Expansion of the blood volume causes increased salt and water excretion in the urine. This is partly due to an inhibition of aldo-sterone secretion, as previously described. However, it is also caused by increased secretion of a natriuretic hormone, a hormone that stimulates salt excretion (natrium is Latin for sodium)—an action opposite to that of aldosterone. The natri-uretic hormone has been identified as a 28-amino-acid polypep-tide called atrial natriuretic peptide (ANP), also called atrial natriuretic factor. Atrial natriuretic peptide is produced by the atria of the heart and secreted in response to the stretching of the atrial walls by increased blood volume. In response to ANP action, the kidneys lower the blood volume by excreting more of the salt and water filtered out of the blood by the glomeruli. Atrial na-triuretic peptide thus functions as an endogenous diuretic.
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