When the blood pressure drops below a certain level, the kidneys step in to bring it back up with the help of the RAAS system. Renin is an enzyme the kidneys create that, with the help of angiotensin and aldosterone, regulates blood pressure. The enzyme converts the liver hormone angiotensinogen to angiotensin I, which becomes angiotensin II, a peptide that increases blood pressure by narrowing blood vessels. Angiotensin II stimulates the secretion of aldosterone, a hormone secreted by the adrenal glands that sit atop each kidney. This further increases blood pressure to normal levels.

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