Describe how the kidney maintains the medullary concentration gradient.

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College Answer

A useful introduction could include a definition of medullary concentration gradient
and its function. The answer was expected to describe the roles of sodium, chloride
and urea in the countercurrent multiplier and the features of the vasa recta
countercurrent exchange system.
The 3 main areas that needed to be addressed to pass this question included:
[1] The loops of Henle with their water permeable descending limbs and water
impermeable ascending limbs, which actively remove solutes from the tubular
lumen. The counter current multiplier system.
[2] The vasa recta which run parallel to the loops of Henle and are permeable to
water and solute and have low flow. This allows the medullary concentration
gradient to be maintained. The counter current exchange mechanism.
[3] The role of urea which is concentrated in the medulla by mechanisms which
involve changes in permeability to urea in different regions of the tubules partly
influenced by the effects of antidiuretic hormone. `
Some candidates elected to draw the loop of Henle and vasa recta together with the
movement of various solutes and water and answer the question from it.
Unfortunately mistakes in these diagrams only confused their answers further.
Syllabus: Section D1, 2c
Recommended sources: Principles of Physiology for the Anaesthetist, Power and
Kam, page 234