FUNCTIONS OF THE HEART
The heart is a hollow pear-shaped muscular organ placed between the lungs in the middle of the chest that pumps blood through the body, supplying cells with oxygen and nutrients. It is attached to the breastbone by special connective tissues called ligaments. The apex (blunt point of the lower edge of the heart) lies on the diaphragm, pointing toward the left. The apex pulses with every beat of the heart. This is what you feel when you hold your hand to your heart. The heart has four cavities: a small upper cavity (atrium) and a large lower cavity (ventricle) on each side. The adult human heart is approximately the size of a fist. In an average adult, it is about five inches long and three and a half inches across at its broadest part, and it weigh less than a pound.
It is hollow, four-chambered, muscular organ that performs the major function of the cardiovascular system: pumping blood to all of the tissues of the body. Every tissue in the body depends on the proper functioning of the heart. Disease or injury to this major organ can greatly affect the function of the body system. Cardiovascular or heart disease takes many forms. Some forms cause a few nuisance problems, but many cause lifestyle changes. Some people are born with heart disease, but many develop it later in life. Eighty percent of all heart attacks stem from cardiovascular diseases, primarily coronary arteries that are diseased. Other common causes of heart failure include cardiomyopathy, hypertension, and valvular heart disease. During the twentieth century great advances have been made in developing effective treatments for heart disease. Medications, devices, and surgical procedures have been developed which are capable of prolonging life, reducing symptoms, and eliminating the causes of many disease processes. There is still much progress to be made and development continues.
Blood returns to the resting heart through veins that empty into the right and left atria. As the atria fill and the pressure in them rises, the AV valves open to admit the blood into the ventricles. The ventricles become about 80% filled during this time. Contraction of the atria wrings out the final 20% of the 80 milliliters of blood the ventricles will receive, on average, in a resting person. These events occur while the ventricles are relaxing; a period called ventricular diastole.
After a slight delay, both ventricles contract this period of contraction is known as ventricular systole. Contraction of each ventricle increases the pressure within each chamber, causing the AV valves to forcefully close and thereby preventing blood from backing up into the atria. Immediately after the AV valves close, the pressure in the ventricles forces the semilunar valves open so that blood can be pushed out into the arterial system.
The right and left pulmonary arteries deliver oxygen depleted blood to the right and left lungs. These return blood to the left atrium of the heart via the pulmonary veins. The aorta and all its branches are systemic arteries, carrying oxygen-rich blood from the left ventricle to all parts of the body. The coronary arteries are the first branches off the aorta; these supply the heart muscle itself. Other systemic artery branch from the aorta as it makes an arch above the heart, and as it descends and traverses the thoracic and abdominal cavities. These branches provide all body organs with oxygenated blood. The blood from the body organs, now lower in oxygen, returns to the heart in the systemic veins. These eventually empty into two major veins: the superior vena cava, which drains the upper body, and the inferior vena cava, which drains the lower body. These veins empty into the right atrium and thereby complete the systemic circulation.