Osmoregulation

Blood

Heart

Nerves

Gonads

Muscle

Kidney

D3.3.9
Role of the loop of Henle

Osmoregulation is the control of the water balance of the blood, tissue or cytoplasm of a living organism

It occurs within the loop of Henle, which is the part of a nephron that descends into the medulla

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 (water will leave the filtrate via osmosis)
The ascending limb of the loop of Henle is permeable to salts but not water (sodium ions are actively transported out of the filtrate)
This means that as the loop descends into the medulla, the interstitial fluid becomes more salty and hypertonic (has a higher osmotic concentration)

Additionally, the vasa recta blood network that surrounds the loop of Henle flows in the opposite direction to the filtrate (counter-current)

This means that salts released from the ascending limb are drawn down into the medulla, further establishing a salt gradient

D3.3.10
Osmoregulation by water reabsorption in the collecting ducts

Once the filtrate has passed through the distal convoluted tubule, it drains into a collecting duct

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 causing vesicles containing aquaporins to fuse with the membrane of the cells lining the duct
This increases the amount of aquaporins lining the collecting duct, meaning more water is reabsorbed into the bloodstream
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