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Urine formation looks like a list of names: glomerulus, PCT, loop of Henle, DCT, collecting duct. Questions on it test something else. They ask which segment is permeable to water, why the filtrate gets diluted on its way up, or why GFR and urine volume are so different. The way to answer them is to follow one drop of filtrate through the nephron and ask what happens to it at each step. This post does that, checks the main numbers with a calculation, lists the errors that cost marks, and gives you a short practice routine.
What happens during glomerular filtration?
Filtration is the first step, and it is not selective. NCERT's chapter on excretory products and their elimination explains that glomerular capillary blood pressure pushes blood through three layers: the endothelium of the glomerular capillaries, the epithelium of Bowman's capsule, and a basement membrane between them. The podocytes of Bowman's capsule leave tiny gaps called filtration slits. Almost everything in plasma passes through except the proteins, which is why the process is called ultrafiltration.
The kidneys filter about 1100 to 1200 ml of blood a minute, roughly one-fifth of the blood pumped out by each ventricle. The volume of filtrate formed per minute is the glomerular filtration rate (GFR), about 125 ml a minute in a healthy person. A region called the juxtaglomerular apparatus (JGA) helps keep GFR steady: a fall in GFR makes its cells release renin, which raises glomerular blood flow and brings GFR back to normal.
Worked example: how much of the filtrate is reabsorbed?
NCERT says a GFR of 125 ml a minute equals 180 litres a day, and that a person passes about 1.5 litres of urine a day. Check both claims yourself.
Minutes in a day: 60 × 24 = 1440.
Filtrate per day: 125 ml × 1440 = 180,000 ml = 180 litres.
Fraction excreted: 1.5 ÷ 180 = 0.0083, or about 0.8 per cent.
Fraction reabsorbed: 100 − 0.8 = 99.2 per cent. This matches NCERT's statement that nearly 99 per cent of the filtrate must be reabsorbed.
The PCT's share: NCERT says the PCT reabsorbs 70 to 80 per cent of electrolytes and water. Taking the water share as 70 to 80 per cent of 180 litres gives 126 to 144 litres a day in one segment.
Two errors are common here. Students multiply by 60 × 60 instead of 60 × 24, or forget to convert millilitres to litres and get 180,000 litres. Writing units on every line catches both.
What does each part of the tubule do?
Follow the filtrate in order. All of this is from NCERT's section on the function of the tubules.
Proximal convoluted tubule (PCT): lined with brush-border cuboidal epithelium that increases surface area. It reabsorbs nearly all essential nutrients and 70 to 80 per cent of electrolytes and water. It also secretes H+ and ammonia and absorbs HCO3- to keep pH and ionic balance.
Descending limb of the loop of Henle: permeable to water but almost impermeable to electrolytes. Water leaves, so the filtrate becomes more concentrated as it moves down.
Ascending limb: impermeable to water but lets electrolytes out. The filtrate becomes more dilute as it moves up. Reabsorption here is minimal, but this limb keeps the medulla salty.
Distal convoluted tubule (DCT): conditional reabsorption of Na+ and water, plus reabsorption of HCO3- and secretion of H+, K+ and NH3.
Collecting duct: large amounts of water can be reabsorbed here to make concentrated urine. It also lets small amounts of urea into the medulla and secretes H+ and K+.
How does the kidney concentrate urine?
The loop of Henle and the vasa recta beside it carry fluid in opposite directions in their two limbs, which NCERT calls the counter-current mechanism. NaCl from the ascending limb and urea from the collecting tubule build an osmolarity gradient in the medulla, from about 300 mOsmol per litre in the cortex to about 1200 in the inner medulla. Water then leaves the collecting duct into this salty tissue. The ratio 1200 ÷ 300 = 4 matches NCERT's statement that human urine can be nearly four times as concentrated as the initial filtrate.
Hormones adjust the result. ADH increases water reabsorption in the later parts of the tubule, the renin-angiotensin mechanism raises blood pressure and GFR, and atrial natriuretic factor acts as a check on it. Our post on hormonal feedback loops explains how the ADH loop switches itself off.
What is tubular secretion for?
Secretion moves substances from the blood into the filtrate. NCERT names H+, K+ and ammonia, and says secretion helps keep the ionic and acid-base balance of body fluids. If a question asks how the kidney helps control blood pH, secretion of H+ and reabsorption of HCO3- is the answer.
Which mistakes cost marks?
"Filtration is selective." It is not; NCERT calls it non-selective. Selection happens in reabsorption and secretion.
Swapping the limbs. Descending: water out, salts stay. Ascending: salts out, water stays. Remember that the filtrate concentrates going down and dilutes going up.
"All reabsorption is active." NCERT says glucose, amino acids and Na+ are reabsorbed actively, while nitrogenous wastes are absorbed passively and water passively in the early segments.
"ADH makes urine dilute." ADH increases water reabsorption, so urine becomes more concentrated and smaller in volume.
A practice routine for this chapter
The NEET (UG) 2026 syllabus lists urine formation, osmoregulation, renin-angiotensin, ANF, ADH and diabetes insipidus in this unit, so prepare each as a step you can explain.
Draw the nephron from memory and label each segment with one line: what goes in, what goes out.
Explain each step aloud. In a study by Chi and colleagues (1994), eighth-grade students asked to explain each line of a passage on the circulatory system to themselves gained more from pretest to posttest than students who read it twice.
Redo the calculation with different numbers, for example a GFR of 100 ml a minute.
Self-test with NCERT's end-of-chapter questions before checking answers.
If the counter-current idea still feels abstract, study it as one concept at a time. The Fermi Concept Library covers every concept in biology, each with its own lesson, so you can work through osmolarity and then the counter-current mechanism in order.
Today, draw the nephron on a blank page and write what the filtrate gains or loses at each segment.
