Urine formation involves three processes : glomerular filtration, tubular reabsorption and tubular secretion.
(a) Glomerular Filtration : (i) Walls of glomerular capillaries and Bowman's capsule are very thin and ‘ semipermeable. (ii) They allow water and small molecules in the blood to pass across the capillary wall through tiny spaces between the cells of the Bowman's capsule, (iii) The blood pressure is high in the glomerular capillaries because the efferent arteriole, which drains them, is narrower than the , afferent arteriole, which supplies them, (iv) This pressure is about 75 mm. Hg. in man. The net force
moving the fluid from the glomerular capillaries
called the filtration pressure, 25 mm. Hg. Filtration of fluid under pressure is termed ultrafiltration.
(v) The filtered out fluid is known as glomerular
filtrate or capsular filtrate or ultra- filtrate, (vi)
The ultrafiltrate contains sodium, potassium,
magnesium and calcium ions, glucose, amino acids
and some other useful materials such as vitamins along with urea, uric acid; ammonia and some other wastes such as creatinine and ketone bodies and a large amount of water.
(b) TubularReabsorption:FromtheBowman'scapsule, the glomerular filtrate passes into the tubule and
flows through it to the collecting duct. During this course, its composition, osmotic pressure and pH change due to selective reabsorption of materials
Proximal Convoluted Tubule : (i) The cells lining , the proximal convoluted tubule, (ii) Mitochondria
power the active transport of nutrient molecules back into the blood, (iii) Microvilli increase the surface for reabsorption, (iv) The cells reabsorption : glucose, almost all amino acids, most of the inorganic ions (Na+, K+, Cl-), much of the water as well as
some urea, uric acid and creatinine from the filtrate and secrete them back into the blood capillaries around the tubule, (v) Reabsorption takes place in two ways : back diffusion and active transport.
(B) Henle's Loop : (i) In the ascending limb of Henle's loop the remaining 25% of K+ and some Cl- and Na+ ions are actively reabsorbed, (ii) The ascending limb is impermeable to water. Therefore, no water is reabsorbed, (iii) The Na+ leaving the ascending limb passively diffuses into the descending limb because now its concentration is higher in the interstitial fluid than in the descending limb.
(C) Distal Convoluted Tubule, Collecting Tubule and Collecting Duct : These regions of the nephron actively reabsorb some Na+ from the filtrate, and in exchange excrete some K+ in the urine. Some Cl- is reabsorbed by diffusion from the DCT.
(D) Tubular Secretion: (i) Cells of the distal convoluted
tubule and collecting duct of the nephron excrete additional waste from the blood-stream into the filtrate by active transport, (ii) This process of tubular secretion adds urea, creatinine, hippuric acid, potassium and hydrogen ions (H+), ammonia (NHj) and a little uric acid, (iii) Removal of hydrogen ions and ammonia from the blood helps to maintain the pH of the blood, (iv) When the blood pressure and consequently the filtration pressure drop below a certain level, filtration stops and urine is formed by tubular secretion only