Anatomy of the Heart
The essential function of the heart is to pump blood to various parts of the body. The mammalian heart has four chambers: right and left atria and right and left ventricles. The two atria act as collecting reservoirs for blood returning to the heart while the two ventricles act as pumps to eject the blood to the body. As in any pumping system, the heart comes complete with valves to prevent the back flow of blood. Deoxygenated blood returns to the heart via the major veins (superior and inferior vena cava), enters the right atrium, passes into the right ventricle, and from there is ejected to the pulmonary artery on the way to the lungs. Oxygenated blood returning from the lungs enters the left atrium via the pulmonary veins, passes into the left ventricle, and is then ejected to the aorta.

The large valve in the foreground is the tricuspid valve that prevents backflow from the right ventricle to the right atrium.

Pumping Action of the Heart
The pumping action starts with the simultaneous contraction of the two atria. This contraction serves to give an added push to get the blood into the ventricles at the end of the slow-filling portion of the pumping cycle called “diastole.” Shortly after that, the ventricles contract, marking the beginning of “systole.” The aortic and pulmonary valves open and blood is forcibly ejected from the ventricles, while the mitral and tricuspid valves close to prevent backflow. At the same time, the atria start to fill with blood again. After a while, the ventricles relax, the aortic and pulmonary valves close, and the mitral and tricuspid valves open and the ventricles start to fill with blood again, marking the end of systole and the beginning of diastole. It should be noted that even though equal volumes are ejected from the right and the left heart, the left ventricle generates a much higher pressure than does the right ventricle.

Electrical Activity of the Heart
When vertebrate muscles are excited, an electrical signal (called an “action potential”) is produced and spreads to the rest of the muscle cell, causing an increase in the level of calcium ions inside the cell. The calcium ions bind and interact with molecules associated with the cell’s contractile machinery, the end result being a mechanical contraction. Even though the heart is a specialized muscle, this fundamental principle still applies.

One thing that distinguishes the heart from other muscles is that the heart muscle is a “syncytium,” meaning a meshwork of muscle cells interconnected by contiguous cytoplasmic bridges. Thus, an electrical excitation occurring in one cell can spread to neighboring cells. Another defining characteristic is the presence of pacemaker cells. These are specialized muscle cells that can generate action potentials rhythmically.

Under normal circumstances, a wave of electrical excitation originates in the pacemaker cells in the sinoatrial (S-A) node, located on top of the right atrium. Specialized muscle fibers transmit this excitation throughout the atria and initiate a coordinated contraction of the atrial walls. Meanwhile, some of these fibers excite a group of cells located at the border of the left atrium and ventricle known as the atrioventricular (A-V) node. The A-V node is responsible for spreading the excitation throughout the two ventricles and causing a coordinated ventricular contraction.




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