The heart is one of the major parts of our body. It is as small as your fist but does a major job in our body. The heart delivers blood throughout our bodies, by arteries, capillaries and veins. It is important for us to understand how it works and how to take care of our heart.
The heart is a strong, muscular pump that continuously works around the clock. The heart can be seen as the power needed for life. Each day the average heart "beats," or expands and contracts, 100,000 times and pumps about 2,000 gallons of blood. In an average lifetime, the human heart beats more than 2.5 billion times.
The heart is divided into four chambers. The upper two are the right and left atria ; the lower two, 
the right and left ventricles. The heart also has four valves that open and close to allow blood to flow in only one direction when the heart contracts (beats).
The four heart valves are:
Each valve has a set of flaps (also called leaflets or cusps). The right side of the heart receives dark, bluish blood from the superior and inferior vena cava. It pumps this blood to the lungs where waste gas (carbon dioxide) is removed and oxygen is picked up. The bright red oxygenated blood returns to the left side of the heart which pumps it out through the aorta to be distributed by smaller arteries to all parts of the body.
The cardiovascular system, comprising both the heart and the blood vessels, circulates blood throughout the body. It carries essential supplies of food and fuel to every living cell and exchanges them for potentially harmful waste products. The adult circulation consists of thousands of miles of tubing containing about 10 pints (4.7l) of blood. The blood is kept flowing round the body by the pumping action of the heart.
Blood is made up of a pale yellow liquid (plasma) containing dissolved nutrients and wastes, plus blood cells, hormones, proteins and other substances. Most numerous of these cells are the disk-shaped red blood cells. Their color comes from the presence of the substance hemoglobin, which combines with oxygen.
When red corpuscles charged with oxygen approach body cells, the oxygen is delivered in exchange for the waste product carbon dioxide. Other two-way transportation of materials takes place between the body cells and the plasma, and all unwanted substances are carried away in the blood for excretion by the kidneys, lungs, and liver. Plasma also contains white blood cells, which help to fight infection, and platelets, which are involved in blood clotting.
In its passage through the body, blood is carried in tubes known as arteries and veins. Most arteries transport oxygen-rich (oxygenated) blood, whereas most veins transport carbon dioxide-rich (deoxygenated) blood. The largest artery is the aorta, which stems directly from the heart. The aorta and other large arteries have thick walls lined with muscle. Blood flow is assisted by the contraction of this muscle and the impetus given by the heartbeat. The "push" from the heart can be felt as the pulse wherever large arteries run near the body surface.
As they penetrate the tissues, arteries split into narrow branches called arterioles, which in turn divide into capillaries. It is through the very thin capillary walls that the blood gives up its oxygen and nutrients and receives carbon dioxide and wastes. Deoxygenated blood in the capillaries flows into narrow veins (venules), and then into veins. The two largest veins, the venae cavae, return this blood to the heart. Veins have thin walls compared with those of arteries, and blood moves through the veins much more slowly. Blood flow in the veins is assisted by the action of muscles in surrounding tissues, and backflow is prevented by one-way valves.
The walls of arteries and veins comprise the same four tissues, but arteries have more muscle to withstand more blood pressure. Blood flow in veins is aided by nearby muscles; valves prevent backflow. Blood flow in the heart is controlled by valves. The tricuspid valve controls flow from right atrium to right ventricle; the mitral valve controls flow from left atrium to left ventricle. Valves also guard the entrances to the aorta and pulmonary artery in order to prevent backflow of blood after the heart contracts.
The deoxygenated blood delivered to the heart along the veins is no use to body cells until it has been recharged with oxygen. To ensure reoxygenation, the circulation has a second "loop." In this part of the system, blood rich in carbon dioxide travels from the heart along the pulmonary artery to the lungs, where carbon dioxide is exchanged for oxygen breathed in. The pulmonary artery is the only artery to carry deoxygenated blood. The newly oxygenated blood is carried back to the heart along the pulmonary vein, the only vein to transport oxygenated blood.
The heart is a muscular organ about the size of a clenched fist. The structure and action of the heart are designed to serve the two loops of the circulation. Inside, the heart is divided vertically by a muscular wall. On each side of this wall is an upper chamber (atrium) and a thicker, lower chamber (ventricle). Blood moves through each side of the heart systematically. Deoxygenated blood is delivered into the right atrium. It then enters the right ventricle, from where it is pumped out into the pulmonary artery and to the lungs. Oxygenated blood returning in the pulmonary veins flows into the left atrium. This blood enters the left ventricle and is then pumped into the aorta for circulation.
The flow of blood in each side of the heart is controlled by a series of valves. The pumping action of the heart is achieved by the contraction of the cardiac muscle, of which the heart is largely composed. The rhythm of the heartbeat is regulated by bursts of electrical impulses sent out by a concentration of specialized heart tissue called the pacemaker.
Under the influence of the pacemaker, the heart of an adult at rest beats at a rate of 60 to 80 beats a minute. The pacemaker also helps to ensure the correct sequence of activities during each heartbeat; first the two atria contract, followed rapidly by the ventricles. The powerful contraction of the ventricles pushes blood into the aorta and pulmonary artery. This period of contraction (systole) is followed by a period of relaxation (diastole), during which the heart refills. The complete sequence is accompanied by electrical activity of the muscle, which can be monitored as an electrocardiogram (EKG).
An electrocardiogram (EKG) records the waves of nerve impulses of each heartbeat. The P wave starts just before the atria contract; the QRS wave indicates the contraction of the ventricles; and the T wave indicates the recovery period before the next contraction.
Aorta is the largest artery in the body, supplying blood to the organs alongside its course. It starts at the left ventricle of the heart, then arches upward and backward, giving off branch arteries to the heart muscles, head, and arms. The aorta then runs down the back of the chest, in front of the spine and esophagus (gullet), to reach the abdomen. Just above the pelvis, the aorta divides into the common iliac arteries to the legs
Vena cava is one of the two veins (superior vena cava and inferior vena cava) that drain blood from all parts of the body (except the heart) into the right atrium of the heart. The superior vena cava returns blood from the head, neck, and arms to the heart. The inferior vena cava returns blood from the chest, abdomen, and legs to the heart.
Researchers in recent years have made great strides in the understanding and treatment of high blood pressure (hypertension). High blood pressure is a major risk factor for heart attack, heart failure, stroke, and kidney failure. Heart disease and stroke are the first and third leading causes of death, respectively, in the United States, according to the National Heart, Lung, and Blood Institute. Significant increases in hypertension awareness, treatment and control rates have occurred since the 1960s. Despite those advances, the declines in stroke and heart attack in the U.S. during the last 30 years have leveled off and congestive heart failure and kidney failure are rising.
Dr. Koop
American Heart Association: High Blood Pressure
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