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 Deficit

- ECF deficit

The main locations in the body where exchanges of salt and water occur
are skin, lungs, alimentary tract, and kidneys.

l. LUNGS - The water lost by the lungs averages 400 ml per day and
results from simple evaporation. This loss is constant and unaffected by other factors. NO salt accompanies this water.

2. SKIN - Water is lost as perspiration in order to "air-condition" the body. The body is cooled by evaporation of water. This may not be helpful in maintaining total water balance. Salts and traces of urea are lost with the water.

3. ALIMENTARY TRACT - The daily total volume of alimentary secretions may reach the staggering volume of 8,200 ml with all but l50 ml reabsorbed in the intestines. It is no wonder that protracted vomiting or diarrhea can cause death in a matter of hours without fluid replacement. Large amount of salts may also be lost.

4. KIDNEYS - Refer to Fluid Exchange kidneys and review the factors controlling urine volume.


Dehydration occurs when the loss of water and salt is greater than intake. Water and salt depletion never occur separately although one or the other usually predominates.

ECF Deficit: When salt depletion is greater, the extracellular compartments respond by excretion of an amount of fluid which corresponds to the amount of salt lost. After this compensation extracellar fluid and electrolytes are in balance.

Lab Test: Therefore, the hematocrit value is again used to monitor this condition. If the normal %HCT is 40, than a value of 45% HCT indicates an ECF deficit. The volume is less therefore the red blood cell amount which is constant give a higher % in a smaller volume.

ICF Deficit: When water depletion is predominant, the greatest fluid loss is sustained by the intracellular compartment. Initially, water loss leads to an increase in electrolytes in the extracellular compartment. This leads to an osmotic flow of water from the intracellular compartment. This is the reverse of the normal situation at #4 in the figure.

Lab Test: The sodium concentration is an indirect measure of the fluid conditions in the intracellular compartment. If the normal value of sodium is 140 mmole/L, than what does a value of 155 mmoles/L indicate? Again the higher value indicates that for the same amount of sodium it is not in a more concentrated condition, which means that the volume of water is less. So a higher sodium value indicates an ICF Deficit.

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Principles: For extracellular dehydration more salt is depleted than water. For intracellular dehydration more water is depleted than salt

Any loss of alimentary contents,whether by vomiting, suction, or diarrhea interferes with normal reabsorption of secretions containing both water and salts. Osmosis occurs into the GI tract rather than the normal active transport process out of it. An extracellular deficit results. Acid-base balance is also usually upset under these conditions as well.

2. HEAT EXHAUSTION: Salt output in the sweat is greater than oral intake. The kidney responds by conserving salt but the urine volume remains normal until fluid loss attains balance with salt loss. The plasma fluid-salt balance is restored at the expense of contracting the plasma volume. Muscular weakness results from an excessive loss of intracellular potassium, which is drawn into the intracellular compartment to compensate for the sodium.

3. BURNS AND SHOCK: Relatively more salt than water is lost as a reaction to burns or shock. The extracellular compartment is dehydrated by external losses or an osmotic shift of water into the intracellular compartment at #4 in figure on the left.

4. EXCESSIVE DIURETIC DRUGS: Diuretic drugs stimulate the kidney to excrete more salt and water than normal.

5. EXCESSIVE WATER LOSS: Fever and diabetes insipidus results in water loss due to a decrease in vasopressin and an increase in urine volume.

6. INSUFFICIENT WATER INTAKE: Water is unavailable or patient cannot swallow.

QUES. 30: Which of the above represent ECF or ICF deficit? What lab value would indicate each?

QUES. 29: Explain why drinking salt water will lead to death by dehydration?
Answer: Kidneys cannot prevent build-up of salt in the extracellular compartment. Therefore, water is drawn by __________ from the cells into the _____________. This results in __________urine volume. Finally for every quart of sea water drunk a quart and a half of urine is produced.