Sodium (Na) is the major cation (positively charged ion) in the extracellular fluid and plays a variety of fundamental roles within the body.

• Sodium plays a large role in fluid balance, with different variations of sodium determining the shift of water by osmosis from one area of the body to another. See Cellular Control of Water Distribution
• Along with chloride and bicarbonate, sodium is important for the proper regulation of the acid-base balance of the body.
• As an active transport mechanism in the form of Na, K –ATPase, sodium is essential for the passage of metabolic materials through cell walls.
• Sodium, along with potassium, is responsible for balancing the response of nerves to stimulation, travel of nerve impulses to muscles, and muscle contraction.

The body easily absorbs sodium into the intestine during digestion. Sodium is excreted primarily through the kidney under the control of the hormone aldosterone (see Organismal Control of Water Distribution.

Sodium's role in the balance of fluid and electrolytes was discussed in the previous section. Reduction in extracellular sodium concentration, or hyponatremia, can result from a shift of water from the cell due to accumulation of solutes, body water retention, a loss of sodium, or a shift of sodium into cells. Abnormal serum sodium levels can affect the action of the muscles, especially of the heart.

The estimated minimal requirement for Sodium intake, as set by the Food and Nutrition Board of the National Academy of Sciences, is 500 milligrams (mg) per day for adults and children 10 year of age and older. The minimal requirement for infants ranges from 120 to–200 mg. For children under 10 years of age the requirement is in the range of 225 to 400 mg. An intake of 2 to 3 grams per day is recommended.

Dietary sodium primarily comes from table salt and products containing high levels through processing. Sodium is also found in most food and other sources including milk, meat, eggs and vegetables.