Question
Describe the structure of a nephron. Explain the three steps of urine formation — glomerular filtration, tubular reabsorption, and tubular secretion.
(NCERT Class 11, high-frequency NEET question)
Solution — Step by Step
A nephron is the structural and functional unit of the kidney. Each kidney has about 1 million nephrons. A nephron consists of:
- Bowman’s capsule (Glomerular capsule): A cup-shaped structure that encloses the glomerulus. The inner layer has specialised cells called podocytes.
- Glomerulus: A tuft of capillaries formed from the afferent arteriole. Blood is filtered here.
- Proximal Convoluted Tubule (PCT): Highly coiled tube lined with brush-border (microvilli) cells for maximum reabsorption.
- Henle’s Loop: U-shaped tube with a descending limb (thin, permeable to water) and ascending limb (thick, impermeable to water, actively transports salts).
- Distal Convoluted Tubule (DCT): Site of conditional reabsorption regulated by hormones.
- Collecting Duct: Collects urine from multiple nephrons, final concentration happens here.
Blood enters the glomerulus via the afferent arteriole (wider) and leaves via the efferent arteriole (narrower). The difference in diameter creates high hydrostatic pressure in the glomerulus.
This pressure forces water, glucose, amino acids, urea, salts, and other small molecules through the filtration membrane into Bowman’s capsule. Large molecules like proteins and blood cells are retained.
The filtrate is called glomerular filtrate. About 125 mL/min (180 L/day) is filtered — this is the Glomerular Filtration Rate (GFR).
About 99% of the glomerular filtrate is reabsorbed back into the blood. This happens along the tubule:
- PCT: Maximum reabsorption — glucose (100%), amino acids, , , , , and ~65% of water are reabsorbed here. Uses active transport and passive diffusion.
- Henle’s Loop: Descending limb reabsorbs water (by osmosis). Ascending limb actively pumps out and (no water reabsorption here — impermeable to water).
- DCT and Collecting Duct: Conditional reabsorption of water (regulated by ADH) and (regulated by aldosterone).
Certain substances are actively secreted from the peritubular capillaries into the tubular fluid. This includes:
- and ions (helps maintain blood pH and ionic balance)
- Urea, uric acid, and creatinine
- Drug metabolites and toxins
Tubular secretion mainly occurs in the PCT and DCT. It is crucial for removing substances that were not filtered at the glomerulus or need to be eliminated in greater quantities.
Why This Works
Urine formation is a three-step filtration-reabsorption-secretion system that balances efficiency with selectivity. The glomerulus casts a wide net (filtering almost everything small), and the tubules selectively recover what the body needs. Secretion acts as a safety net for substances the body must eliminate.
The counter-current mechanism in the Loop of Henle creates a concentration gradient in the medulla, which is essential for producing concentrated urine — especially when the body needs to conserve water.
For NEET, remember these numbers: GFR = 125 mL/min, daily filtrate = 180 L, daily urine output = 1.5-2 L. This means 99% reabsorption. The juxtamedullary nephrons (15%) with long loops of Henle are responsible for concentrating urine.
Common Mistake
Students often confuse the descending and ascending limbs of Henle’s loop. The descending limb is permeable to water but not salts, while the ascending limb is permeable to salts but not water. Mixing these up leads to incorrect explanations of the counter-current mechanism.
Another frequent error: writing that “all glucose is reabsorbed in the collecting duct.” Glucose reabsorption happens almost entirely in the PCT. In diabetics, when blood glucose exceeds the renal threshold (~180 mg/dL), the PCT cannot reabsorb all the glucose, leading to glycosuria (glucose in urine).