Renin is the hormone released by the Juxtaglomerular (JG) cells in response to a fall in glomerular blood flow.

The Juxtaglomerular cells are specialized cells located in the afferent arterioles of the kidney. These cells function as mechanoreceptors and chemoreceptors, sensing changes in blood pressure and sodium levels.

The release of renin is part of the renin-angiotensin-aldosterone system (RAAS), which plays a crucial role in regulating blood pressure, fluid balance, and electrolyte homeostasis.

Function of Renin:
Renin plays a pivotal role in this system by initiating a cascade of events that ultimately leads to the increase in blood pressure and restoration of glomerular filtration rate. The main functions of renin include:

1. Conversion of Angiotensinogen to Angiotensin I: Renin acts on angiotensinogen, a protein produced by the liver, and converts it into angiotensin I, an inactive precursor.

2. Conversion of Angiotensin I to Angiotensin II: Angiotensin I is then converted into angiotensin II by the angiotensin-converting enzyme (ACE) present in the lungs. Angiotensin II is a potent vasoconstrictor that causes the constriction of blood vessels, leading to an increase in systemic vascular resistance and blood pressure.

3. Stimulation of Aldosterone Release: Angiotensin II also stimulates the release of aldosterone from the adrenal glands. Aldosterone acts on the distal tubules and collecting ducts of the kidney, promoting the reabsorption of sodium and water and the excretion of potassium. This results in an increase in blood volume and blood pressure.

4. Stimulation of ADH Release: Additionally, angiotensin II stimulates the release of Antidiuretic Hormone (ADH) from the posterior pituitary gland. ADH acts on the collecting ducts of the kidney, increasing water reabsorption and concentrating the urine.

Overall, the release of renin by the Juxtaglomerular cells in response to a fall in glomerular blood flow is a crucial step in maintaining blood pressure and fluid balance in the body.