The control of water and salt levels in the body. The correct water balance between cells and tissue fluid must be maintained to prevent problems with osmosis.
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How is water gained?
Food, metabolic reactions, drinking.
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How is water lost?
Urine, sweat, faeces, exhaled breath.
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What are osmoreceptors?
Neurosecretory cells- can release hormones into the blood. Found in the hypothalamus. They are sensitive to the water potential of the blood that passes through the hypothalamus.
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What do the neurosecretory cells produce?
The hormone ADH. The osmoreceptors then pass it to the posterior pituitary gland in the brain, where it can be released into the blood when needed.
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What is ADH?
A very small polypeptide made up of just 9 amino acids. A hormone.
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What effect does ADH have on the kidneys?
The cells lining the walls of the collecting ducts in the nephrons respond to the ADH in the blood. These have receptors specific to ADH in their membrane which detect it.
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What happens when the ADH binds to the receptors in the kidneys?
Causes chain of enz.-catalysed reactions in cell. Increase in 2nd messenger cyclic AMP. Causes vesicles containing water-permeable channels to move to membrane & insert channels (aquaporins). Collecting duct walls more permeable to H2O- leaves cell
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Describe what happens when the water potential in the blood is too high.
Water potential in blood increases. Detected by osmoreceptors in hypothalamus. Less ADH secreted by the pituitary gland. Walls of collecting duct less permeable to water. Large volume of dilute urine formed. Normal water potential in blood.
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Describe what happens when the water potential in the blood is too low.
Water potential in blood decreases. Detected by osmoreceptors in hypothalamus. More ADH secreted by pituitary gland. Walls of collecting duct more permeable to water. Small volume of concentrated urine produced. Normal water potential in blood.
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