BMAT Biology – Answers The Respiratory System 1. The human respiratory system a) Nasal Cavity: The main route by which air enters the respiratory system. No gaseous exchange takes place here. There is a good blood supply, which raises the temperature of cold air and preventing it from disrupting the environment of the lung. The passages are covered in hairs and secrete mucus, which filters the air. b) Larynx: This uses the flow of air across it to produce sounds. In humans this has lead to speech. c) Trachea: This is the major airway leading down to the chest cavity. It is lined with columnar epithelial cell, including mucus-secreting cells. The cilia of the epithelia cells beat to move mucus, trapped organisms and dust away from the lungs. Inhalation of tobacco smoke stops the cilia beating. d) Bronchus: Similar in structure to the trachea, but they divide to form bronchioles. The left divides in two and the right in three. e) Bronchioles: Small tubes which spread through the lungs and end in alveoli. Larger bronchioles have cartilage rings; unlike lose of 1mm or less. Their main function is as an airway, but a small amount of gas exchange may also occur. As they get smaller the lining epithelium changes from columnar to flattened cuboidal, making diffusion of gases more likely. f) Alveoli: The main site of gaseous exchange in the lungs. 2. Inspiration: Expiration: It is an active, energy-using process. It is mainly a passive process The muscles around the diaphragm contract The muscles around the diaphragm relax so that it and as a result it is lowered and flattened. moves up into its resting dome shape The external intercostal muscles contract to The external intercostal muscles relax and the rib raise rib cage up and out cage moves down and in. In forced expiration, the internal intercostal muscles also contrast to lower The volume of the chest cavity increases the rib cage. The pressure in the cavity decreases, so it is The volume of the chest cavity decreases. lower than that of the atmospheric air outside The pressure in the cavity increases, so it is greater Air rushes into the trachea and the lungs than that of the outside air. inflate Air moves out of the lungs through the trachea and the lungs deflate. 3. In gas exchange, carbon dioxide diffuses out of the plasma and into the alveoli and oxygen diffuses into blood plasma. It then passes down a concentration gradient and into erythrocytes. Oxygen binds to haemoglobin to maintain this concentration gradient, allowing oxygen to continue to diffuse in, as the concentration of oxygen in erythrocytes is always lower than in the blood plasma. In respiring tissues, oxygen disassociates from oxyhaemoglobin, allowing oxygen to diffuse out of the erythrocytes and to the respiring cells. 4. The human respiratory system is adapted to optimise the ex 5. change of gases in the lungs by diffusion: Large Surface Area: To supply all the respiratory needs of the organism. The alveoli provide an enormous surface area for gas exchange. The average adult has alveoli giving a surface of 100m2. Thin surface: So that diffusion is rapid. The walls of the alveoli are only once cell thick as are the wall of the capillaries that run beside them. This means that gases only have to travel a very short distance. Permeable Surfaces: Gases must be able to travel freely through. Moist: Surface gases diffuse very rapidly in solution Steep Diffusion Gradient: this is maintained by the blood flowing constantly through the capillaries so that blood full of oxygen is constantly being replaced with blood that is deoxygenated. The air within the alveoli is constantly being refreshed through breathing 6. Carbon dioxide is produced in the cells as a product of respiration and passes into the plasma and red blood cells. Here it combines with water to form carbonic acid, which is catalysed by the enzyme carbonic anhydrase. This carbonic acid dissociates to give hydrogen ions and hydrogen carbonate ions. CO2 + H2O H2CO3 HCO3- + H+ Haemoglobin accepts hydrogen ions to form haemoglobinic acid. They bind to amino acid chains in the haemoglobin molecule. This brings about a distortion of the molecule, which decreases its affinity for oxygen (Bohr affect). This
allows haemoglobin to act as a buffer. Hydrogen ions pass out of the red blood cell as chloride ions move in. This is called the chloride shift. Therefore, the carbon dioxide is transported through the blood stream in the RBC. 7. Glucose + Oxygen → Carbon Dioxide + Water (+ energy) C6H12O6 + 6O2 → 6CO2 + 6H2O + energy (2880kJ) Glucose is produced when food is broken down and is carried in the blood plasma for cells. Oxygen from the lungs is carried in red blood cells to cells. Carbon dioxide is a waste product breathed out by lungs. Water is excreted, but some of it remains in the body as it is essential for cells. Energy released from respiration is used to synthesise ATP (adenosine tri-phosphate) from ADP. ATP transfers chemical energy from the energy rich substances in the cell to the cell's energy requiring reactions. When ATP breaks down to ADP, the energy created is used by the cell for processes such as active transport and muscle contraction. ATP is known as the “energy currency” for living things. 8. Anaerobic respiration is when glucose is broken down using enzymes, without the use of oxygen. However, it has a slower release of energy and produces a poisonous by-product. In mammalian muscle cells this is lactic acid. Lactic acid can result in a pain or stitch; therefore anaerobic respiration can only be maintained for a short period of time. Glucose →Lactic Acid + Energy (150kJ) C6H12O6 → 2C3H6O3 + Energy The Circulatory System 9. The cardiac cycle: The right atrium receives deoxygenated blood from the vena cava and pumps it to the right ventricle through the tricuspid valve. The right ventricle pumps blood into the pulmonary artery through the pulmonary valve (which prevents backflow). The left atrium receives oxygenated blood from the pulmonary veins and contracts, forcing blood into the left ventricle through the bicuspid valve. The left ventricle pumps blood into the aorta. The left ventricle has a thicker wall because it must push the blood out with enough force to carry it round the body, the right ventricle only has to pump blood to the lungs, and very high pressure might damage the delicate lung structure. 10. The heart is made up of two sides: The right side receives blood from the body and pumps it to the lungs The left side of the heart receives blood from the lungs and pumps it to the body. The blood in each side of the heart does not mix with the blood from the other side. The heart is made of cardiac muscle, which has special properties – it can carry on contracting regularly without resting or getting fatigued. Cardiac muscle is supplied with glucose and oxygen via the coronary arteries, which deliver blood directly into the wall of the heart. 11. There are four valves in the heart itself: The tricuspid and bicuspid valves (the atrioventricular valves) are forced open when pressure builds in the atria, as they are filled with blood. This allows the ventricles to also begin to fill with blood. When the atria are full, they contract forcing all their remaining blood into the ventricles. Semi-lunar valves are at the start of the pulmonary artery and the aorta preventing the back flow of blood into the ventricles, and also prevent blood flowing into the veins, when moving from the atria to the ventricles. The tough tendinous cords (heartstrings) make sure the valves are not turned inside out by the great pressure exerted when the ventricles contract. 12. There are four vessels that connect to the heart and allow blood to flow to and from the heart: 1. Superior vena cava: Carries blood returning from the upper body to the heart 2. Inferior vena cava: Carries blood returning from the lower body to the heart 3. Pulmonary artery: carries blood from the right side to the heart to the lungs – it is the only artery that carries deoxygenated blood 4. Pulmonary vein: Carries blood from the lungs to the left side of the heart – only vein that carries oxygenated blood 5. Aorta: Contains blood destined for the whole body. It has a wide lumen. 13. The heart muscle (cardiac muscle) is myogenic meaning it is self-exciting. The heartbeat is initiated in a specialised area of muscle in the right atrium called the sinoatrial node (SAN) or the pacemaker as it determines the basic rate of heartbeat. The SAN starts the waves of depolarisation, which spread out over the 2 atrial walls so that they
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contract. There is a band of fibres between the atria and ventricles, which have a high electrical resistance so the waves cannot spread from the atria to the ventricles. The atrio-ventricular node (AVN) in the septum does conduct the electrical impulses. Once the wave reaches the AV node, it is delayed there before being passed on to the Purkinje (also called Purkyne) fibres in the inter-ventricular septum. The excitation is passed to the apex of the heart and then through the ventricle walls. This causes the ventricles to contract from the base upwards ensuring that the blood is forced up and out in the vessels leaving the heart. The delay at the AV node allows enough time for all of the blood in the atria to fill their respective ventricles. The events that occur during atrial systole: Both the atria contract and blood passes down to the ventricles The atrio-ventricular valves (bi and tricuspid valves) open when the pressure in the ventricles falls below that of the atria, but the semi-lunar valves are closed to prevent blood flowing into the aorta and pulmonary artery. 70% of the blood flows passively down to the ventricles so the atria do not have to contract a great amount. The events that occur during ventricular systole: The atria relax. The ventricle walls contract, forcing the blood out. (0.13 sec after atrial systole) The pressure of the blood in the ventricles exceeds atrial pressure, forcing the atrio-ventricular valves to shut (producing the heart sound ‘lub’) At 0.2 seconds, when the ventricular pressure exceeds the pressure in the arteries, the semi lunar valves are forced open Blood passes into the aorta and pulmonary arteries. The events that occur during diastole: The ventricles relax. Pressure in the ventricles falls below that in the arteries causing the semi lunar valves to shut. This produces the second heart sound, ‘dub’. During diastole, all the muscle in the heart relaxes, atria start to fill with blood again The amount of blood pumped around the body is called the cardiac output, and depends on two factors: The stroke volume – the volume of blood pumped by the left ventricle in each heart beat. A typical value for an adult at rest is 75 ml. The heart rate – the number of times the heart beats per minute. A typical value for an adult at rest is 70 bpm. Cardiac Output = Stroke Volume x Heart Rate Blood plasma: is a straw-coloured liquid and is the main component of blood. It is made up of 90% water and 10% dissolved materials, e.g. proteins, lipids and salts. The blood plasma: Transports digested food products from the small intestine to where they are needed for use or storage Transports food molecules from storage area to cells that need them Transports excretory products from cells to the organs that excrete them Transports hormones from where they are made to where they cause changes in the body. Helps to maintain a steady body temperature by carrying heat around the system Acts are a buffer to pH changes Erythrocytes: Shaped like biconcave discs, which gives them a large surface area for rapid oxygen diffusion. There are approximately 5 million per mm3 of blood. They contain haemoglobin, a red pigment that carries oxygen and gives them their colour. They are formed in the red bone marrow of the short bones, e.g. ribs. Mature erythrocytes do not contain a nucleus so they have more room for transporting oxygen and have a limited life of about 120 days. Leucocytes: These are much larger than erythrocytes, but they can squeeze and change their shape. They produce antibodies, which engulf MOs and carry out phagocytosis. There are around 4000-11000 cells per mm3 of blood. They all contain a nucleus and have colourless cytoplasm. There are at least 5 types, most of which are formed in the white bone marrow of the long bones such as the humerus. Platelets: are tiny fragments of large cells called megakaryocytes, which are found in the bone marrow. There are about 150,000 – 400,000 platelets per mm3 of blood. They consist of cytoplasm fragments surrounded by cell membrane. Platelets play an important role in the clotting process which helps to prevent blood loss.
DNA, Genes and Cell Division 23. Nucleic acids are made up of units called mononucleotides. Mononucleotides are joined in a condensation reaction to form a polynucleotide, each time giving off water. Each mononucleotide is composed of: The phosphate group = H3PO4 The sugar = Deoxyribose (a pentose sugar) There are four different nitrogen containing bases and these fall into two types a) Purines (double ringed bases) – Guanine and Adenine b) Pyrimidines (single ringed bases) – Cytosine and Thymine 24. Gametes are formed by a different process of cell division to normal cells called meiosis, to bring about the reduction in chromosome number. Meiosis is a reduction division and it occurs only in the sex organs. 25. In meiosis diploid cells divide once to give two diploid cells, and then once more to give four different haploid cells, each with a sing copy of each chromosome, which creates genetic variability. Normal body cells contain two copies of each chromosome and are called diploid, however if two diploid cells combined to form a new individual the offspring would have four sets of chromosomes, losing the characteristic number for the species and this would increase for each generation. The avoid this, haploid nuclei are formed with one set of chromosomes within the gametes. Sexual reduction occurs when two haploid nuclei fuse to form a new diploid cell called a zygote. 26. At fertilisation, the nuclei of the sperm and an egg join to form the zygote. The zygote contains 23 pairs of chromosomes - 23 single chromosomes from the sperm, and 23 single chromosomes from the egg, thereby creating the correct number of 46 chromosomes for all body cells. It also means the zygote contains a complete set of chromosomes from each parent. The zygote then divides by mitosis to produce a cluster of cells called an embryo. The embryo continues to develop by mitosis to eventually become an adult individual. 27. Mitosis is a type of cell division in somatic cells. Mitosis occurs wherever more cells are needed. It produces two new cells that are identical to each other, and to the parent cell. The process of growth and division is called the cell cycle. Each new cell has identical sets of chromosomes as the parent cell, the same number of chromosomes as the parent’s cell and therefore the same genes as the parent cell. 28. Phases of mitosis: Prophase: The nucleus shrinks and the centrioles move to opposite poles. Spindle fibres begin to form. Chromosomes condense. By the end of this stage, each can be seen to be made up of two sister chromatids joined at a centromere. The nuclear membrane disintegrates. Metaphase: The spindle fibres finish growing across the cell. The chromosomes line up on the equator of the spindle attaching to a spindle at their centromere. Anaphase: The centromeres divide in two. The divided centromeres repel one another and the chromatids begin to move apart. The spindle activity pulls the chromatids apart and the separated chromatids move to opposite poles of the cell. Telophase: The chromosomes begin to uncoil and a nuclear membrane begins to form around each set of chromosomes. The spindle fibres begin to disintegrate.
Patterns of Inheritance: 29. Phenotype: The physical expression of the genes an organism has. This is partly the result of the genetic information (genotype) passed from parents to their offspring, and partly the effect of the environment in which the organism lives. 30. Genotype: The genes that an organism has, e.g. Bb or BB. 31. Homologous pair: A matching pair of chromosomes – one comes from the mother and one from the father. They carry the same genes in the same sequence. 32. Dominant: This describes alleles that will always be shown in the phenotype if they are present. 33. Recessive: A recessive allele is one that is only expressed if both copies of the gene are that allele 34. Homozygous: When both copies of as gene are the same allele, e.g. bb or BB 35. Heterozygous: When the two copies of a gene are different, e.g. Bb 36. Autosomal Dominant: Inheritance of an autosomal dominant trait must appear in every generation. If neither of the parents have the trait, it cannot be passed onto the children. If the dominant gene is inherited the abnormality will always be expressed in the phenotype. In punnet squares, if one homozygous affected parent, 100% children are affected. If one heterozygous parent affected, 50% of children are affected. 37. Autosomal Recessive: The trait does not appear in every generation. For the gene to be expressed in the phenotype, they must be homozygous, if heterozygous they will be a carrier. In punnet squares: Homozygous affected parent (nn) x homozygous unaffected parent = 100% carrier Homozygous affected parent x heterozygous unaffected = 50% carrier, 50% affected Two heterozygous parents = 50% carriers, 25% unaffected, 25% affected 38. The two X chromosomes in a female contain many paired genes, but the Y chromosome in a male lacks partners for most of the genes on its mate (the X). Thus any recessive genes on the X will show up more often in males. The unapried genes on the X are called sex-linked genes. In terms of inheritance, a male must pass on his Y gene to his sons and his X gene to his daughter, so if he has the disease on his X chromosome, then all his daughters will inherit the affected gene but none of his sons. Affected females have a 50% chance that each son and each daughter will inherit the affected gene. 39. X-Linked Recessive: This sort of trait is more likely to be manifested in a male as they only have on X chromosome. A female can show the trait but only if they inherit an affected allele from both parents – this meant that they must have an affected father. 40. X linked dominant alleles are rare. An affected male cannot have an X-affected sons, but his daughters are all affected. Affected females have a 50% chance of a son being affected and a 50% chance of a daughter being affected. If an affected male, has an affected mother then she must be dominant for the trait. If the mother is not affected then she must be a carrier. 41. This is a diagram that shows all the members of a family indicating their sex and whether or not they have the disease being investigated. By looking at these diagrams it is possible to see how a trait is passed on from generation to generation and analyse it. From the relationships within the pedigree, the genotypes of certain individuals may be deduced. Some common patterns of inheritance are: Mostly males affected = X linked abnormality Males and females affected equally = autosomal Every generation affected = dominant for abnormality Skips one or more generations = recessive for the abnormality
Digestive System 42. After food is ingested, it passes through the digestive system, gradually being broken down into simple soluble substances by digestion. Food enters by the mouth and then passes through the: Pharynx: A cavity at the back of the mouth where the mouth cavity and nasal cavities meet. When food is swallowed the soft palate closes the nasal cavities and the epiglottis closes the trachea. Oesophagus: The tube down which food travels to the stomach. A piece of swallowed food is a bolus Stomach: A large sac in which the early stages of digestion occur. Its lining has many folds which to let it expand. Some substances, e.g. water, pass through its wall into blood vessels. Small intestine: The main site of digestion. It is a coiled tube with three parts - the dudodenum, jejunum and ileum. Villi project inwards from its lining into which most of the food is absorbed. The lacteal absorbs recombined fat particles. Large intestine: A thick tube receiving waste from the small intestine. It consists of the caecum, colon, rectum and anal canal. The colon contains bacteria, which break down any remaining food and make some important vitamins. Most of the water in the waste passes through the colon walls into blood vessels. This leaves faeces, which is pushed out of the body via the rectum, anal canal and anus. 43. Carboyhydrates are digested with salivary amylase to disaccharides, e.g. maltose, and monosaccharides. 44. Proteins are digested with pepsin, trypsin and chymotrypisin in the stomach to form polypeptides and amino acids. 45. Fats are digested with bile salts and pancreatic amylase, trypsin and lipase (to fatty acids and glycerol). Most absorption of nutrients takes place in the small intestine via the blood capillaries of villi (large surface area). The Nervous System 46. An animal’s response to a stimulus is coordinated by their central nervous system (CNS). The CNS consists of the brain and the spinal cord. It gathers information about, and responds to, changes in the environment. 47. The peripheral nervous system consists of sensory neurons, which carry impulses from receptors to the CNS and motor neurons, which carry impulses from the CNS to effectors. These nerves link the brain and spinal cord to every other part of the body. 48. Receptors respond to a stimulus and send impulses along sensory neurons to the CNS. The CNS coordinates the information and sends impulses along motor neurons to the effectors that bring about a response. The sequence is: Stimulus Motor neuron Receptor Effector Sensory neuron Response Central nervous system 49. Reflex reactions in humans are controlled by the reflex arc. When the safety of an organism demands a very quick response, the signals may be passed directly from a sensory neuron, via a relay neurone, to a motor neurone for instant, unthinking action. This is a reflex action. 50. The autonomic nervous system controls the automatic functions of the body that maintain stable internal conditions, i.e. homeostasis. For example, respiration, heart rate, blood pressure, temperature and salt-water balance. The hypothalamus of the brain regulates many of the bodies autonomic systems. 51. The iris is an opaque disc of tissue, with blood vessels and central hole (pupil). It contains muscle fibres: Circular muscle: contract to decrease pupil size (in bright light) Radial muscle: contract to increase pupil size in dim light The dilation or constriction of the pupil is by autonomic reflex arc. 52. The pupil reflex is where the pupil of the eye gets larger in dim light and smaller in bright light. The eye needs to control the amount of light entering it in different light conditions. Photoreceptors contained in the retina measure intensity, wavelength and position of light. Impulses are relayed via ganglion cells to the optic nerve, which transmits impulses to the brain. In dim conditions, more light is allowed to enter so that a clear image can be formed on the retina. In bright conditions, less light is allowed to enter so that the retina is not damaged. 53. The retina is the innermost layer of tissue at the back of the eyeball, made up of a layer of pigment and a nervous layer consisting of millions of sensory neurons and their fibres. The image on the retina is inverted and results from the refraction of light at the cornea and fine adjustment at the lens – the image is sharpest near to the centre of the retina at the fovea. 54. The focus of the lens can be altered from parallel light to a near object by the accommodation reflex. The ciliary muscles are responsible for changing the shape of the lens:
Near object = ciliary muscles contract = suspensory ligaments loose = more convex lens (fatter) = more diffraction Distant object = ciliary muscles relaxed = suspensory ligaments taut = less convex lens (thinner) = less diffraction The Endocrine System 55. The endocrine system is made up of a series of hormone glands that produce hormones. Hormones are the chemical messengers of the body. The hormone glands produce and release their own hormone that affects particular organs of the body. The bloodstream receives the hormones directly from the glands and carries them to their particular organ. Hormones respond more slowly than the nervous system, but the effects are longer lasting. 56. Glands in the body: Pineal gland - the pineal gland produces several important hormones including melatonin. Melatonin influences sexual development and sleep-wake cycles. The pineal gland connects the endocrine system with the nervous system in that it converts nerve signals from the sympathetic system of the peripheral nervous system into hormone signals. Pituitary gland – produces ADH which controls blood water level by triggering uptake of water in the kidneys. It also produces hormones for the ovaries such as follicle stimulating hormone (FSH) which triggers egg ripening and oestrogen production in the ovaries and lutenising hormone (LH) which triggers egg release and progesterone production in the ovaries. Thyroid gland – produces thryoxine which controls the rate of metabolism Thymus gland – produces thymosin for T-lymphocytes in immunity Pancreas – produces insulin which controls blood sugar levels Adrenal glands – produces adrenaline which prepares the body for rapid activity by increasing the heart rate and level of sugar in blood and diverting blood to muscles and brain Ovary – Produces progesterone which maintains the lining of the womb by suppressing FSH production in the pituitary gland. It also produces oestrogen which controls puberty and the menstrual cycle in females. This stimulates the production of LH and suppresses the production of FSH in the pituitary gland. Testes – Produces testosterone which controls puberty in males 57. Negative feedback ensures that, in any control system, changes are reversed and returned back to the set level. Negative feedback keeps our body temperature at a constant 37°C. If we get too hot, blood vessels in our skin vasodilate (become larger) and we lose heat and cool down. If we get too cold blood vessels in our skin vasoconstrict (become smaller), we lose less heat and our body warms up. The other factors also controlled in the body by negative feedback are blood oxygen levels, glucose levels and salt levels. 58. Positive feedback magnifies change and promotes any deviation from the norm. Detection of a change stimulates the change, for example, the start of the menstrual cycle = rise in pituitary FSH = ovaries produce more oestrogen = rise in pituitary LH = ovaries produce more oestrogen = LH surge. 59. Several hormones control this cycle, which includes controlling the release of an egg each month from an ovary, and changing the thickness of the uterus lining. These hormones are secreted by the ovaries and pituitary gland. FSH: The hormone FSH is secreted by the pituitary gland. FSH makes two things happen: - it causes an egg to mature in an ovary - it promotes the growth of ovarian follicles that release the hormone oestrogen Oestrogen: The hormone oestrogen is secreted by the ovaries. Oestrogen: - it stops FSH being produced - so that only one egg matures in a cycle - causes the endometrium to build to full thickness - it stimulates the pituitary gland to release the hormone LH LH: The hormone LH cause: the matured oocyte to be released from the follicle into the Fallopian tube mid cycle where the cilia assist its movement. - The empty follicle to form the corpus luteum, which secretes oestrogen and progesterone Progesterone: level rises to maintain gestation and inhibit FSH which prevents further ovulation If the egg is not fertilized the corpus luteum breaks down 60. The oral contraceptive, 'the pill', greatly reduces the chances of mature eggs being produced. The pill contains oestrogen, or oestrogen and progesterone. These hormones inhibit the production of FSH, which in turn stops eggs maturing in the ovaries. However, some women who take the pill may suffer from changes in weight, mood and blood pressure, as a result of the hormones in it. There is also a chance of an increased risk of developing blood clots. On the other hand, there is evidence of a decreased risk of developing cancer of the uterus or ovaries. 61. If a couple are having difficulty conceiving a child because the quantity or quality of the man’s sperm is poor then in vitro fertilisation - or IVF - can be used. This is where the egg is fertilised outside the woman’s body and then
implanted back into her uterus. As FSH and LH can be used to encourage the production of several mature eggs at once, they’re used as part of IVF to increase the number of eggs available for fertilisation.
The Urinary System 62. The urinary system is designed to remove waste products such as urea, as well as excess ions and water from our
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blood. The urinary system is made up of the kidneys, ureters, urinary bladder and urethra. There are three main areas inside the kidney: Cortex – the outer layer Medula – the middle layer which consists of cone-shaped areas of tissue Renal Pelvis – the inner-most part of the kidney. This connects to the ureter and is where all the urine comes together from around kidney There are three blood vessels connected to the kidney: The renal vein carries the filtered blood away from the kidney The renal artery carries blood to the kidney for filtering The ureter is a tube that carries the urine produced by the kidney to be collected in the bladder Inside a kidney – Nephrons: the tiny filtering units of the kidneys. Each consists of a renal corpuscle and a uriniferous tubule Renal corpuscles: bodies that filter fluids out of the blood. Each consists of a glomerulus and a Bowman’s capsule Glomerulus: a ball of coiled up capillaries at the centre of each renal corpuscle. The capillaries branch from an arteriole entering the corpuscle and re-unite to leave the corpuscle as an efferent arteriole. Bowman’s capsule: The outer part of each renal corpuscle. It is a thin-walled sac around the glomerulus. Uriniferous tubules: long tubes, each one leading from a Bowman’s capsule. Each has three main parts – the proximal convoluted tubule, the loop of Henle and the distal convoluted tubule – and has many capillaries twined around it. These are branches of the efferent arteriole and they re-unite to form larger blood vessels carrying blood from the kidney. Blood is brought to the kidneys to be filtered, and then returned, to be circulated around the body: I. Glomerular Filtration: Blood builds up a high pressure in the glomerulus causing all the small molecules including water, salt, glucose and urea are forced through into the Bowman’s capsule. The blood cells and proteins stay. This mixture is called the glomerial filtrate. II. Tubular reabsorption: This flows down the first coiled tubule where the kidneys then reabsorb by the process of osmosis, diffusion and active transport all of the glucose, some water and as much salt as the body needs, putting them back into the blood. III. Water is then reabsorbed at the loop of henle IV. At the second coiled tubule, some more water may be reabsorbed, depending on the concentration of ADH present in the blood. V. Tubular secretion: This leaves some water and salt, and all of the urea, which is now called urine. The urine passes down the collecting duct into the ureter where it flows to the bladder, where it is stored prior to being excreted from the body. ADH controls the concentration of our urine. ADH is produced by the pituitary gland that is situated just below the brain. The pituitary gland monitors the concentration of the blood plasma. It releases ADH into the bloodstream, which travels in the blood to the kidneys. The more concentrated the plasma, the more ADH is released into the blood. When the ADH reaches the kidneys, it causes them to reabsorb more water. This keeps more water in the body and produces more concentrated urine. When the plasma is more dilute, less ADH is released into the
bloodstream. This allows more water to leave the kidneys, producing more dilute urine. This is an example of a negative feedback system.
