Urea Recycling
The thick ascending limb of the loop of Henle, distal tubule and cortical collecting ducts (top and middle portions of collecting ducts) are impermeable to urea. When ADH concentration of plasma is very high, water reabsorption from the distal parts of the nephron takes place at a faster rate, resulting in urine concentration.
The innermost portion of the collecting duct contains many membrane transporters for urea called UT-As, upregulated by vasopressin. The concentration difference favours urea diffusion out of the duct into the interstitial fluid, creating a high urea concentration in the inner medullary interstitial fluid. When the kidney is forming maximum concentrated urine, the urea contributes to about 50% of the osmolarity of the medullary interstitium (Fig. 9.9). A portion of the urea reabsorbed from the inner medullary ducts diffuses into the thin loop of Henle and is re-circulated. This re-circulation helps the concentration of urea to be excreted so that excretion is possible with minimum water loss. A high-protein diet always increases the capacity to concentrate urine because of the comparatively more urea formation as a nitrogenous waste. Malnutrition always reduces the ability to concentrate urine.Know More.......
Shipwreck Victims
The maximum osmotic concentration of the renal medulla is 1200 mOsm/L. The seawater has an osmolarity between 1000 and 1200 mOsm/L with a sodium chloride concentration of about 3.0-3.5%. If an individual drinks 1 L of seawater with an osmotic concentration of 1200 mOsmol/L, that would provide a total sodium intake of 1200 mOsm/L. So, for every litre of seawater drunk, a volume of 2 L of urine would be voided out to get rid of 1200 mOsm of solutes from the body in addition to other obligatory solutes such as urea. The shipwreck victims would have a net fluid loss of 1 L for every litre of seawater drunk, explaining the rapid dehydration in such individuals.
Fig. 9.9 Urea recycling. The urea is transported from the collecting duct to the medullary interstitium, and from there, 50% is re-circulated through the thin limb of the loop of Henle
9.8