Heart Rate, Stroke Volume, and Cardiac Output, Essay Example

You are walking down an old mottled sidewalk of a quiet suburban street. It’s lined with tall trees, big yards, dappled shade, lawn ornaments, and singing birds. You are daydreaming, with a very old song going through your head: And I love to live so pleasantly, live this life of luxury . . . And then you see it. It’s big and off its chain. And now it’s headed straight for you, gaining speed, barking furiously, eyes alive with blazing hatred. He won’t bite! shouts the owner.

The situation you have found yourself in is called fight or flight, and an examination of it will explain what is happening to your brain and heart. First, although the body prefers to have advanced warning of physical and mental situations (like it has when you get ready for your regular workout), if it is healthy enough it can react instantly when it has to, like yours is doing.

Your optic and cochlear (eye and ear) nerves, parts of the central nervous system, registered the sight and sound of the dog to your brain. Your brain is connected to the rest of your body via the central nervous system, which is basically your spine. It can be thought of as sort of a trunk line, like that of a telephone or train system — a shared main route, in this case for communication. Your spine is connected in turn to your peripheral nervous system. Both systems are electrical. The send and receive electrical signals. Your peripheral system receives information from your body — its skin, bone, and muscles, which register sensations based on what is happening to them: pain for bad things, pleasure for good things. It then sends that information to the spine and then to the brain, which considers it, and sends messages back: reduce the pain, increase the pleasure. In a fraction of a second it’s going to be sending a message to your heart, legs, and arms. But your brain knows enough about charging, barking dogs to make sure that this message gets priority. Priority means the release of some powerful chemical messengers called hormones. Two of the principal ones are adrenalin (also known as epinephrine) and cortisol. In excess, they have the effect of putting the autotonic nervous system on hold for the duration of the crisis that caused the brain to release a more than usual amount of them. The autotonic system regulates automatic bodily functions — such as breathing during sleep, digestion, and various regular glandular functions. By contrast, the heart is put into high gear in order to supply the muscles quickly with more oxygen and energy, to fight or flee. The heart is one of those muscles, which is a good thing to know.

Looking down at your own chest, your heart has a top half, consisting of a left and right atrium, and a bottom half consisting of a left and right ventricle. Between heartbeats, the right atrium receives oxygen-lacking blood from the body and the left atrium receives blood with oxygen from the lungs. (The power source of both movements is the previous beat.) With the next beat (called the diastole) blood passes from the atriums to the ventricles. With the second beat (the systole), it passes from the ventricles to the lungs, body, and, importantly, the heart itself. (Both beats together constitute the cardiac cycle.) But the present situation with the rampaging dog is not autotonic, and so adrenaline has been released (via the brain) from the adrenal glands. Almost instantly, it reaches the heart muscle, called the myocardium.

We come equipped with two adrenal glands. One sits atop each kidney. In addition to adrenaline, the adrenal glands produce the hormones testosterone and cortisol. (President John F. Kennedy suffered from Addison’s Disease, which affects the adrenal glands. While president he secretly took drugs to treat the condition to hide his illness from the public (Maugh II, 2009).

Finally we come to the matter of heart rate, stroke volume, and cardiac output. As you might imagine, all three are going to increase when the adrenaline hits the heart muscle. And that’s now. The blood vessels have already begun to narrow, increasing blood pressure.

The heart rate of an adult in reasonably good condition averages about 60–80 beats per minute. The better shape you are in, the lower it gets, and the worse your shape the higher. Middle- and older-aged adults in poor physical condition, finding themselves in a fight-or-flight situation, are in danger of death from a heart attack, apart from the immediate threat. Assuming your heart is able to withstand the stress, and depending on the situation, it will reach a maximum heart rate, estimated with a formula of 220 minus your age in years. In contrast to fight-or-flight, regular exercise (like the comfortable walk you were on a few minutes ago) will produce a steady-state heart rate. That kind of exercise also produces an anticipatory response in the heart as it readies itself for the expected extra work, a mechanism that is unrelated to whether you expect to enjoy that added work or not. But you are not in that kind of situation (ASPCA, 2012).

The stroke volume is the quantity of blood pumped (in milliliters) per heartbeat from the left ventricle. As with heart rate, the better the condition of the heart, the higher stroke volume it is capable of pumping. But given the limited size of a given heart, there is a natural limit to maximum stroke volume. Once the heart reaches it, that volume will plateau for the duration of the exercise or exhaustion. However, due to what is known as the Frank-Starling mechanism, during exercise the ventricle wall does expand enough to cause a significant rebounding contraction, which in turn causes a greater contraction of the heart, which in turn forces the blood from the heart with greater force than during normal rates (McArdle; Katch, 2000).

Cardiac output, being a matter of measuring a liquid volume, can mathematically be measured with a simple formula: SV x HR, meaning stroke volume times heart rate. The quantity is measure in liters per minute. Cardiac output is proportional to exercise intensity, but, again through the use of adrenaline, cardiac output is also conditional upon a foreseen need.

Your heart is now racing, and your muscles are primed. Ditto for Fido. Good luck.

References

ASPCA. (2012). Aggression in Dogs. Retrieved from http://www.aspcabehavior.org/articles/49/Aggression-in-Dogs.aspx

Maugh II, T. (2009). John F. Kennedy’s Addison’s disease was probably caused by rare autoimmune disease. Retrieved from http://articles.latimes.com/2009/sep/05/science/sci-jfk-addisons5

McArdle W., Katch F. (2000). Essentials of Exercise Physiology: Philadelphia, PA: Lippincott Williams & Wilkins.