Question

Explain how renin/angiotensin/aldosterone regulate arteriolar resistance and blood pressure. How can ACE inhibitors reduce hypertension?

Answer 1

The renin–angiotensin–aldosterone system (RAAS) is a hormone system that regulates blood pressure and fluid and electrolyte balance, as well as systemic vascular (arteriolar) resistance.

When renal blood flow is reduced, juxtaglomerular cells in the kidneys convert the precursor prorenin (already present in the blood) into renin and secrete it directly into circulation.

Plasma renin then carries out the conversion of angiotensinogen, released by the liver, to angiotensin I.

Angiotensin I is subsequently converted to angiotensin II by the angiotensin-converting enzyme (ACE) found on the surface of vascular endothelial cells, mostly present in the  lungs.

Angiotensin II is a potent vasoconstrictive peptide that causes blood vessels to narrow, resulting in increased blood pressure.

Angiotensin II also stimulates the secretion of the hormone aldosterone from the adrenal cortex.

Aldosterone causes the renal tubules to increase the reabsorption of sodium and water into the blood, while at the same time causing the excretion of potassium to maintain electrolyte balance. This increases the volume of extracellular fluid in the body, which also increases blood pressure.

This whole mechanism occurs only when the kidney and the baro-receptors in the arch of aorta has sensed low blood pressure.

When there is enough rise in the blood pressure, a negative feed-back mechanism occurs where in the renin secretion is stopped and later the aldosterone release aswell resulting in inhibiting the rise of blood pressure any further.

If this mechanism is altered, any changes will lead to hypertension.

ACEI- these are Angiotensin- Converting-Enzyme-Inhibitors, these prevent the conversion of angiotensin I to angiotensin II, which is a potent vasoconstrictive peptide.This results in inhibition of aldosterone release from the adrenal cortex aswell along with reduction of blood pressure.

EXAMPLES- captopril, enalapril, lisiopril etc.

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