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Physiology 3120 Study Guide - Body Fluid

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Anita Woods

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Weekly Thoughts 2
MODULE 2 - Body Fluids
We looked at the internal environment in module 1 and learnt that it is important to
maintain conditions at relatively constant levels. Recall that the internal environment is
everything "inside" the body but does not include the cells. The internal environment is
the "space" the cells live in.
It is extremely important to maintain the chemical composition and fluid environment of
this space so the cells can function properly - you don't want to have too much water or
too little, you don't want to have too much of one ion and too little of another. Any
change in these values will result in a "not-so-ideal" internal environment, which will
cause cells to function improperly.
Body Fluids
The average human is made up of 60% water or 42 liters (for a 70Kg person). This water
is found in 3 different compartments in the body - the intracellular compartments
(everything inside the cells), the interstitial compartment (everything outside the cells but
not in the plasma) and the plasma. Both the plasma and the interstitial compartment can
be combined into the extracellular compartment (EVERYTHING outside the cells
including plasma).
Most of the water (67%) is found inside the cells (the intracellular compartment). The
interstitial compartment contains roughly 26% of the total body fluid while plasma
contains the remaining 7%.
Chemical Composition
The concentration of ions inside the cell is quite different to the outside of the cell. For
example: Na+ is found in a high concentration outside the cell and a low concentration
inside the cell. K+ is the other way around - its concentration is high on the inside and
low on the outside. Cl- and Ca++ are similar to Na+ in that they are found in a high
concentration outside the cell and very low inside. A way I like to remember this concept
is the salty (Na+) banana (K+), Na+ is higher on the outside of the cell (sodium and Cl-)
and K+ is higher on the inside of the cell (banana).
You will NOT be expected to memorize the exact concentration of these ions (the
numbers on page 2.5 of body fluids). What you absolutely should remember is the
relative amounts of these ions inside and outside the cell. Therefore, you should
COMMIT TO MEMORY the picture on page 2.7 of Body Fluids! That is - there is lots
of Na+, Ca++ and Cl- outside the cell and little inside, and that most of the potassium (K+)
is found inside the cell relative to the outside. We will look at the movement of these ions
(called diffusion) in the next module.
What causes the difference in ion distribution? - the cell membrane, of course. Why?
Because it is selectively permeable. We will look at this in the next module as well.
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