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How is urine formed in the nephron?

Class 11 BiologyNCERT chapter: Excretory Products and their Eliminationइसे हिंदी में पढ़ें

Exam-ready answer

Urine is formed in the nephron by three processes. In glomerular filtration, blood pressure filters plasma (except proteins) into Bowman's capsule. In reabsorption, tubular cells take back nearly 99 per cent of the filtrate, including glucose, amino acids, Na⁺ and water. In tubular secretion, H⁺, K⁺ and ammonia are added to the filtrate.

Explanation

Glomerular filtration is the first step. Blood pressure in the glomerular capillaries forces fluid through three layers: the endothelium of the glomerular blood vessels, the epithelium of Bowman's capsule and the basement membrane between them. Almost all plasma constituents except proteins pass into the capsule, so this is called ultrafiltration. The glomerular filtration rate (GFR) is about 125 mL per minute, or 180 litres per day.

Since only about 1.5 litres of urine is excreted daily, nearly 99 per cent of the filtrate is reabsorbed. The proximal convoluted tubule (PCT) reabsorbs nearly all essential nutrients and 70–80 per cent of electrolytes and water. Glucose, amino acids and Na⁺ are reabsorbed actively, while nitrogenous wastes move passively. The descending limb of Henle's loop is permeable to water, while the ascending limb is impermeable to water but allows electrolytes through.

The distal convoluted tubule (DCT) carries out conditional reabsorption of Na⁺ and water and also reabsorbs HCO₃⁻. Tubular cells secrete H⁺, K⁺ and ammonia into the filtrate, which helps maintain the ionic and acid-base balance of body fluids. Finally, the collecting duct reabsorbs large amounts of water to produce concentrated urine, which drains into the renal pelvis.

Points to remember

  1. Three processes: glomerular filtration, reabsorption and secretion.
  2. Filtration membrane: glomerular endothelium, Bowman's capsule epithelium (podocytes) and basement membrane; the process is ultrafiltration.
  3. GFR ≈ 125 mL/min (180 L/day); urine ≈ 1.5 L/day, so nearly 99% of the filtrate is reabsorbed.
  4. PCT reabsorbs nearly all nutrients and 70–80% of electrolytes and water; the ascending limb of Henle's loop is impermeable to water.
  5. Tubular secretion of H⁺, K⁺ and NH₃ maintains ionic and acid-base balance.

Exam tip

Draw a labelled nephron and mark where filtration, reabsorption and secretion take place; quote GFR as 125 mL/min (180 L/day).

Questions students also ask

What is glomerular filtration rate (GFR)?
GFR is the amount of filtrate formed by both kidneys per minute. In a healthy person it is about 125 mL per minute, which is about 180 litres per day. It is regulated by mechanisms such as the juxtaglomerular apparatus (JGA).
Why does urine not normally contain glucose?
Glucose passes into the filtrate during glomerular filtration, but the proximal convoluted tubule reabsorbs it completely by active transport. So the urine of a healthy person normally has no glucose. Glucose in urine (glycosuria) can be a sign of diabetes mellitus, and such a person should see a doctor.
What is the role of the collecting duct?
The collecting duct runs from the cortex to the inner medulla. It reabsorbs large amounts of water to make urine concentrated, lets small amounts of urea into the medullary interstitium to keep its osmolarity high, and secretes H⁺ and K⁺ to maintain pH and ionic balance.

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