The heart is a muscular and hollow organ, responsible for pumping blood through the blood vessels by rhythmic, repeated contractions. The term cardiac (as in cardiology) means “related to the heart” and comes from the Greek kardia, for “heart.” The heart is composed of cardiac muscle, an involuntary muscle tissue which can only found within this organ. It consists of four chambers, the two upper atria & the two lower ventricles.

The human heart is normally situated just slightly to the left of the middle of the thorax, underneath the breastbone. Most people visualise the heart to be further to the left as the left ventricle is stronger (as it pumps to all the body parts) than the right ventricle.
The apex of the heart is the blunt point that points down and left. A stethoscope can be placed directly over this area, so that heart rate can be measured. The physical location is between the sixth and seventh rib, just to the left of the sternum.
In normal adults, the mass of the heart is 8-11 oz (250-350 g), but extremely diseased hearts can be up to 2 lb (1kg) in mass, due to hypertrophy….

The right-side of the heart collects de-oxygenated blood, in the right atrium, from the body and pumps it, via the right ventricle, into the lungs (pulmonary circulation), so that carbon dioxide can be released and the blood be re-oxygenated (gas exchange). This happens through a passive process called diffusion.
The left side collects oxygenated blood from the lungs into the left atrium. From the left atrium the blood moves to the left ventricle which pumps it out to the body.
On both sides, the lower ventricles are thicker and stronger than the upper atria.

Regulation of the cardiac cycle

The heart is self exciting, because it has its own conducting system. This is unique in the human body, for example skeletal muscle requires either conscious or reflex nervous stimuli.

The heart rate can be changed by nervous or hormonal influences such as exercise or the perception of danger.

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