Excretory Products and their Elimination
Urine formation, osmoregulation
- Urine formation is the result of 3 important processes
(i) Glomerular filtration (or) ultra filtration.
(ii) Selective reabsorption
(iii) Tubular secretion.
- The glomerular capillary blood pressure causes blood filtration through podocytes that are arranged in an intricate manner to leave some minute spaces called filtration slits (or) slit pores.
- Blood is filtered finely such that all components of plasma except the proteins pass onto the Bowman's capsule and hence it is referred as ultra filtration.
- Glomerular ultrafiltration does not require the energy expenditure by cells of kidney.
- To prevent the loss of water, nutrients and ions from the body, 99% of the filtrate is reabsorbed.
- Water and urea undergoes passive transport whereas glucose and amino acids are reabsorbed by active transport. Reabsorption of Na2+ is by both active and passive..
- About 70-90% of water and electrolytes are reabsorbed in PCT which is termed as obligatory water reabsorption.
- Water is reabsorbed in all tubular parts except the ascending limb of Henles' loop.
- PCT helps to maintain pH and ionic balance of body fluids by selective secretion of H+ ions, NH3 and k ions into the filtrate.
- DCT is also capable of selective secretion of hydrogen and potassium ions and NH3 to maintain the PH and sodium -potassium balance in blood.
- Descending limb of Henles' loop is permeable to water but impermeable to electrolytes by which the filtrate becomes hypertonic to plasma of blood.
- As the ascending limb of Henle's loop is impermeable to water by its nature and its permeable to Cl-, k+ and Na+ and its partially permeable to urea and the filtrate becomes hypotonic to blood plasma.
- In DCT, there is an active reabsorption of Na+ ions from filtrate due to influence of aldosterone. Also, water is re-absorbed under the influence of ADH (Anti-Diuretic Hormone) and hence the fluid becomes isotonic to the blood plasma.
- In CT, further water gets re-absorbed making the filtrate more concentrated to make it hypertonic to blood plasma as the entire duet is permeable to water.
- After all the above process is complete, the filtrate is now called urine which is hypertonic to blood plasma.
- Tubular secretion refers to the addition of selected materials from the blood to ultra filtrate to increase its volume which occurs through active transport in DCT and PCT.
- Filtrate flow in Henles' loop is in opposite direction forming a counter current pattern with the flow of blood in vasa recta.
- The close proximity between loop and vasa recta helps in maintaining an increased osmolarity towards the inner medullary interstitum, i.e, 300 mos mol L-1 in the cortex to about 1200 mos mol L-1 in the medulla, which is caused by NaCl and urea.
- Human kidneys can produce urine nearly 4 times concentrated than the initial filtrate formed.
Watch this video for the topic Urine formation, osmoregulation Part-1 from 0:30 to 30:55
Watch this video for the topic Urine formation, osmoregulation Part-2 from 0:07 to 16:33
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