Describe the role of the renin-angiotensin-aldosterone system in hypertension.

The renin-angiotensin-aldosterone system (RAAS) plays a critical role in regulating blood pressure by increasing blood pressure through vasoconstriction. When blood pressure drops, the kidneys release renin, which then converts angiotensinogen, a plasma protein produced by the liver, into angiotensin I. Angiotensin I is further converted to angiotensin II, primarily in the lungs by the enzyme angiotensin-converting enzyme (ACE).

Angiotensin II has several important effects that contribute to hypertension. It is a potent vasoconstrictor, meaning it narrows the blood vessels, which directly increases systemic vascular resistance and thus raises blood pressure. In addition to this effect on blood vessels, angiotensin II also stimulates the secretion of aldosterone from the adrenal glands. Aldosterone promotes sodium and water retention in the kidneys, further increasing blood volume and, consequently, blood pressure.

In summary, the mechanism by which RAAS contributes to hypertension is primarily through its action of vasoconstriction and its ability to increase blood volume, both of which elevate blood pressure significantly.