Heart

The heart is a muscular pump that transports blood throughout the body. As an essential part of an organism's respiratory system, the heart circulates blood through the lungs. It exchanges carbon dioxide (gas) for oxygen which it distributes to the rest of the body. The heart is made of tough cardiac muscle that never needs to rest.

All animals that have blood and a circulatory system also need a version of a heart to move that blood throughout the body. The design and complexity of a heart is generally related to the complexity of an organism and to the type of lifestyle it leads. For example, an earthworm's simple system needs nothing more than two pulsating tubes running up and down its body. In vertebrates (animals with a backbone), there is also a degree of complexity depending on the structure of the vertebrate's body. For example, a fish's heart has two separate chambers. One is called an "atrium" that receives blood after it has circulated through the fish's body. The other is called a "ventricle" that forces the blood received from the atrium over its gills. Here the exchange of carbon dioxide and oxygen takes place. Still higher up the vertebrate ladder, the amphibian, like the frog, heart has three chambers, while birds and mammals have four chambers.

THE HUMAN HEART

High-energy animals, like birds and mammals that move about quickly and maintain a constant body temperature, need a very efficient heart. Such a heart can keep oxygen-rich blood entirely separate from blood that has surrendered all its oxygen (deoxygenated blood). The human heart is a good example of how high efficiency is achieved. The human heart is actually two hearts or pumps. One serves the lungs and the other serves the body. Located at the center of the chest cavity behind the breastbone, the heart is about the size of a clenched fist. The surface of the heart is covered with a number of small arteries, known as coronary arteries. These supply the heart's muscle fibers with oxygen-rich blood. The inside of the heart is divided into four chambers. The two at the top, called the atria (singular, atrium), are the receiving chambers. The two at the lower half are called the ventricles and are the pumping chambers. Each atrium is separated from a ventricle by a valve that allows blood to travel only in one direction and prevents any backup. The right and left sides of the heart are separated by a thick wall of muscle called the septum. Blood flows from the right atrium into the right ventricle. The right ventricle pumps the blood to the lungs where it leaves its waste (carbon dioxide) and receives oxygen. From the lungs, blood flows into the left atrium and then to the powerful left ventricle which pumps it throughout the rest of the body.

The heart is made of tireless cardiac muscle that beats, or contracts and relaxes, an average of seventy-two times a minute. This beating is known as the pulse rate. The heart beats in a regular sequence due to a complex electrical network. This network stimulates the muscle fibers and make the chambers contract or pump in the proper sequence. The heart also regulates blood pressure, or the pressure exerted by flowing blood against the inside of the walls of the veins and arteries through which it is flowing.

The heart is always able to adjust to physical and chemical changes in the body. Its beating strength, rate, and blood pressure are affected by the needs and demands of the body. The heart also responds to chemical changes in the body, such as the increased demands placed upon it during strenuous exercise. In this example, waste products must be eliminated at a much faster rate than when resting. Hormones, the body's chemical messengers, affect the rate at which the heart beats, and the heart is able to react appropriately to all types of stress, whether physical or emotional.

HEART DISEASES AND DISORDERS

Finally, a variety of diseases and conditions can affect the heart. An infection can inflame the sac around the heart (pericarditis), the lining of the heart (endocarditis), or the cardiac muscle itself (pericarditis). The electrical impulses that make the heart beat regularly can be upset. This is called arrhythmia. Also, the arteries that supply the heart muscle can become clogged. This can lead to a heart attack in which a part of the heart muscle dies due to a lack of a proper blood supply. High blood pressure and clogged arteries can also make the heart have to work harder, causing it to enlarge. This condition is known as congestive heart failure, and if not treated, is fatal.

[See alsoCirculatory System ]