Fluid /Electrolyte Balance  Elmhurst College
Fluid Exchange Processes Fluid Exchange kidneys Fluid Deficit  Chemistry Department
Fluid Exchange in Tissues Fluid Excess    Virtual ChemBook

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Fluid Exchange Processes in Tissues

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.


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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The intracellular fluid is hypertonic with respect to the interstitial fluid, therefore, water moves into the cells by osmosis at #4. Look at the electrolyte concentrations - 165 in the interstitial space and 205 in the cells. Water moves by osmosis to the higher concentration electrolyte area. A balance occurs when the water distribution by body weight is l5% and 40% respectively for the interstitial and intracellular compartments.

The major ions (potassium and phosphate, etc.) in the cells can only get there by active transport #5. Wastes, excess ions, and excess water are moved out of the cells by active transport #6. Any change in salt (NaCl) concentration in the interstitial compartment, will cause water movement into or out of the cells.