Understanding:
• The loop of Henle maintains hypertonic conditions in the medulla
Osmoregulation is the third of three processes by which blood is filtered and urine is formed
- Osmoregulation is the control of the water balance of the blood, tissue or cytoplasm of a living organism
Osmoregulation occurs in the medulla of the kidney and involves two key events:
- The loop of Henle establishes a salt gradient (hypertonicity) in the medulla
- Anti-diuretic hormone (ADH) regulates the level of water reabsorption in the collecting duct
Establishing a Salt Gradient
- The function of the loop of Henle is to create a high solute (hypertonic) concentration in the tissue fluid of the medulla
- The descending limb of the loop of Henle is permeable to water but not salts
- The ascending limb of the loop of Henle is permeable to salts but not water
- This means that as the loop descends into the medulla, the interstitial fluid becomes more salty and hypertonic
- Additionally, the vasa recta blood network that surrounds the loop of Henle flows in the opposite direction (counter-current)
- This means that salts released from the ascending limb are drawn down into the medulla, further establishing a salt gradient
Creating Hypertonicity in the Medulla
Understanding:
• ADH controls reabsorption of water in the collecting duct
Water Reabsorption
- As the collecting duct passes through the medulla, the hypertonic conditions of the medulla will draw water out by osmosis
- The amount of water released from the collecting ducts to be retained by the body is controlled by anti-diuretic hormone (ADH)
- ADH is released from the posterior pituitary in response to dehydration (detected by osmoreceptors in the hypothalamus)
- ADH increases the permeability of the collecting duct to water, by upregulating production of aquaporins (water channels)
- This means less water remains in the filtrate, urine becomes concentrated and the individual urinates less (i.e. anti-diuresis)
- When an individual is suitably hydrated, ADH levels decrease and less water is reabsorbed (resulting in more dilute urine)
- Remember: ADH is produced when you Are DeHydrated
Role of ADH in Water Reabsorption
