HEART STRUCTURE

Each person's heart is about the size of the person's fist. A newborn baby's heart weighs about 2/3 ounce (19 grams). An adult's heart weighs from 9 to 11 ounces (255 to 312 grams). The heart lies near the middle of the chest, between the lungs. The heart lies closer to the front of the chest than to the back and slightly to the left side. Muscular walls. The heart consists chiefly of muscle. Heart muscle, also called myocardium or cardiac muscle, forms the walls of the heart as well as the septum, a wall that divides the left and right sides of the heart. All the muscles contract and relax, thereby pushing blood through the heart. A membrane called the epicardium covers the outer surface of the heart. Another membrane, the pericardium, surrounds the epicardium.

It completely encloses the heart and extends above the blood vessels that emerge from the top of the heart. A slippery fluid between the epicardium and the pericardium enables the heart to contract smoothly. Heart muscle differs from the other muscles of the body--skeletal and smooth muscles. Skeletal muscles, such as those in the arms and legs, have long fibers with alternate dark and light bands called striations. We can consciously control the skeletal muscles. Smooth muscles form the walls of the stomach, intestines, and most other internal organs. The muscles lack striations, and we do not consciously control them.

They work automatically. Heart muscle has striations like skeletal muscle. But it contracts and relaxes automatically like smooth muscle. In addition, heart muscle cells act as one cell. When one heart muscle cell contracts or relaxes, the cells around it do the same. For that reason, the heart beats continuously and rhythmically throughout a person's life. Chambers. The septum divides the heart lengthwise, and valves divide it crosswise. Each side of the art thus has two chambers, one above the other. A thin membrane called the endocardium lines each chamber. The top chambers, called the right atrium and left atrium, receive and collect blood returning to the heart through the veins.

After the atria (plural of atrium) have filled with blood, they contract and squeeze blood into the lower chambers, called the right ventricle and left ventricle. After the ventricles have filled, they contract and pump blood out of the heart through the arteries. The ventricles have extremely thick walls. The ventricles, which must squeeze blood from the heart, are much larger and stronger than the atria. Blood vessels. Blood enters and leaves the heart through several major vessels. Blood from the body flows into the right atrium through the body's two largest veins. The superior vena cava brings blood from the head and arms. The inferior vena cava carries blood from the trunk and legs.