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 and by absorption of \tt HCO_{3}^{-} from it and so the filtrate becomes isotonic to blood plasma.
  • 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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