Question

Water reabsorption

		occurs between the collecting duct and the peritubular capillaries in the medulla.
		is influenced by hormones, such as vasopressin.
		occurs to the greatest extent in the proximal convoluted tubule.
		none of these (a, b, c) are correct.
		all of these (a, b, c) are correct.

PAH (para-amino hippuric acid) is often injected into patients to test maximum kidney function (i.e. clearance rates). That is because PAH is

		filtered, not reabsorbed, and secreted
		not filtered, not reabsorbed, but secreted
		filtered and reabsorbed
		filtered, reabsorbed, but not secreted

Answer 1

Answer:

Answer 1. Option B (is influenced by the hormones, such as ADH), Option C (occurs to the greatest extent in the proximal convoluted tubule)

Explanation: Nearly 180 liters of the glomerular filtrate is produced per day by the nephron of the kidney whereas the amount of urine we excrete per day is around 1-1.5 liters. This signifies that the human urine is hypertonic is nature and thus the maximum of the filtrate needs to be reabsorbed by the body by different methods like osmosis, active transport, and facilitated diffusion.

ADH (anti diuretic hormone) also called as vasopressin gets released from the posterior lobe of the pituitary gland and it acts on the cells of the distal convoluted tubule and collecting ducts leading them absorb water. Thus, true that hormone ADH remains involved in the water reabsorption.

Also, out of the total filtrate formed, nearly 65% of the total filtrate gets actively reabsorbed in the PCT (proximal convoluted tubule) and it is due to this reason that the PCT is known to have a huge number of mitochondria which provides the sufficient amount of energy to mediate a heavy reabsorption of water within the PCT.

Also, peritubular capillaries lies in near proximity to the proximal and distal convoluted tubules, whereas the blood vessel branch which is located within the medulla of the nephron is the vasa recta. The reabsorptions of the water and other ions done within the medullary region of the nephron are returned back to the body through the vasa recta.

Answer 2. Option A (Filtered, not reabsorbed, and secreted)

Explanation: PAH; also known as para-amino hippuric acid is a very important diagnostic agent and is widely used in the determination of the renal efficiency pertaining to the rate of determination of the renal plasma formation.

It is a derivative of the hippuric acid and needs to be injected into the human body due to its absence within the human body. The reason why this particular compound can be employed for the determination of the renal plasma is that this compound can be effectively filtered from the renal corpuscle of the nephron thus forming a major constituent of the filtrate. Also the compound does not get reabsorbed by our body and likewise gets also secreted by the renal tubules.

The concentration of the compound is then calculated in terms of measuring its amount in the blood within the arteries which has to be filtered to that of its amount present within the urine flow. And thus using these parameters, the renal plasma flow rate can be calculated.