Hormonal Regulation

When a person needs water, they will secrete ADH (anti-diuretic hormone) and excrete more concentrated urine. ADH acts on the walls of the collecting ducts to make them more permeable to water. Since the fluid of the medulla is very concentrated with ions, water will flow out of the collecting ducts if the walls of the collecting duct are water permeable and allow osmosis. If no ADH is present, the walls of the collecting ducts do not permit osmosis, and the urine will remam dilute.

Another hormone that regulates urine formation is the steroid hormone aldosterone. Aldosterone is secreted in response to low extracellular sodium and acts on the distal tubule to increase the resorption of sodium from the urinary filtrate. The resorption of sodium causes water to be removed from the filtrate by osmosis as well, reducing the urine volume and increasing the volume of the extracellular fluids of the body. Aldosterone is another means the body can use to conserve water as well as sodium.

This animation (Audio - Important) describes hormonal regulation.

The Concentrating Urine - The Mammalian Kidney Tutorial provides an excellent explanation of kidney function.

REVIEW: The basic functional unit of the mammalian kidney filters the blood and actively secretes and resorbs specific solute molecules. What is the name of this unit?

REVIEW: In the nephron, the glomerulus filters blood, after which the filtrate - water and solutes - enters Bowman/s capsule. From Bowman's capsule, the filtrate passes through several regions of a long tubule. Out of which region are most of the solutes and water removed from the tubule?