There are two main components of the circulatory system. There is the series of tubes, called the vascular system, that carries oxygen and nutrients to all of the trillions of cells that make up the human body. The other component is the heart, which is the machine of the circulatory system. It is a muscular organ that is approximately the size of a man’s fist. The proper role of the heart in maintaining circulation of the blood was first presented in the middle of the seventeenth century by Sir William Harvey, an English physician. Dr. Harvey was the first to understand that the heart functioned as a pump that circulates blood throughout the body. Prior to that time, the generally accepted theory was that blood was continuously being produced by the heart and the liver; first proposed by the ancient Roman physician, Galen. The heart’s role in circulation is accomplished because of its unique properties.
First, the heart is composed of a specialized muscle fiber, called cardiac muscle. It is striated as skeletal muscle, but is unique because it is structured to form branches, instead of bundles as the skeletal muscle in our arms and legs. In addition, unlike other muscle tissue, it automatically contracts in a rhythmical fashion. These contractions are regulated by an internal pacemaker called the sinoatrial node, or SA node. The SA node discharges regular electrical impulses that enables the cardiac muscle to contract simultaneously at a rate which usually ranges from sixty to one hundred beats per minute. During exercise, the heart beats faster to enable more oxygen to be transported to the working muscle.
The section of the heart with the thickest wall of muscle is the left ventricle, which pumps oxygenated blood throughout the body. Next is the right ventricle, which pumps deoxygenated blood to the lungs, where it picks up oxygen, then returns it to the heart, where it enters the left atrium. The atria are the collection chambers for the blood returning to the heart, while the ventricles are responsible for pumping the blood to the tissue.
To ensure that blood flows in one direction, the heart has four valves. The mitral, or bicuspid valve is located in between the left atrium and left ventricle. It prevents blood from backing up into the left atrium when the left ventricle contracts. The tricuspid valve is located in between the right ventricle and right atrium. It prevents blood from backing up into the right atrium when the right ventricle contracts. The aortic valve sits above the aorta, the main artery that carries blood from the left ventricle, and the pulmonic valve is positioned in front of the pulmonary artery, which carries blood pumped from the right ventricle to the lungs. Every time the heart contracts, it pumps approximately sixty to seventy cubic centimeters, or four to seven quarts of blood in one minute.
There are a number of conditions that can affect cardiac function. Elevated levels of cholesterol in the blood can clog the coronary arteries that supply oxygen and nutrients to cardiac muscle. If the coronary arteries become occluded, a myocardial infarction, or heart attack can result. Cigarette smoking has also been shown to cause narrowing of the coronary vessels, as can high levels of prolonged stress. Proper diet and exercise have been shown to effectively maintain normal cardiac function. Diseases that affect the conduction pathways that regulate the heartbeat will also disrupt function. Any conduction defects can be detected by performing a diagnostic test called an electrocardiogram, or ECG. For example, if the SA node isn’t functioning properly, then an irregular heart beat, or arrhythmia can result. If medication doesn’t work, an electronic pacemaker may be implanted to regulate heart rate.
Advances in modern medicine have enabled physicians to correct many of the problems that can impede normal cardiac function. Surgical procedures can replace damaged valves, or restore coronary circulation. The need for many of these procedures, however, can be averted if one adopts a healthy lifestyle.
