Fluid Exchange Processes

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.

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. 


 

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.

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

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

A:  hemolysis

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

A:  crenation