Controlling blood water potential

• Water is essential to keep the body functioning, so the amount of water in the blood (and so the water potential of the blood) needs to be kept constant
• mammals excrete urea and other waste products in solution, which means water is lost during excretion. water is also lost in sweat
• the kidneys regulate the water potential of the blood (and urine), so they body has the right amount of water - osmoregulation
• • if the water potential of the blood is too low (the body is dehydrated) more water is reabsorbed by osmosis into the blood from the tubules of the nephrons. this means the urine is more concentrated, so less water is lost during excretion
• • if the water potential of the blood is too high (the body is too hydrated), less water is reabsorbed by osmosis into the blood from the tubules of the nephrons. this means the urine is more dilute, so more water is lost during excretion
• • water is reabsorbed into the blood along almost all of the nephron, but regulation of water potential mainly takes place in the loop of Henle, DCT and collecting duct. The volume of water reabsorbed by the DCT and collecting duct is controlled by hormones
• • The loop of Henle is located in the medulla (inner layer) of the kidneys
  • made up of 2 limbs
  • descending limb
  • ascending limb
  • the limbs control the movement of sodium ions so that water can be reabsorbed by the blood
• Na+ is a sodium ion - these help to establish the water potential that drives the reabsorption of water from the glomerular filtrate back into the blood
• water moves out by osmosis of:

  • the descending limb
  • the DCT
  • the ascending limb
• 1. near the top of the ascending limb, Na+ ions are actively pumped out into the medulla. The ascending limb is impermeable to water, so the water stays inside the tubule. This creates a low water potential in the medulla bc there is a high concentration of ions
• 2. bc there is a lower water potential in the medulla than in the descending limb, water moves out of the descending limb (which is permeable to water) into the medulla by osmosis. This makes the glomerular filtrate more concentrated (the ions can't diffuse out - the descending limb isn't permeable to them) - the water in the medulla is reabsorbed into the blood through the capillary network
• 3. near the bottom of the ascending limb Na+ ions diffuse out into he medulla, further lowering the water potential in the medulla (the ascending limb is impermeable to water, so it stays in the tubule)
• 4. water moves out of the distal convoluted tubules by osmosis and is reabsorbed into the blood
• 5. the first three stages massively increase the ion concentration in the medulla, which lowers the water potential. This causes water to move out of the collecting duct by osmosis. As before, the water in the medulla is reabsorbed into the blood through the capillary network
• the volume of water reabsorbed into the capillaries is controlled by changing the permeability of the DCT and the collecting duct
• Antidiuretic hormone:

  • water potential of the blood is monitored by cells called osmoreceptors in part of the brain called the hypothalamus
  • when water potential of the blood decreases, water will move out of the osmoreceptor cells by osmosis
  • this causes the cells to decrease in volume
  • this sends a signal to other cells in the hypothalamus - sends a signal to the posterior pituitary gland
  • causes posterior pituitary to release ADH into the blood
• • ADH molecules bind to receptors on the plasma membrane of cells in the DCT and collecting duct
  • when this happens, protein channels called aquaporins are inserted into the plasma membrane
  • these channels allow water to pass through via osmosis, making the walls of the DCT and collecting duct more permeable to water
  • this means more water is reabsorbed from these tubules into the medulla and into the blood by osmosis
  • a small amount of concentrated urine is produced - means less water is lost from the body
• ADH changes the water content of the blood when it is too low or too high
• Dehydration - blood water content is too low:

  • dehydration is what happens when you lose water e.g. by sweating during exercise, so the water content of the blood needs to be increased
  • the water content of the blood drops, so its water potential drops
  • this is detected by osmoreceptors in the hypothalamus
  • the posterior pituitary gland is stimulated to release more ADH into the blood
  • more ADH means that the DCT and collecting duct are more permeable, so more water is reabsorbed by osmosis
  • small amount, high conc. urine produced - less water lost
• Hydration - blood water content is too high:

  • if you are hydrated - you have taken in lots of water, so the water content of the blood needs to be reduced
  • the water content of the blood rises, so its water potential rises
  • this is detected by the osmoreceptors in the hypothalamus
  • the posterior pituitary gland releases less ADH into the blood
  • less ADH means that the DCT and collecting duct are less permeable, so less water is reabsorbed into the blood by osmosis
  • large amount of dilute urine is produced and more water lost
• ADH is a protein, like hormones - once it has had its effect, it travels in the bloodstream to the liver where is is broken down
• different animals have different length loops of Henle - longer an animal's loop of Henle, the more water they can reabsorb from glomerular filtrate
• Diuresis is when lots of dilute urine is produced - ADH is so called bc it causes a small amount of conc urine to be produced - the opposite of diuresis