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 Excess - Edema


Fluid and electrolyte disturbances are characteristically "multiple situations". Individual imbalances rarely exist alone. Imbalances may involve fluid, electrolytes, and pH simultaneously. Nonetheless this section will examine imbalances caused predominantly by a single factor since the "multiple situations" are quite difficult to understand.

 Exchange Process Effected Clinical Condition
 1. Increased blood pressure results in increased filtration from plasma. Hypertension
 2. Increased permeability of capillaries results in too large a loss of ions and protein from
Local inflammation caused by injury.
 3. Loss of plasma protein results in lower osmotic pressure in plasma. Kidney disease (cirrhosis);
 4. Retention of salts especially total body sodium caused by decreased excretion in urine from low blood pressure prevents effective filtration. Increased water is retained to maintain osmotic pressure. Kidney disease associated with low blood pressure; Congestive heart failure
 5. High venular pressure prevents return of water by osmosis to plasma.  Phlebitis, obesity, varicose veins
 6. Blockage of lymphatic drainage  Local inflammation.

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Abnormal Conditions which Affect the Fluid Balance

QUES. 18: Traumatic shock results from a leakage of protein from capillaries caused by sudden, severe injury, burns, or major surgery. Loss of protein means osmotic pressure relations are upset. If osmotic pressure drops to 11 mm, what happens at #2? What happens to the blood plasma volume?

QUES. 19: In dehydration, loss of water from the plasma causes a relative increase in protein concentration, this causes an increase in osmotic pressure in the capillaries (for example OP = 20 at #2). What happens to the water balance in the interstitial compartment.

QUES. 20: Water intoxication may result when large amounts of water are drunk on a hot day after loss of water and salts in perspiration. The electrolyte concentration in the interstitial compartment may drop to 150 meq/l. What happens in the cells at #4 as a result? How would the ingestion of salt along with water prevent this from occurring?

QUES. 26: In cases of malnutrition, a decrease in plasma protein lowers the osmotic pressure in the capillary to 11. What happens as a result at #2?

QUES. 27: Merely standing in one position causes the venular hydrostatic pressure to increase to 20 above the protein osmotic pressure at #2. What happens?


Lab Tests for ECF Fluid Excess or Deficit:

An extracellular fluid (NaCl + water) excess causes the compartment to become expanded. This most commonly occurs when kidneys are not functioning properly as in renal disease or decreased renal blood flow. No electrolyte test will indicate this condition.

The hematocrit value may be used to indicate extracellular fluid excess and will be somewhat low.

(Hematocrit (HCT): Measures a volume percentage of red blood cells in the plasma.

Normal value : Females = 37-47%; Males = 40=54%)

Why should the HCT value be low? If the normal HCT value is 40% and the normal plasma fluid content is 3 liters, what is the HCT % if the plasma fluid is now expanded to 3.3 liters?


First find the volume of red cells (40% of total plasma).

3 L x 40 / 100 = 1.7 L of red blood cells.

Find the new percentage of red blood cells at the increased volume.
% HCT = 1.7 L / 3.3 L x 100 = 36.3% HCT

Therefore, a lower percent value for HCT means an INCREASE in the fluid volume. Low %HCT means ECF excess.

A height %HCT means an ECF Deficit.



Cell volume is regulated by changes in the amount of total body water. All fluids taken into and excreted from the body pass through extracellular compartment. Osmosis and filtration distribute this in all compartments. For every liter taken in or excreted l/3 goes to or from the ECF compartment and 2/3 goes to or from the ICF compartment. See Question 20: above.

Intracellular fluid excess is caused primarily by excessive water intake or an increase in vasopressin hormone. Interestingly, most cases occur in the hospital following trauma, surgery, or anesthesia. These conditions stimulate the release of vasopressin which in turn decreases urine volume and increases retention of water. Forced fluids without salt could aggravate the situation.

Lab Test for ICF Fluid Excess - Measure the sodium concentration:

Serum sodium electrolyte values are the best indicators of ICF excess or deficit even though sodium is found and measured in the ECF compartment. For a condition with excess water the value is less than normal for much the same reason as the hematocrit values.

If the normal concentration of sodium is l40 meq/l or l40 mmoles/l
and 17 liters is the volume in the ECF compartment, what is the new concentration of Na if one Liter of excess water is ingested? The calculation gives a value of 132 mmoles/L

Again a lower concentration of sodium means that it is present in a larger volume. Low sodium = ICF excess.