Question
Explain the three processes of urine formation — glomerular filtration, tubular reabsorption, and tubular secretion. What is GFR, and why is it clinically important?
(NEET + CBSE Class 11 — high-weightage topic)
Solution — Step by Step
Blood enters the glomerulus (a tuft of capillaries) under high pressure. Small molecules (water, glucose, amino acids, urea, ions) pass through the glomerular membrane into Bowman’s capsule. Large molecules (proteins, blood cells) are too big to pass — they stay in the blood.
The filtrate formed is called primary urine or glomerular filtrate. About 125 mL/min of filtrate is produced — this is the GFR (Glomerular Filtration Rate), totalling about 180 litres/day.
We obviously do not urinate 180 L/day — most of this filtrate is reabsorbed.
As the filtrate flows through the nephron tubule:
| Region | What is reabsorbed | Mechanism |
|---|---|---|
| PCT (Proximal Convoluted Tubule) | ~65% water, glucose, amino acids, Na⁺, HCO₃⁻ | Active transport + osmosis |
| Loop of Henle | Water (descending), NaCl (ascending) | Creates concentration gradient |
| DCT (Distal Convoluted Tubule) | Na⁺, water (hormone-dependent) | ADH and aldosterone regulate |
| Collecting duct | Water (if ADH present) | Facultative reabsorption |
All glucose and amino acids are reabsorbed in the PCT — none should appear in final urine.
Certain substances are actively secreted from blood into the tubule:
- H⁺ ions (to maintain blood pH)
- K⁺ ions (to maintain electrolyte balance)
- Urea and uric acid (extra waste)
- Drug metabolites and toxins
This happens mainly in the PCT and DCT. Secretion is the opposite of reabsorption — it adds substances TO the urine.
Nephron Function Flowchart
flowchart TD
A["Blood enters glomerulus"] -->|"High pressure filtration"| B["Glomerular filtrate in Bowman's capsule"]
B -->|"Contains: water, glucose, urea, ions"| C["PCT: reabsorbs 65% water, all glucose, amino acids"]
C --> D["Loop of Henle: creates medullary gradient"]
D -->|"Descending: water out"| E["Ascending: NaCl out"]
E --> F["DCT: hormone-regulated reabsorption + secretion"]
F --> G["Collecting duct: final water reabsorption via ADH"]
G --> H["Final urine: ~1.5 L/day"]
A -.->|"GFR = 125 mL/min = 180 L/day"| B
H -.->|"99% of filtrate reabsorbed"| I["Only 1% becomes urine"]Why This Works
Urine formation is a three-step process: first, filter everything (non-selective), then reabsorb what the body needs (selective), and finally secrete extra waste. This is more efficient than a selective filtration system because the kidney does not need to “know” every toxic substance — it filters everything and then selectively takes back only the useful molecules.
The 180 L/day filtration with 99% reabsorption means the blood is cleaned thoroughly. A drop in GFR indicates kidney disease — this is why GFR is the primary clinical marker for kidney function.
Common Mistake
Students say “kidneys filter 1.5 litres of blood per day” because we produce about 1.5 L of urine. The kidneys actually filter about 180 litres per day — 99% is reabsorbed. Also, if glucose appears in urine (glycosuria), it means blood glucose has exceeded the reabsorption capacity of the PCT (renal threshold ~180 mg/dL) — this is a sign of diabetes mellitus. NEET tests this concept regularly.