Tubule Functions (Reabsorption and Secretion)

NCERT grounding

NCERT Class XI Biology, Chapter 16 (Excretory Products and their Elimination), devotes section 16.3 — "Function of the Tubules" — to the four functional segments of the nephron: the proximal convoluted tubule (PCT), Henle's loop with its descending and ascending limbs, the distal convoluted tubule (DCT) and the collecting duct. Section 16.2 frames the larger logic: of the 180 litres of filtrate formed per day, "nearly 99 per cent" must be reabsorbed by these tubular segments, while tubular cells simultaneously secrete H+, K+ and ammonia to defend acid-base balance.

Segment-by-segment tubule function

Once Bowman's capsule has received the ultrafiltrate, every solute and every water molecule must be individually triaged. The tubule is not a passive drainpipe — it is a polarised epithelium with regional specialisations of membrane proteins, permeability and energy budget. Reabsorption is the bulk movement of water and solutes from the tubular lumen back into the peritubular capillary blood. Secretion is the opposite vector: substances move from the peritubular blood (or are synthesised in the tubular cell) into the filtrate. Together, these two flows sculpt the final urine.

Three principles structure the entire segment-by-segment discussion. First, the proportion of filtrate handled drops sharply from the PCT (bulk reabsorption) to later segments (fine-tuning). Second, the apical brush border, mitochondrial density and tight-junction "leakiness" of each segment match its workload. Third, hormonal control is concentrated downstream — aldosterone, ADH and atrial natriuretic factor act mostly on the DCT and collecting duct, not on the PCT.

Proximal convoluted tubule (PCT)

The PCT is the longest and most metabolically active segment, lined by a simple cuboidal epithelium with a dense apical brush border of microvilli. This brush border massively increases the surface area available for transport. PCT cells also pack their cytoplasm with mitochondria, because much of the reabsorption here is primary or secondary active transport that depends on the Na+/K+ ATPase on the basolateral membrane.

NCERT summarises the segment in a single key sentence: "Nearly all of the essential nutrients, and 70–80 per cent of electrolytes and water are reabsorbed by this segment." Practically, this means:

• Glucose — 100% reabsorbed at normal plasma concentrations (SGLT2 cotransporter at the apical membrane, GLUT2 at the basolateral surface).
• Amino acids — virtually 100% reabsorbed by family-specific Na+-coupled symporters.
• Na+ and Cl- — about 65–70% reabsorbed, mostly isotonically.
• HCO3- — about 80–90% reabsorbed via H+ secretion and intracellular carbonic anhydrase.
• Water — about 70–80% reabsorbed passively, following solutes (obligatory water reabsorption).

Alongside this bulk reclamation, the PCT also secretes H+ and ammonia into the filtrate and reabsorbs bicarbonate from it — the first line of renal pH control. Filtrate leaving the PCT is therefore reduced in volume but still iso-osmotic with plasma (~300 mOsm/L).

Loop of Henle

Henle's loop hands off most water/solute reclamation duties already done by the PCT and instead specialises in building the medullary osmotic gradient. NCERT is precise here: reabsorption is "minimum in its ascending limb," but the loop "plays a significant role in the maintenance of high osmolarity of medullary interstitial fluid." The two limbs are functionally opposite.

The net effect: very little volume is reabsorbed across the loop as a whole, but the salt dumped by the ascending limb raises medullary osmolarity from ~300 mOsm/L in the cortex to ~1200 mOsm/L at the inner medullary tip. That gradient is what the collecting duct exploits later. Detailed gradient mechanics — vasa recta exchange, urea recycling — are picked up in the sibling note linked below.

Distal convoluted tubule (DCT)

By the time filtrate reaches the DCT, the heavy lifting is done. The DCT performs conditional reabsorption — the volumes and electrolytes it reclaims depend on circulating hormone signals rather than on bulk osmotic pull. This is the segment where the body fine-tunes blood pressure, blood volume and pH from minute to minute.

NCERT lists three DCT functions: conditional reabsorption of Na+ and water (under aldosterone and, in the late DCT, ADH); reabsorption of HCO3-; and selective secretion of H+, K+ and NH3 to maintain pH and the Na+/K+ balance of blood. Aldosterone (from the adrenal cortex, downstream of angiotensin II) is the key driver — it inserts Na+ channels and Na+/K+ ATPase into late DCT cells, pulling Na+ back into blood and pushing K+ out into the filtrate.

Collecting duct

The collecting duct runs from the cortex deep into the medulla, traversing the very osmotic gradient that the loop of Henle built. This is where the body decides — under ADH command — whether to make a concentrated, low-volume urine or a dilute, high-volume one. NCERT describes the duct as the segment from which "large amounts of water could be reabsorbed... to produce a concentrated urine."

Three functions matter for NEET. First, ADH-gated water reabsorption: ADH inserts aquaporin-2 channels into the apical membrane of collecting duct principal cells, letting water flow down the osmotic gradient into the medullary interstitium and onward into the vasa recta. Second, urea recycling: the inner medullary collecting duct is permeable to urea, allowing small amounts to enter the interstitium and reinforce the gradient (a key contribution alongside NaCl). Third, fine pH and K+ control: intercalated cells of the collecting duct secrete H+ or HCO3- depending on systemic acid-base status, and continue the selective secretion of K+ begun in the DCT.

Tubular secretion overview

Reabsorption gets the headlines, but tubular secretion is what makes the urine an actively crafted fluid rather than a mere leftover of filtration. NCERT puts it crisply: "the tubular cells secrete substances like H+, K+ and ammonia into the filtrate. 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." Different segments contribute different secreted species. The PCT is the major site of H+ and NH3 secretion. The DCT and collecting duct add K+ secretion under aldosterone, plus further H+ secretion via intercalated cells.

Two pharmacological footnotes are worth knowing because they recur as one-line NEET facts. Some weak organic acids and bases (penicillin, p-aminohippuric acid, creatinine) are also actively secreted by PCT cells — which is why creatinine clearance slightly overestimates GFR. And many diuretics (loop diuretics like furosemide, thiazides) act on segment-specific transporters in the ascending limb or DCT, blocking reabsorption to increase urine volume.

Worked examples

Common confusion & NEET traps