The whole process of mechanism of urine formation occurs in three stages and these are:-
(i) Glomerular ultra filtration
(ii) Tubular reabsorption
(iii) Tubular secretion.
Explanation of the mechanism of urine formation are given below
The first step in urine formation is the glomerular filtration. The water, inorganic and organic compounds of low molecular weight present in the plasma are filtered by the wall of the capillaries of the glomerulus as the capillaries are highly permeable. The filtration occurs th’Jugh three layers i.e. the endothelium of glomerular capillaries, the epithelium of Bowman’s capsule and the basement membrane between these two layers. Water, inorganic salts viz. Na+, K+, Cl-, HCO3- and organic compounds such as urea, uric acid, glucose, amino acids can pass through the wall of the capillaries very easily except plasma protein, RBC and WBC. This is called the glomerular flltrate. Chemically this fluid has almost similar components of plasma but without any plasma protein.
The glomerular filtrate is formed mainly due to hydrostatic pressure of the glomerular capillaries which is 70-90 mm. Hg. It also depends upon the oncotic pressure of the plasma which is about 30 mmHg. and the volume of fluid pressure within the glomerular capsules is about 15 mm. Hg. Therefore, the pressure realizing the glomerular filtration is the difference between glomerular capillary pressure and sum total of oncotic pressure and intracapsular pressure of fluid. Hence the filtration pressure is 70 – (30 + 15) = 25 mm. Hg.
Read about The human excretory system
The amount of filtrate formed by the kidneys per minute is called glomerular filtration rate (GFR). About 130 ml. of glomerular filtrate is formed per minute in both the kidneys which amounts to l70~180 litres in a day. This large filtration capacity is possible because of rich blood supply to the kidneys, the specialized structure and the large filtration surface of the glomerular capillaries and also the relatively high blood pressure within them. However, this fluid is greatly reduced and modified and the amount of urine excreted per day is only 1.5 litres.
Nearly 99 per cent of the filtrate has been reabsorbed by the renal tubules before it reaches the ureter. This is possible because large amount of water of the filtrate with some inorganic and organic ions are reabsorbed as the fluid passes through the tubule. This is called reabsorption. Glucose, amino acids, Na+ etc. of the filtrate are reabsorbed in the renal tubules by active absorption while nitrogeneous wastes and to some extent water are absorbed passively in the initial segments of the renal tubule.
The kidneys have an elaborate mechanism of controlling the glomerular filtration rate. Juxta glomerular apparatus (JGA), a special sensitive region formed by the cellular modifications in the distal convoluted tubules and the afferent arterioles control the glomerular filtration rate. These cells of JGA stimulate the secretion of renin if the GFR falls below normal which causes more blood flow to the glomerular and thereby increasing the glomerular filtration rate.
To reduce the volume of the urine as mentioned above, most of the glomerular filtrate is absorbed in the renal tubules. The epithelial cells of the different parts of the renal tubule reabsorbed the filtrate either by active or passive mechanisms. Glucose, amino acids, Na+ etc. are reabsorbed actively while the nitrogenous wastes and water are absorbed by passive mechanism. The reabsorption of substances from the glomerular filtrate into the blood differs in different segments of the nephron. Some parts of the tubule also secrete H+, K+ and ammonia into the filtrate and thereby maintaining the ionic and acid balance of the body.
This is the process of mechanism of urine formation