Question

Describe the mechanism (in mammals) by which blood ultrafiltrate is passed from the circulatory system to kidney tubules and can then be diluted below blood plasma osmolality. Include a description of the Single Effect (the mechanism that establishes the concentration gradient around the Loop of Henle) in your answer.

Answer 1

When blood is passing through glomerular capillaries, it is subjected to lot of blood pressure. Because of this is lot of plasma of blood will leave the blood and enters in to the lumen of the Bowman's capsule. This is called the glomerular filtrate. The filtrate is same as the blood plasma, but has no plasma proteins. Filtrate first passes through PCT , where 75% of the filtrate is absorbed. Glucose, aminoacids , vitamins, etc are completely absorbed in to the blood. But Minerals like, Na, K, Ca, Cl ions are not completely absorbed in PCT.

From PCT filtrate enters in to Henle's loop where the water absorptin occurs and it helps in concentrating urine. The filtrate becomes hypertonic as it passes through descending limb of loop of Henle. It becomes hypotonic as it passes through ascending limb of loop of Henle. Mineral absorption by active transport makes the filtrate hypotonic in ascending limb of Henle's loop.

Filtrate which is hypotonic passes through DCT of nephron. Here lot of water gets absorbed based on the body requirement. The DCT leads in to collecting duct. Here also lot of water and minerlas get absorbed based on body requirement. Absorption is under the control of ADH and aldosterone. ADH produced by posterior pituitary increases water reabsorption in to blood making urine concentrated. Water absorption always occurs by the process of osmosis. But if the blood water concentration is high, water needs to be eliminated from the blood along with urine. In such a case, ADH is not produced. This prevents water reabsorption from filtrate in to blood. So, filtrate becomes more dilute than the blood because of the excess water in it ,reducing the osmolarity much less than blood.

Henle's loop helps in keeping the medullary interstitium concentrated. A mechanism called counter current system seen in henles loop and vasa recta near the Henle's loop show the phenomena of counter current system. Counter current system keeps the ions in the medulla and takes away water from the medulla. Ions keep accumulating to more and more in medulla, keeping its concentration high. That helps in water reabsorption.