Urine Formation
சிறுநீர் உருவாக்கம்
Out of about 180 litres of filtrate formed each day, only about 1.5 litres leaves as urine — nearly 99% is reabsorbed. That one number explains why tubules matter more than the filter.
The idea
Urine formation involves three processes: glomerular filtration, reabsorption and secretion. On average 1100-1200 ml of blood is filtered by the kidneys per minute, about one-fifth of the blood pumped out by each ventricle. Glomerular capillary blood pressure filters blood through three layers — the endothelium of glomerular blood vessels, the epithelium of Bowman's capsule and a basement membrane between them; the epithelial cells of Bowman's capsule (podocytes) leave minute slit pores. Almost all constituents of plasma except proteins pass into the lumen of Bowman's capsule, so this is ultrafiltration. The glomerular filtration rate (GFR) in a healthy person is about 125 ml/minute, i.e. 180 litres per day. Since urine released is about 1.5 litres, nearly 99% of the filtrate is reabsorbed by the renal tubules. Glucose, amino acids and Na+ are reabsorbed actively; nitrogenous wastes are absorbed passively and water passively in the initial segments. During urine formation the tubular cells secrete substances like H+, K+ and ammonia into the filtrate; tubular secretion helps maintain the ionic and acid-base balance of body fluids, and the secretion of H+ ions keeps human urine usually acidic.
Why it matters
Filtering almost everything and then taking back what is useful lets the kidney remove any unwanted substance without needing a separate sensor for each one.
NEET lens
Numbers NEET loves: renal blood flow 1100-1200 ml/min, GFR 125 ml/min = 180 L/day, urine 1.5 L/day, ~99% reabsorbed. Three filtration layers; podocytes with slit pores; filtrate = plasma minus proteins. Active reabsorption: glucose, amino acids, Na+. Urine is acidic because H+ is actively secreted into the filtrate.
Clinical significance
Doctors estimate GFR from blood creatinine to stage chronic kidney disease. Protein in urine signals damage to the glomerular filter, because healthy glomeruli keep plasma proteins in the blood.
Introduction
Urine is formed in three steps — glomerular filtration, tubular reabsorption and tubular secretion.
Core concept
Filtration: glomerular capillary blood pressure; 3 layers — glomerular endothelium, epithelium of Bowman's capsule (podocytes, slit pores), basement membrane between them; filtrate = plasma minus proteins (ultrafiltration). Renal blood flow ~1100-1200 ml/min (~1/5 of cardiac output per ventricle). GFR ≈ 125 ml/min ≈ 180 L/day. Urine ≈ 1.5 L/day → ~99% reabsorbed. Reabsorption: active — glucose, amino acids, Na+; passive — nitrogenous wastes, water (initial segments). Secretion: H+, K+, ammonia into filtrate → ionic and acid-base balance; H+ secretion makes urine acidic. GFR is autoregulated by the JGA (see Regulation of Kidney Function).
Go deeper
Filtrate in Bowman's capsule is essentially protein-free plasma. Because glomerular pressure drives filtration, a big fall in blood pressure lowers GFR.
From Class 10
Class 10: the nephron filters blood and the tubule reabsorbs useful substances. Now learn the numbers and the three steps.