Cardiovascular Function: Heart Failure Is a Problem for Patients -- and for Evolutionary Theory
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." Dr. Glicksman practices palliative medicine for a hospice organization.
Since the body is made up of matter, it must live within the laws of nature. These laws demand that the body have enough energy to propel the amount of blood needed through the circulatory system to give its cells what they need to live. To do this, the heart must have enough power to overcome forces like inertia, friction, and gravity, which naturally prevent blood from moving anywhere.
The last few articles in this series have shown that the heart is a pump with its own blood supply, efficient one-way valves, and an electrical system that coordinates muscle contraction. In addition, experience teaches that the body uses the autonomic nervous system to control how hard and fast the heart pumps so it can meet its metabolic needs. Moreover, medical science has been able to determine the numerical values of these metabolic needs and how much blood the heart would have to pump out per minute to achieve them. The normal cardiac output at complete rest is about five liters per minute, but with maximal activity, the type our earliest ancestors would have needed to do to survive, the cardiac output needs to be at least 25 liters per minute.
Evolutionary biology may use its imagination to explain how human life and the cardiovascular system came into being, but it does so without taking into account the specific numbers that heart function must achieve so the body can survive. That's like trying to explain how the blueprints for a car and its assembly came about without taking into account its performance on the road. In other words, they explain how human life looks without taking into account how it actually works.
We have already seen that for the heart to perform well enough for our earliest ancestors to survive, their coronary arteries would need to have been wide enough to accommodate enough blood flow. How do we know this? Clinical experience shows that people with coronary artery disease and restricted blood flow to the heart muscle are incapable of performing the kinds of activities that would have been needed for survival.
We have also seen that for the heart to have been able to perform well enough for our earliest ancestors to survive, the valves within it would need to have been open wide enough to let enough blood flow forward and close tight enough to not let any blood go backwards. How do we know this? Because clinical experience shows that people with thickened valves, causing restricted forward blood flow, and/or weakened valves, causing leakage of blood backwards, have reduced cardiac efficiency, leading to weakness and shortness of breath often with limited activity. Now let's consider heart muscle function itself and its effect on survival capacity.
As you may recall, the cardiac cycle is made up of one-third systole and two-thirds diastole. During the diastolic phase, the heart relaxes and the ventricles fill up with blood. At rest, the volume of blood in the ventricle at the end of diastole (EDV) is usually about 120 mL. Systole then begins with ventricular contraction and the amount of blood pumped out with each heart beat is called the stroke volume (SV). At rest, the SV is usually about 70 mL. The ejection fraction (EF) is the ratio between the SV and EDV, showing what percentage of blood is pumped out of the ventricle with each heartbeat. The EF is a measure of how well the ventricle contracts. At rest the EF is usually about 60 percent (70/120) and the heart rate (HR) is about 72 beats per minute. The cardiac output (CO) is the amount of blood flowing out of the heart every minute. It can be calculated by the formula: CO = SV x HR. This means that the CO is directly related to the SV and HR. If the SV and HR both rise, so does the CO, and if they both fall, so does the CO.
Using this formula we can see that, at rest, the CO is usually about 5 L/min (70 times 72). Finally, with extreme levels of activity, the autonomic nervous system causes the heart to pump harder and faster. With maximum stimulation the EDV and EF can rise so the SV can be about 125 mL and the HR can rise to 200 beats per minute. This results in a CO of 25 L/min (125 times 200). Clinical experience teaches that if the heart of our earliest ancestors could not have achieved these high levels of cardiac output, they never could have survived to reproduce. How do we know this? Heart failure.
When the heart cannot meet the metabolic needs of the body it is said to be in heart failure. This common condition can involve either the left or right ventricle, or both at the same time. Left ventricular failure can also be systolic, with diminished contractility during systole, or diastolic, with reduced relaxation and filling of the ventricle during diastole. Coronary artery disease, resulting in diminished blood flow, and frequently damage to the heart muscle, is the commonest cause of systolic heart failure. The hallmark of this condition is an EF below normal, often with a lower SV, because the myocardium can't contract well. Coronary artery disease, hypertension, and aortic stenosis, which causes the heart muscle to thicken, are common causes of diastolic heart failure. This muscle thickening results in stiffening of the ventricular walls and limits the entry of blood during diastole which lowers the EDV and the SV as well.
Most people with heart failure can function well at rest. And for those who have mild heart failure, the heart can compensate by increasing the heart rate and size of the ventricular cavity, allowing for activities like slow walking. But for people with significant heart failure, increased levels of activity are physically impossible because their CO can't match their body's metabolic needs. In other words, real numbers can lead to debility.
People with heart failure must live relatively sedentary lives. They are incapable of performing at the high levels of activity that our earliest ancestors would need to have been able to perform to win the battle for survival. When trying to explain how human life came into being, there should be some discussion about how the heart just so happens to have the right amount of ventricular contractility and relaxation to allow its output to meet the metabolic needs of the body.
So far in this series I've demonstrated how impaired coronary blood flow, valve dysfunction, and heart failure can each lead to significant debility, and we have discussed related problems posed for evolutionary explanations of our biological origins. However, there is still one more cardiac problem to consider when it comes to how real numbers can result in debility. That's what we'll look at next time.
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