HSEB Notes on Urine Formation
Lesson : Excretion and Osmoregulation
Class : 12
Three basic steps 1.Glomerular filtration or ultra filtration 2. Selective reabsorption 3. Tubular secretion
Glomerular filtration smaller diameter of efferent arteriole than that of afferent arteriole increases the blood pressure in Glomerulus. Hydrostatic pressure of about 55 mm of Hg is developed which can force out the different substances of blood into the lumen of Bowman’s capsule. This is assisted by the thin and closely applied walls of Bowman’s capsule and walls of capillaries. The hydrostatic pressure is opposed by osmotic pressure of about 30 mm of Hg and filtrate pressure of about 15 mm of Hg. The net filtration pressure is of about 10 mm of Hg.
About 125 ml of Glomerular filtrate is formed from 1200 ml of blood by both kidneys every minute. In one day 180 liters of glomerular filtrate is formed. The glomerular filtrate is composed of water, glucose, amino acids, urea, uric acid, hormones vitamins mineral salts etc. The composition of filtrate is similar to blood except in having larger molecules of protein.
Selective reabsorption of 180 liters of glomerular filtrate per day, only 1.5 liter is put out as urine. 99 % of filtrate is reabsorbed. Out of 125 ml produced every minute 124 ml is returned and 1 ml is put as urine. Active absorption usually glucose, amino acids are returned by active absorption. It takes place against the gradient at the expense of energy. Passive absorption normally Sodium chloride, water, urea, ammonia, ketone bodies reabsorbed by osmosis.
At the PCT, the filtrate is isotonic. From the descending limb water is returned so as the filtrate comes down it becomes hypertonic. At the ascending limb, as it impermeable to water and as salts are removed actively, the filtrate becomes hypotonic again. Much water is removed from the DCT and collecting tubule, the urine becomes hypertonic finally. Tubular secretion If the blood in the capillaries at the DCT contains any urea, uric acid, creatinine, hippuric acid K+ , H+ , ammonia etc these can be directly put into the tubule. K+ is exchanged for Na+.
Composition of urine normally urine does not contain glucose and amino acids. It contains 95 % of water; rest of the 5 % is solute. The largest component by weight other than water is urea which account for 2 %, salts 2 % approx, 0.3 % of uric acid and traces of creatinine, creatine, ammonia etc. Urine is slightly acidic about pH 6. urochrome gives the yellowish color to the urine. The characteristic odor is due to urinod. If the urine is allowed to stand for short time also, it gives the strong smell of ammonia due to action of bacteria to change urea into ammonia.
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