Salt is Life

Why do we need to eat salt to live?

The two commonest salts in your diet are sodium chloride (table salt) and potassium chloride (salt substitute).  They are called ionic compounds because each atom carries an electrical charge.  The sodium (Na) and potassium (K) atoms are positive ions because they each give up one electron to the chlorine (Cl) atoms which are negative ions.  When sodium chloride (NaCl) and potassium chloride (KCl) enter your body they dissolve within your water and the positive and negative ions separate from each other, becoming Na+ ions, K+ ions and Cl- ions. 

The water in your body is located either inside your cells or outside your cells, and if outside your cells, is either in between and surrounding them or within your blood.  The chemical make-up of the water within your cells is totally different than it is for the water outside your cells.  The water inside your cells has a high potassium and low sodium content whereas the water outside your cells has a high sodium and low potassium content.  Scientists know that this difference in chemical content between the water inside and outside your cells must always be maintained for life to survive.  Since there are ion channels within the plasma membrane that allow Na+ ions to leak in, and K+ ions to leak out, the cell has about a million sodium-potassium pumps in its plasma membrane to send most of them back to where they came from.  This action requires a lot of work and it’s been estimated that, at complete rest, about one-quarter to one-half of your body’s energy is used for the sodium-potassium pumps to maintain the chemical content of the water inside and outside your cells.

Sodium is vital for life because it sets how much water stays outside your cells to make up the blood within your circulation.  Here’s why.  Due to the laws of nature, as the sodium-potassium pumps push Na+ ions out of your cells, water goes out with them.  Without enough Na+ ions outside your cells, there wouldn’t be enough water there either.  This means that there would be no blood volume, no blood pressure, no blood flow, and no life.  It also means that the sensors in your cardiovascular system that detect blood volume, blood pressure and blood flow, also inform the body about its sodium content.          

Potassium is vital for life because it allows all of your cells, especially the ones in your heart, nervous system and muscles to work properly.  Here’s why.  Despite the work of the sodium-potassium pumps in the plasma membrane, some K+ ions leak out of the cell while some Na+ ions leak back in.  The amount of K+ ions lost in this manner is greater than that of the Na+ ions gained.  This net loss of positive ions from the cell makes the inside of the plasma membrane carry a negative electrical charge and the outside a positive one.  This difference in the electrical charge across the cell’s plasma membrane is called the resting membrane potential.  Your heart, nervous system and muscles can’t work properly unless the resting membrane potential is maintained at a certain level.  And this level is directly related to the difference between the K+ ion content of the water inside and outside your cells.   

Although your body always loses sodium and potassium through the gastrointestinal system and the skin (particularly with perspiration) the main way these ions are lost is through the kidneys.  As your kidneys filter water out of your blood they also filter out the sodium and potassium ions dissolved within it.  If the kidneys couldn’t bring back any of this filtered sodium, you would die in about one-half hour, and if none of the potassium, in about a day.  It turns out that, under proper control, the kidneys bring back about 99.5% of the sodium and 90% of the potassium they filter out each day.  So, to make sure you don’t run out of sodium or potassium, you must have salt (sodium and potassium chloride) in your diet.  But how much is enough and how does your body make sure you don’t have too much, as this can cause problems as well?

Most of the sodium you take into your body goes into the water outside your cells and most of the potassium goes into the water inside your cells. Through various sensors, the body monitors its blood volume, blood pressure and blood flow and, in so doing, provides information about its sodium content.  Also, specialized cells in the adrenal glands seem to be able to detect the ratio between the levels of Na+ and K+ ions in the blood.  The information from all of these sources eventually affects the release of an adrenal hormone called aldosterone

Aldosterone goes to the hypothalamus and tells the body to bring in salt and it also attaches to specific aldosterone receptors in the kidneys and tells them how much sodium to bring back into the body and how much potassium to release.  In general, the less blood volume, blood pressure and blood flow, and the lower the ratio between Na+ and K+ ions, the more aldosterone is sent out to bring back more sodium into the body and release more potassium.  And the more blood volume, blood pressure and blood flow, and the higher the ratio between Na+ and K+ ions, the less aldosterone is sent out to release more sodium from the body and bring more potassium back in.  In this way, your body makes sure it has the right amount of sodium and potassium no matter how much salt you eat, but nonetheless, that’s why you have to eat salt to live.  

Three Questions for Mr. Darwin

    1. Where did the information come from to produce enough sodium-potassium pumps for each of my cells, and tell them what to do, so my body could control its distribution of water inside and outside its cells and their chemical content as well?

    2. How did life anticipate its need to control sodium and potassium and so produce all the parts needed (sensors for blood volume, blood pressure, blood flow, the ratio of Na+ and K+ ions in the blood, adrenal cells to produce aldosterone and the presence of specific receptors on the kidney cells) to perform these tasks and how did it survive each step along the way?

    3. How do my adrenal glands know how much aldosterone to send out for a given ratio of Na+ and K+ ions in the blood and where did the information come from to teach my kidneys how to respond properly? 


Also see Dr. Glicksman's Series on

"Beyond Irreducible Complexity"

"Exercise Your Wonder"

Howard Glicksman M. D. graduated from the University of Toronto in 1978. He practiced primary care medicine for almost 25 yrs in Oakville, Ontario and Spring Hill, Florida. He now practices palliative medicine for a Hospice organization in his community. He has a special interest in how the ethos of our culture has been influenced by modern science’s understanding and promotion of what it means to be a human being.

Comments and questions are welcome.

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