NCERT grounding
Section 16.1 of NCERT Class XI Biology, Excretory Products and their Elimination, defines the nephron in a single sentence that NEET treats as canon: "Each kidney has nearly one million complex tubular structures called nephrons, which are the functional units." The same paragraph divides every nephron into two parts — the glomerulus and the renal tubule — and then unfolds the tubule into the Bowman's capsule, proximal convoluted tubule, Henle's loop, distal convoluted tubule and collecting duct. NCERT also fixes the cortex–medulla geometry: the Malpighian corpuscle, PCT and DCT sit in the cortex, while the loop of Henle dips into the medulla. The juxtaglomerular apparatus is introduced in §16.2 as a regulator of glomerular filtration rate, but it lives on the nephron and is therefore structural too.
Anatomy of a single nephron
A nephron is a long, looped tubule with a filtering bulb at one end and an opening into a collecting duct at the other. Functionally, it carries out three operations — ultrafiltration, reabsorption and secretion — distributed along its length. Structurally, NCERT splits it into the renal corpuscle (the filter) and the renal tubule (the processor). Both sit inside the same kidney lobe, but the parts of the tubule are partitioned between cortex and medulla in a way that NEET tests as a near-annual fact. This section walks through each component in the order filtrate flows.
Nephrons per kidney
NCERT § 16.1 states "nearly one million complex tubular structures" per kidney. This is the structural and functional unit count that appears repeatedly in PYQ stems and assertion–reason items.
The renal corpuscle (Malpighian body)
The renal corpuscle is the head of the nephron — a spherical filter that lies entirely within the renal cortex. It has two components fused into one structure: a tuft of capillaries called the glomerulus and a surrounding double-walled cup called Bowman's capsule. NCERT states that "Glomerulus alongwith Bowman's capsule, is called the malpighian body or renal corpuscle." Blood enters the glomerulus through an afferent arteriole — a fine branch of the renal artery — and leaves through an efferent arteriole. The efferent then breaks up again to form the peritubular network and, in juxtamedullary nephrons, the vasa recta.
The geometry of the two arterioles is itself a NEET fact: the efferent arteriole has a smaller diameter than the afferent, which throttles outflow and forces glomerular capillary blood pressure upward. That elevated pressure is what drives ultrafiltration across the three filtration layers — the capillary endothelium, the basement membrane, and the visceral epithelium of Bowman's capsule made of podocytes whose interlocking pedicels leave filtration slits through which plasma escapes into the capsular space.
Figure 1. Schematic of a juxtamedullary nephron. Renal corpuscle (cortex), PCT, descending and ascending limbs of the loop of Henle (medulla), DCT and collecting duct. Note the afferent arteriole drawn wider than the efferent, the macula densa contact at the JGA, and the vasa recta running parallel to the loop. Cortical nephrons have the same parts but a short loop and absent or reduced vasa recta.
Proximal convoluted tubule (PCT)
Immediately after Bowman's capsule the tubule throws itself into a tight coil called the proximal convoluted tubule. It is the longest segment of the nephron and lies entirely within the renal cortex. NCERT § 16.3 fixes two histological facts that NEET asks repeatedly: the PCT is lined by simple cuboidal epithelium bearing a dense apical brush border of microvilli that multiplies the absorptive surface area. Functionally — although structure is our subject — this lining reabsorbs nearly all glucose and amino acids, 70–80 percent of electrolytes and water, and selectively secretes H⁺ and ammonia into the filtrate.
Loop of Henle
Beyond the PCT the tubule straightens, narrows, and plunges into the medulla as the hairpin-shaped loop of Henle. NCERT explicitly distinguishes the two limbs by their wall and permeability properties. The descending limb is thin-walled and permeable to water but almost impermeable to electrolytes; as filtrate moves downward into an increasingly hyperosmotic medullary interstitium, water leaves and the filtrate concentrates. The hairpin turns, and the ascending limb, which becomes thick-walled, is impermeable to water but actively or passively extrudes NaCl into the interstitium; this is the engine of the corticomedullary osmotic gradient. By the time the filtrate re-enters the cortex it is dilute relative to plasma.
Distal convoluted tubule (DCT)
The ascending limb continues as another highly coiled tubular region — the distal convoluted tubule — which, like the PCT, lies in the cortex. It is shorter than the PCT and its epithelium lacks the dense brush border. The DCT is the site of conditional reabsorption of Na⁺ and water under aldosterone and ADH, and it selectively secretes H⁺, K⁺ and NH₃ to fine-tune blood pH. Structurally, the DCT is where the nephron returns to the vicinity of its own glomerulus and brushes against the afferent arteriole — the contact zone that NCERT calls the juxtaglomerular apparatus.
Collecting duct
DCTs of many nephrons drain into a common straight tube called the collecting duct. NCERT describes it as a long duct that "extends from the cortex of the kidney to the inner parts of the medulla." Many collecting ducts then converge and open into the renal pelvis via the medullary pyramids and calyces. Although strictly speaking it is shared between nephrons rather than part of any single nephron, the collecting duct is treated as the nephron's terminal segment for NEET purposes because the final concentration of urine — under the action of ADH and the medullary osmotic gradient — happens here.
Cortical vs juxtamedullary nephrons
Not every nephron is built the same way. NCERT names two structural classes based on where the renal corpuscle sits and how far the loop of Henle reaches. Cortical nephrons account for roughly 85 percent of the population — their corpuscles lie in the outer cortex and their loops of Henle extend only a little way into the medulla. Juxtamedullary nephrons, the remaining ~15 percent, sit close to the corticomedullary junction; their long loops plunge deep into the inner medulla and are paralleled by a well-developed vasa recta. Because urinary concentration depends on the medullary osmotic gradient, the juxtamedullary nephrons are the ones that make concentrated urine possible — a point NEET 2024 (Q.189) tested directly.
Cortical nephron
~85%
of all nephrons
- Renal corpuscle in the outer cortex
- Loop of Henle short — barely enters medulla
- Vasa recta absent or highly reduced
- Primary role: filtration and routine reabsorption
Juxtamedullary nephron
~15%
of all nephrons
- Renal corpuscle deep in cortex, near medulla
- Loop of Henle long — runs deep into inner medulla
- Vasa recta well developed, parallel to Henle
- Primary role: counter-current concentration of urine
Blood supply — peritubular capillaries and vasa recta
The efferent arteriole leaving the glomerulus does not rejoin the venous circulation immediately; instead it breaks into a second capillary bed wrapped around the renal tubule — the peritubular capillaries. NCERT describes this as a "fine capillary network around the renal tubule." In juxtamedullary nephrons, a slender U-shaped vessel called the vasa recta runs in parallel to the loop of Henle deep into the medulla. The flow of blood in vasa recta is counter-current to filtrate flow in the adjacent Henle's loop. NCERT pins down the key fact: vasa recta is absent or highly reduced in cortical nephrons.
Juxtaglomerular apparatus (JGA)
The juxtaglomerular apparatus is a small but conceptually load-bearing piece of the nephron. NCERT § 16.2 introduces 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 matter: the macula densa — tightly packed columnar cells of the DCT wall where it touches its own glomerulus — and the JG cells — modified smooth muscle cells in the wall of the afferent arteriole. A fall in glomerular filtration rate triggers JG cells to release renin, which initiates the renin–angiotensin cascade and restores GFR.
Reading rule. Every part of the nephron has a position (cortex vs medulla), a lining cell type, and a permeability signature. NEET stems usually attack one of those three facets.
Renal corpuscle
Position: cortex.
Walls: capillary endothelium + basement membrane + podocyte epithelium with filtration slits.
Vessels: wide afferent in, narrower efferent out.
PCT
Position: cortex.
Lining: simple cuboidal with apical brush border (microvilli).
Function tag: bulk reabsorption + selective H⁺/NH₃ secretion.
Loop of Henle
Position: medulla.
Descending: thin, water-permeable, electrolyte-impermeable.
Ascending: thick, water-impermeable, NaCl out.
DCT + collecting duct
Position: DCT in cortex; collecting duct spans cortex to inner medulla.
Control: aldosterone and ADH set water and Na⁺ reabsorption.
JGA contact: DCT touches own afferent arteriole.
Figure 2. Renal corpuscle and the juxtaglomerular apparatus. The afferent arteriole is drawn visibly wider than the efferent — the diameter mismatch is what raises glomerular capillary pressure. The DCT loops back to touch its parent afferent arteriole at the macula densa; modified smooth muscle cells in the arteriole wall here are the renin-secreting JG cells.
Worked examples
Which segment of the nephron lies entirely in the renal medulla?
The loop of Henle. NCERT § 16.1 places the Malpighian corpuscle, PCT and DCT in the cortex, while the loop of Henle "dips into the medulla." The collecting duct also enters the medulla but spans cortex to inner medulla, so it is not "entirely" medullary. Both limbs of the loop of Henle, in contrast, are medullary structures.
In a cortical nephron, which of the following is absent or highly reduced — peritubular capillaries, vasa recta, glomerulus, or Bowman's capsule?
The vasa recta. NCERT states unambiguously: "Vasa recta is absent or highly reduced in cortical nephrons." The glomerulus, Bowman's capsule and a peritubular capillary network are present in every nephron irrespective of class. The vasa recta is the U-shaped capillary that parallels a long loop of Henle and therefore exists meaningfully only in juxtamedullary nephrons.
Why is the glomerular capillary blood pressure higher than in most capillary beds?
Because the glomerulus is fed by a wide afferent arteriole and drained by a narrower efferent arteriole. The mismatch in diameters creates a high outflow resistance, so pressure rises inside the capillary tuft. This elevated hydrostatic pressure is the driving force for ultrafiltration across the three filtration layers — capillary endothelium, basement membrane, and the podocyte epithelium of Bowman's capsule.
Where exactly does the juxtaglomerular apparatus form?
At the point of contact between the distal convoluted tubule and the afferent arteriole of the same nephron. NCERT § 16.2 defines the JGA as "a special sensitive region formed by cellular modifications in the distal convoluted tubule and the afferent arteriole at the location of their contact." The DCT's contact cells form the macula densa; the afferent arteriolar wall provides renin-secreting JG cells.