Which three hormonal systems regulate human kidney function?

Three feedback systems regulate the kidneys. First, the hypothalamic osmoreceptor-ADH axis controls water reabsorption from the late distal tubule and collecting duct by adjusting release of antidiuretic hormone (vasopressin) from the neurohypophysis. Second, the juxtaglomerular apparatus drives the renin-angiotensin-aldosterone system, which raises blood pressure by vasoconstriction (angiotensin II) and increases Na+ and water reabsorption in the distal tubule (aldosterone). Third, the cardiac atria release atrial natriuretic factor (ANF) when stretched, producing vasodilation and a fall in blood pressure. ANF acts as a check on the renin-angiotensin mechanism.

Where is ADH produced and where is it stored before release?

Antidiuretic hormone is a nonapeptide synthesised in the magnocellular neurons of the supraoptic and paraventricular nuclei of the hypothalamus. It is then packaged into neurosecretory granules that travel down the axons of these neurons into the posterior pituitary (neurohypophysis), where it is stored until osmoreceptor stimulation triggers its release into the bloodstream. NCERT names this site of release as the neurohypophysis. ADH is not made in the posterior pituitary; the posterior pituitary only stores and releases it.

What activates the JG cells to release renin?

Three stimuli converge on the juxtaglomerular cells of the afferent arteriole. A fall in glomerular blood flow, glomerular blood pressure or glomerular filtration rate stretches the JG cells less and triggers renin release. A drop in NaCl delivery to the macula densa cells of the distal tubule provides a second tubular signal. Sympathetic activation of beta-1 adrenergic receptors on the JG cells provides a neural signal. NCERT lists the first cluster — fall in glomerular blood flow, blood pressure or GFR — as the trigger for the renin-angiotensin mechanism.

What does angiotensin II do?

Angiotensin II is the active octapeptide of the cascade. It is a powerful vasoconstrictor — it constricts systemic arterioles and, to a lesser extent, the efferent arteriole of the glomerulus, raising glomerular blood pressure and GFR. It activates the zona glomerulosa cells of the adrenal cortex to release aldosterone, which then increases Na+ and water reabsorption from the distal parts of the tubule. Angiotensin II also stimulates ADH release and triggers thirst. The combined effect is restoration of blood volume and blood pressure.

Where is ANF released from and what is its effect on the kidney?

Atrial natriuretic factor is a peptide hormone released by stretched myocardial cells in the atria of the heart, particularly the right atrium, when blood volume or blood pressure rises. ANF causes vasodilation, which lowers systemic arterial pressure. At the kidney it promotes loss of Na+ (natriuresis) and water in the urine and it inhibits renin release and aldosterone secretion. ANF therefore acts as a physiological brake on the renin-angiotensin mechanism, opposing its salt-and-water-retaining effects.

Why does a decrease in blood pressure not release ANF?

ANF is released only when the atria are over-distended by a rise in blood volume or venous return. A fall in blood pressure or blood volume produces the opposite — reduced atrial stretch — so ANF is not released. The same fall does, however, stimulate ADH (via osmoreceptors and baroreceptors), renin (via JG cells) and downstream aldosterone. This is the principle tested in the NEET 2017 question on which hormone is not released when blood pressure or volume falls.

Kidney Regulation — ADH, RAAS & ANF — NEET Notes

Q: How does ADH prevent diuresis?

ADH binds V2 receptors on the basolateral membrane of cells lining the late distal convoluted tubule and the collecting duct. This signals the insertion of aquaporin-2 water channels into the luminal membrane. The wall, which was nearly water-impermeable, becomes water-permeable, so water moves osmotically out of the filtrate into the hyper-osmotic medullary interstitium maintained by the counter-current mechanism. The filtrate left behind is concentrated and the volume of urine falls — diuresis is prevented. Through V1 receptors on arteriolar smooth muscle, ADH also causes mild vasoconstriction, raising blood pressure and glomerular flow.

NCERT grounding

NCERT Class 11, Chapter 16, Section 16.5 lays the entire content of this subtopic in five short paragraphs. The chapter states that the functioning of the kidneys is "efficiently monitored and regulated by hormonal feedback mechanisms involving the hypothalamus, JGA and to a certain extent, the heart." Each of the three players named in that sentence corresponds to one hormonal axis: the hypothalamus to ADH, the juxtaglomerular apparatus (JGA) to renin and the downstream renin-angiotensin-aldosterone cascade, and the heart to atrial natriuretic factor (ANF). NEET 2017, 2019, 2020 and 2023 have all drawn on exactly those three boxes.

"The functioning of the kidneys is efficiently monitored and regulated by hormonal feedback mechanisms involving the hypothalamus, JGA and to a certain extent, the heart."

NCERT Class 11 Biology, Section 16.5

Three regulatory loops in one frame

Every regulatory loop in this section follows the same logic. A receptor in the body senses a deviation from the set point — usually a drop in blood volume, a rise in plasma osmolarity, or a fall in GFR. An effector hormone is released that pushes the kidney to retain or release more water and salt, or that changes the diameter of blood vessels. The result restores the set point and switches the receptor signal off. Negative feedback closes the loop.

The three NCERT loops at a glance. Each row maps a receptor → hormone → kidney effect → net effect on blood pressure and urine output. Memorising the four-step arrow is more useful than memorising prose.

ADH axis

Sensor: hypothalamic osmoreceptors

Hormone: ADH (vasopressin) from neurohypophysis

Renal action: water reabsorption in late DCT & collecting duct

Side effect: mild vasoconstriction → ↑BP

Renin-angiotensin-aldosterone

Sensor: JG cells of afferent arteriole

Hormones: renin → angiotensin II → aldosterone

Renal action: Na⁺ & water reabsorption in distal tubule

Side effect: systemic vasoconstriction → ↑↑BP

ANF brake

Sensor: stretch receptors in cardiac atria

Hormone: atrial natriuretic factor (ANF)

Renal action: Na⁺ & water excretion ↑; inhibits renin

Net effect: vasodilation → ↓BP

Osmoreceptors and the ADH axis

NCERT opens Section 16.5 with the osmoreceptor-ADH loop. Osmoreceptors in the hypothalamus respond to changes in blood volume, body fluid volume and ionic concentration. When plasma osmolarity rises — for example after sweating, vomiting or limited water intake — these receptors fire and instruct the hypothalamus to release antidiuretic hormone (ADH), also called vasopressin, from the neurohypophysis (the posterior pituitary). NCERT places ADH release at this exact site, which is a frequent NEET pinning point.

ADH travels in the blood to the nephron and binds receptors on cells of the late distal convoluted tubule and the collecting duct. The wall of these segments is normally near-impermeable to water. ADH binding inserts aquaporin water channels into the luminal membrane, so water now moves osmotically out of the filtrate into the hyper-osmotic medullary interstitium. The filtrate left behind is concentrated. The NCERT statement is simply that "ADH facilitates water reabsorption from latter parts of the tubule, thereby preventing diuresis."

Two consequences of this water reabsorption are worth marking. First, urine output falls — the meaning of anti-diuretic. Second, plasma volume rises, plasma osmolarity falls back towards normal, and the osmoreceptor signal switches off. ADH release is suppressed and the feedback loop closes. NCERT puts this as: "An increase in body fluid volume can switch off the osmoreceptors and suppress the ADH release to complete the feedback."

Figure 1 — Osmoreceptor-ADH-collecting-duct loop

Figure 1. Negative-feedback control of water balance by the osmoreceptor-ADH axis. A rise in plasma osmolarity (or fall in volume) triggers ADH release from the neurohypophysis; ADH inserts aquaporins into the collecting duct, water is reabsorbed, plasma volume rises and the osmoreceptors switch off. ADH also produces a mild vasoconstrictor effect via V1 receptors, raising blood pressure and GFR.

NCERT also flags a second action: "ADH can also affect the kidney function by its constrictory effects on blood vessels. This causes an increase in blood pressure. An increase in blood pressure can increase the glomerular blood flow and thereby the GFR." This is the action that justifies the alternative name vasopressin. NEET 2023 Q.199 turned on this exact statement: option D, "ADH causes increase in blood pressure," was a correct statement (option E, "ADH is responsible for decrease in GFR," was the wrong one — ADH increases GFR by raising blood pressure).

JGA and the renin-angiotensin-aldosterone system

The juxtaglomerular apparatus (JGA) is a microscopic regulatory unit lying where the distal convoluted tubule comes back to touch the afferent arteriole of its own glomerulus. NCERT describes it as "a special sensitive region formed by cellular modifications in the distal convoluted tubule and the afferent arteriole at the location of their contact." Two cell populations dominate the JGA — modified smooth-muscle cells of the afferent arteriole called JG cells (granular cells), which store and release renin, and the chemosensitive macula densa cells of the distal tubule, which sample tubular NaCl. NEET classes this whole structure as the JGA.

NCERT states the trigger crisply: "A fall in glomerular blood flow/glomerular blood pressure/GFR can activate the JG cells to release renin." Renin is an enzyme, not a hormone in the classical sense. Once in the blood it cleaves circulating angiotensinogen (a plasma protein from the liver) to angiotensin I. As angiotensin I passes through the pulmonary circulation, angiotensin-converting enzyme (ACE) clips off two more residues to give the active octapeptide angiotensin II.

The renin-angiotensin-aldosterone cascade

NCERT Section 16.5 · NEET 2017, 2020

Step 1
↓ GFR / BP sensed

JG cells of afferent arteriole detect fall in glomerular blood flow, blood pressure or GFR.
JGA
Step 2
Renin released

JG cells release the proteolytic enzyme renin into the blood.
Enzyme, not hormone
Step 3
Angiotensinogen → AI

Renin cleaves plasma angiotensinogen from the liver to give angiotensin I (decapeptide).
Inactive
Step 4
AI → AII via ACE

Angiotensin-converting enzyme (mostly in lung capillaries) clips AI to angiotensin II.
Active octapeptide
Step 5
Effects of AII

Powerful vasoconstrictor → ↑BP, ↑glomerular pressure, ↑GFR. Stimulates adrenal cortex.
↑↑BP
Step 6
Aldosterone

Adrenal cortex releases aldosterone → Na⁺ and water reabsorption in distal parts of tubule.
↑Volume → ↑BP

NCERT names two distinct downstream effects of angiotensin II. First, it is "a powerful vasoconstrictor" — it constricts systemic arterioles, raising arterial blood pressure, and it constricts the efferent arteriole of the glomerulus more than the afferent, so glomerular capillary pressure and GFR are restored even though renal flow may not be. Second, angiotensin II "activates the adrenal cortex to release Aldosterone." Aldosterone is a steroid hormone from the zona glomerulosa of the adrenal cortex that "causes reabsorption of Na+ and water from the distal parts of the tubule." This Na⁺-with-water pull raises blood volume, which, multiplied by the vasoconstriction angiotensin II is already producing, restores both blood pressure and GFR. NCERT bundles these effects under the name Renin-Angiotensin mechanism.

Distal-tubule actions in this subtopic

ADH inserts aquaporins to reabsorb water. Aldosterone reabsorbs Na⁺ and water. ANF promotes Na⁺ and water excretion. All three converge on the distal tubule and collecting duct — they are the kidney's hormonal control surface.

Why "renin" is not the same as "rennin"

NEET style guides distinguish renin (kidney enzyme that cleaves angiotensinogen) from rennin (a gastric milk-curdling enzyme found in young mammals). NCERT spells the renal enzyme as renin throughout Section 16.5. Mis-spelling it as "rennin" in a match-the-column has cost rank points in the past; the safe rule is one n in the middle for the kidney enzyme.

Atrial natriuretic factor — the brake

The third loop counter-balances the second. NCERT states: "An increase in blood flow to the atria of the heart can cause the release of Atrial Natriuretic Factor (ANF). ANF can cause vasodilation (dilation of blood vessels) and thereby decrease the blood pressure. ANF mechanism, therefore, acts as a check on the renin-angiotensin mechanism."

ANF is a peptide hormone synthesised and stored in the atrial myocardial cells, most prominently the right atrium. When venous return rises — for example after intravenous fluid, after a high-salt meal that expands plasma volume, or in a recumbent patient — the atrial wall is stretched. Stretch-activated channels and mechanoreceptors in these cells trigger ANF release. At the kidney, ANF dilates afferent arterioles (transiently raising GFR), inhibits Na⁺ reabsorption in the medullary collecting duct, and inhibits renin secretion from JG cells. The net effect is natriuresis — loss of Na⁺ and water in the urine — and systemic vasodilation. Both lower blood pressure.

RAAS vs ANF — opposing hands on blood pressure

Renin-Angiotensin-Aldosterone

↑ BP

activated by fall in BP / GFR

Trigger: ↓ GFR, ↓ glomerular blood flow / BP at JG cells
Angiotensin II is a powerful systemic vasoconstrictor
Aldosterone → Na⁺ and water reabsorption in distal tubule
Net: blood volume up, blood pressure up, GFR restored

Atrial Natriuretic Factor

↓ BP

activated by atrial stretch (↑ volume)

Trigger: atrial wall stretch from ↑ blood volume
Vasodilator — dilates arterioles, lowers peripheral resistance
Promotes Na⁺ and water excretion (natriuresis)
Inhibits renin and aldosterone — brakes RAAS

The clinical and exam relevance of this opposition is exactly what NEET 2017 Q.129 tested. A fall in blood pressure or blood volume will not release ANF — because ANF needs atrial over-distension. The same fall does release ADH, renin and (downstream) aldosterone. Knowing the direction of the stimulus for each hormone is the single most reliable lever for NEET multi-statement questions on Section 16.5.

Integrated response to volume loss

Picture a marathon runner late in a race on a hot day. Cumulative water loss in sweat has reduced plasma volume by several per cent and the remaining plasma is hyper-osmotic from concentrated electrolytes. Three loops fire at once.

Coordinated response — falling plasma volume

all three NCERT axes acting together

Sensor
Hypothalamus

Osmoreceptors fire (↑ plasma osmolarity).
→
ADH released

Water reabsorbed at collecting duct; urine concentrated; mild vasoconstriction.
Sensor
JG cells

Detect ↓ glomerular blood flow, ↓ pressure, ↓ GFR.
→
RAAS fires

Renin → AI → AII → vasoconstriction + aldosterone → Na⁺ & H₂O retained.
Sensor
Atria

Less stretch — ANF release is suppressed; brake is off.
Out
Net

Plasma volume restored, BP restored, urine minimum-volume, maximally concentrated.

Once the runner finishes and drinks several litres of water, the polarity flips. Plasma osmolarity falls, osmoreceptors are switched off, ADH release stops and the collecting duct becomes water-impermeable again, allowing a flood of dilute urine. Plasma volume rises, the atrial wall is stretched, and ANF is released. Systemic vasodilation lowers blood pressure. JG cells, which see normal GFR again, stop releasing renin; aldosterone falls; Na⁺ is excreted. Within a few hours the body is back at its set point. This integrated picture is the way NEET examiners increasingly write multi-statement questions on Section 16.5.

Figure 2 — Three regulatory loops on one nephron

Figure 2. A single nephron with the three regulatory loops overlaid. ADH acts on aquaporin-mediated water reabsorption at the late DCT and collecting duct. Aldosterone reabsorbs Na⁺ together with water from the distal tubule. ANF promotes Na⁺ and water excretion at the collecting duct and inhibits renin release at the JGA. Renin is released by JG cells of the afferent arteriole when GFR or glomerular blood pressure falls.

Worked examples

Worked example 1

A patient loses 2 litres of plasma in haemorrhage. State the direction (↑/↓/no change) in which each of the following changes within the next thirty minutes: (i) ADH release, (ii) renin release, (iii) angiotensin II in plasma, (iv) aldosterone, (v) ANF release, (vi) urine volume.

Plasma volume falls and plasma osmolarity rises slightly (haemorrhage loses isotonic plasma, but the body responds to both volume and osmolarity sensors). (i) ADH release ↑ — osmoreceptors fire and baroreceptors potentiate release. (ii) Renin release ↑ — JG cells detect a fall in glomerular blood flow and pressure. (iii) Angiotensin II ↑ — substrate (angiotensinogen) is constitutive, renin is the rate-limiting step. (iv) Aldosterone ↑ — secondary to angiotensin II's action on the adrenal cortex. (v) ANF ↓ — atria are under-filled, no stretch, no ANF. (vi) Urine volume ↓ — concentrated by ADH and reduced by Na⁺-and-water reabsorption from aldosterone.

Worked example 2

Which one of these is NOT a direct action of angiotensin II? (a) Vasoconstriction of systemic arterioles. (b) Stimulation of aldosterone release from adrenal cortex. (c) Insertion of aquaporins in collecting-duct cells. (d) Stimulation of thirst.

Option (c). Aquaporin insertion at the collecting duct is the direct downstream action of ADH, not angiotensin II. Angiotensin II is a powerful systemic vasoconstrictor (a), it stimulates the adrenal cortex to release aldosterone (b — NCERT's exact wording), and it stimulates thirst centres in the hypothalamus (d). It can secondarily enhance ADH release, but it does not act directly on aquaporins.

Worked example 3

A long-distance swimmer drinks 3 litres of pure water immediately on finishing. Within an hour, blood pressure has dropped slightly. Account for the change in renin, ANF, ADH and urine volume.

Plasma volume has risen and plasma osmolarity has fallen. Osmoreceptors are switched off, so ADH release falls; the collecting duct becomes water-impermeable and urine volume rises (a dilute diuresis). The atrial wall is stretched by the increased venous return, so ANF release rises; ANF dilates arterioles, lowers blood pressure and inhibits renin and aldosterone. JG cells therefore release less renin, angiotensin II falls and aldosterone falls. Net effect: a large dilute urine, lower BP for a short time, set point restored.

Worked example 4

NCERT (Class 11, Chapter 16, Section 16.5) names the sequence as renin → angiotensinogen → angiotensin I → angiotensin II. Identify the substrate, the enzyme of the first step, and where the second step (conversion of AI to AII) takes place.

Substrate of step 1: angiotensinogen (a plasma α₂-globulin secreted by the liver). Enzyme of step 1: renin (released from JG cells of the afferent arteriole of the kidney). The conversion of angiotensin I to angiotensin II occurs predominantly in the pulmonary capillary endothelium, catalysed by angiotensin-converting enzyme (ACE). NCERT names the cascade but does not name ACE; competitive exams treat ACE as standard application.

Kidney Regulation — ADH, RAAS & ANF