렌인, or "Renin," is an enzyme produced by the kidneys that plays a crucial role in regulating blood pressure. It is part of the renin-angiotensin-aldosterone system (RAAS), which helps maintain fluid balance and blood pressure in the body. When blood pressure drops, the kidneys release renin, which then converts angiotensinogen, a protein produced by the liver, into angiotensin I. Angiotensin I is further converted into angiotensin II, a powerful vasoconstrictor that narrows blood vessels, increasing blood pressure. Additionally, angiotensin II stimulates the release of aldosterone from the adrenal glands, promoting sodium and water retention, which also contributes to raising blood pressure.