Sodium chloride, more commonly known as table salt, contains two essential minerals that your cells require in order to maintain life, health and function. Without salt in your diet, you can't survive. The typical American diet, however, contains more than enough salt to meet your needs -- and often contains too much.
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Sodium chloride, with the chemical formula NaCl, is an ionic salt. This means that it's made up of positively charged particles of sodium interspersed with negatively charged particles of chloride. The particles aren't chemically bonded together; instead, they're held together by electrostatic forces, which are the physical attractions between unlike charges. In water, the sodium and chloride particles separate from each other, which is critical to the separate functions of sodium and chloride in the human body.
One of the simplest important roles of sodium chloride in the human body is in maintaining fluid balance. Osmosis is the name for the movement of water through permeable membranes toward areas of higher salt -- or other soluble particle -- concentration. Sodium and chloride in your blood and cells help keep you from losing too much water. For instance, when your kidneys filter blood, they take up sodium and chloride to keep them from being lost in the urine, which also helps draw water out of the urine and back into the blood.
Sodium is critical to establishing and maintaining a charge differential across each cell's membrane. Every cell is negatively charged inside, as there's more sodium outside the cell than inside, and sodium particles are positively charged. This is called the "resting membrane potential," and allows cells to communicate with one another. Nerve cells and muscle cells depend on the resting membrane potential to function; without it, nerve cells couldn't send messages and muscle cells couldn't contract.
While you need sodium chloride in your body to stay healthy, you don't necessarily need to add it to your diet. There's plenty of sodium chloride in the food most Americans eat; in fact, most people actually get too much salt in their food. Overdoing the salt can lead to high blood pressure and increased risk of cardiovascular disease. As such, while you need salt to survive, you shouldn't assume that more is better.