What Does Your Heart Do?

by Katrina Cain

This article is for Mike

We all know the simple answer; it pumps blood, but how exactly does it work. Let’s piggy back on a red blood cell and follow its journey as it leaves the lungs with a fresh supply of oxygen. But first, understand there are many components that make up our blood including plasma, red and white blood cells, platelets and nutrients from our food.

Freshly rejuvenated with a  supply of oxygen from the alveoli in our lungs, the rbc enters the left atrium of the heart via the pulmonary veins. Although undetectable when checking your own pulse; your heart actually has two sets of contractions; atrial and ventricular systole. The left atrium contracts to force the newly oxygenated blood into the left ventricle via the mitral valve; while at the same time, the right atrium contracts to force deoxygenated blood into the right ventricle en route to the lungs. During ventricular systole, the left ventricle contracts, forcing the oxygen saturated rbc into the superior (upper) and inferior (lower) aorta for distribution through your body. There are special valves that close to prevent back flow into the atrium during the powerful ventricular systole. Some people have a disorder called a heart murmur, where their mitral valve doesn’t close properly and blood from the ventricle flows back into the atrium. In aortic stenosis, the aortic valve doesn’t close completely and blood doesn’t properly leave the ventricle forcing the heart into a number of compensatory mechanisms, some of which cause ventricular hypertrophy; an enlargement of heart muscle. Simply put, a muscle working hard will get bigger, much like weight lifters building body muscle mass.

The aorta immediately branches into the ascending and descending aortas; to distribute blood to the upper and lower body.  At rest, the blood flow percentages to the body are as follows: 4-5% is used by the heart muscle itself, 21-22% is used by the kidneys, 18% is used by the skeletal muscles (during exercise, the skeletal muscles use 71-72% of our total blood flow), 7% by our skin, 25% by our viscera, 13% by our brain (consider that the brain only makes up 1/50th the total mass of our bodies), 11-12% by the rest of our bodies.

Our red blood cells exchange oxygen for carbon dioxide; a waste product of cellular respiration. This carbon dioxide needs to be transported back to our lungs via our veins for us to breath out. The easiest way to remember which vessel carries which type of blood is to associate our arteries with “away”; in that arteries take blood away from the heart. Veins transport blood back to the right atrium of the heart via the superior and inferior vena cava. Veins always bring blood to the heart. Our red blood cell has exchanged its oxygen for carbon dioxide and now needs to return to the lungs for refueling.  From the right atrium, into the right ventricle and back to the lungs via the pulmonary arteries, the indispensable red blood cell has completed one full circuit through our bodies.

Interestingly, the “lub dub” sounds heard through the doctor’s stethoscope are the closing of the valves of the heart; the bicuspid (a.k.a mitral) and tricuspid valves that separate the right and left atrium from the right and left ventricle, and the semilunar valves that prevent back flow into the ventricles once the blood has been forced out into the aorta and pulmonary arteries.

What is an Electrocardiogram or ECG?

The ECG measures electrical impulses traveling through the heart. The heart contracts based on a signal generated by the sinoatrial node (SA), and continued by the atrioventricular node (AV). The SA node is our pacemaker; it sets the normal sinus rhythm of our heart. The electrical impulse travels to the AV node which conducts this impulse from the atria to the ventricles. This sinus rhythm is what causes the contraction of the heart muscle. While, the cells that generate the electrical impulse are heart muscle cells, they, themselves do not contract. To me, this is the beauty of the body at work; real magic.

The normal sinus rhythm is represented by a series of peaks and depressions on graph. The P-wave is the current traveling between the SA and the AV Nodes. It is worth noting that the spikes on an ECG reading don’t represent the physical contraction of the heart muscle, but rather the flow of electrical impulse through the myocardium (“myo” means muscle and “cardium” means heart). The QRS segment is the depolarization of the ventricles and the T-wave is the repolarization of the ventricles – allowing them a brief state of rest before the next impulse occurs. Doctors can diagnose potential heart problems and their approximate location based on disruptions in these waves.

Whenever I think about how people abuse their bodies with substances, I marvel at the heart’s marvelous capacity for being both an incredibly precise and terribly complicated machine and one of incredible power. If you live to be 80, your heart will beat between 2 and 3 billion times. There is no machine I can think of more efficient than that!

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