Three processes: Glomerular filtration , reabsorption & secretion.
1. Glomerular filtration (ultrafiltration)
– The glomerular capillary blood pressure causes filtration of blood through 3 layers, i.e. endothelium of glomerular blood vessels, epithelium of Bowman’s capsule & a basement membrane between these 2 layers.
– The epithelial cells (podocytes) of the Bowman’s capsule are arranged in an intricate manner leaving some minute spaces called filtration slits (slit pores).
– Almost all constituents of the blood plasma except the proteins pass onto the lumen of the Bowman’s capsule.
– About 1100-1200 ml of blood is filtered by the kidneys per minute. It constitutes 1/5th of the blood pumped out by each ventricle of the heart in a minute.
– The amount of glomerular filtrate formed per minute is called Glomerular filtration rate (GFR).
– Normal GFR = 125 ml/minute, i.e., 180 litres /day.
– 180 litres of glomerular filtrate is produced daily. But about 99% of this is reabsorbed by the renal tubules.
So normal volume of urine released is 1.5 liters.
– From the filtrate, glucose, amino acids, Na+, etc. are reabsorbed actively and nitrogenous wastes are absorbed passively.
Passive reabsorption of water occurs in the initial segments of the nephron.
– PCT reabsorbs most of the nutrients and 70-80% of electrolytes & water. Simple cuboidal brush border epithelium of PCT increases surface area for reabsorption.
– Loop of Henle maintains high osmolarity of medullary interstitial fluid. Descending limb is permeable to water but almost impermeable to electrolytes. This concentrates the filtrate. In the ascending limb, minimum reabsorption occurs. It is impermeable to water but allows the transport of electrolytes.
So, the filtrate gets diluted. – In DCT, conditional reabsorption of Na+ & water takes place.
– Collecting duct extends from the cortex to the inner parts of the medulla. It reabsorbs a large amount of water to concentrate urine. It also allows the passage of small amounts of urea into the medullary interstitium to keep up the osmolarity.
3. Tubular Secretion
– Cells of PCT & DCT maintain ionic (Na-K balance) and acid-base balance (pH) of body fluids by selective secretion of H+, K+ & NH3 into the filtrate and absorption of HCO3- from it.
– Collecting duct maintain pH and ionic balance of blood by the secretion of H+ and K+ ions.
Mechanism of concentration of the filtrate
– Henle’s loop & vasa recta help to concentrate the urine.
– The flow of filtrate in the 2 limbs of Henle’s loop and the flow of blood through the 2 limbs of vasa recta are in opposite directions (counter current pattern). This is called Counter current mechanism.
– Due to the counter current and proximity between Henle’s loop & vasa recta, osmolarity increases from cortex (300 mOsmolL-1) to the inner medullary interstitium (1200 mOsmolL-1).This gradient is caused by NaCl & urea.
– NaCl is transported by ascending limb of Henle’s loop that is exchanged with descending limb of vasa recta.
NaCl is returned to interstitium by ascending limb of vasa recta. Similarly, small amount of urea enters the thin segment of the ascending limb of Henle’s loop which is transported back to the interstitium by the collecting tubule.
Thus electrolytes and urea are retained in the interstitium and maintain a concentration gradient (interstitial gradient) in medullary interstitium.
It enables easy passage of water from collecting tubule to concentrate the filtrate (urine).
– Thus DCT & collecting duct produce urine four times concentrated than the initial filtrate formed (i.e., 300 mOsmolL-1 to 1200 mOsmolL-1)