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Section B : Control of the Internal Environment 1. Structure of the Human Urinary System The human urinary system is composed of the kidneys, renal artery, renal vein, ureter, bladder and urethra. 2. Role of the Mammalian Kidney (a) Osmoregulation Mammals need to constantly take in water to counteract that water lost by sweat, breath, urine and faeces. To counteract this loss of water, mammals gain water by eating, drinking and metabolic reactions (respiration occurs in every cell of our bodies). To remain healthy, the volume of water we lose everyday must be replaced by an equal volume of water. If we do not maintain the balance of water within the body, the major organs of our body will stop working properly, eventually we would die. Water gain Water loss 1500 ml drinking 600 ml eating 500 ml sweat 450 ml breath 400 ml chemical reactions 1450 ml urine 100 ml faeces i.e. 2500 ml water lost = 2500 ml water gained. The kidneys, working closely with the brain, are responsible for maintaining the delicate balance of water within the body. The regulation of the quantity of water in the human body is known as osmoregulation. The kidneys, together with the other organs of the urinary system, are resposnible for osmoregualtion within a mammal's body. Depending on how much water we gain in a 24 hour period, the brain will alter kidney function to maintain the balance of water within the body (about 70% of our body mass is water). (b) Urea and Urine • • • • • The body uses urine as a way of getting rid of nitrogenous waste. Urine is a solution of urea. Urea is produced by the body, as a waste product from breaking down excess amino acids in the liver. Deamination of amino acids produces urea. Urea travels to the kidneys, dissolved in the blood. Once at the kidneys, urea is filtered from the blood. The kidneys dispose of the filtered urea as urine, a solution of urea. (c) Structure and function of the kidneys Structure: a longitudinal section through a kidney. (see your own notes) Within the cortex / medulla area of a kidney are millions of functional units, known as nephrons. Each of these very small structures perform the primary role of the kidney – the production of urine. A nephron is made up of several parts, each part fulfilling a different role in urine production. Structure of a nephron (see your own notes) Function of the nephron: Production of urine in the kidneys involves the filtration of the blood, followed by reabsorption of useful subtances back into the blood from the filtrate. Filtration: • Blood enters the kidney by the renal artery. The renal artery divides thousands of times, until we are left with a single capillary to supply a single glomerulus for each nephron. • • • • A glomerulus is a knot of capillaries, providing a very large surface area for filtration of small soluble molecules from the blood into the Bowman's Capsule. Small soluble molecules such as glucose, salts, water and urea that can pass through the capillary wall into the Bowman's Capsule. Large molecules, such as plasma protein, and blood cells cannot move through the walls of the capillaries. Filtration will only occur at the glomerulus / Bowman's Capsule if the blood present in the capillaries of the glomerulus is at a high enough pressure: • • Blood arriving at the kidney comes through the renal artery, via the aorta, straight from the heart. Hence, it is at a high pressure. A bottle neck is created, as the capillaries arriving at a glomerulus are wider than those forming the glomerulus. Therefore, the same volume is forced into a small space, creating a higher pressure. Reabsorption: • As the filtrate produced at the glomerulus passes through the tubules of the nephron, useful substances are reabsorbed back into the blood stream. • All glucose, some salts and a lot of water are reabsorbed as the filtrate passes along the tubule of the nephron. • • • • • • No waste products, such as urea, are reabsorbed into the blood. The cells of the kidney show selective reabsorption, to carry this out they require energy which they get from respiration. Over 99 % of the water filtered from the blood, at the glomerulus, is returned to the blood before the filtrate enters the collecting duct. The filtrate produced by each nephron collects at the pelvis of each kidney. From here, the filtrate enters the ureter, before travelling onto the bladder. It is only once the filtrate enters the ureter that it is called urine. Once in the bladder, urine is stored ready for expulsion from the body along the urethra. • The capillaries leaving each nephron contain purified blood. • The capillaries join up together, eventually forming the renal vein. • The renal vein leaves the kidney, containing only purified blood. (d) Negative Feedback Control • • • Changes in the water concentration of the blood are monitored by the hypothalamus. Osmoreceptors in the hypothalamus are stimulated by changes in water concentrations of the blood. If we decrease the volume of water we take in, or increase the amount we sweat, the water concentration of our blood will decrease. • A decrease in water concentration will trigger the release of increased AntiDiuretic Hormone (ADH) by the pituitary gland. • ADH increases the permeability of the kidney tubules and collecting duct. This increase in permeability allows more water • • to be reabsorbed from the kidney tubules into the blood. If more water is reabsorbed from the kidney tubules into the blood, a smaller volume of more concentrated urine will be produced. If we increase the volume of water we take in, or decrease the amount we sweat, the water concentration of our blood will increase. • An increase in the water concentration of the blood will result in less AntiDiuretic Hormone (ADH) will be released by the pituitary gland. • A decreased concentration of ADH will decrease the permeability of the kidney tubules and collecting duct. This will not allow less water to be reabsorbed from the kidney tubules into the blood. • If less water is reabsorbed from the kidney tubules into the blood, a larger volume of more dilute urine will be produced. (e) Osmoregulation in Marine and Freshwater Bony Fish (i) Marine Bony Fish • The tissues of a marine bony fish are hypotonic to the surrounding salt water. • Marine bony fish constantly face the risk of dehydration as water moves from the fish to the surrounding water, by osmosis. • During osmosis, water moves from an area of higher water concentration (in the tissues of the fish) to an area of lower water concentration (in the surrounding water) down a concentration gradient. • To counteract the excessive loss of water from its body, marine bony fish: • drink plenty of sea water. • actively excrete salt from their tissues. • produce small volumes of concentrated urine. (ii) Freshwater Bony Fish • The tissues of a freshwater bony fish are hypertonic to the surrounding freshwater. • Freshwater bony fish constantly face the problem of water moving into their bodies from the surrounding freshwater by osmosis. • During osmosis, water moves from an area of higher water concentration (in the surrounding water) to an area of lower water concentration (in the tissues of the fish) down a concentration gradient. • To counteract the excessive gain of water, by its body, freshwater bony fish excrete large volumes of dilute urine.