How is urine concentrated before its excretion?

How is urine concentrated before its excretion?

Higher vertebrates, including man, have evolved a counter current mecha­nism to excrete hypertonic urine (urine is more concentrated than the blood) for conserving body water, necessary for land life. As the filterate passes through the ascending limb of Henle’s loop, sodium is actively transported into the interstitial tissue fluid and chloride ions pass out passively.

The sodium and chloride ions naturally increase in the interstitial fluid. Some of sodium and chloride ions from the interstitial fluid, due to their higher concentration, passively diffuse into the descending limb of the loop of Henle.

The cyclic movement of sodium and chloride ions from the as­cending limb to the descending limb via., interstitial fluid and from the descending limb to the ascending limb through the loop of Henle helps to establish and maintain a high concentration is lowest near the cortex and highest deep in the medulla.

Composition of Normal Urine:

Urine is mainly water urea and sodium chloride water constitutes 96% urea 2% and Other metabolic products 2%. Stale urine has the smell of ammonia. Specific gravity is 1.005 to 1.030.

Urea is one of the end products of protein metabolism. It is prepared from amino acids which are delaminated in the liver. The normal blood urea is 30 mg per 100 ml of blood. Uric acid normal level – 2 to 3 mg per 100 ml, of blood 1.5 to 2 mg being excreted daily in the urine. Creatinine is a waste product of creatine in muscles. Other products of metabolism in­clude purine bodies, oxalates, phosphates, sulphates and urates. Electro­lytes of salts such as sodium, potassium chloride are excreted to balance the amounts taken by mouth.