The human body is comprised of 60% water. Two-thirds of the body’s water are inside cells, or intracellular, and one-third is extracellular or outside the cells. Of this, 25% is inside the blood vessels and 75% is interstitial fluids.
The fluid is kept inside and outside the cell compartments by osmotic forces which are contributed by positive and negative ions. Potassium is the major positive ion in the intracellular fluid and sodium is the major cation in the extracellular fluid.
What we normally measure from the blood is what is seen in the serum. It is extracellular fluid. Normal sodium in the cell is 12 meq. Normal sodium in the extracellular fluid is 140 meq. Normal potassium intracellularly is 140 meq, and extracellularly is 3.5-5 meq. Osmotic forces exerted by these ions control the fluid in and out of the cells. Concentration of combined solute in water is called osmolality. This is the amount of solute per liter of water. Plasma osmolality can be measured and or calculated by formula. Normal body osmolality is 270-290 milliosmals per kilogram. Sodium is the major determinant of osmolality.
Water crosses freely across the membranes from areas of low solute concentration to areas of high solute concentration. The passage of electrolytes, like sodium and potassium, are restricted across the cell membranes by osmotic pressure. Body fluids and electrolytes are normally maintained within a narrow range. The kidneys are the main organs which control body fluids.
Sodium is a component of salt. Two and a half grams of salt provides one gram of sodium. All body fluids contain sodium, including blood and sodium is necessary to maintain fluid balance.
Functions of sodium include:
- regulation of fluid balance
- conduction of electrical impulses in nerves and muscles
Excess sodium in diet is absorbed readily and is regulated by the kidneys through excretion of urine. Sodium is present in much of every food, especially in processed foods. Requirement of sodium per day is 1.6grams. Too much sodium can lead to hypertension, heart disease and stomach cancer.
Potassium is an important ion in the body which contributes to a multitude of body functions, such as fluid balance, muscle contraction, synthesis of nucleic acid, nerve conduction, protein synthesis, and control of hear beat.
Functions of potassium
- -regulation of heart rhythm
- -blood pressure control
- -protein and nucleic acid synthesis
- -fluid balance
- -nerve conduction and muscle contraction
- maintenance of acid-base balance
Potassium is the principle intracellular cation.
Normal daily requirements:
- 0-3 yrs is 750-800 mg/day
- 4-10 yrs- 1100-2000 mg/day
- adults 3100-3500 mg/day
Sources of potassium include lima beans, bananas, avocados, tomatoes, raisins, cauliflower, and peas. Potassium supplementation is not recommended in healthy individuals. People who are taking diuretics and heart medications may need potassium supplementation. This should be done under the direction of a physician.