- The urea is then carried to the kidneys through blood for its elimination as urine.
- According to cushny (1917) the mechanism of urine formation occurs in three stages namely.
ii. Selective reabsorption
iii. Tubular secretion
- The glomerular capillaries are highly permeable and are also provided with small perforations of about 0.1 n in diameter.
- However, substances having high molecular weight between 67,000 and above can not pass through them. Therefore macromolecular proteins like plasma proteins and cellular components like blood cells can not pass through them.
- Inorganic salts (Na++, K++, Cl-, HCO3, etc.), water, urea, uric acid, glucose, aminoacids etc. pass out easily into the lumen of Bowman's capsule.
- This selective process of filtration of some of the substances present in the blood due to hydrostatic pressure is called ultrafiltration or glomerular filtration.
- The fluid thus formed is called glomerular filtrate or primary urine.
- The blood pressure in the glomerular capillaries is 70 - 90 mm Hg.
- The osmotic pressure of plasma proteins is about 30 mm Hg while the pressure produced by the filtrate in the malphigian capsule is about 20 mm Hg.
- The pressure responsible for glomerular filtration averages to 70 - (30 + 20) = 20 mm Hg.
- About 130 ml of glomerular filtrate is formed per minute in both the kidneys.
- The total quantity of glomerular filtrate is about 150 to 170 litres per day.
- If this entire fluid was allowed to be excreted out, the body would soon undergoes dehydration that endangers the life. However, this fluid is greatly modified and reduced in volume by reabsorption upto 99%.
- Thus the rate of glomerular filtration is 130 ml/ min. while the rate of urine flow from both the kidneys to ureters is only 1ml/min.
- Thus in addition to filtration, reabsorption is another important factor in the mechanism of urine formation.
- When the glomerular filtrate passes through the uriniferous tubule, most of the water and number of substances undergo reabsorption.
- This is essential because the glomerular filtrate contains number of useful substances.
- The body can not afford to loose such important substance like water, salts, glucose, and aminoacids which are vital to the animal.
- Hence these substances are reabsorbed in the uriniferous tubule.
- The reabsorption of certain substances is regulated by their concentration in the blood.
- For example, glucose is absorbed completely if the blood glucose level is not above 150 to 180 mg per 100 ml.
- Those substances whose reabsorption is governed by their concentration in the blood are called threshold substances.
- Most aminoacids, vitamins, sodium, potassium, calcium, chlorides etc. are reabsorbed as completely as possible. However, other substances like ammonia, urea, uric acid are reabsorbed in much smaller amounts.
- Sulphates and creatinine are not reabsorbed at all and are almost eliminated.
- Such a type of reabsorption is called selective reabsorption.
- In certain fishes the kidney is aglomerular and excretion entirely depends on the process of tubular secretion.
- These animals form urine by means of secretion of substances from the blood into the lumen of uriniferous tubules.
- This process is called Tubular secretion.
- The secretory activity of the mammalian kidneys is however not well marked.
- Some of the substances which can be secreted by the mammalian uriniferous tubules are creatinine, para amino hippuric acid and diodrast.
- To the final form of urine, epithelial cells and mucous may be added before it is conveyed to the urinary bladder by the contraction of wall of ureters.
- When about 250 to 300 ml. of urine has collected the pressure reaches to about 180 mm H2O.
- The sensory nerve endings in the wall of urinary bladder are stimulated and there arises a sense of fulness and desire to urinate.
- The expulsion of urine to outside is called Micturition.
- During passing out of the urine the sphincter muscles of the bladder relax and muscles of the walls contract.
- Thus urine is carried away to the urinogenital canal and then to outside through urinogenital aperture.