Countercurrent multiplication system

The upstream multiplication system is a system that allows the formation of hypertonic marrow with an osmolarity gradient growing from the cortex to the depth of the kidney marrow. In humans, it acts in the kidney marrow, specifically in the part of the nephron called the Henle loop.

Henle loop
Henle's loop consists of a descending arm and an ascending arm, which have a parallel arrangement. The individual parts of the loop are separated by an interstitium and differ in their permeability.

The descending arm is highly permeable to water, but poorly permeable to solutes. On the contrary, the ascending arm is almost impermeable to water, but there is an active transport of Na + and Cl - from the tubular fluid to the interstitium.

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The mechanism
Isoosmotic tubular fluid flows into the descending arm of Henle's loop. The surrounding interstitium is hyperosmotic, so there is a passive transfer of water from the tubule to the interstitium. The tubular fluid thickens.

The highest osmotic concentration is reached by the tubular fluid at the top of the loop. In the ascending arm, there is an active transport of solutes from the tubular fluid to the interstitium, which conditions the formation of hypertonic marrow (there is no water resorption). The tubular fluid is gradually diluted and the hypoosmotic fluid leaves the Henle loop.

Meaning
Due to this arrangement, a strongly hyperosmotic interstitium is formed at the top of the loop, which draws water (in the case of opening aquaporins by ADH ) from the distal tubule and collecting duct. It ensures the production of concentrated urine. The longer the Henle loop (juxtamedullar nephrons in the kidney marrow), the more concentrated urine the nephron is able to produce.

Energy is consumed during the active resorption of solutes from the ascending arm of the Henle loop. However, transport always takes place against a constant small gradient, so energy consumption is relatively low.

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