Intelligent Design Icon Intelligent Design
Medicine Icon Medicine

Understanding Cardiovascular Function: How Water Stays Within the Circulation

Dollarphotoclub_54589973.jpg

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.jpgOnce oxygen (from the lungs) and salt, glucose, and other nutrients (from the gastrointestinal system) enter the blood, it’s the job of the cardiovascular system to get them to the cells. However, to do this requires that there be enough blood within the circulation. The previous article in this series showed that an adequate blood volume requires not only sufficient water, but in general, a 2/3:1/3 relationship between the intracellular and extracellular fluid and a 80:20 relationship between the interstitial fluid and the plasma.

Moreover, we saw that it is the sodium-potassium pumps within the plasma membrane of the cells that allow the body to take control to maintain the 2/3:1/3 ratio in order to follow the rules of diffusion and osmosis. Now we need to find out what innovation maintains the 80:20 relationship between the interstitial fluid and the plasma. To begin to solve this problem we must first look at how the cardiovascular system works and how some of the laws of nature affect its function.

The cardiovascular system consists of the heart and the blood vessels, and is responsible for circulating the blood throughout the body. The blood travels from the left side of the heart, through the arteries to the capillaries where the exchange of chemicals takes place between the circulation and the interstitial fluid. The interstitial fluid then exchanges chemicals with the cells, acting as a bridge between the cells and the circulation.

On leaving the capillaries, the blood travels in the veins, picking up water, salt, glucose, and other nutrients from the gastrointestinal system, on its way to the right side of the heart. The blood then travels from the right side of the heart to the lungs where it picks up oxygen and drops off carbon dioxide. It then returns to the left side of the heart and the circuit is repeated.

Since blood consists of matter, and thus has mass, it is subject to the law of inertia, which is the tendency of an object at rest to remain at rest. That means the blood requires a source of energy to power it to where it needs to go.

The left side of the heart uses energy to pump the blood into the arteries. This generates a hydrostatic pressure, which is the force that the blood applies against the walls of the blood vessel as it flows through it. Think of it like the air pressure in a tire or the pressure you feel against the walls of a pipe as water flows through.

The blood flows through the arteries and arterioles, using up energy. By the time it travels from the heart to the capillaries, the hydrostatic pressure has dropped from about 100 units (at the heart) to about 35 units (at the capillary). As the blood then leaves the capillary, the pressure drops to about 15 units. The walls of the capillaries contain tiny pores and as the blood moves through them the hydrostatic pressure pushes water out of the blood into the interstitial fluid. It’s like squeezing boiled potatoes through a ricer.

In fact, as blood continues to flow into the capillaries, this pressure can force an amount of water equal to the total plasma volume out of the circulation within just a few minutes. It’s obvious that if the body didn’t have a way to force most of this squeezed out water immediately back into the circulation, human life would not exist.

The force that opposes filtering water out of the circulation by the hydrostatic pressure is osmosis. Plasma mainly consists of water (90 percent) and chemicals in solution such as glucose and sodium. But it also contains plasma proteins that perform various functions.

The main plasma protein that provides this osmotic effect is albumin. Remember, a solute exerts an osmotic pull on water across a membrane based on its inability to leave that solution. The total protein content of blood is much higher than in the interstitial fluid. And since the protein can’t pass through the capillary wall, this means that, by the power of osmosis, water naturally tends to move from the interstitial fluid back into the circulation.

This osmotic pressure exerted by albumin can be measured and is equal throughout the capillary. In fact, a normal plasma level of albumin provides an osmotic pull (to bring water back into the circulation) of about 25 units.

Recall that the hydrostatic pressure squeezing water out of the circulation at the start of the capillary is about 35 units, while at the end it’s about 15 units. This means that when the blood enters the capillary, the net flow of water is +10 units (35 – 25), meaning that it’s moving from the circulation into the interstitial fluid. When it leaves the capillary, the net flow of water is -10 units (15 – 25), meaning that it’s moving from the interstitial fluid back into the circulation.

Thus albumin helps the body bring back most of the water that is squeezed out of the capillaries by the hydrostatic pressure that moves blood through the cardiovascular system. Of course, the filtering of water through the capillary is dependent on many other factors as well. However, this provides a basis for understanding why having the right amount of albumin in the blood is vital for maintaining the circulation.

So, in following the rules, the body must take control with its innovation of albumin to help preserve the 80:20 relationship between the plasma and the interstitial fluid. This is needed so the body can have enough blood volume and the circulation can adequately feed the cells.

As we’ve noted before, when it comes to life and death, real numbers have real consequences. The normal range of albumin is 3.5-5.0 units and nobody really knows how the body controls its production nor how the liver knows how much to make to keep the body alive. Clinical experience shows that when severe malnutrition or certain liver, kidney, or gastrointestinal disorders cause the serum albumin to drop below 2.0 units, the osmotic pull of water back into the circulation is so diminished that it allows a lot of water to stay inside the interstitial fluid, forming what is called edema.

Edema of the tissues results in organ malfunction. When it affects the lungs it can result in respiratory failure and death. Because water tends to move out of the blood into the interstitial fluid, the blood volume drops as well, resulting in severe weakness, fatigue, and dizziness on standing. In fact, a plasma albumin level that is less than 1.0 unit is thought to be incompatible with life.

It’s mainly the sodium-potassium pumps in the plasma membrane of your cells and the albumin in your blood that together make sure the water in your body is properly distributed. Despite the fact that medical science really doesn’t understand how the body controls the production of albumin or knows how much it should make, evolutionary biologists reassure us that all of this came about by chance and the laws of nature alone.

But, what in the body controls its total water content? After all, even if water is properly distributed in our body, if we don’t have enough water, we die. That is a subject we’ll begin to tackle next time.

Image: abhijith3747 / Dollar Photo Club.