nderstanding Cardiovascular Function: Evolutionary Biologists Face a Catch-22
Howard Glicksman September 11, 2015 11:16 AM
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.
The last several articles in this series have looked at the heart: its structure, how it works, how it is controlled, and how it helps the body meet its metabolic needs, given the laws of nature. We also reviewed the four main conditions that cause the heart to malfunction resulting in significant debility: coronary artery disease, valvular heart disease, heart failure, and some of the arrhythmias.
But what exactly happens to make the heart suffer cardiac arrest? It's important to realize that although the conditions mentioned in the last several articles can predispose a person to die from cardiac arrest, they are not the final event that actually causes it. Understanding how life came into being requires understanding how the body dies of cardiac arrest as well. It's like knowing all the different ways a car can "die" and then working backwards to figure out how to prevent them from happening. If you think about it, that's exactly how the control systems in the body are set up: to prevent not only respiratory but also cardiac arrest.
Cardiac arrest is the sudden stoppage of effective cardiac output, where the heart is no longer able to provide enough blood flow to sustain life. Clinical assessment will show that the person is unconscious and unresponsive to tactile stimulation and not breathing because the brain does not have enough blood flow for it to work properly. In addition, a pulse will not be palpable and no heart sounds will be heard because the heart is not pumping blood effectively enough.
There are basically three different conditions that can cause cardiac arrest. One is called asystole, (flat-line) where there is no electrical activity and no ventricular contraction. This results in no systolic function and therefore no cardiac output and blood flow within the circulatory system. Another is called pulseless with electrical activity(formerly called electromechanical dissociation), where there is electrical activity but no mechanical response from the ventricles. This is effectively like asystole in that there is no systolic function and therefore zero cardiac output and blood flow within the circulation.
The third are the malignant and life-threatening ventricular arrhythmias such as ventricular tachycardia, ventricular flutter, and ventricular fibrillation. Ventricular tachycardia (100-250 bpm) and ventricular flutter (150-300 bpm) are very rapid and regular rhythms that can severely limit cardiac output and blood flow within the circulation. If prolonged, these tachyarrhythmias usually degenerate into ventricular fibrillation. Ventricular fibrillation is an extremely rapid ventricular arrhythmia (400-600 bpm) in which there is electrical disorganization and an absence of coordinated ventricular contraction. Essentially, the ventricle quivers and is unable to effectively pump blood, leading to minimal, if any, cardiac output and blood flow within the circulatory system.
The malignant and life-threatening ventricular arrhythmias take place due to electrical irritability within the heart muscle and are most often associated with coronary artery disease and/or left ventricular failure. If a person dies unexpectedly from a cardiac arrest, it is called sudden cardiac death and is most often due to one of these arrhythmias. In fact, it is unfortunate that the first symptom for 25 percent of people with coronary artery disease is sudden cardiac death.
There are many different conditions that can lead to the other two causes of cardiac arrest, which effectively results in no systolic function and zero cardiac output. Both of these conditions require a massive failure in cardiac function. Although it is the sinus node that naturally paces the heart (usually at about 60-100 bpm), there exists a hierarchy of other cells below it that can also automatically depolarize and pace the heart, albeit at lower rates.
So, if the sinus node malfunctions, the AV node takes over with what is called a junctional rhythm, usually at a rate of 40-60 bpm. And if the AV node malfunctions, the specialized conduction tissue below it automatically paces the heart at even a lower rate of 20-40 bpm. People who experience this usually feel very tired and weak and have problems even walking slowly. However, when there is no systolic function, whether there is electrical activity or not, something catastrophic must have happened. Here are a few of the usual culprits.
It is important to remember that all of the parts of the heart are made up of cells, each of which must control its volume and chemical content and require certain chemicals to work properly. So, either a major blood flow problem to the heart or the lungs or a generalized metabolic problem in the body can lead to a sudden loss of systolic function. If not reversed, ventricular fibrillation usually degenerates into asystole because there is limited, if any, blood flow going through the coronary arteries. This results in the death of most of the heart cells.
A sudden lack of sufficient coronary blood flow caused by the blockage of either a major vessel or several branches, due to a clot (thrombosis), can do the same thing. A major blockage of blood flow in the pulmonary arteries, due to clots coming from the leg or pelvic veins (pulmonary embolism), not only drastically reduces the blood level of oxygen, but also the return of blood to the left side of the heart and leads to asystole.
Severely low blood pressure (hypotension) often due to low blood volume (hypovolemia), from either excessive blood loss (hemorrhage) and/or excessive water loss (dehydration) results in limited blood flow to the heart cells, which leads to cell malfunction and death from asystole. Very low blood levels of oxygen (hypoxia) and/or glucose (hypoglycemia) prevents the cells from getting the energy they need to function properly and this is another cause of the total absence of heart function. High levels of hydrogen ion (acidosis) are toxic to cells and high or low blood levels of potassium (hyperkalemia or hypokalemia) can cause nerve and muscle cell malfunction, so these conditions can also result in cardiac arrest due to asystole. Also, the core temperature of the body affects its metabolism and how well the enzymes in the cells work, so when the body is extremely cold (hypothermia) this can result in a lack of heart function as well.
In seeking to understand how human life came into being, we need to consider whether an explanation is within the credibility zone. In trying to explain how the heart is able to provide the blood flow the body needs, evolutionary biologists are faced with a Catch-22. Not only is the heart made up of cells, but so are all the organs and tissues involved in providing the control mechanisms needed to allow the heart to function in the first place. In other words, the heart is dependent on its own function for survival because all of the systems in the body that control chemicals like oxygen, glucose, water, potassium, hydrogen ion, and things like clotting, blood pressure, and temperature, are also made up of cells that are dependent on the heart providing enough blood flow.
The universal human desire to know the truth requires us to go beyond just describing how life looks, as evolutionary biology does. We must dig deeper. Seeing what is required for the body to maintain proper heart function, and how easily that can fail, you wonder how anyone could believe that such an intricate system, co-dependent on so many other controls in the body, could have come about without intelligent agency.
As our exploration of heart function has shown, the body must take control to follow the rules imposed on it by the laws of nature. That is because real numbers are a matter of life and death. So far in this series I have shown how the body controls its blood volume and heart function. But that is still not enough to explain how the body is able to do what it needs to do to live. As I noted above, the blood pressure is another factor that needs to be addressed.
Even if the heart is working properly and there is enough blood volume, debility and death can still take place if the blood pressure isn't just right. What is the blood pressure, how is it controlled, and what can make it drop too low or rise too high? That's what we'll begin to consider next time.
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