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 human body consists of trillions of cells made up of atoms and molecules that, in the way of all matter, must follow the laws of nature. These laws demand that the cell have enough energy, water, and various chemicals to do what it must to live and work properly.
The cells get what they need from the blood, which travels throughout the body in the cardiovascular system. The fluid component of blood is made up of water. The lungs may bring in oxygen, which is grabbed by hemoglobin to be transported to the cells. But it’s the red blood cells (which contain hemoglobin) that require the water in blood to be able to float within the circulation to get the oxygen to where it’s needed.
By the same token, the gastrointestinal system may bring in salt, glucose, and other chemicals and place them in the bloodstream. But, many of these chemicals must first be dissolved within the water contained in blood before they can be sent to the tissues. So, not only must the body make sure its cells have the right amount of water, it must also be sure that there’s enough water within the blood that travels inside the circulation as well.
Therefore, to begin to understand how the cardiovascular system works you must first understand where water is located in the body and how the laws of nature affect its movement. For if there isn’t enough water in the body or it’s not distributed properly, the body dies.
Water is the body’s most abundant molecule, making up about 60 percent by weight. The water in our body is located in one of two places: either inside or outside our cells. Two-thirds of our body’s water is located within its trillions of cells and is called the intracellular fluid. The remaining one-third is located outside the cells and is called the extracellular fluid.
About 80 percent of this extracellular water is located between the cells and is called the interstitial fluid while about 20 perent is located within the blood and is called the plasma.
The water in the body acts as a solvent for the chemicals that are vital for life. These chemicals in solution include glucose, sodium, potassium, and many different proteins. The amount of water within the cell, the interstitial fluid, or the plasma is called its volume. And the total number of chemical particles dissolved within a given amount of the volume is called the total chemical concentration. If water enters the cell, the interstitial fluid, or the plasma, its volume rises and its total chemical concentration falls. Conversely, if water leaves the cell, the interstitial fluid, or the plasma, its volume falls and its total chemical concentration rises.
To live and work properly the cell must keep its volume and total chemical concentration relatively constant. However, if not resisted by the sodium-potassium pumps within the plasma membrane, diffusion and osmosis would not only drastically alter the chemical concentration inside the cell, but would also make so much water enter that it would die by explosion.
Moreover, given that the water being naturally pulled into the cell would be coming from the extracellular fluid, this also means that without the sodium-potassium pump there wouldn’t be enough water in the plasma. In other words, in following the rules, the body must take control with its innovation of the sodium-potassium pump to preserve the 2/3:1/3 relationship between the intracellular and extracellular fluid. As we’ve seen repeatedly in this series, when it comes to life and death, real numbers have real consequences, and it’s this 2/3:1/3 ratio that allows the cells to maintain their proper volume and total chemical concentration while at the same time making sure there’s enough blood in the circulation.
Within the context of this 2/3:1/3 relationship, the laws of nature also demand that when there’s a difference in the total chemical concentration on either side of the plasma membrane, water will naturally move from the one of lower concentration to the one with higher concentration. This occurs by the power of osmosis. When there’s a change in the total chemical concentration outside or inside the cell, osmosis naturally makes water move from inside the cell to outside the cell or vice versa, to make sure the total chemical concentration on either side of the plasma membrane is the same.
In fact, osmosis affects not only how water moves between the cells and the interstitial fluid, but also between the interstitial fluid and the plasma. The cells lining the capillaries allow water to move back and forth between the circulation and the interstitial fluid. This water is then able to move back and forth between the interstitial fluid and the cell. So one can see that the interstitial fluid acts as a bridge between the cells and the circulation.
Now that you understand how the laws of nature determine where water moves within the body, and how the sodium-potassium pump preserves the important 2/3:1/3 relationship between the intracellular and extracellular fluid, we need to consider another problem.
Recall that 80 percent of the extracellular fluid is in between the cells and 20 percent is in the plasma within the blood. If water can naturally move across biological membranes, like the ones surrounding the capillaries, what’s to stop most of it from going from the blood into the interstitial fluid?
Such a massive loss of water from the circulation would render the cardiovascular system unable to adequately feed the tissues, resulting in death. It’s the sodium-potassium pump that preserves the 2/3:1/3 relationship between the intracellular and extracellular fluid.
But what innovation maintains the 80:20 relationship between the interstitial fluid and the plasma? That’s what we’ll look at next time.
Image: adimas / Dollar Photo Club.