1. Ultrafiltration:
Walls of glomerular capillaries and Bowman’s capsule are very thin and semipermeable in nature. Hence, they act as ultrafilters.
The glomerular hydrostatic pressure or the capillary pressure is the main driving force that tends to move fluid out of the glomeruli. It is exerted by the blood while passing through the glomerulus.
Due to this high filtration pressure in the blood of glomerulus, a part of water and dissolved constituents of blood like nitrogenous wastes, glucose, amino acids, mineral ions, etc., are filtered out in the Bowman’s capsule. This forms the glomerular filtrate. Filtration under high pressure is called ultrafiltration.
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During ultrafiltration, nearly all the liquid component of blood comes out of glomerulus and passes into the Bowman’s capsule. The rest of the blood, left behind in the glomerulus after ultrafiltration is very thick (because of the liquid part has been removed) and is carried forward by the efferent arteriole. The glomerular filtration rate in adults is 120 mL per minute and the filtrate produced daily amounts to about 172 litres.
2. Tubular Reabsorption or Selective Absorption:
Our body cannot afford to lose several useful substances that are filtered in the glomerular filtrate. As this filtrate flows through the nephric tubule, several useful substances are reabsorbed through the wall of the renal tubule (nephric tubule) and returned to the blood flowing in peritubular capillaries. Water is reabsorbed by osmosis. Selective reabsorption of glucose, amino acids and salts also takes place.
i. In the PCT, 65-80 per cent of the filtrate is reabsorbed. It includes water, glucose, amino acids and minerals like Na\ K+ and Ca++.
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ii. In the descending limb, 5 per cent water is reabsorbed.
iii. In the ascending limb, only minerals (Na+, K+, and Ch) are reabsorbed. The ascending limb is impermeable to water.
iv. In the DCT and collecting ducts, Na+ is reabsorbed under the influence of hormone aldosterone and water is reabsorbed under the influence of hormone ADH (Antidiuretic hormone).
3. Tubular Secretion:
This process is the converse of tubular reabsorption. The cells of the renal tubule also remove wastes from blood and pass into the filtrate by the process of secretion. Tubular secretion removes ammonia, urea, uric acid, creatinine and hippuric acid from the blood.
4. Hormonal Control of Urine Formation:
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Urine formation is under the control of two hormones aldosterone and antidiuretic hormone (ADH).
i. Antidiuretic hormone (ADH) released from the posterior lobe of pituitary gland, increases the permeability of the renal tubule to water. When the blood plasma is concentrated, more ADH is secreted and more water is reabsorbed in the urinary tubules.
Hence, urine passed out from body is concentrated, for example in summers. When the blood plasma is diluted, less ADH is secreted and less water is reabsorbed in the urinary tubule. In this case, dilute urine is passed out of the body, for example in winters.
ii. Aldosterone, a hormone released from the adrenal cortex, influences reabsorption of sodium and potassium. This hormone regulates the amount of sodium and potassium that is to be retained in the blood and is to be excreted.