• Homeostasis is the maintenance of constant or steady state conditions within the body
  • Homeostatic control of body systems is essential for many reasons such as:

-providing the optimum pH and temperature for enzyme reactions

-avoiding osmotic problems in cells and body fluids

  • A major homeostatic organ in mammals is the kidney
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The kidneys

  • The kidney has two very important functions:

>Excretion - the removal of the toxic waste from metabolism such as urea and creatinine

>Osmoregulation - regulates water potential of the blood through controlling the volume and concentration of urine produced

  • The kidney is composed of about a million small tubules, called nephrons, organised within distinct layers - the outer cortex and the inner medulla
  • Each nephron is supplied with blood vessels, which form a knot of capillaries, the glomerulus, enclosed by the cup-shaped Bowman's capsule, and which surround all other parts of the nephron
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Excretion from the kidneys

Each nephron produces urine as follows:

  • Ultrafiltration takes place from the glomerulus into the Bowman's capsule. Small molecules are filtered, by the basement membrane of the glomerulus endothelium, under high hydrostatic pressure. The glomerular filtrate is hypotonic to the blood as it lacks plasma proteins
  • In the proximal convoluted tubule, glucose, amino acids and some ions are actively transported back into the blood during selective reabsorption. Water follows by osmosis and the filtrate becomes isotonic to the blood
  • The loop of Henle creates hypertonic conditions in the medulla, giving a decreasing water potential gradient from the boundary with the cortex to the inner medulla. This enables water to be withdrawn from the collecting ducts if they are permeable to water
  • Before the filtrate passes down the collecting duct, it travels through the distal convoluted tubule where further ions are actively reabsorbed
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  • Antidiuretic hormone (ADH) makes the wall of the distal convoluted tubule and collecting ducts permeable to water by causing water channel proteins, called aquaporins, to open
  • ADH is produced in the hypothalamus but stored in the posterior lobe of the pituitary gland from where it is secreted into the blood when hypothalamic osmoreceptors detect that the blood water potential has fallen below the norm. ADH enables water to be reabsorbed through the tubule walls so that hypertonic urine is produced
  • If the water potential of the blood rises (more dilute), it is detected by the osmoreceptors in the hypothalamus and ADH release is reduced. The walls of the DCT and collecting duct become less permeable and less water is reabsorbed back into the blood. Larger volumes of more dilute urine are produced and the water potential of the blood falls.
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