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).