Question

Arrange the following events occurring in Renin-Angiotensin mechanism in the correct order :
A. Increase in blood pressure and Glomerular filtration rate.
B. Reabsorption of $Na^+$ and water from distal parts of tubule due to Aldosterone.
C. Fall in Glomerular filtration rate.
D. Vasoconstriction by Angiotensin II and release of Aldosterone.
E. Renin converts Angiotensinogen into Angiotensin I, followed by Angiotensin II.
Choose the correct answer from the options given below :

• A. C, E, D, B, A
• B. A, C, E, B, D
• C. A, D, B, E, C
• D. C, A, B, D, E

Hint:

Logic Tip: The mechanism is a classic negative feedback loop. The problem (Fall in GFR = C) must be at the very beginning, and the solution to the problem (Increase in GFR = A) must be at the very end. Only Option (1) follows this logic!

Answer 1

The Correct Option is A

Step 1 : Understanding the Question:
This question asks us to identify the correct chronological sequence of the Renin-Angiotensin-Aldosterone System (RAAS). This is a vital homeostatic mechanism used by the body to restore blood pressure and kidney filtration when they drop below normal levels.
Step 2 : Key Formulas and Approach:
The RAAS follows a standard biological cascade:
Stimulus (Low BP/GFR) $\rightarrow$ Sensor (JG cells) $\rightarrow$ Enzyme (Renin) $\rightarrow$ Intermediate Activation (Angiotensin I and II) $\rightarrow$ Effector (Vasoconstriction and Aldosterone) $\rightarrow$ Target Organ Action $\rightarrow$ Correction of stimulus.
Step 3 : Detailed Explanation:

Step 1: The Initial Stimulus (C): The process is triggered when there is a Fall in Glomerular Filtration Rate (GFR) or a drop in blood pressure/blood volume. This change is detected by the macula densa and juxtaglomerular (JG) cells of the kidney.

Step 2: Enzyme Release and Conversion (E): The JG cells release the enzyme Renin into the blood. Renin acts on a plasma protein called angiotensinogen (produced by the liver) to convert it into Angiotensin I. Subsequently, Angiotensin-Converting Enzyme (ACE) converts Angiotensin I into the highly active Angiotensin II.

Step 3: Hormonal and Vascular Action (D): Angiotensin II is a powerful vasoconstrictor that increases systemic blood pressure. It also acts on the adrenal cortex to stimulate the release of Aldosterone.

Step 4: Tubular Reabsorption (B): Aldosterone acts on the distal parts of the renal tubule (DCT and collecting duct), causing the reabsorption of $Na^+$ and water. This increases blood volume and restores the osmotic balance.

Step 5: Final Homeostatic Result (A): As blood volume and pressure rise, there is an Increase in blood pressure and GFR, returning the system to its set point and completing the negative feedback loop.

Step 4 : Final Answer:
The logical chronological sequence is C $\rightarrow$ E $\rightarrow$ D $\rightarrow$ B $\rightarrow$ A. This exact order describes the step-by-step restoration of kidney function, as shown in option (A).