The Ranin-Angiotensin Aldosterone System

Let us Learn about Ranin-Angiotensin Aldosterone System.

This system helps in homeostatic condition of Na⁺ and H₂O level. Aldosterone (mineralocorticoid) is secreted from zona glomeru­losa of adrenal cortex. Aldosterone is se­creted under conditions of low arterial blood pressure. One way to correct low blood pres­sure is to increase the extracellular fluid (ECF) volume.

Na⁺ (and CI^–) within the ex­tracellular fluid exert an osmotic “hold” on water. This effect, combined with increased reabsorption of water, increases the volume of extracellular fluid (ECF) and therefore in­creases arterial blood pressure.

When low blood pressure and low Na⁺ level are detected, specialized cells of the kidney, called Juxtaglomerular cells (modi­fied smooth muscle cells in the arterioles associated with glomerulus) secrete a sub­stance Renin. The juxtaglomerular cells are sensitive to stretch. In case of low blood pres­sure, they stretch less and secrete renin ex­cessively.

Renin interacts enzymatically with a large protein, called Angiotensinogen, which is produced by the liver and is found in the blood. Renin cleaves a peptide bond of angiotensinogen to produce a decapeptide, called angiotensin-l.

In the circulation, angiotensin-l is converted into angiotensin II by the membrane bound enzyme, called angiotensin converting enzyme (ACE), which is produced by the endothelial cells of blood vessels of lungs. ACE cleaves 2 amino acids from angiotensin I to make the octapeptide hormone angiotensin-ll.

In one hand angiotensin II stimulates cells of zona glomerulosa to secrete aldos­terone. It enters its target cells (renal tubu­lar cells) and binds to cytoplasmic receptor (intracellular receptor) within the cells, and stimulates them to synthesize new Na⁺-K⁺– ATPase pumps.

This pump actively trans­ports K⁺ into the epithelial cells and Na⁺ out of the cell. Thus reabsorption of Na⁺ takes place through cells of distal convo­luted tubule.

On the other hand, angiotensin-II causes constriction of systemic arterioles, stimu­lates secretion of vasopressin and promotes thirst and drinking. All of these actions con­tribute to raising the extracellular fluid vol­ume and therefore raising arterial blood pressure (Fig. 5.5).