Question

Which one of the following autoregulatory mechanisms is employed by the kidney when glomerular filtration rate is reduced?

• A. Levels of renin, angiotensin I and II and aldosterone are increased.
• B. Levels of renin and aldosterone are reduced.
• C. Levels of renin are increased, while those of angiotensin I and II and aldosterone are reduced.
• D. Levels of angiotensin I and II are increased, while that of aldosterone are reduced.

Hint:

Remember that the RAAS pathway is a positive cascade.
An increase in the starting enzyme (renin) leads directly to sequential increases in all subsequent downstream products (angiotensins and aldosterone) to restore blood pressure.

Answer 1

The Correct Option is A

Step 1 : Understanding the Question:
The question asks about the physiological hormonal response mechanism triggered by mammalian kidneys when there is a reduction in the Glomerular Filtration Rate (GFR).

Step 2 : Key Formulas and Approach:
The approach involves examining the homeostatic feedback loop governed by the Renin-Angiotensin-Aldosterone System (RAAS), which regulates blood volume, systemic blood pressure, and GFR.

Step 3 : Detailed Explanation:

• Triggering the RAAS: A fall in GFR or blood pressure is detected by the juxtaglomerular (JG) cells of the kidney.

• Release of Renin: In response, the JG cells release the enzyme renin directly into the bloodstream.

• Activation Cascade: Renin converts the circulating plasma protein angiotensinogen into angiotensin I.

• Angiotensin I is then converted into angiotensin II by the Angiotensin-Converting Enzyme (ACE) present in lung capillaries.

• Action of Angiotensin II and Aldosterone: Angiotensin II acts as a powerful vasoconstrictor to increase glomerular blood pressure.

• Simultaneously, angiotensin II stimulates the adrenal cortex to release the hormone aldosterone.

• Aldosterone promotes the reabsorption of sodium ions ($Na^+$) and water from the distal tubules, increasing blood volume and blood pressure, which helps restore GFR back to normal levels.

• Consequently, during this autoregulatory response, the levels of renin, angiotensin I, angiotensin II, and aldosterone are all increased.

Step 4 : Final Answer:
Therefore, the autoregulatory response involves an increase in the levels of all these components, as described in Option (A).