Question

In what circumstances do the kidneys secrete renin, and what is its purpose? (p. 426)

Short answer

Answer: The kidneys secrete renin under conditions such as low blood pressure, low blood volume, low sodium concentration, and decreased sympathetic nervous system activity. The main purpose of renin secretion is to increase blood pressure and maintain electrolyte balance by converting angiotensinogen into angiotensin I and then angiotensin II, which constricts blood vessels and stimulates aldosterone secretion, promoting sodium reabsorption in the kidney.

Step-by-step solution

Understand the function of the kidneys

The kidneys are essential organs that help regulate blood pressure, maintain osmotic balance, and filter waste products from the blood. They play a crucial role in maintaining homeostasis in our bodies.

Introduce the Renin-Angiotensin System (RAS)

The Renin-Angiotensin System (RAS) is a hormone system that plays a role in regulating blood pressure and electrolyte balance. It involves the secretion of renin by special kidney cells called juxtaglomerular cells.

Identify the circumstances leading to renin secretion

The kidneys secrete renin under certain conditions. These circumstances include low blood pressure, low blood volume, low sodium (Na+) concentration, and a decrease in sympathetic nervous system activity. These trigger the juxtaglomerular cells to secrete renin.

Explain the purpose of renin secretion

The main purpose of renin secretion is to increase blood pressure and maintain electrolyte balance. Renin does this by converting angiotensinogen (a protein produced in the liver) into angiotensin I, which is further converted to angiotensin II by angiotensin-converting enzyme (ACE). Angiotensin II, a potent vasoconstrictor, causes blood vessels to constrict, effectively increasing blood pressure. Additionally, angiotensin II stimulates the secretion of aldosterone, a hormone that promotes sodium reabsorption in the kidney, which helps to maintain electrolyte balance and further increase blood pressure by increasing blood volume.