by Eleanor Neal

Created on: December 18, 2008

Overview of the heart.

In order for the various systems, organs, muscles and tissues of our bodies to function properly, they all need nutrients, and they all need wastes removed. The vehicle for both of these processes is blood; the engine causing the blood to pump around our body, ferrying nutrients and wastes, is the heart.

The heart is found slightly to the left of the vertical midline of the chest, at approximately the level of your armpit. If you clench your fist, you will have a rough idea of the size of this vital, conical pyramid shaped organ comprising four chambers, through which blood must flow.

On the outside of this organ is a covering called the epicardium. This is comprised of epithelium (a lining of cells), capillaries (fine blood vessels), and nerves. Underneath the epidcardium likes the muscle tissue of the heart: the myocardium comprised of single, striated cells the myofibrils of which are branched and interlocked with adjacent muscle fibres. This ensures the heart beats as a single unit, without rupture to the organ. This is the part of the heart that contracts to move blood around the heart, and subsequently around the body. On the inside of the heart is a protective lining called the endocardium. Endocardium is comprised of muscle fibres, epithelial cells, connective tissues and blood vessels.

The heart is divided into four chambers, which can be imagined as a four paned window. The chambers of the heart are arranged so that two atria (or chambers) sit above two ventricles (chambers). Feeding in to the right atrium are two major blood vessels, the superior vena cava and the inferior vena cava, that fill the right atrium with deoxygenated blood from the body. When the heart contracts, the right atrium squeezes blood past the gate joining the right atrium and the right ventricle (the tricuspid valve) filling the right ventricle with deoxygenated blood. The tricuspid valve is, as its name suggests, made up of three cusplike valves.

As the heart contracts, the right ventricle contracts, causing blood to move from this ventricle along the blood vessel feeding into the lungs (the pulmonary trunk). The tricuspid valve to closes, preventing backflow of blood from the right ventricle into the right atrium, and the gate between the right ventricle and the pulmonary trunk (the pulmonary semilunar valve) opens allowing blood to flow out to the lungs. When the ventricle relaxes, the three half-moon shaped constituents of the pulmonary semilunar valve prevent the blood from flowing back down into the right ventricle.

In the lungs, the blood flows through a fine capillary network where it is oxygenated. This oxygenated blood flows from the lungs through the blood vessels feeding back into the left atrium of the heart, (the pulmonary veins). When the heart contracts, the gate between the left atrium and the left ventricle (the bicuspid or mitral valve) opens allowing the newly oxygenated blood to flow into the left ventricle. The bicuspid valve closes upon relaxing, preventing backflow of blood; simultaneously the gate between the left ventricle and the blood vessel leaving the left ventricle (the aortic semilunar valve) opens, allowing the oxygenated blood to flow out into the various blood vessels of the body along which it travels to supply the systems, organs, muscles, tissues and cells with oxygen. Once this blood has been deoxygenated again, it comes back again to cycle through the heart: right atrium, right ventricle, lungs, left atrium, left ventricle, body, right atrium, right ventricle, lungs, left atrium, left ventricle body and so on

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