ScienceHomeostasis
Homeostasis is the maintainence of a constant internal environment such as inside the body. This includes a constant level of water. The control of water content is largely the job of the kidneys.
A step by step guide to what happens in the kidneys
- Ultrafiltration forces water and small molecules out of the blood in the Bowman’s capsule under very high pressure.
- Salt ions are pushed out of both the tubules by active transport.
- This causes water to move out of the descending limb by osmosis.
- At the bottom of the loop of Henlé the fluid within the tubules has become very concentrated due to the loss of water.
- However, the ascending limb of the loop is impermeable to water so no more water is lost although more salt ions are pumped out making the filtrate more dilute.
- The level of water in the blood is measured by osmoreceptors in the hypothalamus. If the water level is low a hormone called ADH is released.
- The ADH controls the permeability of the collecting duct making it more permeable to water.
- This means more water is absorbed into the blood and the urine is more concentrated.