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Blood plasma contains water, ions, nutrient molecules (glucose,
amino acids, protein, lipids), and waste molecules. These substances
can be transported from the blood capillaries to the interstitial
fluid to the cells and back again by diffusion, filtration, osmosis,
and active transport. Figure at the left summarizes the concentrations,
pressures, and processes which occur normally between the three
compartments.
Blood pressure (BP) and osmotic pressure (OsP) are given in
millimeters of pressure. The electrolyte concentration are given
in meq/l = milliequivalent per liter. A milliequivalent is similar
to millimoles which are 1/1000th of a mole. The units for our
purposes are not important - only the numbers themselves.
PLASMA & INTERCELLULAR BALANCE:
Filtration occurs at #l because the blood pressure at 32 is
greater on the arterial side of the capillary than the osmotic
pressure of 12. Water, ions, and nutrients are forced into the
interstitial space. The fact that the protein remains in the
blood plasma creates the differential in electrolyte concentration
175 in the plasma and 165 in the interstitial compartment.
The capillary wall is not very permeable to proteins, therefore,
on the venous side - site #2 the lower blood pressure allows
water to return to the plasma by osmosis. Wastes and other ions
excreted by the cells return to the plasma by diffusion or dialysis
#2.
The net concentration does not change but the specific chemicals
are different depending on position in the capillary. Materials
which cannot return to the blood stream return via the lymph
system #3. Essentially, the water balance in the plasma and interstitial
compartments depends on blood pressure and plasma protein concentration.
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