Page 10 - LN- EXCRETION

Collecting Duct
This duct extends from the cortex of the kidney to the inner parts of the medulla and is
highly permeable to water. Thus, a considerable amount of water is reabsorbed here under
the influence of ADH to produce concentrated filtrate. Sodium is also reabsorbed here
under the influence of aldosterone.
CT (Collecting Tubule) allows passage of small amounts of urea into the medullary
interstitium to maintain the osmolarity. It also plays an important role in the maintenance of
+
+
pH and ionic balance of blood by the selective secretion of H and K ions.
Therefore, the filtrate is now called urine. Thus, urine is isotonic to medullary fluid and
hypertonic to blood.
Concentrations of important ions and other substances in the blood are controlled by
regulating water levels.

Topic 2 Excretion : Various Controlling Mechanisms and Disorders

Counter Current Mechanism
Kidney of higher vertebrates (such as mammals, birds including man) has the ability of
absorbing more and more water from tubular filtrate (in the Henle’s loop region) to make