Ans: Urine Formation: is an aqueous solution containing water (96%), urea (2%) and other dissolved substances (2%). The process of urine formation involves glomerular filtration, selective reabsorption and tubular secretion.
1. Glomerular filtration: Blood containing nitrogenous wastes flows into the glomerulus through afferent arteriole with a pressure of 60 mm Hg. It is opposed by Bowman's capsular hydrostatic pressure of 18 mm Hg and glomerular blood colloidal osmotic pressure of 32 mm Hg (18 + 32 = 50 mm Hg). The net filtration pressure is 10 mm Hg in the glomerular capillaries. It causes the filtration of blood through filtration membrane. This process of pressure filtration through glomerular capillaries in Bowman's capsule is known as glomerular filtration.
Filtration membrane is formed by three layers, namely, endothelial cell layer of glomerular capillaries, basement membrane and layer of podocytes of Bowman's capsule.
Blood is filtered through minute pores and fenestrae of filtration membrane due to net filtration pressure (NFP) and the process is also called ultra filtration which is a non energy consuming (i.e., passive) process.
RBC, WBC and plasma proteins having high molecular weight are unable to pass out during glomerular filtration.
The resultant filtrate is called primary urine or glomerular filtrate. It is hypotonic to cortical fluid. Glomerular filtrate contains water, amino acids, glucose, salts of Na, K, Ca, Mg etc., urea creatinine, toxins etc., The glomerular filtrates passes through the renal tubule where selective reabsorption and tubular secretion takesplace.
2. Selective reabsorption: The cells of renal tubule absorb the essential substances from the glomerular filtrate by active transport, passive transport etc. The process of this absorption is unregulated in PCT and LOH and is regulated in DCT.
a. In PCT: The cuboidal cells with brush boarders in PCT absorb about 70% - 80% electrolytes and water along with all nutrients (glucose, amino acids etc.,). Water is absorbed by osmosis (obligatory water absorption). Na+ ions, glucose, amino acids and other essential substances are absorbed by active transport. To maintain pH and ionic balance of body fluids, cells of PCT secretes H+ and ammonia into the glomerular filtrate. At the end of PCT, glomerular filtrate is isotonic to cortical fluid.
b. In LOH:
i. In descending limb: In this part about 15% of water is absorbed by osmosis. Little amount of urea diffuses into the descending limb and thus the concentration of glomerular filtrate increases and is isotonic to medullary fluid. ii. In ascending limb: This part is impermeable to water Na+, Cl−, Mg++, Ca++,
K+, HCO3 − etc., are reabsorbed in this region and H+ ions are secreted. Hence the glomerular filtrate progressively diluted and at the end of ascending limb, it is isotonic to medullary fluid.
c. In DCT: In this region, water is absorbed under the influence of Vasopressin (Anti Diuretic Hormone - ADH) of pituitary gland (Facultative water absorption). Under the influence of aldosterone (adrenal hormone) Na+ ions are reabsorbed. Moreover, K+ and HCO3 − are absorbed and H+ ions and K+ ions are secreted. As
a result, glomerular filtrate becomes isotonic to cortical fluid.
d. In collecting duct: Little quantity water is absorbed in this part under the regulation of ADH. In addition Na+ ions are absorbed and H+ ions are secreted. As a result, at the end of collecting duct, the glomerular filtrate is hypertonic to blood and is called urine.
3. Tubular secretion (Augmentation): During urine formation cells of renal tubule secrete H+ ions, K+ ions, ammonia hippuric acid etc., into the tubule. This process is called tubular secretion.
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