocytosis through the plasma membrane on the inner side of the villus epithelium cells
7. they then either enter the lacteal and are carried away in the lymph, or enter the blood capillaries in the villi.
Glucose:
1. cannot pass through the plasma membrane by simple diffusion - it is a polar molecule and thus hydrophobic
2. sodium-potassium pumps in the inwards-facing part of the plasma membrane pump sodium ions by active transport from the cytoplasm to the interstitial spaces inside the villus and potassium ions in the opposide direction
3. this creates a low concentration of sodium ions inside villus epithelium cells
4. sodium-glucose co-transporter proteins in the microvilli transfer a sodium ion and a glucose molecule together from the intestinal lumen to the cytoplasm of the epithelium cells
5. this type of facilitated diffusion is passive but it depends on the concentration gradient of sodium ions created by active transport
6. glucose channels allow glucose to move by facilitated diffusion from the cytoplasm to the interstitial spaces inside the villus and on into blood capillaries in the villus.
APPLICATION:
A6.1.1 Processes occurring in the small intestine that result in the digestion of starch and transport of the products of digestion to the liver.
Starch digestion illustrates some important processes including catalysis, enzyme specificity and membrane permeability. Starch is a macromolecule, composed of many alpha-glucose monomers linked together in plants by condensation reactions. It is a major constituent of plant-based foods such as bread, potatoes and pasta. Starch molecules cannot pass through membranes so much be digested in the small intestine to allow absorption.
All of the reactions involved in the digestion of starch are exothermic (release heat energy), but without a catalyst they happen at very slow rates. There are two types of molecule in starch:
- amylose has unbranched chains of alpha-glucose linked by 1,4 bonds;
- amylopectin has chains of alpha-glucose linked by 1,4, bonds and some 1,6 bonds that make the molecule branched.
STAGE 1:
STAGE 2:
STAGE 3:
STAGE 4:
A6.1.2 Use of dialysis tubing to model absorption of digested food in the intestine.
A6.1.1 Processes occurring in the small intestine that result in the digestion of starch and transport of the products of digestion to the liver.
Starch digestion illustrates some important processes including catalysis, enzyme specificity and membrane permeability. Starch is a macromolecule, composed of many alpha-glucose monomers linked together in plants by condensation reactions. It is a major constituent of plant-based foods such as bread, potatoes and pasta. Starch molecules cannot pass through membranes so much be digested in the small intestine to allow absorption.
All of the reactions involved in the digestion of starch are exothermic (release heat energy), but without a catalyst they happen at very slow rates. There are two types of molecule in starch:
- amylose has unbranched chains of alpha-glucose linked by 1,4 bonds;
- amylopectin has chains of alpha-glucose linked by 1,4, bonds and some 1,6 bonds that make the molecule branched.
STAGE 1:
- The enzyme that begins the digestion of both forms of starch is amylase
- Saliva contains amylase but most starch digestion occurs in the small intestine, catalysed by pancreatic amylase
- Any 1,4 bond in starch molecules can be broken by this enzyme, as long as there is a chain of at least 4 glucose monomers
- Amylose is therefore digested into a mixture of two- and three-glucose fragments called maltose and maltotriose.
STAGE 2:
- Because of the specificity of its active site, amylase cannot reak 1,6 bonds in amylopectin
- Fragments of the amylopectin molecule containing a 1,6 bond that amylase cannot digest are called dextrins
- Digestion of starch is completed by three enzymes in the membranes of microvilli on villus epithelium cells
- Maltase, glucosidase and dextrinase digest maltose, maltotriose and dextrin into glucose.
STAGE 3:
- Glucose is absorbed into villus epithelium cells by co-transport with sodium ions
- It them moves by facilitated diffusion into the fluid in interstitial spaces inside the villus
- The dense network of capillaries close to the epithelium ensures that glucose only has to travel a short distance to enter the blood system
- Capillary walls consist of a single layer of thin cells, with pores between adjacent cells, but these capillaries have larger pores than usual, aiding the entry of glucose.
STAGE 4:
- Blood carrying glucose and other products of digestion flows through villus capillaries to venules in the sub-mucosa of the wall of the small intestine
- The blood in these venules is carried via the hepatic portal vein to the liver, where excess glucose can be absorbed by liver cells and converted to glycogen for storage
- Glycogen is similar in structure to amylopectin, but with more 1,6 bonds and therefore more extensive branching.
A6.1.2 Use of dialysis tubing to model absorption of digested food in the intestine.
S6.1.1 Production of an annotated diagram of the digestive system.
S6.1.2 Identification of tissue layers in transverse sections of the small intestine viewed with a microscope or in a micrograph.
S6.1.2 Identification of tissue layers in transverse sections of the small intestine viewed with a microscope or in a micrograph.
Beside is a longitudinal section through the wall of the small intestine. Folds are visible on the inner surface and on these folds are finger-like projections called villi.
serosa - an outer coat
muscle layers - longitudinal muscle and inside it circular muscle
sub-mucosa - a tissue layer containing blood and lymph nodes
mucosa - the lining of the small intestine, with the epithelium that absorbs nutrients on its inner surface.
Essential idea: The blood system continuously transports substances to cells and simultaneously collects waste products.
6.2 The blood system
U6.2.1 Arteries convey blood at high pressure from the ventricles to the tissues of the body.
Arteries are vessels that convey blood from the heart to the tissues of the body. The main pumping chambers of the heart are the ventricles. They have thick strong muscle in their walls that pumps blood into the arteries, reaching a high pressure at the peak of each pumping cycle. The artery walls work with the heart to facilitate and control blood flow. Elastic and muscle tissue in the walls are used to do this.
Elastic tissue contains elastic fibres, which store the energy that stretches them at the peak of each pumping cycle. Their recoil helps propel the blood on down the artery. Contraction of smooth muscle in the artery wall determines the diameter of the lumen and to some extent the rigidity of the arteries, thus controlling the overall flow through them.
Both the elastic and muscular tissues contribute to the toughness of the walls, which have to be strong to withstand the constantly changing and intermittently high blood pressure without bulging outwards (aneurysm) or bursting. The blood's progress along major arteries is thus pulsatile, not continuous. The pulse reflects each heartbeat and can easily be felt in arteries that pass near the body surface e.g. in the wrist or neck.
Each organ of the body is supplied with blood by one or more arteries, e.g. each kidney is supplied by a renal artery and the liver by the hepatic artery. The powerful, continuously active muscles of the heart itself are supplied with blood by coronary arteries.
U6.2.2 Arteries have muscle and elastic fibres in their walls.
The wall of the artery has several layers:
- tunica externa = a tough outer layer of connective tissue
- tunica media = a thick layer containing smooth muscle and elastic fibres made of the protein elastin
- tunica intima = a smooth endothelium forming the lining of the artery
U6.2.3 The muscle and elastic fibres assist in maintaining blood pressure between pump cycles.
The blood entering an artery from the heart is at high pressure. The peak pressure reached in an artery is called the systolic pressure It pushes the wall of the artery outwards, widening the lumen and stretching elastic fibres in the wall, thus storing potential energy.
At the end of each heartbeat the pressure in the arteries falls sufficiently for the stretched elastic fibres to squeeze the blood in the lumen. This mechanism saves energy and prevents the minimum pressure inside the artery, called the diastolic pressure, from becoming too low. Because it is relatively high, blood flow in the arteries is relatively steady and continuous although driven by a pulsating heart.
The circular muscles in the wall of the artery form a ring so when they contract, in a process called vasoconstriction, the circumference is reduced and the lumen is narrowed. Vasoconstriction increases blood pressure in the arteries. Branches of arteries called arterioles have a particularly high density of muscle cells that respond to various hormone ad neural signals to control blood flow to downstream tissues. Vasoconstriction of arterioles restricts blood flow to the part of the body that they supply and the opposite process, called vasodilation, increases it.
U6.2.4 Blood flows through tissues in capillaries with permeable walls that allow exchange of materials between cells in the tissue and the blood in the capillary.
Capillaries are the narrowest blood vessels with diameter of about 10um. They branch and rejoin repeatedly to form a capillary network with a huge total length. Capillaries transport blood through almost all tissues in the body. Two exceptions are the tissues of the lens and the cornea in the eye which must be transparent so cannot contain any blood vessels. The density of capillary networks varies in other tissues but all active cells in the body are close to a capillary.
The capillary wall consists of one layer of very thin endothelium cells, coated by a filter-like protein gel, with pores between the cells. The wall is thus very permeable and allows part of the plasma to leak out and form tissue fluid. Plasma is the fluid in which the blood cells are suspended. Tissue fluid contains oxygen, glucose and all other substances in blood plasma apart from large protein molecules, which cannot pass through the capillary wall. The fluid flows between the cells in a tissue, allowing the cells to absorb useful substances and excrete waste products. The tissue then re-enters the capillary network.
The permeabilities of capillary walls differ between tissues, enabling particular proteins and other large particles to reach certain tissues but not others. Permeabilities can also change over time and capillaries repair and remodel themselves continually in response to the needs of the tissues that they perfuse.
U6.2.5 Veins collect blood at low pressures from the tissues of the body at return it to the atria of the heart.
Veins transport blood from capillary networks back to the atria of the heart. By now the blood is at much lower pressure than it was in the arteries. Veins do not therefore need to have as thick a wall as arteries and the wall contains far fewer muscle and elastic fibres. They can therefore dilate to become much wider and thus hold more blood than arteries. Around 80% of a sedentary person's blood is in the veins though this proportion falls during vigorous exercise.
Blood flow in veins is assisted by gravity and by pressures exerted on them by other tissues especially skeletal muscles. Contraction makes a muscle shorter and wider so it squeezes on adjacent veins like a pump. Walking, sitting or even just fidgeting greatly improves venous blood flow.
Each part of the body is served by one or more veins, e.g. blood is carried from the arms in the subclavian veins and from the head in the jugular veins. The hepatic portal vein is unusual because it does not carry blood back to the heart. It carries blood from the stomach and intestines to the liver. It is regarded as a portal vein rather than an artery because the blood it carries is at low pressure so it is relatively thin.
U6.2.6 Valves in veins and the heart ensure circulation of blood by preventing back flow.
Blood pressure in veins is sometimes so low that there is a danger of back flow towards to capillaries and insufficient return of blood to the heart. To maintain circulation, veins contain pocket valves, consisting of three cup-shaped flaps of tissue.
- If blood starts to flow backwards, it gets caught in the flaps of the pocket valve, which fill with blood, blocking the lumen of the vein.
- When blood flows towards the heart, it pushes the flaps to the sides of the vein. The pocket valve therefore opens and blood can flow freely.
These valves allow blood to flow in one direction only and make efficient use of the intermittent and often transient pressures provided by muscular and postural changes. They ensure that blood circulates in the body rather than flowing to and fro.
U6.2.7 There is a separate circulation for the lungs.
Pulmonary Circulation
Systemic Circulation
U6.2.8 The heart beat is initiated by a group of specialized muscle cells in the right atrium called the sinoatrial node.
The heart is unique in the body as its muscles can contract without stimulation from motor neurons. The contraction is called myogenic, meaning that it is generated in the muscle itself. The membrane of a heart muscle cell depolarises when the cell contracts and this activates adjacent cells, so they also contract. A group of cells therefore contracts almost simultaneously at the rate of the fastest.
The region of the heart with the fastest rate of spontaneous beating is a small group of special muscle cells in the wall of the right atrium, called the sinoatrial node. These cells have few of the proteins that cause contraction in other muscles cells, but they have extensive membranes. The sinoatrial node therefore initiates each heartbeat, because the membranes of its cells are the first to depolarise in each cardiac cycle.
U6.2.9 The sinoatrial node acts as a pacemaker.
Because the sinoatrial node initiates each heartbeat, it sets the pace of the beating of the heart and is often called the pacemaker. If it becomes defective, its output may be regulated or even replaced entirely by an artificial pacemaker. This is an electronic device, placed under the skin with electrodes implanted in the wall of the heart that initiate each heartbeat in place of the sinatrial node.
U6.2.10 The sinoatrial node sends out an electrical signal that stimulates contraction as it is propagated through the walls of the atria and then the walls of the ventricles.
U6.2.11 The heart rate can be increased or decreased by impulses brought to the heart through two nerves from the medulla of the brain.
U6.2.12 Epinephrine increases the heart rate to prepare for vigorous physical activity.
The sinoatrial node also responds to epinephrine in the blood, by increasing the heart rate. This hormone is also sometimes called adrenalin and is produced by the adrenal glands. The secretion of epinephrine is controlled by the brain and rises when vigorous physical activity may be necessary because of a threat or opportunity. So ephinephrine has the nickname "fight of flight hormone".The Vagus nerve causes the heart rate to slow down through the release of acetylcholine, which has an inhibitory action on the heart rate.
APPLICATION:
A6.2.1 William Harvey’s discovery of the circulation of the blood with the heart acting as the pump.
Harvey demonstrated that blood flow is unidirectional, with valves to prevent backflow. He also showed that the rate of flow through major vessels was far too high for blood to be consumed in the body after being pumped out by the heart, as earlier theories proposed. It must therefore return to the heart and be recycled. Harvey showed that the heart pumps blood out in the arteries and it returns in veins. He predicted the presence of numerous fine vessels too small to be seen with contemporary equipment that linked arteries to veins in the tissues of the body.
Blood capillaries are too narrow to be seen with the naked eye or with a hand lens. Microscopes had not been invented by the time that Harvey published his theory about the circulation of blood in 1628. It was not until 1660, after his death, that blood was seen flowing from arteries to veins through capillaries as he had predicted.
A6.2.2 Pressure changes in the left atrium, left ventricle and aorta during the cardiac cycle.
0.0 - 0.1 seconds:
- the atria contract causing a rapid but relatively small pressure increase, which pumps blood from the atria to the ventricles, through the open atrioventricular valves
- the semilunar valves are closed and blood pressure in the arteries gradually drops to its minimum as blood continues to flow along them but no more is pumped in.
0.1 - 0.15 seconds:
- the ventricles contract, with a rapid pressure build up that causes the atrioventricular valves to close
- the semilunar valves remain closed.
0.15 - 0.4 seconds:
- the pressure in the ventricles rises above the pressure in the arteries so the semilunar valves open and blood is pumped from the ventricles into the arteries, transiently maximising the arterial blood pressure
- pressure slowly rises in the atria as blood drains into them from the veins and they fill.
0.4 - 0.45 seconds:
- the contraction of the ventricular muscles wanes and pressure inside the ventricles rapidly drops below the pressure in the arteries, causing the semilunar valves to close
- the atrioventricular valves remain closed.
0.45 - 0.8 seconds:
- pressure in the ventricles drops below the pressure in the atria so the atrioventricular valves open
- blood from the veins drains into the atria and from there into the ventricles, causing a slow increase in pressure.
NOTE: diastolic pressure = specifically the minimum arterial pressure during relaxation and dilatation of the ventricles of the heart when the ventricles fill with blood.
NOTE: systolic pressure = the highest arterial blood pressure of a cardiac cycle occurring immediately after systole of the left ventricle of the heart
A6.2.3 Causes and consequences of occlusion of the coronary arteries.
Causes
Consequences
U6.2.1 Arteries convey blood at high pressure from the ventricles to the tissues of the body.
Arteries are vessels that convey blood from the heart to the tissues of the body. The main pumping chambers of the heart are the ventricles. They have thick strong muscle in their walls that pumps blood into the arteries, reaching a high pressure at the peak of each pumping cycle. The artery walls work with the heart to facilitate and control blood flow. Elastic and muscle tissue in the walls are used to do this.
Elastic tissue contains elastic fibres, which store the energy that stretches them at the peak of each pumping cycle. Their recoil helps propel the blood on down the artery. Contraction of smooth muscle in the artery wall determines the diameter of the lumen and to some extent the rigidity of the arteries, thus controlling the overall flow through them.
Both the elastic and muscular tissues contribute to the toughness of the walls, which have to be strong to withstand the constantly changing and intermittently high blood pressure without bulging outwards (aneurysm) or bursting. The blood's progress along major arteries is thus pulsatile, not continuous. The pulse reflects each heartbeat and can easily be felt in arteries that pass near the body surface e.g. in the wrist or neck.
Each organ of the body is supplied with blood by one or more arteries, e.g. each kidney is supplied by a renal artery and the liver by the hepatic artery. The powerful, continuously active muscles of the heart itself are supplied with blood by coronary arteries.
U6.2.2 Arteries have muscle and elastic fibres in their walls.
The wall of the artery has several layers:
- tunica externa = a tough outer layer of connective tissue
- tunica media = a thick layer containing smooth muscle and elastic fibres made of the protein elastin
- tunica intima = a smooth endothelium forming the lining of the artery
U6.2.3 The muscle and elastic fibres assist in maintaining blood pressure between pump cycles.
The blood entering an artery from the heart is at high pressure. The peak pressure reached in an artery is called the systolic pressure It pushes the wall of the artery outwards, widening the lumen and stretching elastic fibres in the wall, thus storing potential energy.
At the end of each heartbeat the pressure in the arteries falls sufficiently for the stretched elastic fibres to squeeze the blood in the lumen. This mechanism saves energy and prevents the minimum pressure inside the artery, called the diastolic pressure, from becoming too low. Because it is relatively high, blood flow in the arteries is relatively steady and continuous although driven by a pulsating heart.
The circular muscles in the wall of the artery form a ring so when they contract, in a process called vasoconstriction, the circumference is reduced and the lumen is narrowed. Vasoconstriction increases blood pressure in the arteries. Branches of arteries called arterioles have a particularly high density of muscle cells that respond to various hormone ad neural signals to control blood flow to downstream tissues. Vasoconstriction of arterioles restricts blood flow to the part of the body that they supply and the opposite process, called vasodilation, increases it.
U6.2.4 Blood flows through tissues in capillaries with permeable walls that allow exchange of materials between cells in the tissue and the blood in the capillary.
Capillaries are the narrowest blood vessels with diameter of about 10um. They branch and rejoin repeatedly to form a capillary network with a huge total length. Capillaries transport blood through almost all tissues in the body. Two exceptions are the tissues of the lens and the cornea in the eye which must be transparent so cannot contain any blood vessels. The density of capillary networks varies in other tissues but all active cells in the body are close to a capillary.
The capillary wall consists of one layer of very thin endothelium cells, coated by a filter-like protein gel, with pores between the cells. The wall is thus very permeable and allows part of the plasma to leak out and form tissue fluid. Plasma is the fluid in which the blood cells are suspended. Tissue fluid contains oxygen, glucose and all other substances in blood plasma apart from large protein molecules, which cannot pass through the capillary wall. The fluid flows between the cells in a tissue, allowing the cells to absorb useful substances and excrete waste products. The tissue then re-enters the capillary network.
The permeabilities of capillary walls differ between tissues, enabling particular proteins and other large particles to reach certain tissues but not others. Permeabilities can also change over time and capillaries repair and remodel themselves continually in response to the needs of the tissues that they perfuse.
U6.2.5 Veins collect blood at low pressures from the tissues of the body at return it to the atria of the heart.
Veins transport blood from capillary networks back to the atria of the heart. By now the blood is at much lower pressure than it was in the arteries. Veins do not therefore need to have as thick a wall as arteries and the wall contains far fewer muscle and elastic fibres. They can therefore dilate to become much wider and thus hold more blood than arteries. Around 80% of a sedentary person's blood is in the veins though this proportion falls during vigorous exercise.
Blood flow in veins is assisted by gravity and by pressures exerted on them by other tissues especially skeletal muscles. Contraction makes a muscle shorter and wider so it squeezes on adjacent veins like a pump. Walking, sitting or even just fidgeting greatly improves venous blood flow.
Each part of the body is served by one or more veins, e.g. blood is carried from the arms in the subclavian veins and from the head in the jugular veins. The hepatic portal vein is unusual because it does not carry blood back to the heart. It carries blood from the stomach and intestines to the liver. It is regarded as a portal vein rather than an artery because the blood it carries is at low pressure so it is relatively thin.
U6.2.6 Valves in veins and the heart ensure circulation of blood by preventing back flow.
Blood pressure in veins is sometimes so low that there is a danger of back flow towards to capillaries and insufficient return of blood to the heart. To maintain circulation, veins contain pocket valves, consisting of three cup-shaped flaps of tissue.
- If blood starts to flow backwards, it gets caught in the flaps of the pocket valve, which fill with blood, blocking the lumen of the vein.
- When blood flows towards the heart, it pushes the flaps to the sides of the vein. The pocket valve therefore opens and blood can flow freely.
These valves allow blood to flow in one direction only and make efficient use of the intermittent and often transient pressures provided by muscular and postural changes. They ensure that blood circulates in the body rather than flowing to and fro.
U6.2.7 There is a separate circulation for the lungs.
- Humans and other mammals have two different circulations of blood (blood is pumped twice)
- One circulation (systemic circulation) goes from the left ventricle to the rest of the body and back to the right atrium
- The second circulation (pulmonary circulation) goes from the right ventricle to the lungs and returns to the left atrium of the heart
- Fish for example have a single circulation however the lungs used by mammals for gas exchange are supplied with blood by a separate circulation - this is because blood capillaries in the lungs cannot withstand high pressures so blood is pumped to them at relatively low pressure.
Pulmonary Circulation
- Blood flows from the right atrium into the right ventricle through the R atrioventricular valve (mitral or bicuspid valve). The right atrium contracts when the ventricle is almost full in order to push the rest of the blood into the ventricle.
- The right ventricle contracts sending the blood out of the ventricle (past a semi-lunar valve), through the pulmonary arteries to the lungs.
- The atrioventricular valve shuts preventing back flow into the atrium.
- The blood flows through capillaries obtaining oxygen from the lungs and returning to the heart by the pulmonary veins; which empty into the left atrium.
- This blood is actually returning to the heart from the lungs at the same time as the blood that returns to the right atrium from the rest of the body.
Systemic Circulation
- The blood then flows into the left ventricle through an L atrioventricular valve (tricuspid valve).
- The left ventricle contracts, sending the blood through another semi-lunar valve and out through the biggest artery in the body called the aorta.
- Again the atrioventricular valve shuts, preventing backflow into the atrium.
- The oxygenated blood flows to all the tissues and organs in the body to be used in aerobic respiration - arteries -> arterioles -> capillaries.
- Blood then flows from the capillaries to the numerous venules and then through the different veins in the body.
- These will all eventually 'dump' the blood into the inferior and superior vena cava.
- Blood returns to the right atrium of the heart flowing from the inferior vena cava (blood from lower body) and the superior vena cava (blood coming upper body and head).
- NOTE: both ventricles contract at the same time sending blood to the lungs and the other parts of the body.
U6.2.8 The heart beat is initiated by a group of specialized muscle cells in the right atrium called the sinoatrial node.
The heart is unique in the body as its muscles can contract without stimulation from motor neurons. The contraction is called myogenic, meaning that it is generated in the muscle itself. The membrane of a heart muscle cell depolarises when the cell contracts and this activates adjacent cells, so they also contract. A group of cells therefore contracts almost simultaneously at the rate of the fastest.
The region of the heart with the fastest rate of spontaneous beating is a small group of special muscle cells in the wall of the right atrium, called the sinoatrial node. These cells have few of the proteins that cause contraction in other muscles cells, but they have extensive membranes. The sinoatrial node therefore initiates each heartbeat, because the membranes of its cells are the first to depolarise in each cardiac cycle.
U6.2.9 The sinoatrial node acts as a pacemaker.
Because the sinoatrial node initiates each heartbeat, it sets the pace of the beating of the heart and is often called the pacemaker. If it becomes defective, its output may be regulated or even replaced entirely by an artificial pacemaker. This is an electronic device, placed under the skin with electrodes implanted in the wall of the heart that initiate each heartbeat in place of the sinatrial node.
U6.2.10 The sinoatrial node sends out an electrical signal that stimulates contraction as it is propagated through the walls of the atria and then the walls of the ventricles.
- When the pacemaker cells contract, because they are myogenic, they cause the muscle cells around them to contract as well spreading the action potential across the cardiac tissue of the heart - this electrical signal that spreads throughout the walls of the atria can happen because there are interconnections between adjacent fibres across which the electrical signal can be propagated (the fibres are also branched, so each fibre passes the signal on to several others)
- It takes less than a tenth of a second for all cells in the atria to receive the signal
- This causes the right and the left atria to contract, pushing the remaining blood into the ventricles
- This electrical signal reaches another node called the AV node, causing a slight delay (approx. 0.1 seconds) before the signal is sent out to the rest of the heart, causing the ventricles to contract slightly later than the atria.
- The electrical impulses are conducted by tiny bundles of muscle fibres called Purkinje Fibres, collectively known as the bundles of His.
U6.2.11 The heart rate can be increased or decreased by impulses brought to the heart through two nerves from the medulla of the brain.
- The rate of the pacemaker can be affected by nerves connected to the medulla region of the brain - the sinoatrial node responds to signals from outside the heart (from the cardiovascular centre)
- These two nerves braches act rather like the throttle and brake of a car
- Low blood pressure, high levels of CO2 (low pH) and low levels of oxygen, stimulate the heart to increase its rate and therefore deliver more oxygen to the tissues and remove more carbon dioxide
- High blood pressure, low levels of CO2 (high pH) and high levels of oxygen, cause the heartrate to slow down
- Cardiac nerves which are part of the sympathetic nervous system cause the heart rate to increase
- The Vagus nerve that is part of the parasympathetic nervous system causes the heart rate to slow down
- The medulla of the brain controls most of the autonomic functions of the body such as breathing, heart rate and blood pressure.
U6.2.12 Epinephrine increases the heart rate to prepare for vigorous physical activity.
The sinoatrial node also responds to epinephrine in the blood, by increasing the heart rate. This hormone is also sometimes called adrenalin and is produced by the adrenal glands. The secretion of epinephrine is controlled by the brain and rises when vigorous physical activity may be necessary because of a threat or opportunity. So ephinephrine has the nickname "fight of flight hormone".The Vagus nerve causes the heart rate to slow down through the release of acetylcholine, which has an inhibitory action on the heart rate.
APPLICATION:
A6.2.1 William Harvey’s discovery of the circulation of the blood with the heart acting as the pump.
Harvey demonstrated that blood flow is unidirectional, with valves to prevent backflow. He also showed that the rate of flow through major vessels was far too high for blood to be consumed in the body after being pumped out by the heart, as earlier theories proposed. It must therefore return to the heart and be recycled. Harvey showed that the heart pumps blood out in the arteries and it returns in veins. He predicted the presence of numerous fine vessels too small to be seen with contemporary equipment that linked arteries to veins in the tissues of the body.
Blood capillaries are too narrow to be seen with the naked eye or with a hand lens. Microscopes had not been invented by the time that Harvey published his theory about the circulation of blood in 1628. It was not until 1660, after his death, that blood was seen flowing from arteries to veins through capillaries as he had predicted.
A6.2.2 Pressure changes in the left atrium, left ventricle and aorta during the cardiac cycle.
0.0 - 0.1 seconds:
- the atria contract causing a rapid but relatively small pressure increase, which pumps blood from the atria to the ventricles, through the open atrioventricular valves
- the semilunar valves are closed and blood pressure in the arteries gradually drops to its minimum as blood continues to flow along them but no more is pumped in.
0.1 - 0.15 seconds:
- the ventricles contract, with a rapid pressure build up that causes the atrioventricular valves to close
- the semilunar valves remain closed.
0.15 - 0.4 seconds:
- the pressure in the ventricles rises above the pressure in the arteries so the semilunar valves open and blood is pumped from the ventricles into the arteries, transiently maximising the arterial blood pressure
- pressure slowly rises in the atria as blood drains into them from the veins and they fill.
0.4 - 0.45 seconds:
- the contraction of the ventricular muscles wanes and pressure inside the ventricles rapidly drops below the pressure in the arteries, causing the semilunar valves to close
- the atrioventricular valves remain closed.
0.45 - 0.8 seconds:
- pressure in the ventricles drops below the pressure in the atria so the atrioventricular valves open
- blood from the veins drains into the atria and from there into the ventricles, causing a slow increase in pressure.
NOTE: diastolic pressure = specifically the minimum arterial pressure during relaxation and dilatation of the ventricles of the heart when the ventricles fill with blood.
NOTE: systolic pressure = the highest arterial blood pressure of a cardiac cycle occurring immediately after systole of the left ventricle of the heart
A6.2.3 Causes and consequences of occlusion of the coronary arteries.
Causes
- Artery walls become damaged as fat (low-density lipoproteins) are deposited under the endothelium and fibrous tissue builds up (called atheroma)
- LDLs contain fats and cholesterol, accumulating and causing phagocyes to become attracted by signals from endothelium cells and smooth muscle
- The phagocytes engulf the fats and cholesterol by endocyctosis and grow very large
- Smooth muscle cells migrate th form a tough cap over the atheroma
- The artery wall bulges into the lumen narrowing it and thus impeding blood flow
- Can result from a poor diet, over-eating, constant high blood glucose levels, obesity, diabetes or smoking
- The flow of blood is impeded and the heart has to work harder (beat faster) to pump blood to the tissue, increasing blood pressure
- The smooth lining of the arteries begins to break down and form lesions called atherosclerotic plaques
- Platelets can bind to these lesions, causing an inflammatory response creating a blood clot
- The blood clot formed is called a thrombus and an embolus if it breaks free to travel through the bloodstream.
Consequences
- If an embolus breaks free, it can get stuck in a smaller arteriole and cause a blockage of blood supply to that tissue, eventually causing that tissue to die
- If this happens to the coronary arteries or arterioles in the heart, and enough of the tissue is deprived of oxygen, a myocardial infarction (heart attack) can occur
- If an embolus reaches the brain, and enough of the brain is deprived of oxygen and nutrients, a stroke can occur
- If coronary arteries are damaged, by-pass surgery can be performed, that takes an artery typically from a patient’s leg , replacing the damaged coronary artery
- Coronary Angioplasty (balloon angioplasty) can be an alternative to a by-pass operation. A catheter (with attached balloon) is inserted in the arm or the leg of a patient and is guided to the obstructed artery by x-ray and television monitors
- A harmless dye is injected into the patient to determine exactly where the blockage is
- The balloon is inflated to reestablish blood flow stretching the arterial wall and squashing the plaques.
SKILL:
S6.2.1 Identification of blood vessels as arteries, capillaries or veins from the structure of their walls.
S6.2.1 Identification of blood vessels as arteries, capillaries or veins from the structure of their walls.
S6.2.2 Recognition of the chambers and valves of the heart and the blood vessels connected to it in dissected hearts or in diagrams of heart structure.
- the heart has two sides, left and right, that pump blood to the systemic and pulmonary circulations
NOTE: systemic = the part of blood circulation that carries oxygenated blood away from the heart, to the body, and returns deoxygenated blood back to the heart
NOTE: pulmonary = the part of blood circulation which carries oxygen-depleted blood away from the heart, to the lungs, and returns oxygenated blood back to the heart
- each side of the heart has two chambers, a ventricle that pumps blood out into the arteries and an atrium that collects blood from the veins and passes it to the ventricle
- each side of the heart has two valves, an atrioventricular valve between the atrium and the ventricle and a semilunar valve between the ventricle and the artery
- oxygenated blood flows into the left side of the heart through the pulmonary veins from the lungs and out through the aorta
- deoxygenated blood flows into the right side of the heart through the vena cava and out in the pulmonary arteries.
- the heart has two sides, left and right, that pump blood to the systemic and pulmonary circulations
NOTE: systemic = the part of blood circulation that carries oxygenated blood away from the heart, to the body, and returns deoxygenated blood back to the heart
NOTE: pulmonary = the part of blood circulation which carries oxygen-depleted blood away from the heart, to the lungs, and returns oxygenated blood back to the heart
- each side of the heart has two chambers, a ventricle that pumps blood out into the arteries and an atrium that collects blood from the veins and passes it to the ventricle
- each side of the heart has two valves, an atrioventricular valve between the atrium and the ventricle and a semilunar valve between the ventricle and the artery
- oxygenated blood flows into the left side of the heart through the pulmonary veins from the lungs and out through the aorta
- deoxygenated blood flows into the right side of the heart through the vena cava and out in the pulmonary arteries.
Essential idea: The human body has structures and processes that resist the continuous threat of invasion by pathogens.
6.3 Defence against infectious disease
UNDERSTANDINGS:
U6.3.1 The skin and mucous membranes form a primary defence against pathogens that cause infectious disease.
Pathogen = microbes that can cause disease
- the skin provides a physical barrier against the entry of pathogens and protection against physical and chemical damage
- sebaceous glands are associated with hair follicles and they secrete a chemical called sebum which maintains skin moisture and slightly lowers skin pH (this inhibits the growth of bacteria and fungi)
- mucous membranes are a thinner and softer type of skin - found in nasal passages and other airways, the head of the penis and foreskin and the vagina
- the mucus that these areas of skin secrete is a sticky solution of glycoproteins
- mucus acts as a physical barrier - pathogens and harmful particles are trapped in it and either swallowed or expelled
- mucus also has antiseptic properties because of the presence of the anti-bacterial enzyme lysozome
- skin and muscous membranes are examples of non-specific immunity
U6.3.2 Cuts in the skin are sealed by blood clotting.
- when the skin is cut, blood vessels in it are severed and start to bleed
- the bleeding usually stops after a short time because of a process called clotting
- the blood emerging from a cut changes from being a liquid to semi-solid gel
- this seals up the wound and prevents further loss of blood and blood pressure
- clotting is also important because cut breach the barrier to infection provided by the skin
- clots prevent entry of pathogens until new tissue has grown to heal the cut
U6.3.3 Clotting factors are released from platelets.
- bleeding clotting involves a cascade of reactions, each of which produces a catalyst for the next reaction
- blood clots very rapidly
- it is important that clotting is under strict control, because if it occurs inside blood vessels the resulting clots can cause blockages
- the process of clotting only occurs if platelets release clotting factors
- platelets are cellular fragments that circulate in the blood
- they are smaller than both red and white blood cells
- when a cut or other injury involving damage to blood vessels occurs, platelets aggregate at the site forming a temporary plug
- they then release the clotting factors that trigger off the clotting process
U6.3.4 The cascade results in the rapid conversion of fibrinogen to fibrin by thrombin.
- the cascade of reactions that occurs after the release of clotting factors from platelets quickly results in the production of an enzyme called thrombin
- thrombin in turn converts the soluble protein fibrinogen into the insoluble fibrin
- the fibrin forms a mesh in cuts that traps more platelets and also blood cells
- the resulting clot is initially a gel, but if exposed to the air it dries to form a hard scab
U6.3.5 Ingestion of pathogens by phagocytic white blood cells gives non-specific immunity to diseases.
- if microorganisms get past the physical barriers of skin and mucous membranes and enter the body, white blood cells provide the next line of defense
- there are many different types of white blood cell
- some are phagocytes that squeeze out through pores in the walls of capillaries and move to sites of infection
- there they engulf pathogens by endocytosis and digest them with enzymes from lysosomes
- when wounds become infected, large numbers of phagocytes are attracted, resulting in the formation of a white liquid called pus
U6.3.6 Production of antibodies by lymphocytes in response to particular pathogens gives specific immunity.
- proteins and other molecules on the surface of pathogens are recognised as foreign by the body and they stimulate a specific immune response
- any chemical that stimulates an immune response is referred to as an antigen
- the specific immune response is the production of antibodies in response to a particular pathogen
- the antibodies bind to an antigen on that pathogen
_________________________________________________________
- antibodies are produced by types of white blood cells called lymphocytes, specifically B-lymphocytes
- each lymphocyte produces just one antibody, but our bodies can produce a vast array of different antibodies
- this is because we have small numbers of lymphocytes for producing each of the many types of antibody
- therefore there are too few lymphocytes initially to produce enough antibodies to control a pathogen that has not previously infected the body
- however antigens on the pathogen stimulate cell division of the small group of lymphocytes that produce the appropriate type of antibody
- a large clone of lymphocytes called plasma cells are produced within a few days and they secrete large enough quantities of the antibody to control the pathogen and clear the infection
- antibodies are large proteins that have two functional regions - a hypervariable regions that binds to a specific antigen and another region that helps the body to fight the pathogen in one or a number of ways, including 1) making a pathogen more recognisable to phagocytes so they are more readily engulfed and 2) preventing viruses from docking to host cells so that they cannot enter the cells
- antibodies only persist in the body for a few weeks or months and the plasma cells that produce them are also gradually lost after the infection has been overcome and the antigens associated with it are no longer present
- however some of the lymphocytes produced during an infection are not active plasma cells and instead become memory cells that are very long-lived and provide rapid production of the antibody if the body is infected again by the same pathogen
U6.3.7 Antibiotics block processes that occur in prokaryotic cells but not in eukaryotic cells.
- an antibiotic is a chemical that inhibits the growth of microorganisms
- most antibiotics are antibacterial
- they block processes that occur in prokaryotes but not in eukaryotes and can therefore be used to kill bacteria inside the body without causing harm to human cells
- the processes targeted by antibiotics are bacterial DNA replication, transcription, translation, ribosome function and cell wall formation
- many antibacterial antibiotics were discovered in saprotrophic fungi
- these fungi compete with saprotrophic bacteria for the dead organic matter on which they both feed
- by secreting antibacterial antibiotics, saprotrophic fungi inhibit the growth of their bacterial competitors
- e.g. penicillin - produced by some strain of the Penicillium fungus, but only when nutrients are scarce and competition with bacteria would be harmful
- the first antibiotic discovered by Alexander Fleming was identified as penicillin
- Fleming was working on a culture of disease-causing bacteria when he noticed the spores of little green mold (fungi) on one of his culture plates
- he observed that the presence of the mold killed or prevented the growth of the bacteria by excreting antibacterial antibiotics
U6.3.8 Viruses lack a metabolism and cannot therefore be treated with antibiotics. Some strains of bacteria have evolved with genes that confer resistance to antibiotics and some strains of bacteria have multiple resistance.
- since viruses lack their own metabolism, they have to use the chemical processes of a cell from a host that they infect
- they are unable to reproduce on their own and cannot perform protein synthesis, transcription and other metabolic functions
- antibiotics work by blocking these vital processes in bacteria, killing the bacteria, or stopping them from multiplying
- since viruses do not perform their own metabolic reactions antibiotics such as penicillin and streptomycin, are ineffective in treating viral infections, as the virus relies on the host cell's enzymes for ATP synthesis thus the antibiotic would damage the human cell
- therefore treating viruses with antibiotics is not only useless and ineffective, it can also create antibiotic resistance in bacterial strains eg. Methicillin-resistant Staphylococcus aureus
Antibiotic resistance is an avoidable problem. These measures are required:
- doctors prescribing antibiotics only for serious bacterial infections
- patients patients completing courses of antibiotics to eliminate infections completely
- hospital staff maintaining high standards of hygiene to prevent cross-infection
- farmers not using antibiotics in animal feeds to stimulate growth
- pharmaceutical companies developing new types of antibiotic - no new types have been introduced since the 1980s
UNDERSTANDINGS:
U6.3.1 The skin and mucous membranes form a primary defence against pathogens that cause infectious disease.
Pathogen = microbes that can cause disease
- the skin provides a physical barrier against the entry of pathogens and protection against physical and chemical damage
- sebaceous glands are associated with hair follicles and they secrete a chemical called sebum which maintains skin moisture and slightly lowers skin pH (this inhibits the growth of bacteria and fungi)
- mucous membranes are a thinner and softer type of skin - found in nasal passages and other airways, the head of the penis and foreskin and the vagina
- the mucus that these areas of skin secrete is a sticky solution of glycoproteins
- mucus acts as a physical barrier - pathogens and harmful particles are trapped in it and either swallowed or expelled
- mucus also has antiseptic properties because of the presence of the anti-bacterial enzyme lysozome
- skin and muscous membranes are examples of non-specific immunity
U6.3.2 Cuts in the skin are sealed by blood clotting.
- when the skin is cut, blood vessels in it are severed and start to bleed
- the bleeding usually stops after a short time because of a process called clotting
- the blood emerging from a cut changes from being a liquid to semi-solid gel
- this seals up the wound and prevents further loss of blood and blood pressure
- clotting is also important because cut breach the barrier to infection provided by the skin
- clots prevent entry of pathogens until new tissue has grown to heal the cut
U6.3.3 Clotting factors are released from platelets.
- bleeding clotting involves a cascade of reactions, each of which produces a catalyst for the next reaction
- blood clots very rapidly
- it is important that clotting is under strict control, because if it occurs inside blood vessels the resulting clots can cause blockages
- the process of clotting only occurs if platelets release clotting factors
- platelets are cellular fragments that circulate in the blood
- they are smaller than both red and white blood cells
- when a cut or other injury involving damage to blood vessels occurs, platelets aggregate at the site forming a temporary plug
- they then release the clotting factors that trigger off the clotting process
U6.3.4 The cascade results in the rapid conversion of fibrinogen to fibrin by thrombin.
- the cascade of reactions that occurs after the release of clotting factors from platelets quickly results in the production of an enzyme called thrombin
- thrombin in turn converts the soluble protein fibrinogen into the insoluble fibrin
- the fibrin forms a mesh in cuts that traps more platelets and also blood cells
- the resulting clot is initially a gel, but if exposed to the air it dries to form a hard scab
U6.3.5 Ingestion of pathogens by phagocytic white blood cells gives non-specific immunity to diseases.
- if microorganisms get past the physical barriers of skin and mucous membranes and enter the body, white blood cells provide the next line of defense
- there are many different types of white blood cell
- some are phagocytes that squeeze out through pores in the walls of capillaries and move to sites of infection
- there they engulf pathogens by endocytosis and digest them with enzymes from lysosomes
- when wounds become infected, large numbers of phagocytes are attracted, resulting in the formation of a white liquid called pus
U6.3.6 Production of antibodies by lymphocytes in response to particular pathogens gives specific immunity.
- proteins and other molecules on the surface of pathogens are recognised as foreign by the body and they stimulate a specific immune response
- any chemical that stimulates an immune response is referred to as an antigen
- the specific immune response is the production of antibodies in response to a particular pathogen
- the antibodies bind to an antigen on that pathogen
_________________________________________________________
- antibodies are produced by types of white blood cells called lymphocytes, specifically B-lymphocytes
- each lymphocyte produces just one antibody, but our bodies can produce a vast array of different antibodies
- this is because we have small numbers of lymphocytes for producing each of the many types of antibody
- therefore there are too few lymphocytes initially to produce enough antibodies to control a pathogen that has not previously infected the body
- however antigens on the pathogen stimulate cell division of the small group of lymphocytes that produce the appropriate type of antibody
- a large clone of lymphocytes called plasma cells are produced within a few days and they secrete large enough quantities of the antibody to control the pathogen and clear the infection
- antibodies are large proteins that have two functional regions - a hypervariable regions that binds to a specific antigen and another region that helps the body to fight the pathogen in one or a number of ways, including 1) making a pathogen more recognisable to phagocytes so they are more readily engulfed and 2) preventing viruses from docking to host cells so that they cannot enter the cells
- antibodies only persist in the body for a few weeks or months and the plasma cells that produce them are also gradually lost after the infection has been overcome and the antigens associated with it are no longer present
- however some of the lymphocytes produced during an infection are not active plasma cells and instead become memory cells that are very long-lived and provide rapid production of the antibody if the body is infected again by the same pathogen
U6.3.7 Antibiotics block processes that occur in prokaryotic cells but not in eukaryotic cells.
- an antibiotic is a chemical that inhibits the growth of microorganisms
- most antibiotics are antibacterial
- they block processes that occur in prokaryotes but not in eukaryotes and can therefore be used to kill bacteria inside the body without causing harm to human cells
- the processes targeted by antibiotics are bacterial DNA replication, transcription, translation, ribosome function and cell wall formation
- many antibacterial antibiotics were discovered in saprotrophic fungi
- these fungi compete with saprotrophic bacteria for the dead organic matter on which they both feed
- by secreting antibacterial antibiotics, saprotrophic fungi inhibit the growth of their bacterial competitors
- e.g. penicillin - produced by some strain of the Penicillium fungus, but only when nutrients are scarce and competition with bacteria would be harmful
- the first antibiotic discovered by Alexander Fleming was identified as penicillin
- Fleming was working on a culture of disease-causing bacteria when he noticed the spores of little green mold (fungi) on one of his culture plates
- he observed that the presence of the mold killed or prevented the growth of the bacteria by excreting antibacterial antibiotics
U6.3.8 Viruses lack a metabolism and cannot therefore be treated with antibiotics. Some strains of bacteria have evolved with genes that confer resistance to antibiotics and some strains of bacteria have multiple resistance.
- since viruses lack their own metabolism, they have to use the chemical processes of a cell from a host that they infect
- they are unable to reproduce on their own and cannot perform protein synthesis, transcription and other metabolic functions
- antibiotics work by blocking these vital processes in bacteria, killing the bacteria, or stopping them from multiplying
- since viruses do not perform their own metabolic reactions antibiotics such as penicillin and streptomycin, are ineffective in treating viral infections, as the virus relies on the host cell's enzymes for ATP synthesis thus the antibiotic would damage the human cell
- therefore treating viruses with antibiotics is not only useless and ineffective, it can also create antibiotic resistance in bacterial strains eg. Methicillin-resistant Staphylococcus aureus
Antibiotic resistance is an avoidable problem. These measures are required:
- doctors prescribing antibiotics only for serious bacterial infections
- patients patients completing courses of antibiotics to eliminate infections completely
- hospital staff maintaining high standards of hygiene to prevent cross-infection
- farmers not using antibiotics in animal feeds to stimulate growth
- pharmaceutical companies developing new types of antibiotic - no new types have been introduced since the 1980s
APPLICATION:
A6.3.1 Causes and consequences of blood clot formation in coronary arteries.
There are some well-known factors that are correlated with an increased risk of coronary thrombosis and heart attacks:
- smoking
- high blood cholesterol concentration
- high blood pressure
- diabetes
- obesity
- lack of exercise
A6.3.2 Florey and Chain’s experiments to test penicillin on bacterial infections in mice.
The story of Sir Alexander Fleming's discovery of penicillin is well known. In 1929 he discovered a mould growing on a glass dish in his laboratory which appeared to kill the bacteria he was cultivating. In his follow-up studies, the crude penicillin broth that he had extracted from the mould was non-toxic to rabbits and mice.1 But it rapidly disappeared from their blood, and it seemed to work very slowly in the test tube.
These results led Fleming to believe that penicillin would only be useful as an antiseptic for surface infections rather than as a powerful antibiotic for general infections. After this, little came of his discovery, although a few patients with eye infections were successfully treated by the application of impure extracts of penicillin broth in the 1930s.
The enormous death toll from septic infections led to a great interest in developing antibiotics at the beginning of the 1940s. One of the substances tested by researchers was Fleming's crude penicillin broth.
Howard Florey and Ernst Chain, searching for potential antibiotics at Oxford University in 1940, used the mouse protection test. This animal test was first described in 1911 and was in routine use from 1927. In the test, Florey and Chain injected eight mice with a lethal suspension of bacteria, and four of these were also given penicillin.2 The four mice which received penicillin lived and all the rest died, giving definite proof that penicillin worked as an antibiotic against serious bacterial infections. It was this test which set Florey, Chain, Heatley and others on the long road to purifying and mass producing penicillin.
In 1945, Alexander Fleming, Ernst Chain and Howard Florey received the Nobel Prize for the discovery and development of penicillin.
A6.3.3 Effects of HIV on the immune system and methods of transmission.
The production of antibodies by the immune system is a complex process and includes different types of lymphocyte, including helper T-cells. The human immunodeficiency virus (HIV) invades and destroys helper T-cells. The consequence is a progressive loss of the capacity to produce antibodies. In the early stages of infection, the immune system makes antibodies against HIV. If these can be detected in a person's body, they are said to be HIV-positive.
HIV is a retrovirus that has genes made of RNA and uses reverse transcriptase to maake DNA copies of its genes once it has entered a host cell. The rate at which helper T-cells are destroyed by HIV varies considerably and can be slowed down by using anti-retroviral drugs. In most HIV-positive patients antibody production eventually becomes so ineffective that a group of opportunistic infections strike, which would be easily fought off by a healthy immune system. Several of these are normally so rare that they are marker diseases for the later stages of HIV infection, for example Kaposi's sarcoma. A collection of several diseases or conditions existing together is called a syndrome. When the syndrome of conditions due to HIV is present, the person is said to have acquired immune deficiency syndrome (AIDS).
AIDS spreads HIV infection. The virus only survives outside the body for a short time and infection normally only occurs if there is blood to blood contact between infected and uninfected people. Methods of transmission include:
- sexual intercourse, during which minor abrasions to the mucous membrane of the penis and vagina cause minor bleeding
- transfusion of infected blood, or blood products such as Factor VIII
- sharing of hypodermic needles by intravenous drug users
A6.3.1 Causes and consequences of blood clot formation in coronary arteries.
- Coronary arteries are arteries that branch from the aorta and supply oxygen to the heart.
- Individuals that have coronary heart disease sometimes form blood clots in these arteries
- If the arteries are blocked, that part of the heart becomes deprived of oxygen and vital nutrients.
- The heart can no longer produce the amount of ATP (through aerobic respiration) needed for the heart to work properly
- The individual is therefore at a high risk of having a possible fatal heart attack
- Atherosclerosis is a disease of the arteries characterised by the deposition of plaques of fatty material on their inner walls - atherosclerosis causes occlusion in the coronary arteries and where atheroma develops the endothelium of the arteries tends to become damaged and roughened; especially, the artery wall is hardened by deposition of calcium salts
- Patches of atheroma sometimes rupture causing a lesion
- Coronary occulsion, damage to the capillary epithelium, hardening of arteries and rupture of atheroma all increase the risk of coronary thrombosis
- This blocking of the arteries can lead to a heart attack
There are some well-known factors that are correlated with an increased risk of coronary thrombosis and heart attacks:
- smoking
- high blood cholesterol concentration
- high blood pressure
- diabetes
- obesity
- lack of exercise
A6.3.2 Florey and Chain’s experiments to test penicillin on bacterial infections in mice.
The story of Sir Alexander Fleming's discovery of penicillin is well known. In 1929 he discovered a mould growing on a glass dish in his laboratory which appeared to kill the bacteria he was cultivating. In his follow-up studies, the crude penicillin broth that he had extracted from the mould was non-toxic to rabbits and mice.1 But it rapidly disappeared from their blood, and it seemed to work very slowly in the test tube.
These results led Fleming to believe that penicillin would only be useful as an antiseptic for surface infections rather than as a powerful antibiotic for general infections. After this, little came of his discovery, although a few patients with eye infections were successfully treated by the application of impure extracts of penicillin broth in the 1930s.
The enormous death toll from septic infections led to a great interest in developing antibiotics at the beginning of the 1940s. One of the substances tested by researchers was Fleming's crude penicillin broth.
Howard Florey and Ernst Chain, searching for potential antibiotics at Oxford University in 1940, used the mouse protection test. This animal test was first described in 1911 and was in routine use from 1927. In the test, Florey and Chain injected eight mice with a lethal suspension of bacteria, and four of these were also given penicillin.2 The four mice which received penicillin lived and all the rest died, giving definite proof that penicillin worked as an antibiotic against serious bacterial infections. It was this test which set Florey, Chain, Heatley and others on the long road to purifying and mass producing penicillin.
In 1945, Alexander Fleming, Ernst Chain and Howard Florey received the Nobel Prize for the discovery and development of penicillin.
A6.3.3 Effects of HIV on the immune system and methods of transmission.
The production of antibodies by the immune system is a complex process and includes different types of lymphocyte, including helper T-cells. The human immunodeficiency virus (HIV) invades and destroys helper T-cells. The consequence is a progressive loss of the capacity to produce antibodies. In the early stages of infection, the immune system makes antibodies against HIV. If these can be detected in a person's body, they are said to be HIV-positive.
HIV is a retrovirus that has genes made of RNA and uses reverse transcriptase to maake DNA copies of its genes once it has entered a host cell. The rate at which helper T-cells are destroyed by HIV varies considerably and can be slowed down by using anti-retroviral drugs. In most HIV-positive patients antibody production eventually becomes so ineffective that a group of opportunistic infections strike, which would be easily fought off by a healthy immune system. Several of these are normally so rare that they are marker diseases for the later stages of HIV infection, for example Kaposi's sarcoma. A collection of several diseases or conditions existing together is called a syndrome. When the syndrome of conditions due to HIV is present, the person is said to have acquired immune deficiency syndrome (AIDS).
AIDS spreads HIV infection. The virus only survives outside the body for a short time and infection normally only occurs if there is blood to blood contact between infected and uninfected people. Methods of transmission include:
- sexual intercourse, during which minor abrasions to the mucous membrane of the penis and vagina cause minor bleeding
- transfusion of infected blood, or blood products such as Factor VIII
- sharing of hypodermic needles by intravenous drug users
Essential idea: The lungs are actively ventilated to ensure that gas exchange can occur passively
6.4 Gas exchange
UNDERSTANDINGS:
U6.4.1 Ventilation maintains concentration gradients of oxygen and carbon dioxide between air in alveoli and blood flowing in adjacent capillaries.
- all organisms absorb one gas from the environment and release a different one
- this process is called gas exchange
- leaves absorb carbon dioxide to use in photosynthesis and release oxygen produced by this process
- humans absorb oxygen for use in cell respiration and release the carbon dioxide produced by this process
- terrestrial organisms exchange gases with the air
- in humans, gas exchange occurs in small air sacs called alveoli inside the lungs
UNDERSTANDINGS:
U6.4.1 Ventilation maintains concentration gradients of oxygen and carbon dioxide between air in alveoli and blood flowing in adjacent capillaries.
- all organisms absorb one gas from the environment and release a different one
- this process is called gas exchange
- leaves absorb carbon dioxide to use in photosynthesis and release oxygen produced by this process
- humans absorb oxygen for use in cell respiration and release the carbon dioxide produced by this process
- terrestrial organisms exchange gases with the air
- in humans, gas exchange occurs in small air sacs called alveoli inside the lungs
- gas exchange happens by diffusion between air ini the alveoli and blood flowing in the adjacent capillaries
- the gases only diffuse because there is a high concentration gradient: the air in the alveolus has a higher concentration of oxygen and a lower concentration of CO2 than the blood in the capillary
- to maintain these concentration gradients fresh air must be pumped into the alveoli and stale air must be removed
- this process is called ventilation
U6.4.2 Type I pneumocytes are extremely thin alveolar cells that are adapted to carry out gas exchange.
- the lungs contain huge numbers of alveoli with a very large total surface area for diffusion
- the wall of each alveolus consists of a single layer of cells, called the epithelium
- most of the cells in the epithelium are Type I pneumocytes
- they are flattened cells, with the thickness of only about 0.15um of cytoplasm
- the wall of the adjacent capillaries also consists of a single layer of very thin cells
- the air in the alveolus and the blood in the alveolar capillaries are therefore less than 0.5um apart
- the distance over which oxygen and carbon dioxide has to diffuse is therefore very small, which is an adaptation to increase the rate of gas exchange
U6.4.3 Type II pneumocytes secrete a solution containing surfactant that creates a moist surface inside the alveoli to prevent the sides of the alveolus adhering to each other by reducing surface tension.
- Type II pneumocytes are rounded cells that occupy about 5% of the alveolar surface area
- they secrete a fluid which coats the inner surface of the alveoli
- this film of moisture allows oxygen in the alveolus to dissolve and then diffuse to the blood in the alveolar capillaries
- it also provides an area from which carbon dioxide can evaporate into the air and be exhaled
- the fluid secreted by the Type II pneumocytes contains a pulmonary surfactant
- its molecules have a structure similar to that of phospholipids in cell membranes
- they form a monolayer on the surface of the moisture lining the alveoli, with the hydrophilic heads facing the water and the hydrophobic tails facing the air
- this reduces the surface tension and prevents the water from causing the sides of the alveoli to adhere when air is exhaled from the lungs
- this helps to prevent collapse of the lung
- premature babies are often born with insufficient pulmonary surfactant and can suffer from infant respiratory distress syndrome
- treatment involves giving the baby oxygen and also one or more doses or surfactant, extracted rom animal lungs
U6.4.4 Air is carried to the lungs in the trachea and bronchi and then to the alveoli in bronchioles.
- air enters the ventilation system through the nose or mouth and then passes down the trachea
- this has rings of cartilage in its wall to keep it open even when air pressure inside is low or pressure in surrounding tissues is high
- the trachea divides to form two bronchi, also with walls strengthened with cartilage
- one bronchus leads to each lung
- inside the lungs the bronchi divide repeatedly to form a tree-like structure of narrower airways, called bronchioles
- the bronchioles have smooth muscle fibres in their walls, allowing the width of these airways to vary
- at the end of the narrowest bronchioles are groups of alveoli, where gas exchange occurs
U6.4.5 Muscle contractions cause the pressure changes inside the thorax that force air in and out of the lungs to ventilate them.
-
- the gases only diffuse because there is a high concentration gradient: the air in the alveolus has a higher concentration of oxygen and a lower concentration of CO2 than the blood in the capillary
- to maintain these concentration gradients fresh air must be pumped into the alveoli and stale air must be removed
- this process is called ventilation
U6.4.2 Type I pneumocytes are extremely thin alveolar cells that are adapted to carry out gas exchange.
- the lungs contain huge numbers of alveoli with a very large total surface area for diffusion
- the wall of each alveolus consists of a single layer of cells, called the epithelium
- most of the cells in the epithelium are Type I pneumocytes
- they are flattened cells, with the thickness of only about 0.15um of cytoplasm
- the wall of the adjacent capillaries also consists of a single layer of very thin cells
- the air in the alveolus and the blood in the alveolar capillaries are therefore less than 0.5um apart
- the distance over which oxygen and carbon dioxide has to diffuse is therefore very small, which is an adaptation to increase the rate of gas exchange
U6.4.3 Type II pneumocytes secrete a solution containing surfactant that creates a moist surface inside the alveoli to prevent the sides of the alveolus adhering to each other by reducing surface tension.
- Type II pneumocytes are rounded cells that occupy about 5% of the alveolar surface area
- they secrete a fluid which coats the inner surface of the alveoli
- this film of moisture allows oxygen in the alveolus to dissolve and then diffuse to the blood in the alveolar capillaries
- it also provides an area from which carbon dioxide can evaporate into the air and be exhaled
- the fluid secreted by the Type II pneumocytes contains a pulmonary surfactant
- its molecules have a structure similar to that of phospholipids in cell membranes
- they form a monolayer on the surface of the moisture lining the alveoli, with the hydrophilic heads facing the water and the hydrophobic tails facing the air
- this reduces the surface tension and prevents the water from causing the sides of the alveoli to adhere when air is exhaled from the lungs
- this helps to prevent collapse of the lung
- premature babies are often born with insufficient pulmonary surfactant and can suffer from infant respiratory distress syndrome
- treatment involves giving the baby oxygen and also one or more doses or surfactant, extracted rom animal lungs
U6.4.4 Air is carried to the lungs in the trachea and bronchi and then to the alveoli in bronchioles.
- air enters the ventilation system through the nose or mouth and then passes down the trachea
- this has rings of cartilage in its wall to keep it open even when air pressure inside is low or pressure in surrounding tissues is high
- the trachea divides to form two bronchi, also with walls strengthened with cartilage
- one bronchus leads to each lung
- inside the lungs the bronchi divide repeatedly to form a tree-like structure of narrower airways, called bronchioles
- the bronchioles have smooth muscle fibres in their walls, allowing the width of these airways to vary
- at the end of the narrowest bronchioles are groups of alveoli, where gas exchange occurs
U6.4.5 Muscle contractions cause the pressure changes inside the thorax that force air in and out of the lungs to ventilate them.
-