The Human Heart

The heart's considered a muscle because its cells and tissues have the ability to contract, exerting the force that pumps the blood. The heart is the muscular pump of the circulatory system. A three-layer sac called the pericardium surrounds the internal parts of the heart. It weighs about 12 ounces, measures 6" long by 4" wide and beats 2 million times during the average life span. A healthy heart is capable of pumping 7000 quarts of blood a day through 60,000 miles of blood vessels. At rest the heart pumps 4-5 liters per minute and can increase the flow to 20-30 liters during strenuous exercise. It only gets about one second of rest between beats. Even though it's 1/200th the total body weight it uses 1/20th of the blood supply.

It's also considered an organ because it's a part of the body that's composed of more than one tissue and forms a structural unit that's responsible for a specific function. The internal structure of the heart consists of four chambers, the right and left atriums and the right and left ventricles. 70% of the blood entering the ventricle goes in before the contraction of the atrium forces more into, and pressurizes, the ventricle with the remaining 30%. With ventricular contraction, the tricuspid and mitral valves (A-V) are closed to prevent back flow of blood into the atrium. During the ventricle contraction blood is collecting in the atrium and the semilunar valves (aortic and pulmonary) are pushed open and blood is pumped from the heart into the large arteries. Pressure buildup in the large arteries forces the closing of the semilunar valves. The cycle is repeated when blood builds up in the atrium and forces the A-V valves open.

There are three types of muscle fiber in the heart: arterial and ventricular (both are similar to skeletal muscle) and conductive muscle fibers. Normal heart rate is about 72 beats per minute (BPM). The S-A node in the right atrium controls the heart rate by generating action potentials. The S-A node fibers go into the atrium causing the contraction; the fibers also stimulate the A-V node delaying the ventricular contraction by about 1/10th of a second.