Excretory System and Osmoregulation

Water Balance-Osmoregulation   As part of a homeostatic system, the kidney helps to regulate the amount of water present in the body. Aside from oxygen, water is the most important substance for proper body functioning. An individual can go for weeks without food but will only last 2-3 days without water. The kidney can be used to help retain water in the body when levels are low and it can release excess water when levels are high. A chemical substance called antidiuretic hormone (ADH) works with the kidney to help regulate these levels.

Osmoreceptors located in the hypothalamus region of the brain detect changes in osmotic pressure. When fluid levels are low, the blood solutes become more concentrated, increasing the osmotic pressure. Water moves from the cells into the bloodstream to try to decrease the solute concentration. This causes the cells in the hypothalamus to shrink. The shrinking causes a message to be sent to the pituitary gland to release ADH. Once released, ADH causes the semi-permeable membranes of the nephron tubules to become more permeable to water prompting it to move into the capillary beds and back into the bloodstream.

A second message is sent to create the feeling of thirst, prompting the ingestion of water to increase fluid levels to normal. Once this occurs ADH is no longer released and the permeability of the membrane decreases so more water is released as urine. We are all physically aware of this water balancing. We know that if we drink a lot of liquids, we will need to urinate frequently. Conversely, when we haven’t been drinking a lot, the urine we do release is much a much darker yellow colour indicating a higher concentration of urea.