MICROANATOMY

Detailed explanation-1: -Urea recycling is a process in kidneys that involves the reabsorption and secretion of urea several times, which contributes to the high concentration of solutes in the renal medulla and helps to establish and maintain the corticopapillary gradient.

Detailed explanation-2: -Vasopressin (anti-diuretic hormone) is the key hormone regulating the production of concentrated urine. Vasopressin rapidly increases water and urea transport in the terminal inner medullary collecting duct (IMCD).

Detailed explanation-3: -Urea recycling in the inner medulla also contributes to the osmotic gradient generated by the loops of Henle. Antidiuretic hormone increases water permeability, but not urea permeability in the cortical and outer medullary collecting ducts, causing urea to concentrate in the tubular fluid in this segment.

Detailed explanation-4: -About 40% of the urea filtered is normally found in the final urine, since there is more reabsorption than secretion along the nephron. It is regulated by antidiuretic hormone, which controls the amount reabsorbed in the collecting duct system and secreted into the loop of Henle.