Question

A fall in glomerular filtration rate activates
A) Juxta glomerular cells to release renin
B) Adrenal cortex to release aldosterone
C) Adrenal medulla to release adrenaline
D) Posterior pituitary to release vasopressin

Answer 1

Glomerular filtration rate is the rate of blood flow through the kidneys and tells how well the kidneys are functioning. It is the first step in making urine.

Complete answer:
The glomerular filtration rate is one of the indicators of kidney function (GFR). The flow rate of filtered fluid through the kidney is defined by the glomerular filtration rate. Glomerular filtration rate (GFR) is the amount of fluid filtered per unit time from the renal (kidney) glomerular capillaries into the Bowman capsule.

The differential basal tone of the afferent and efferent arterioles is central to the physiological maintenance of GFR. In other words, the rate of filtration depends on the difference between the higher blood pressure produced by the input or afferent arteriole vasoconstriction versus the lower blood pressure.

Renin is a proteolytic hormone that is secreted by the afferent arterioles of the kidney from the granular juxtaglomerular (JG) cells in the wall. Renin secretion is the rate-limiting step in the formation of angiotensin II and vasoconstrictive peptides, making it an effective regulator of blood pressure and sodium balance.

The induction of renin secretion by juxtaglomerular cells, stimulating the renin-angiotensin-aldosterone system, is one of the effects of the sympathetic nervous system (RAAS). By - sodium reabsorption, which falls with glomerular filtration rate, the RAAS increases extracellular fluid volume. Therefore, this is the correct answer.

Hence the correct answer is option ‘A’.

Note: For normal control of blood pressure and electrolyte homeostasis, the renin angiotensin aldosterone system is critical and is stimulated in low blood pressure or low sodium intake situations.
An increased cAMP concentration intracellularly facilitates renin exocytosis from the JG cells, while exocytosis is inhibited by an increased $Ca^{2+}$ concentration.