Sodium is one of the most important minerals in the body. The kidneys use sodium to maintain the proper balance of water and electrolytes—chemical substances such as sodium and potassium—and sodium is critical for water retention. But this very characteristic can cause problems with blood pressure.
Water and Sodium in the Body
According to "Fluid and Electrolyte Balance: Nursing Considerations," by Norma Metheny, the male human body is about 60 percent water by weight; females have a slightly lower percentage of water because they have more body fat. This water is both extracellular—outside of the body cells—and intracellular, or inside the cells. The human body must have water, but too much water is just as bad for the body as too little, and the kidneys use electrolytes such as sodium, potassium and chloride to maintain balance. Water moves through a cell membrane toward the side that has a higher concentration of sodium; each kidney has a filtration system in which water and electrolytes pass back and forth through cell membranes. Waste products and excess water are filtered from the blood and excreted in the urine, while the sodium necessary for body functions is returned to the bloodstream. While the complete process is much more complex than described, the simplified explanation is that increased sodium in the body can mean increased water both inside the cells and circulating in the blood vessels, and that water causes an increase in blood pressure.
Sodium and Blood Pressure
Humans ingest sodium by eating salt in food and occasionally in medications. Most people will simply excrete excess sodium in the urine, and we also lose sodium and water through perspiration. But about 20 percent of the population has a genetic characteristic that causes sodium retention, which results in water retention, and can increase blood pressure. Eating salt also makes people thirsty, which means they tend to drink more, and this makes the situation worse. Too much salt keeps the amount of fluid circulating in the body higher than it should be, which means increased pressure on the blood vessel walls. The walls thicken and narrow under the continued high pressure, and the heart must pump harder to move fluid around. This increased work makes the heart muscle larger and increases the force of each contraction; the higher pressure damages the blood vessels—and the filtration system—in the kidneys and raises blood pressure.
How Much Salt?
According to the American Association of Kidney Patients, lower salt usage means lower blood pressure. The association recommends less than 5g, or 100 nanomols of sodium daily, which translates into 2,300 mg per day. According to the Cleveland Clinic, salt should be restricted to no more than 1,500mg, which is considerably less—this recommendation originated with the American Heart Association. One teaspoon of salt contains about 2,400mg of sodium. Foods such as bacon, sausage, canned soups, snack foods and pickled foods all tend to be high in salt; the Cleveland Clinic recommends you do not eat them if you have high blood pressure.
Considerations and Warnings
High blood pressure is a potentially serious condition and may need medication as well as dietary changes. While decreasing the amount of salt in your diet may be helpful, if you think or know that you have high blood pressure, always check with a health care professional.