The JGA secretes an enzyme rennin in response to a variety of stimuli such as the decreased flow of fluid and NaCl to macula densa, decreased stretch of JG cells, and increased frequency of nerve impulses in the renal sympathetic nerves. The rennin, after being released into the blood acts on a plasma protein called angiotensinogen secreted by the liver and converting enzyme converts it into angiotensin-II. This is an active hormone having the following functions.
a. It is a vasoconstrictor, that constricts the efferent arterioles to increase the glomerular blood pressure which in turn increases GFR.
b. It stimulates the adrenal cortex to secrete aldosterone. Aldosterone increases reabsorption of Na+ through the kidney tubules followed by osmotic reabsorption of water. Hence, the blood volume increases along with the increase in glomerular blood pressure and GFR.
c. It stimulates the thirst center in the hypothalamus including increased intake of water. This also increases the volume of blood followed by an increase in glomerular blood pressure and GFR.
d. It stimulates neurohypophysis to release ADH which in turn increases reabsorption of water through kidney tubules causing an increase in the volume of blood.
The atrial natriuretic peptide/factor is secreted by the cells of the atria of the heart. The secretion of ANP is induced by increased stretching of atrial chambers by the increased volume of blood. It enhances the excretion of sodium and water by the kidneys. ANP inhibits the release of ADH, aldosterone, and rennin. This is known as a feedback inhibition mechanism.