The Kidneys (Nephron)

Bowman's capsule -- proximal convoluted tubule -- loop of Henle -- distule convoluted tubule -- collecting duct
ultrafiltration
1. 1. The endothelium of the capillaries in the glomerulus is thin. it creates a barrier to only cells.
2. 2. A Basement membrane of collagen fibres lies in the Bowman's capsule. it allows water and small molecules to pass through. proteins are filtered out as they are too large and are repelled by the negative charge on the fibres
  1. small molecules = glucose, amine acids, vitamins and drugs.
3. 3. The epithelium of the Bowman's capsule is made of Podocytes specialised for filtration.
4. filtrate moves into the bowman's capsule due to hydrostatic pressure.
  1. The afferent arteriole is wider than the efferent arteriole
  2. The kidneys are close to the heart.
Selective Reabsorption
1. the inner walls of the proximal convoluted tubule are covered with microvilli to increase surface area for absorption
2. 1. Outer membrane of the cells of the tubule actively pump NA+ into the tissue fluid.
3. 2. The concentration of NA+ is lowered inside the cells.
4. 3. NA+ ions in the filtrate diffuse into the cells of the tubule down their concentration gradient.
5. 4. The NA+ ions carry another molecule with them into the surrounding cells Eg) amino acids
  1. Co- transport
6. 5. The amino acids diffuse down their concentration gradient into the blood.
7. 6. The water potential in the blood is lowered so water moves back into the blood by Osmosis
Water Reabsorption
1. 1. Na and Cl are actively pumped out of the ascending limb.
2. 2. This decreases the water potential in the surrounding tissue.
3. 3. As a result, Na and Cl diffuse into the descending limb down their concentration gradient.
4. 5. As the fluid moves down the descending limb, the concentration of Na and Cl increases. Na and Cl diffuse into the tissue fluid in the cortex, at the bottom of the loop of Henle
5. 4. Some water moves into the tissue fluid by osmosis.
6. 6. The collecting duct lies in the medulla. Therefore, water in the collecting duct moves into the surrounding tissue by osmosis. this reduces the amount of water lost in urination.
7. The longer the Loop of Henle, the more concentrated the salt in the tissue fluid gets, so in organisms with longer loops, more water is absorbed and less is lost in urination.
Osmoregulation
1. 1. Osmoreceptors in the hypothalamus detect a low water potential in the blood.
2. 2. Neurosecretory cells in the hypothalamus produce and release ADH.
3. 3. ADH passes down the axons of the osmoreceptors to the posterior pituitary glands.
4. 4. ADH binds to complementary receptors on the cells of the collecting duct. Enzymes are activated.
5. 5. This stimulates aquaporins to to move from the cytoplasm into the cell membranes, forming channels for water.
6. 6. Therefore, the permeability to water is increased, so more water moves into the tissue fluid and subsequently into the capillaries by osmosis.
7. 7. The water potential in the blood rises, so the hypothalamus stops producing ADH.
Kidney failure
1. causes
  1. diabetes
  2. hypertension
  3. kidney infection
2. treatment
  1. Dialysis: Partially permiable membrane allows the exchange of salt, urea and water between blood and dialysis fluid.
    1. Haemodialysis
      1. Blood from a vein is passed into a machine where it is passed over an artificial dialysis membrane
      2. Heparin is added to avoid clotting.
    2. Peritoneal dialysis
      1. Dialysis solution is poored through a surgically implanted tube, filling the space between the abdominal wall and the organs
      2. After a few hours, the solution is drained and the correct balance in the body should have been restored.
  2. Transplant
    1. positives
      1. freedom and less limited diet
      2. able to travel
    2. negatives
      1. must take immunosepressants
      2. risk of the causation of fluid retention, high blood pressue and infection
      3. Risks of being put under general anaesthetic

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The Kidneys (Nephron)

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