The osmolarity of interstitial fluid in the renal medulla is far greater than th

Question

The osmolarity of interstitial fluid in the renal medulla is far greater than the osmolarity of the blood. That's pretty unusual, physiologically? What contributes to this ability of the kidney to produce an increasingly salty medulla? Please select the one correct answer. 0 a. Filtrate leaving the loop of Henle and entering the distal tubule has a high concentration of Na O b. the structure of the loop of Henle (hairpin turn, proximity of descending and ascending limbs with differential permeability to H20 and Nacl). OC. The renal medulla makes lots of [Na+] O d. The hormone vasopressin (AVP). O e. the structure of the vasa recta (O) f. both b and e O g. all of the above

Explanation / Answer

The loop of Henle in the deeper part of renal medulla is responsible for concentrating the osmolarity. It does so by virtue of its ascending and descending limbs having varying permeability to salt and water. Active reabsorption of sodium ions occurs in the ascending limb. However, the ascending limb is impermeable to water. Here, the interstitial fluid is hyperosmolar relative to the fluid in the tubule.

In the descending limb, the sodium ions cannot be reabsorbed. Water can however, leave the filtrate or interstitial fluid in the tubule. Water is drawn/ attracted towards the high osmolarity of the ascending limb, thereby concentrating the filtrate in the descending limb.

There is a countercurrent flow occuring in the loop of Henle. It causes osmolarity differences to increase when the renal tubule descends into the medulla. The filtrate in the descending limb is more concentrated. However, active absorption of sodium ions in the ascending limb causes the filtrate to be less concentrated. Thus, the ascending limb will transfers osmolarity to the descending limb and interstitial fluid in the medulla.

The vasa recta, capillaries that flow in parallel to the Loop of Henle, has a sluggish blood flow. As a result, the plasma of the vasa recta can equilibrate with the interstitial fluid. The vasa recta plasma will increases its osmolarity when it reaches the medulla, and further decrease it in the ascending limb. As a result, blood flows to the medulla without affecting the osmotic gradient.

Vasopressin is not involved in this mechanism. The renal medulla also doesn’t synthesize sodium.

Option is f (both b and c).

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