Counter Current Multiplier Mechanism
Can you please explain to me the counter-current multiplier
mechanism. I understand that cholride and sodium ions are filtered out
of the ascending loop of Henle into the interstial fluid, however, I'm
not sure exactly what happens from there and how this effects osmotic
pressure gradients in the nephron. Any help would be greatly appriciated.
This mechanism is very complex when it comes to writing a response. You
have to have a strong background in osmotic pressure understanding and the
anatomy of the kidney. It involves the cortex, outer and inner medula in
relationship to the vasa recta, interstitial fluids at two points, the
loop of Henle and the collecting duct. The size of the tubes and the
position in relations to the cortex and medulla is an essential part. I
can suggest some references.
These are excellent sites that address your question.
This function is dependent on the anatomical arrangement of the nephrons and
the vasa recta (blood vessels surrounding the nephron). The descending loop
of Henle carries urine filtrate down ward from the cortex into the medulla.
The ascending loop carries urine filtrate upward from the medulla into the
cortex. This sets up a situation where urine flowing in one tube is running
parallel and counter to fluid flowing in another tube. The descending tube
is permeable to water and impermeable to solutes. The fluid outside of the
tube is more concentrated and water leaves it by osmosis. This causes the
filtrate to become more concentrated. The ascending limb of Henle is
impermeable to water but permeable to solutes such as Na, Cl and urea. As
the filtrate moves up the limb the ions move out making the filtrate less
concentrated. The actual concentration is dependent on the hormone ADH or
anti-diuretic hormone. When ADH is present, water moves out of the
collecting ducts which are past the loops of Henle. This causes the urine to
be more concentrated. When ADH is NOT present, the water stays in the urine
and leaves the kidneys.
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Update: June 2012