Salt, or sodium, is a mineral. It carries a positive charge. Table salt consists of positively charged sodium molecules bound to negatively charged chloride molecules. Sodium is necessary for many bodily functions. It is important for maintaining the electrical charge in cells, and distributing fluid in the body. In health, the body's physiology tightly regulates the sodium levels; certain diseases can disrupt these mechanisms and cause sodium imbalances.
Salt and Fluids
Water does not have a positive or negative charge; it is neutral. However, one end of a water molecule is positive, and one end is negative, which means it is a polar molecule. Sodium carries a strong positive charge. Therefore, many water molecules will be magnetically attracted to sodium, and where sodium travels, water follows. The body can be divided into different compartments with varied amounts of sodium and water. These include the space outside the cells, including within the blood vessels, the space outside the blood vessels, and the space inside of the cells. Sodium levels are much higher outside of cells, which has important implications for physiology.
Fluid Shifts and Osmosis
Sodium is absorbed through the gastrointestinal system and into the blood. Increased salt in the blood vessels encourages water to flow across the blood vessel walls from outside the blood vessels and from the cells. This tendency of water to flow across a membrane from areas of lower concentration of a chemical such as sodium to areas of higher concentration is called osmosis.
Short Term Changes
Too much salt intake will increase the fluid in blood vessels, which raises the blood pressure. It will trigger the brain to increase thirst to balance the increased salt by diluting it in the body. In addition, the kidneys will attempt to rid the body of excess salt through the urine.
Long Term Changes
Over the long-term, the elevated blood pressure will cause the heart to become larger. An enlarged heart is not as effective at pumping blood, and is a form of heart disease. Decreased blood supply to the kidneys causes them to release hormones that signal the body to retain even more salt and water in an attempt to increase the delivery of blood to the kidneys. Unfortunately, this ends up being counterproductive by increasing the strain on the heart. The body becomes salt- and fluid-overloaded, a condition called congestive heart failure.