Understanding Cardiovascular Function: How the Body Controls the Heart
Howard Glicksman July 14, 2015 3:23 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.
Blood circulating throughout the body supplies the cells with what they need to live. But blood is made up of matter and therefore is subject to the laws of nature. These laws state that since blood has mass, energy must be used to move it against the forces of inertia, friction, and gravity.
The last article in this series showed that the heart is a muscular pump that provides the power needed to circulate the blood. It has a right and left side, each with an upper chamber called an atrium, and a lower chamber, called a ventricle. Between the atrium and the ventricle, and the ventricle and its outflow tract are "V"-shaped one-way valves. These ensure that the blood flows in the right direction.
The right side pumps blood through the main pulmonary artery to the lungs, while the left side pumps through the aorta to the rest of the body. The heart also has a natural pacemaker, the sino-atrial node, which automatically stimulates it to contract and controls how often it beats. The electrical signal conducted from the sino-atrial node triggers Na+ ions to flood into the heart muscle cells, and Ca++ ions as well. Together they cause depolarization and heart muscle contraction, the strength of which is dependent on the degree of Ca++ ion entry.
But being able to survive in the world is a dynamic process. Experience teaches that when we work and play hard our heart has to pump faster and harder to give us the energy we need. This functional ability of the heart would have been the difference between our earliest ancestors eating, or being eaten.
Evolutionary biologists seem to assume that life takes place in a vacuum and that just because different organisms have similar structures, those structures must have come into existence through the interaction of chance and the laws of nature alone. In contrast, our experience of human invention shows that when we encounter a mechanism that takes control in overcoming the laws of nature to remain functional, it is more plausible to infer the work of an intelligent designer.
A car moving up a hill must have a mechanism in place, like a driver or cruise control, to provide enough gas to its engine so it can overcome the forces of inertia, friction, and gravity to go over the top and continue on its way. So too, the body must be able to take control of its heart function. Let's see how it does that.
When the body is at complete rest, it is like a car that is idling. The car needs a minimum amount of energy just to keep the pistons in the engine moving while the oil lubricates the moving parts and the anti-freeze circulates to keep everything cool. The same can be said of the body. Even though its muscles are relaxed and its gastrointestinal system is at rest, it still needs a minimum amount of energy to maintain adequate brain, heart, lung, kidney, and liver function.
When the driver wants to speed up, she steps on the accelerator, which immediately directs more gas and air into the engine, releasing more energy, and the car moves faster right away. So too, when you decide to be more active, your heart speeds up and beats harder to provide your muscles with the energy they need. Without this ability to respond immediately to the body's energy needs, the survival of our earliest ancestors would have been impossible.
Gland cells and hormones take several minutes to take effective action, but nerve cells can send signals at about one-third the speed of sound and release neurohormones that take action immediately. The Autonomic Nervous System(ANS), centrally located mainly in the hypothalamus, is the mechanism by which heart function is controlled to match the energy needs of the body. These unconscious actions are involuntary and take place automatically.
At rest, the heart is dominated by the parasympathetic division of the ANS -- the rest and digest component. The parasympathetic nerves send out a neurohormone called acetylcholine which attaches to specific cholinergicreceptors in the heart. One of the effects of acetylcholine is to slow down the entry of Na+ and Ca++ ions into the pacemaker and heart muscle cells. This helps to reduce the heart rate and the force of muscle contraction. When the body becomes more active and/or more anxious and excited, it is thesympathetic division of the ANS -- the flight or fight component -- that takes over to make sure the blood supply to the tissues is adequate. Here's how it works.
As you may recall my saying in previous articles, the first thing you need to take control in such a situation is a sensor that can detect what needs to be controlled. An increase in physical activity is caused by an increase in skeletal muscle contraction. And an increase in skeletal muscle contraction causes a local build-up of certain chemical by-products from the increase in metabolic activity. The body has sensors in the skeletal muscle that can detect its rate of contraction and these chemicals as well.
The second thing you need to take control is something to integrate the data by comparing it with a standard, decide what must be done, and then send out orders. The hypothalamus takes the data from these sensors and from other areas of the brain, and modulates the release of a neurohormone callednorepinephrine from the sympathetic nerves and epinephrine from the sympathetic stimulated adrenal glands.
The third thing you need to take control is an effector that can do something about the situation. Norepinephrine and epinephrine attach to specificadrenergic receptors in the heart, which increases the entry of Na+ and Ca++ions into the cells of the sino-atrial node, the conducting system and heart muscle cells. This causes the heart rate and the force of muscle contraction to increase. In addition, these neurohormones attach to specific adrenergic receptors in the blood vessels to increase the blood flowing back to the right side of the heart from the veins. The combination of these effects brings about a significant increase in the amount of blood the heart pumps to the tissues.
Now that you understand how the body takes control of its heart function, it is important to consider how all of this works in real life. For, when it comes to the laws of nature, real numbers have real consequences. Just as a car's performance depends on its engine size and efficiency, so too, medical science knows that to stay alive the body's heart function must meet certain objective parameters.
Evolutionary biologists imagine how irreducibly complex organs and their control mechanisms might have come into being. But in real life, when dealing with the laws of nature, if the numbers don't add up just right, the body is as good as dead. Next time we'll look at how this applies to heart function and survival.
