14-8 Urine Formation 5: Collecting Duct's Selective Reabsorption(Cambridge AS A Level Biology, 9700)

While detailed reabsorption in the loop of Henle and DCT is not required for Cambridge A-Levels, it's important to know they reabsorb some water and salts. The main focus is on the collecting duct, which selectively reabsorbs water based on the body's water needs. This process is crucial for osmoregulation, the control of blood and tissue fluid water potential. Maintaining optimal water balance is vital as both too much and too little water can be detrimental to body cells, potentially causing them to burst or dehydrate, respectively. This regulation is an example of homeostasis, maintaining a stable internal environment.

Osmoregulation is regulated through a negative feedback mechanism: stimulus, receptor, control center, effector, and response. Changes in blood water potential (stimulus) are detected by osmoreceptors located in the hypothalamus (receptor and control center). The hypothalamus then sends a signal to the posterior pituitary gland. The posterior pituitary gland, located in the brain, decides whether to release Antidiuretic Hormone (ADH) or not. ADH is a chemical signal that travels in the blood and affects the collecting duct in the kidneys.

If the blood's water potential is low (meaning there's too little water), osmoreceptors in the hypothalamus detect this. A signal is sent to the posterior pituitary gland, which releases ADH. ADH travels to the collecting duct, which acts as the effector. The collecting duct becomes more permeable to water, leading to increased reabsorption of water back into the blood. This results in a higher blood water potential and the production of less, more concentrated urine (e.g., tea-colored urine).

Conversely, if the blood's water potential is high (too much water), the osmoreceptors detect this. The hypothalamus sends a signal to the posterior pituitary gland, which reduces or stops the release of ADH. With less ADH, the collecting duct becomes less permeable to water, leading to less water reabsorption into the blood. This allows the excess water to be expelled from the body, resulting in a higher volume of dilute urine.

The collecting duct cells have ADH receptors on their surface, making them sensitive to ADH. Inside these cells, there are vesicles containing aquaporins (channel proteins for water) embedded in their membranes. When ADH binds to its receptors, it triggers a cascade reaction within the cell, activating a phosphoenzyme. This enzyme causes the aquaporin-containing vesicles to move and fuse with the cell surface membrane of the collecting duct. This insertion of aquaporins makes the cell membrane more permeable to water, facilitating increased water reabsorption. When blood water potential normalizes, ADH detaches from the receptors, and the surface membrane folds inwards, internalizing the aquaporins and reducing water permeability.

If you drink too much water, blood water increases, less ADH is released, the collecting duct becomes less permeable, and more urine is produced, bringing water levels back to normal. If you sweat excessively, blood water decreases, more ADH is released, the collecting duct becomes more permeable, and more water is reabsorbed, also returning water levels to normal. This entire process demonstrates how the kidney effectively regulates osmoregulation.