Osmoregulation

• Mammalian plasma concentration is controlled by the pituitary gland and osmoreceptors in the hypothalamus of the brain. Antidiuretic hormone (ADH) is the hormone released by the pituitary that affects the permeability of the kidney tubules to water.
• Osmoregulation

  • change in water potential -> osmoreceptors in hypothalamus ->nerves send to ->pituitary -> ADH sent to -> nephron -> normal water potential restored
• Without ADH, the hypotonic filtrate passes through the collecting duct unchanged, producing high volumes of dilute urine
• The effects of ADH is dependent on two properties of the fluids in the kidney caused by the countercurrent multiplier in the loop of Henle:

  • the filtrate as it reaches the end of the loop has a water potential higher than that of plasma (is more dilute)
  • the medulla has a very low water potential.
• ADH causes the channels to open in the collecting duct, which allows water to pass through. Because the medulla has a very low water potential, water will leave the filtrate by osmosis as it passes through. This produces small quantities of concentrated urine.
• kangaroo rats

  they live in places which are so arid that they are never able to drink water, they obtain water from food and biochemical reactions.

  they need to conserve water by producing very little, very concentrated urine and their kidney structure reflects this.

  they have nephrons with very long loops of Henle to produce a very low water potential in the medulla. the tubules have more microvilli and more mitochondria than in most animals, for efficient reabsorption.
• ADH makes the walls of the collecting duct permeable to water. Water leaves by osmosis and is carried away by capillaries.