Urine Formation — Filtration, Reabsorption, Secretion

NCERT grounding

NCERT Class 11 Biology, Chapter 16, Section 16.2 — Urine Formation — anchors this subtopic to the syllabus. The text opens with the line that all three NEET examiners come back to: "Urine formation involves three main processes namely, glomerular filtration, reabsorption and secretion, that takes place in different parts of the nephron." Filtration occurs at the glomerulus; reabsorption is distributed along the renal tubule with most of the work falling on the proximal convoluted tubule; secretion is performed by tubular epithelial cells along nearly the full length of the tubule and into the collecting duct.

"A comparison of the volume of the filtrate formed per day (180 litres per day) with that of the urine released (1.5 litres), suggest that nearly 99 per cent of the filtrate has to be reabsorbed by the renal tubules."

— NCERT Biology, Class 11, §16.2

Two arithmetic anchors do most of the work in this section. Roughly 1100–1200 mL of blood reach the glomerulus per minute, about one-fifth of cardiac output, and yield 125 mL of filtrate per minute — the glomerular filtration rate (GFR). Multiply by 1440 minutes per day and the kidneys filter 180 litres per day, of which 99 per cent is reabsorbed and 1.5 litres is released as urine. The three steps are the syllabus skeleton onto which the rest of the chapter — tubule functions, the counter-current mechanism, the hormonal regulators ADH, RAAS and ANF — is hung.

The three steps in sequence

Think of the nephron as a single processing line with three operations bolted onto it in series. The first operation, glomerular filtration, is bulk and indiscriminate — it pushes a large volume of plasma minus its proteins into Bowman's capsule. The second, tubular reabsorption, is highly selective and works in the opposite direction, recovering useful solutes and most of the filtered water back into the peritubular blood. The third, tubular secretion, is a targeted in-flow that adds H⁺, K⁺, ammonia and a long list of drug metabolites to the filtrate. Filtration is one-way and crude; reabsorption is one-way and refined; secretion is one-way and corrective. Their net effect is a small volume of urine with composition tuned to the body's homeostatic needs.

All three steps run continuously and simultaneously, but on a single nephron each segment specialises. The renal corpuscle (glomerulus plus Bowman's capsule) does only filtration. The proximal convoluted tubule (PCT) does the bulk of reabsorption and a smaller share of secretion. The loop of Henle reabsorbs water from the descending limb and electrolytes from the ascending limb, contributing little to secretion. The distal convoluted tubule (DCT) does conditional reabsorption of Na⁺ and water plus targeted secretion of H⁺, K⁺ and NH₃. The collecting duct fine-tunes water reabsorption under ADH control and continues secreting H⁺ and K⁺. The overview that follows treats the three operations as discrete steps; the deep per-segment view sits in the sibling page below.

Step 1 · Glomerular filtration — ultrafiltration across three layers

The first step is mechanical. Blood entering the glomerulus through the wide afferent arteriole meets a narrower efferent arteriole on its way out; the size mismatch raises glomerular capillary hydrostatic pressure to roughly 50–55 mm Hg. That pressure is high enough to push plasma — but not red cells, white cells, platelets or plasma proteins — out of the capillary lumen, across the filtration barrier and into Bowman's space. Because the driving force is purely hydrostatic and the membrane is finely porous, NCERT calls this process ultrafiltration.

The filtration barrier has three layers, listed by NCERT in this exact order from blood to filtrate. The first is the fenestrated endothelium of the glomerular capillaries, peppered with 70–90 nm pores that bar formed elements but pass plasma. The second is a continuous glomerular basement membrane (GBM) made of type IV collagen and negatively charged glycoproteins; its mesh excludes molecules above roughly 70 kDa and its negative charge repels anionic plasma proteins such as albumin. The third is the visceral epithelium of Bowman's capsule, formed by specialised cells called podocytes. Podocytes wrap their interdigitating foot processes around the capillaries, leaving narrow gaps called filtration slits or slit pores that are bridged by a thin slit diaphragm.

The net filtration pressure across this barrier is what NCERT summarises as "glomerular capillary blood pressure." More precisely, it is the difference between the outward hydrostatic pressure inside the capillary and the inward forces opposing it — the colloid osmotic pressure of plasma proteins (because proteins are retained, plasma in the glomerulus becomes increasingly oncotic along its length) and the hydrostatic pressure inside Bowman's capsule. Glomerular hydrostatic pressure dominates this balance, which is why a fall in systemic blood pressure rapidly reduces GFR and why a renal artery stenosis can suppress filtration.

GFR and JGA autoregulation

Glomerular filtration rate (GFR) is defined by NCERT as "the amount of the filtrate formed by the kidneys per minute" and is approximately 125 mL min⁻¹ in a healthy adult, i.e. about 180 litres per day. The figure is examined directly as numerical recall and indirectly through factors that change it. Because both kidneys filter such a vast volume, even a modest change in net filtration pressure rapidly alters body fluid composition, so GFR must be held within narrow limits.

The headline regulator is the juxtaglomerular apparatus (JGA) — a "special sensitive region formed by cellular modifications in the distal convoluted tubule and the afferent arteriole at the location of their contact." When systemic pressure drops, glomerular blood flow falls, and JG cells in the wall of the afferent arteriole release renin. Renin initiates the renin-angiotensin cascade that ends in angiotensin II — a powerful vasoconstrictor that raises glomerular blood pressure and so restores GFR. Autoregulation also runs locally through the myogenic response (the afferent arteriole constricts when stretched by a pressure spike) and through tubuloglomerular feedback (macula densa cells in the DCT signal afferent arteriolar tone in response to luminal NaCl). The detailed cascade is the subject of the regulation sibling page.

Step 2 · Tubular reabsorption — recovering 99 per cent of the filtrate

The kidney filters 180 litres of plasma per day but the body holds only about 3 litres of plasma at any one moment. The mismatch makes the second step non-negotiable: roughly 99 per cent of everything filtered has to be put back into the peritubular blood. NCERT calls this process reabsorption and emphasises that "the tubular epithelial cells in different segments of nephron perform this either by active or passive mechanisms."

Reabsorption is selective. Substances that the body needs are recovered nearly completely — essentially all the filtered glucose, all the filtered amino acids, all the filtered vitamins, almost all of the filtered Na⁺, K⁺, Cl⁻, HCO₃⁻, and most of the filtered water. Substances meant to leave the body — urea, creatinine, urate, sulphate — are reabsorbed only partially or not at all, so their concentration rises down the tubule. The selectivity is built into the tubular epithelium: PCT cells carry brush borders and specific carrier proteins for glucose (SGLT) and amino acids; loop cells carry Na⁺-K⁺-2Cl⁻ symporters; DCT cells carry thiazide-sensitive NaCl cotransporters; collecting duct principal cells carry ADH-controlled aquaporins.

Two mechanisms drive reabsorption. Active transport uses ATP-powered pumps — the basolateral Na⁺/K⁺-ATPase is the master pump that creates the Na⁺ gradient on which most other transport hitches a ride. Glucose, amino acids, phosphate and Na⁺ itself are reabsorbed by transporters coupled to this gradient. Passive transport moves substances down their electrochemical or osmotic gradients: water follows the osmotic pull set up by NaCl reabsorption, urea diffuses through urea transporters, and Cl⁻ often follows Na⁺ through paracellular paths. NCERT singles out the example: "substances like glucose, amino acids, Na⁺, etc., in the filtrate are reabsorbed actively whereas the nitrogenous wastes are absorbed by passive transport. Reabsorption of water also occurs passively in the initial segments of the nephron."

Step 3 · Tubular secretion — fine-tuning pH and ionic balance

Filtration and reabsorption together would still leave the body with the wrong urine. Filtration is non-selective, so anything small enough crosses; reabsorption is selective for the substances the kidney has carriers for. Some compounds — drugs, drug metabolites, organic acids, excess H⁺ and K⁺ — need to leave the body but are not adequately removed by filtration alone. The third step plugs this gap. NCERT describes tubular secretion in a single dense sentence: "the tubular cells secrete substances like H⁺, K⁺ and ammonia into the filtrate" and adds that "tubular secretion is also an important step in urine formation as it helps in the maintenance of ionic and acid base balance of body fluids."

Secretion is the mirror image of reabsorption. The same tubular epithelium that pulls Na⁺ and water out of the lumen also pushes H⁺, K⁺ and NH₃ into it. PCT cells secrete H⁺ via Na⁺/H⁺ exchangers, reclaiming filtered HCO₃⁻ as part of bicarbonate buffering. They also secrete ammonia generated from glutamine — a key route for daily acid load disposal. DCT and collecting duct intercalated cells secrete H⁺ through proton pumps (α-cells) or HCO₃⁻ through anion exchangers (β-cells) depending on whether the body is in acidosis or alkalosis. Principal cells of the DCT and collecting duct secrete K⁺ under aldosterone control and reabsorb Na⁺ in exchange. The NEET 2025 stem on PCT and DCT secretion of H⁺, K⁺ and NH₃ tests exactly this picture.

Secretion is also the route by which the kidney clears compounds that are protein-bound in plasma and therefore filter poorly. Many drug metabolites — including penicillins and a long list of organic anions — leave the body almost entirely by tubular secretion in the PCT. NCERT does not detail this pharmacology, but the principle that "tubular secretion helps in the maintenance of ionic and acid base balance" is the conceptual umbrella under which it sits.

Putting the three steps together: the volume that leaves the body is small (~1.5 litres), the composition is tightly controlled, and the timing is continuous. The kidney does not store urine inside the nephron; final urine drains from the collecting ducts into the calyces and pelvis, down the ureters and into the urinary bladder. NCERT places that storage and release in Section 16.6 (micturition); the upstream three-step production line — filter, reabsorb, secrete — is what this page covers.

Worked examples

Common confusion & NEET traps