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Physiology 3120 Lecture Notes - Stem-Loop, Kangaroo Rat, Extracellular Fluid

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Tom Stavraky

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Human Physiology
Monday, December 7, 2009
“Renal VII”
Counter-Current Multiplier System
High osmolarity of the medulla is essential for concentrating urine
Initially, all osmolarities are 300
At the same time, water leaves the descending loop, and salt leaves the ascending loop, so the
interstitium is more concentrated than normal
As the fluid flows, water from descending loop leaves, and salt from the ascending loop leaves,
which forms a gradient
Arrangement of vasa recti
Less abundant than peritubular capillaries
Run in parallel to each other, and anti-parallel to the loop of Henle
Run in hairpin loop, which really maintains concentration gradient
As it moves through the medulla, the blood becomes more concentrated, but never equalizes with
the interstitium because the plasma moves too fast (except at the bottom of the loop
Recycling of urea
Don’t worry about complicated explanation
Urea = organic ion that is a waste product of the body
50% excreted in urine; 50% is kept in the medulla of the kidney, and contributes to medullary
During low protein intake, decreased ability to concentrate urine (can’t make urea)
Epithelial cells in the medulla are in a highly hyper-osmotic solution, so how do they stand these
Cells make osmolytes
Molecules such as inositol are specialized that accumulate in the cell; artificially increase
the cell osmolarity to match the surrounding
Prevents cell from shrivelling
Not all mammals are the same
1. Kangaroo rat
Desert dwelling, so must be able to conserve a lot of water
Has more juxtamedullary nephrons with longer loops of Henle
Can make urine that is very concentrated (6000-8000 mOs)
2. Beaver
Water dwelling
Actually not able to concentrate urine; short loop nephrons, so can’t concentrate the
In situations of dehydration, must drink more water
Acid/Base Balance
Kidneys aren’t the only organs that regulate acid/base (i.e. lungs; actually much faster & efficient than
the kidneys)
If ECF was more acidic, more H excreted, more HCO3 reabsorbed, and vice-versa if the ECF is
more basic
Sources of acid
Food (acidic fruits, etc.)
Metabolic intermediates (pyruvate, lactate)
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