THE HUMAN HEART
The heart drawn on the average Valentine is only a rough representation of the actual structure of the heart. The heart is actually shaped more like an upside-down pear.
The human heart (click here to view anatomical drawing) is primarily a shell. There are four cavities, or open spaces, inside the heart that fill with blood. Two of these cavities are called atria. The other two are called ventricles. The two atria form the curved top of the heart. The ventricles meet at the bottom of the heart to form a pointed base which points toward the left side of the chest. The left ventricle contracts most forcefully, so one can best feel the heart pumping on the left side of the chest.
The left side of the heart houses one atrium and one ventricle. The right side of the heart houses the others. A wall, called the septum, separates the right and left sides of the heart. A valve connects each atrium to the ventricle below it. The mitral valve connects the left atrium with the left ventricle. The tricuspid valve connects the right atrium with the right ventricle.
The top of the heart connects to a few large blood vessels. The largest of these is the aorta, or main artery, which carries nutrient-rich blood away from the heart. Another important vessel is the pulmonary artery which connects the heart with the lungs as part of the pulmonary circulation system.
The two largest veins that carry blood into the heart are the superior vena cava and the inferior vena cava. They are called vena cava because they are the "heart's veins." The superior is located near the top of the heart. The inferior is located beneath the superior.
The heart's structure makes it an efficient, never-ceasing pump. From the moment of development through the moment of death, the heart pumps. The heart, therefore, has to be strong. The average heart's muscle, called cardiac muscle, contracts and relaxes "automatically" about 70 to 80 times per minute. As the cardiac muscle contracts it pushes blood through the chambers and into the vessels. Nerves connected to the heart regulate the speed with which the muscle contracts. When one runs, the heart pumps more quickly. When one sleeps, the heart pumps more slowly.
Considering how much work it has to do, the heart is surprisingly small. The average adult heart is about the size of a clenched fist and weighs about 11 ounces (310 grams). Located in the middle of the chest behind the breastbone, between the lungs, the heart rests in a moistened chamber called the pericardial cavity which is surrounded by the ribcage. The diaphragm, a tough layer of muscle, lies below. As a result, the heart is well protected.
The heart is susceptible to different forms of heart disease which may threaten its ability to perform its vital job in the body. Fortunately, modern medicine can effectively treat most of them in ways that enable patients of all ages to keep living their lives.