Control of blood water potential

• Urine contains:

  • Water
  • Urea
  • Dissolved salts
• Urine DOESN'T contain:

  Proteins & red blood cells-too large to be filtered out

  Glucose-reabsorbed during selective reabsorption
• Ultrafiltration:

  1. blood enters through the afferent arteriole which leads into the glomerulus
  2. the efferent arteriole diameter is smaller than the afferent arteriole leading to high hydrostatic pressure
  3. water and small molecules are forced out of the glomerulus into the bowman's capsule forming glomerular filtrate
  4. the filtrate needs to pass across the podocytes, capillary endothelium, basement membrane and bowman epithelium
  5. large proteins and red blood cells are too big to leave so remain in blood
• Selective reabsorption- useful substances in the filtrate is reabsorbed back into the blood.

  takes place in the proximal convoluted tubule
• Selective reabsorption:

  1. concentration of sodium ions in the PCT is decreased as they are actively transported out of the PCT into the capillaries
  2. forms a concentration gradient, so sodium ions diffuse down the concentration gradient carrying glucose with it (co-transport) into the cells lining the PCT from the lumen
  3. Glucose diffuses from the PCT epithelial cells into the blood stream
  4. This is how all the glucose is reabsorbed
• Loop of Henle:

  1. Mitochondria in the cell walls provide energy to actively transport sodium ions out of the ascending limb
  2. the accumulation of sodium ions outside the nephron lowers the water potential
  3. therefore, water diffuses out via osmosis into the interstitial space and then the blood capillaries
  4. due to all the sodium ions being actively transported out, when the filtrate reaches the distal convoluted tubule it is very dilute.
  5. the filtrate moves out of the DVT and into the collecting duct.
  6. therefore even more water diffuses out.
  7. what remains is transported to form urine.
• Loop of Henle in desert animals:

  • Desert animals will have a longer loop of henle
  • the longer the loop the more sodium ions that are actively transported out
  • resulting in more water being reabsorbed and more concentrated urine