Question

Explain the function of the vasa recta.

Answer 1

Hint: Vasa recta are the capillary networks that supply blood to the medulla, that run parallel to the loops of Henle. They are highly permeable to solute and water.

Complete Answer:
Vasa recta are the series of long loops of thin-walled blood vessels (efferent arterioles) that dip down alongside the loop of Henle in the vertebrate kidney. These capillaries are hairpin-shaped blood vessels and they run parallel to the loops of Henle.

- This hairpin structure turns down the rate of blood flow, which helps in maintaining the osmotic gradient required for water reabsorption.
- Henle’s loop and vasa recta play a significant role in water reabsorption. The flow of filtrate in the two limbs of Henle’s loop is in opposite directions and thus forms a counter current. The flow of blood through the two limbs of the vasa recta is also in a countercurrent pattern.
- The proximity between the Henle’s loop and vasa recta, as well as the countercurrent in them, help in maintaining an increasing osmolarity towards the inner medullary interstitium, i.e., from 300 mOsmolL–1 in the cortex to about 1200 mOsmolL–1 in the inner medulla.
- This gradient is mainly caused by NaCl and urea. NaCl is transported by the ascending limb of Henle’s loop which is exchanged with the descending limb of the vasa recta. NaCl is returned to the interstitium by the ascending portion of the vasa recta.
- Similarly, small amounts of urea enter the thin segment of the ascending limb of Henle’s loop which is transported back to the interstitium by the collecting tubule.
- The above-described transport of substances facilitated by the special arrangement of Henle’s loop and vasa recta is called the countercurrent mechanism. This mechanism helps to maintain a concentration gradient

Note: It is to be noted that concentration gradient is established during the countercurrent mechanism. This counter-current mechanism is maintained by selective permeability in the loops of Henle and passive diffusion in the vasa recta.