Biology · Excretory Products and Their Elimination · NEET
Kidney function is controlled by three hormone systems: ADH (which makes the kidney save water), the Renin-Angiotensin-Aldosterone System or RAAS (which makes the kidney save salt and water to raise low blood pressure), and ANF (which does the opposite and lowers high blood pressure). A simple memory hook: ADH and RAAS both SAVE water to push blood pressure UP; ANF is the "brake" that lets water GO to bring blood pressure DOWN.
Three hormone systems control the kidney: ADH and RAAS save water to raise low blood pressure, while ANF lowers high blood pressure. The bottom line shows the exact RAAS sequence NEET asks you to arrange.
Your doubts, answered
Does ADH increase or decrease the amount of urine?
ADH DECREASES the amount of urine. ADH (also called vasopressin) is released when the body loses water and blood becomes concentrated. It makes the DCT and collecting duct pull back more water into the blood, so less water leaves as urine. This is why ADH prevents diuresis (large urine output). Low ADH does the opposite and gives dilute, high-volume urine.
What is the correct order of the Renin-Angiotensin-Aldosterone mechanism?
The sequence NEET tests is: (1) Fall in glomerular blood pressure or GFR, (2) JG cells sense this and release renin, (3) Renin converts angiotensinogen to Angiotensin I and then to Angiotensin II, (4) Angiotensin II causes vasoconstriction and triggers the adrenal cortex to release aldosterone, (5) Aldosterone causes reabsorption of Na+ and water from the distal parts of the tubule. The net effect is that blood pressure and GFR go back up.
How is aldosterone different from ADH?
Both save water, but the trigger and the main target differ. Aldosterone (from the adrenal cortex, released through RAAS) mainly reabsorbs Na+, and water follows the salt. ADH (from the posterior pituitary) directly increases water reabsorption in the collecting duct. Simple rule: aldosterone chases SALT (Na+), ADH chases WATER.
Why does ANF lower blood pressure while ADH and RAAS raise it?
ANF (Atrial Natriuretic Factor) is released when blood flow to the atria of the heart INCREASES, meaning blood pressure is already high. ANF causes vasodilation (widening of blood vessels), which lowers blood pressure. So ANF is a check on the RAAS pathway: RAAS/ADH raise a low BP, ANF brings a high BP down.
What makes the JG cells release renin?
JG (juxtaglomerular) cells sit in the JGA and act as pressure sensors. A FALL in glomerular blood pressure or GFR activates them to release renin. This is the very first hormonal step of the RAAS pathway, so a drop in BP is the trigger, not high BP.
⚠️ The NEET trap ✗ A decrease in blood pressure or blood volume releases ANF to fix it. ✓ A DECREASE in BP/volume releases renin, angiotensin and ADH (all raise BP). ANF is released when blood flow to the atria INCREASES (high BP) and it lowers BP by vasodilation. 🧠 ANF = the 'too much' hormone. It only shows up when blood volume is HIGH, never when it is low. If the question says BP fell, cross ANF out first.
Real NEET questions
2017
A decrease in blood pressure/volume will not cause the release of
A · Renin
B · Atrial Natriuretic Factor ✓
C · Aldosterone
D · ADH
Solution: A fall in blood pressure or volume triggers the renin-angiotensin system (renin from JG cells, then angiotensin II, then aldosterone) and ADH release to conserve water and raise BP. ANF works oppositely: it is released when blood flow to the atria INCREASES, causing vasodilation to lower BP. So a DECREASE in BP will not cause ANF release.
2020
Which of the following would help in prevention of diuresis?
A · Atrial natriuretic factor causes vasoconstriction
B · Decrease in secretion of renin by JG cells
C · More water reabsorption due to undersecretion of ADH
D · Reabsorption of Na+ and water from renal tubules due to aldosterone ✓
Solution: Diuresis means increased urine output, so preventing it means saving water. Aldosterone (released via RAAS) causes reabsorption of Na+ and water from the distal tubule, retaining water and preventing diuresis. (A) is wrong because ANF causes vasodilation, not vasoconstriction; (B) less renin means less aldosterone; (C) ADH must be OVER-secreted, not under-secreted, to save water.
2023
Arrange the events of Renin-Angiotensin mechanism in correct sequence. (A) Activation of JG cell and release of renin. (B) Angiotensin II activates release of aldosterone. (C) Fall in glomerular blood pressure. (D) Reabsorption of Na+ and water from distal convoluted tubule. (E) Angiotensinogen is converted to Angiotensin I and then to Angiotensin II.
A · (A), (D), (C), (B), (E)
B · (B), (A), (E), (D), (C)
C · (C), (A), (E), (B), (D) ✓
D · (A), (D), (E), (C), (B)
Solution: RAAS is triggered by a fall in glomerular blood pressure (C), which activates JG cells to release renin (A). Renin converts angiotensinogen to angiotensin I then angiotensin II (E). Angiotensin II activates the adrenal cortex to release aldosterone (B), and aldosterone finally causes reabsorption of Na+ and water from the tubule (D). So the order is C, A, E, B, D.
Solved Excretory Products and Their Elimination NEET PYQs
Try the real previous-year questions from this chapter — each with the answer and a full solution.
Which three hormones regulate kidney function in NEET?
ADH (antidiuretic hormone / vasopressin), the RAAS hormones (renin, angiotensin II and aldosterone), and ANF (Atrial Natriuretic Factor). ADH and RAAS conserve water and raise blood pressure; ANF lowers a high blood pressure.
Where is ADH secreted from?
ADH is secreted from the posterior pituitary. Osmoreceptors sense a rise in blood osmolarity (water loss) and trigger ADH release, which increases water reabsorption in the collecting duct.
What is the role of ANF in the kidney?
ANF is released when blood flow to the atria increases (high blood volume). It causes vasodilation, which decreases blood pressure and acts as a check on the RAAS system.
Does aldosterone reabsorb sodium or water first?
Aldosterone primarily drives Na+ (sodium) reabsorption from the distal parts of the tubule, and water follows the sodium by osmosis. So salt is reabsorbed first, then water follows.
