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.

In the last two articles in this series (here and here), I showed that since blood consists of matter and has mass, it must overcome forces including inertia, friction, and gravity to be powered to where it needs to go in the body. The heart is the muscular pump that does this job and its rate and force of contraction are controlled by the autonomic nervous system. Sensors in the muscles detect the increasing physical and chemical changes that take place during exercise and notify the brain. The brain responds by activating the sympathetic division of the autonomic nervous system which sends out more norepinephrine and epinephrine.

These adrenergic neurohormones attach to specific receptors in the cardiovascular system, which results in more blood being pumped out of the heart. But how much is enough? This is something that, obviously, would have been very important for our earliest ancestors to stay alive and be able to reproduce.

When it comes to the laws of nature, real numbers have real consequences. Just ask the firefighters trying to put out a three-story blaze. They must spray enough water over it as fast as they can to prevent it from being destroyed. They do this by using a pump to propel water upwards against inertia, the friction within the hose, and the force of gravity involved in climbing three stories. Their success depends not only on how much water they can apply, but also how fast they can get it there.

If they have enough water but it takes too long to get where it needs to go, the fire will destroy the building. If they can get the water there quickly, but they don’t have enough, then the building will go down in flames. The right amount of water in the right amount of time is what is needed.

Similarly, since our cells need the right amount of chemicals, like oxygen, to live and function properly, the body must send enough blood in the right amount of time to the tissues. Not just any amount will do. It has to be the right amount and it has to get there in time.

Modern evolutionary theory claims to explain how the heart and the control mechanisms involved in its function have come into being but it doesn’t even mention the fact that it has no explanation for how the body knows exactly how fast its blood flow should be able to survive within the laws of nature.

The amount of blood flow in the circulation depends directly on how much blood the heart can pump within a specific amount of time. This is usually measured in liters per minute (L/min) and is called the cardiac output (CO). Just as a fire pump’s flow capacity will determine how effective it is at putting out fires, so too the cardiac output will determine how effective the heart is at providing enough blood flow to the tissues to meet their energy needs.

The cardiac output depends on two things; how much blood the heart pumps out each time it pumps and how fast the heart is beating. The amount of blood pumped out of the heart with each contraction is called the stroke volume (SV) and is measured in milliliters (mL). The number of times the heart beats within a minute is called the heart rate (HR) and is measured in beats per minute (bpm). The relationship between the CO and the SV and HR is CO = SV x HR.

In general, the CO is directly related to the SV and HR in that if the SV and the HR rise so does the CO, and if they drop, so does the CO. It would seem that the body understands this relationship and knows exactly what it needs to do to stay alive and be competitive within the laws of nature. Let’s look at what exactly it must be able to do.

At rest, the average male body needs about 250 mL/min of oxygen (O₂) and the amount of blood in each of his ventricles just before they contract is about 120 mL. The normal ejection fraction, the percentage of blood the heart pumps out with each contraction, is about 60 percent. This means that at rest, the normal stroke volume is about 70 mL/beat (120 x 0.6).

At rest the autonomic nervous system normally sets the heart rate at about 72 bpm. Recall that CO = SV x HR, so by multiplying the SV (70 mL) by the HR (72 bpm) we get a CO of about 5 L/min at rest. The average male has 150 gm of hemoglobin per liter in his blood, which can carry about 200 mL/liter of O₂. In addition, the tissues extract about 25-70 percent of the available O₂ in the blood. If the CO at rest is 5 L/min, this means that the body can provide itself with 1,000 mL/min of O₂ (200 mL/L x 5 L/min). Since it only needs 250 mL/min of O₂ at rest, the body only uses about 25 percent of the O₂ available to it.

The O₂ requirements of higher activity levels are: walking slowly: 500 mL/min, walking quickly: 1,000 mL/min, moderate jogging: 2,000 mL/min, and very quick running (the kind of activity our earliest ancestors needed to survive): 3,500 mL/min. Increasing levels of activity cause the sympathetic nerves to release more norepinephrine and epinephrine, which results in an increase in the venous return of blood to the heart, an increase in the heart rate, and an increase in the force of ventricular contraction.

In people with normal heart function, all of these effects combine to cause a rise in the CO; for slow walking: 7 L/min, fast walking: 12 L/min, and moderate jogging: 18 L/min. However, what cardiac output would our earliest ancestors have needed to stay active enough to win the battle for survival?

Since we know they would have needed 3,500 mL/min of O₂ to do the things they would have needed to survive, their blood normally would have been able to carry about 200 mL/min of O₂, and the maximum amount of the available O₂ their tissues could have removed was about 70 percent, we can calculate what their CO would have to have been. By dividing 3,500 by 0.7 we see their blood would need to have carried 5,000 mL/min of O₂ to be able to use just 3,500 mL/min (70 percent).

Then, by dividing 5,000 mL/min by 200 mL/L, the amount of O₂ each liter of blood can carry, we find that the CO would need to have been at least 25 L/min. This number is identical to the maximum CO for the average male adult, although well-trained athletes can raise their CO to 35 L/min or more.

Recall that at rest the SV is usually 70 mL and the HR is about 72 bpm, generating a CO of 5L/min. But with extreme exercise and maximum stimulation from the sympathetic neurohormones, the SV can rise to about 125 mL and the HR to about 200 bpm, generating a CO of 25 L/min. So it would appear that the system that controls heart function really does know what it is doing.

The mechanism involving the cardiovascular and autonomic nervous systems is irreducibly complex. If any one part is missing or not working properly, the system fails and the body dies. But there’s more. The body needs to have an inherent knowledge of what is required to stay alive. I call this quality natural survival capacity,because cardiac function, controlled by the autonomic nervous system, must have the natural capacity to adequately meet the metabolic needs of the body if it is to survive under the laws of nature.

What happens when it can’t meet these needs? That’s what we’ll consider next time.

Image credit: U.S. Navy photo by Mass Communication Specialist 3rd Class Scott A. Raegen [Public domain], via Wikimedia Commons.