Hyperosmolarity of renal medulla is due to-
Ans. is 'b > a & d' i.e., Na > K & Cl o A countercurrent system is a system in which inflow runs parallel to, counter to, and in close proximity to the outflow for some distance. In the kidney there are two countercurrent mechanisms : - i) Countercurrent multiplier at the loop of Henle which generate high medullary osmotic pressure gradient; and ii) Countercurrent exchanger at vasa recta of medullary capillaires, which helps to maintain the medullary osmotic pressure gradient.Countercurrent multipliero The loop of Henle gives the nephron a peculiar shape. The descending limb, in which the tubular fluid flows towards the hairpin bend (downwords) and the ascending limb, in which the tubular fluid flows 'upwards' lie close to each other. Two such tubes with fluid flowing through them in opposite directions provide an opportunity for the countercurrent multiplication.o The thick ascending limb of loop of Henle is impermeable to water but it actively reabsorbs NaCl. The thin descending limb of loop of Henle is relatively impermeable to solutes but highly permeable to water. The most important cause of the high medullary osmolarity is active transport of sodium0 and co-transport of potassium, chloride and other ions out of the thick ascending limb of loop of Henle0 into the medullary interstitium. Most important among all these is NaCl0, for maintenance of high medullary interstitium.o Consider the Na* is continuously flowing in from glomerulus into the tubule. However, most of the Na+, instead of flowing out, gets trapped in a circular path and recycles between the ascending and descending limbs of the loop of Henle. Consequently, the Na* concentration becomes very high at the tip of the loop.Role of ureao Urea also contributes to the osmalority of medullary interstitium0, especially when the kidney is forming maximally concentrated urine. The medullary region ofthe collecting duct is permeable to urea. Diffusion ofurea from these ducts into the medulla increases the osmolarity of medullary' interstitium. Under the influence of ADH, the permeability of medullary collecting ducts to urea further increases. Moreover. ADH makes these collecting ducts also permeable to water and outward diffusion of wrater. That increases the urea concentration within the duct. That leads to further diffusion of more urea out of duct along concentration gradient. Thus urea makes an important contribution to the genesis of high osmolarity in the renal medulla.Countercurrent exchangero If the blood flowrs through the hyperosmolar medulla as shown in figure A below', it would equilibrate with medullary interstitium and carry away the solutes concentrated in the medulla. However, what actually happens is shown in figure 'BT below. Vasa recta is U shaped (as is the loop of Henle). As the vasa recta dips into the hyperosmolar medulla, the blood equilibrates with the surrounding interstitium and becomes hyperosmolar. Thereafter, as the vasa recta loops around and ascends towards the cortex, the osmolarity of the blood keeps decreasing as it equilibrates with the surrounding interstitium. By the time vasa recta leaves the medulla, the blood in it has the same osmolarity as when it entered the medulla. In other words, the solutes concentrated in the medulla are not wrashed away by the blood flowing through the medulla,o Since the blood flowing through the medulla equilibrates completely with the medullary' inerstitium, the amount of solutes carried away by the blood is flow limited. Hence, the slow rate of blood flow through the vasa recta contributes to the conservation of medullary hyperosmolarity. Conversely, a high blood flow rate through the medulla reduces the medullary hyperosmolarity'.
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Na
Glucose
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Kidneys and Body Fluids