Eat less salt. It’s advice often recommended as a way to reduce blood pressure, but why? And if the body needs sodium (salt) to work properly, how does eating too much of it become unhealthy? In the cardiovascular system, excess sodium changes the body’s physiological processes to encourage high blood pressure, or hypertension. Sodium affects blood volume and the way two key organs—the kidneys and the heart—do their jobs.
Blood is made up of blood cells suspended in plasma—water containing proteins, nutrients, dissolved minerals and cellular waste. The kidneys purify blood by moving the water and everything in it, besides the proteins, out of the bloodstream into its ducts. The nutrients and minerals the body uses, including sodium, are then moved back into the bloodstream. Water is attracted to salt, so it follows sodium back into the bloodstream. The extra minerals and water left behind are filtered out, joined with waste products and excreted as urine.
Eating salt raises the sodium level in the plasma. As a result, more water gets reabsorbed into the blood and the total volume of the blood increases. The heart senses blood volume through how much its chambers are filled. When more blood is present, the heart contracts with greater force and pumps more blood out to the body. This increase in output causes blood pressure to rise.
The kidneys are eventually able to filter out excess sodium into the urine. However, constantly eating a lot of sodium maintains the elevated plasma sodium concentration, slowing down the return to normal blood volume levels and keeping already high blood pressure high.
Limiting dietary salt breaks this cycle. Blood volume decreases, the heart does not pump as strongly and blood pressure falls towards a healthy range. This is why “eat less salt” is heart-healthy advice to remember during American Heart Month and beyond.
Maggie Kuo, PhD, is the former Communications and Social Media Coordinator for APS. Catch more of her writing in the Careers Section of Science Magazine.
Reviewed by Barbara E. Goodman, PhD
Correction (3/16/15): An earlier version had said “The kidneys purify blood by moving plasma and everything in it out of the bloodstream into its duct.” Protein actually remains in the blood vessels and do not filter into the kidneys’ ducts. The text has been edited accordingly.