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

Fluid Exchange Processes

Exchange Processes Between Fluid Compartments

The movement of water and electrolytes between fluid compartments takes place by a variety of processes. Movement of water and electrolytes occurs through membranes and cell walls. The permeability of membranes is controlled by the size of the "pores" or "holes". The size of the pores can be changed in response to pressure and hormones. Some membranes selectively allow passage of certain ions or molecules and exclude all others. A semi-permeable membrane allows only water through it

l. DIFFUSION (Df): Ions and molecules flow from an area of higher concentration to
and area of lower concentration. Dialysis is the specific process of separating diffusible from nondiffusible ions and molecules.

QUES. 15: Hemodialysis is the artificial cleansing of blood when kidneys are not working efficiently. The blood is diverted through a cellophane tube that serves as a dialyzing membrane. On the outside of the cellophane tube is an isotonic solution to all materials that are to be left in the blood. This solution, however, doe not contain waste products. Where do the waste products go by the process of diffusion or dialysis?

2. FILTRATION (FL): Pressure causes water, ions, and molecules to move from an area of higher pressure to an area of lower pressure. The pumping heart causes a blood pressure. The height of a column of water causes hydrostatic pressure.


ELECTROLYTE DISTRIBUTION

 Electrolyte

 Extracellular
meq/liter

 Intracellular
meq/liter

Function

 Sodium  142  10  fluid balance, osmotic
pressure
 Potassium 5 100  Neuromuscular excitability
acid-base balance
 Calcium 5 - bones, blood clotting
 Magnesium 2 123 enzymes
 Total Positive ions  154 205  

Electrolyte Distribution

 Electrolyte

Extracellular
meq/liter

 Intracellular
meq/liter

 Function

 Chloride  105  2 fluid balance, osmotic pressure
 Bicarbonate 24 8 acid-base balance
 Proteins 16 55 osmotic pressure
 Phosphate 2 149 energy storage
 Sulfate 1 - protein metabolism
 Total Negative ions 154 205  

3. ACTIVE TRANSPORT (AcT): Water and/or ions and molecules are carried by a larger molecule or a vacuole on a cell wall. This requires specific enzymes and energy. This transport is in the manner of a "pump" against a concentration difference.

As you look at the table of Electrolyte Distributions, you may wonder how the potassium and phosphate ions get from the extracellular compartment into the cells. Active transport is needed to get potassium ions into the cells since diffusion will not work as the concentration of potassium is highest in the cells.

Anytime there is a concentration differential where you need to go from a low concentration to a high concentration, then active transport mechanisms are needed. The cells have developed many types of these.

 

 

4. OSMOSIS (OS): Water only flows from a dilute solution to a more concentrated solution.

Osmotic pressure is defined as the pressure required to be placed on a solution separated from water by a membrane to prevent osmosis from taking place. Osmosis occurs because there are more molecules of water bombarding the membrane on the pure water side than on the side containing a solution (water molecules plus dissolved molecules).

OSMOTIC PRINCIPLE: Water only flows from a dilute solution to a more concentrated solution. Water flows from a solution of low osmotic pressure (Hypotonic) to a solution of higher osmotic pressure (hypertonic).

If two solutions have identical osmotic pressures, they are isotonic. If one solution has a lower osmotic pressure (lower concentration of salts), it is hypotonic with respect to the other.

In the opposite situation a solution of higher osmotic pressure is hypertonic with respect to the other. In clinical situations the concentrations of solutions used for replacement of body fluids must be carefully controlled. A 0.9% NaCl (normal saline solution) or 5% glucose solutions are isotonic with body fluids.

QUES. 13: If red blood cells are placed in a solution of low osmotic pressure (hypotonic), which direction will water flow in the red blood cells? (Called hemolysis)

QUES. 14: If red blood cells are placed in a l.3% NaCl saline solution (hypertonic), which direction will the water flow in the red cells? (Called crenation)

   

 QUES. 13: If red blood cells are placed in a solution of low osmotic pressure (hypotonic), which direction will water flow in the red blood cells? (Called hemolysis)

QUES. 14: If red blood cells are placed in a l.3% NaCl saline solution (hypertonic), which direction will the water flow in the red cells? (Called crenation)