The formation of urine is a three step process occurring within nephrons, functional units of kidneys. Each kidney contains approximately one million nephrons that filter blood, restore essential nutrients, and eliminate metabolic waste.
It has sequence: Glomerular Filtration, Tubular Reabsorption, and Tubular Secretion.
1. Glomerular Filtration
It begins in renal corpuscle, which consists of a cluster of capillaries called glomerulus surrounded by Bowman’s capsule.
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Pressure Gradient: Blood enters glomerulus under high pressure. It forces water and small solutes out of blood and into capsule space.
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Filtration membrane allows water, glucose, amino acids, and nitrogenous wastes (like urea) to pass through, but it is too small for blood cells and large proteins to escape.
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Fluid collected in Bowman’s capsule is called filtrate. It is blood plasma without proteins.
2. Tubular Reabsorption
As filtrate moves through renal tubule (starting with Proximal Convoluted Tubule), body begins restoring phase. If we excreted everything filtered in first step, we would dehydrate in minutes.
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Proximal Convoluted Tubule (PCT): This is where the reabsorption happens. Nearly all glucose and amino acids, along with about 65% of sodium and water, are pumped back into surrounding capillaries.
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Loop of Henle: It divides into renal medulla. It helps in concentrating urine by reabsorbing water (in descending limb) and salt (in ascending limb).
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Hormonal Control: In Distal Convoluted Tubule and Collecting Duct, hormones like Antidiuretic Hormone (ADH) and Aldosterone determine exactly how much more water or salt should be saved based on body’s current hydration levels.
3. Tubular Secretion
While reabsorption moves substances from the filtrate back into blood, secretion does opposite. It moves substances from blood directly into filtrate.
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Waste Removal: It remove chemicals that were too large to be filtered initially or are present in excess.
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pH Balance: idneys secrete hydrogen ions (H+) and potassium ions (K+) to maintain the body’s acid-base balance and electrolyte equilibrium.
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Once filtrate leaves collecting duct and enters renal pelvis, it is final urine.
Summarize…
| Process | Location | Action | Key Substances Moved |
| Filtration | Glomerulus | Blood -> Filtrate | Water, ions, glucose, urea |
| Reabsorption | Renal Tubule | Filtrate -> Blood | Glucose, amino acids, H2O, Na+ |
| Secretion | Renal Tubule | Blood -> Filtrate | H+, K+, drugs, ammonia |
Excretion
Final urine—consists roughly 95% water and 5% solutes (urea, creatinine, uric acid, and various ions)—flows through ureters to urinary bladder, where it is stored until it is excreted via urethra.