Test: Mechanism of Concentration of the Filtrate and Regulation of Kidney Function (August 23) - NEET MCQ

ADH helps in water reabsorption in DCT and collecting duct making urine hypertonic. ADH is secreted by hypothalamus and released into the blood from the posterior lobe of pituitary gland. ADH is stimulated to be released when osmoreceptors in the hypothalamus detect an increase in the osmolarity of theblood above 300 mosmL^-1.

Aldosterone is secreted by the outer layer of the adrenal gland. When aldosterone is present in the blood, all the Na⁺ in the filtrate is reabsorbed b the epithelial cells of DCT and collecting duct. Retaining Na⁺ raises the osmotic pressure of the blood and reduces water loss from the body. 

The descending limb is permeable to water but almost impermeable to electrolytes (sodium ions) and ascending limb is permeable to electrolytes (sodium ions) but impermeable to water, therefore when the filtrate through the descending limb it becomes hypertonic and as it passes through the ascending limb it becomes hypotonic.

When someone drinks lot of water, the volume of bod fluid increases, thic switches off the osmoreceptors in the body and suppresses the ADH release.

The counter current mechanism in loop of Henle and vasa recta, helps in maintaining an increasing osmolarity of the interstitial fluid i.e. form 300 mOsmoL^-1 in the cortex is about 1200 mOsmolL^-1 in the inner medulla. This gradient is mainly caused by NaCl and urea. This gradient thus helps in easy reabsorption of water from the filtrate in the collecting duct so as to produce hyperosmotic urine.

Changes in blood volume, body fluid volume and ionic concentration activate the osmoreceptors in the body. These receptors further stimulate the hypothalamus to release ADH From the neurohypophysis. ADH increases water reabsorption from DCT and CT, thereby preventing diuresis.

Henle's  loop is a U-shaed part of nephron a start at the end of proximal convoluted tubule are consists of descending limb and ascending limb. Concave current mechanism in Henle's loop and vasa recta to convertrate the urine. Maintenance of concentration gradient in the medullar interstitial fluid by the concave form the filtrate present in the collecting duct so that the concentratin of the filtrate (urine) is increased. Therefore, if Henle's loop was absent from mammalian nephew urine will be more dilute.

An increase in body fluid volume can switch off the osmoreceptors present in the body which further suppresses  the release of ADH from neurohypophysis. ADH affects the Kidney function by its constricting effects on blood vessels. This will increase the blood pressure and an increase in blood pressure will increase the glomerular blood flow and thereby the GFR. A fall in GFR activates the JG cells to release renin which converts angiotensinogen in blood to angiotensin I which is further convert to angiotensin II. Angiotensin II stimulates the  adrenal cortex to release aldesterone.

In DCT (distal convoluted tubule) reabsortion of sodium ions, chloride ions and water occurs.