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Understanding Cardiovascular Function: The Vital Role of Sodium

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Editor’s note: Physicians have a special place among the thinkers who have elaborated the argument for intelligent design. Perhaps that’s because, more than evolutionary biologists, they are familiar with the challenges of maintaining a functioning complex system, the human body. With that in mind, Evolution News & Views is delighted to present this series, “The Designed Body.” For the complete series, see here. Dr. Glicksman practices palliative medicine for a hospice organization.

the-designed-body4.jpgThe laws of nature demand that each cell have enough energy, water, and appropriate chemicals to do what it needs to do to live and work properly. The cells get what they require from the blood, which travels through the body via the cardiovascular system. However, cardiovascular function depends on having sufficient blood volume. That in turn depends on having enough water in the body, which must be properly distributed.

The last several articles in this series considered how the body distributes and controls its water content. The sodium-potassium pumps in the cell’s plasma membrane maintain the 2/3:1/3 ratio between the water inside the cells (intracellular) and the water outside the cells (extracellular). The albumin in the blood maintains the 80:20 ratio between the water surrounding the cells (interstitial) and what is inside the circulation (plasma). Finally, the body uses Anti-Diuretic Hormone to control its water content by properly stimulating the thirst center and adjusting the water loss from the kidneys.

However, having enough water in the circulation is dependent on the presence of another chemical. That chemical is sodium. If the body didn’t have enough sodium, there wouldn’t be enough water in the circulation and there would be no human life.

The chemical name for table salt is sodium chloride and its chemical formula is NaCl. NaCl is an ionic compound, because the two atoms that join to form it each have an electrical charge. Sodium (Na) gives up one of its electrons to chlorine (Cl) and becomes a positively charged sodium ion (Na+). Chlorine gets one extra electron from sodium and becomes a negatively charged chloride ion (Cl).

When table salt is dissolved in water, the Na+ and Cl ions are released from each other. This means that when NaCl enters the intracellular and extracellular fluid it breaks up into Na+ and Cl ions. The number of Na+ ions in a given volume of water is called the Na+ ion concentration.

As Na+ ions, by diffusion, naturally enter the cell through the plasma membrane, the sodium-potassium pumps constantly push them back out again. Due to this action, over 90 percent of the body’s sodium is in the extracellular fluid and its Na+ ion concentration is about ten times that of the intracellular fluid.

Also, due to the high protein level within the cell, as Na+ ions move inside, water tends to go with it. If this water movement into the cell were not opposed, it could seriously increase the volume of the cell, with fatal consequences. Remember that in biology, a solute exerts an osmotic pull on water across a membrane based on its inability to leave that solution.

By forcing Na+ ions out of the cell, the sodium-potassium pumps effectively make Na+ ions unable to leave the extracellular fluid. In doing so, the sodium-potassium pumps make Na+ ions capable of osmotically pulling water out the cell as they leave. In fact, it is an axiom of medicine that wherever Na+ ions go in the body’s fluids, so do water molecules.

This makes the Na+ ion the most important positive ion (cation) in the extracellular fluid. And since the extracellular fluid includes the plasma, this means that the Na+ ion is the most important cation in the blood.

In dealing with the laws of nature, our body is constantly losing sodium, mainly through the gastrointestinal system, perspiration, and the formation of urine. To do its job the digestive system secretes a lot of fluids containing saline (NaCl) and sodium bicarbonate (NaHCO3). Most of the sodium in these fluids is brought back into the body, but some of it is lost through the feces.

The body must also maintain its temperature within a narrow range so its enzymes can work properly. Just like a car engine, our metabolism gives off heat, which affects our temperature. The more active we are, the more heat our cells release. One of the main ways the body controls its temperature is to release heat through perspiration. The more active we are and the hotter and more humid our surroundings, the more water and sodium we lose through this process.

Finally, protein metabolism produces ammonia, which is converted in the liver into the more soluble urea. Just like carbon dioxide, the build-up of ammonia and urea in the body can be toxic. The kidneys continuously filter water, which contains sodium, from the blood. This fluid moves through millions of tubules and becomes more concentrated with urea as it becomes urine. There is a minimum amount of urine, containing sodium, that the body must excrete to rid itself of the toxins (like urea) it produces on a daily basis.

No matter how much sodium is in the body, the sodium-potassium pumps in the plasma membrane constantly push Na+ ions out of the cell and back into the extracellular fluid. The sodium-potassium pumps only know what the cells need to do to survive and are blind to the sodium needs of the body as a whole.

When it comes to the laws of nature, the body, by necessity, is constantly losing sodium through the stool, perspiration, and urine formation. However, the body must take control to be sure it has enough sodium to maintain its blood volume. How does it do this? That’s the question we’ll consider next time.

Image: Salt-works, L�s�, Denmark, by � 2011 by Tomasz Sienicki [user: tsca, mail: tomasz.sienicki at gmail.com] (Photograph by Tomasz Sienicki (Own work)) [GFDL or CC BY 3.0], via Wikimedia Commons.