A: Urine 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 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. The
process of pressure filtration through glomerular capillaries in
Bowman's capsule is known as 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. Glomerular filtrate passes through the renal tubule for
selective reabsorption and tubular secretion.
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 absorption is unregulated in
proximal convoluted tubule (PCT) and loop of Henle (LOH) and is
regulated in distal convoluted tubule (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 secrete 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 descending limb and thus
concentration of renal fluid increases and isotonic to medullary
fluid.
ii. In ascending limb: This part is impermeable to water. Na+, Cl -,
Mg++, Ca++, K+, HCO3 - etc. are absorbed and H+ are secreted in this
part. Hence, glomerular filtrate is progressively diluted and at the
end of ascending limb, it is isotonic to medullary fluid.
c. In DCT: In this part, 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-
ions are absorbed and H+ and K+ ions are secreted. As a result,
glomerular filtrate becomes iosotic to cortical fluid.
d. In collecting duct: Little quantity of 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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