Counter current exchange:
The loop of the nephron increases the efficiency of re-absorption by counter current flow with water and ions being reabsorbed. Generally, the longer the loop of the nephron, the more water and ions that can be reabsorbed. It follows that desert rodents (e.g. kangaroo rat) that form highly concentrated urine have vary long nephron loops, similarly amphibians that are closely associated with aquatic habitats have nephrons that lack a loop.
Figure shows the counter current flow mechanisms for concentrating urine. The process of re-absorption in the proximal convoluted tubule removes some salt (NaCl) and water from the glomerular filtrate and reduces its volume by 25%. However the concentration of salt and urea are still iso osmotic with extra cellular fluid. As the filtrate passes into the ascending limb, sodium (Na+) ions are actively transported out of the filtrate into extra cellular fluid, with chlorine (Cl-) ions following passively. Water cannot flow out of ascending limb because the cells of the ascending limb are impermeable to water.
Thus the salt concentration of the extra cellular fluid becomes very high. The salt flows passively into the descending loop, only to move out again in the ascending loop, creating a recycling of salt through the loop and the extra cellular fluid. Because the flows in the descending limbs are in opposite direction, a counter current gradient in salt is set up. The osmotic pressure of extra cellular brine bath is made even higher because of abundance of urea that moves out of collecting ducts. Finally the distal convoluted tubule empties into the collecting duct which is permeable to urea and concentrated urea in the filtrate diffuses out into surrounding extra cellular fluid. High urea concentration in extra cellular fluid, coupled with high concentration of salt, form, urio brine bath that causes water to move out of the filtrate by osmosis as it moves down the descending limb. Finally many peritubular capillaries surrounding each nephron collect the water and return it to the systematic circulation.
Renal pelvis of mammalian kidney is continuous with ureter that carries urine to urinary bladder. Urine from two ureters (one from each kidney) accumulates in the urinary bladder. Urine leaves the body through single tube, the urethra which opens at the body surface at the end of the Penis (in human male) or just in front of vaginal entrance (in human female). As the urinary bladder fills with urine tension increases in its smooth muscle walls. In response to this tension, a reflex response relaxes sphincter muscles at the entrance to the urethra. This response is called urination. Two kidneys, two ureters, urinary bladder and urethra constitute urinary system of mammals.
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