The heart is a complex muscular organ that pumps blood through the three divisions of the circulatory system: the coronary (vessels that serve the heart), pulmonary (heart and lungs), and systemic (systems of the body). It has four chambers: two atria and two ventricles.

In humans, the heart is about the size of a clenched fist. It is divided into four chambers: two atria and two ventricles. There are one atrium and one ventricle on the right side and one atrium and one ventricle on the left side. The atria are the chambers that receive blood while the ventricles are the chambers that pump blood.

The journey of the blood starts in the right atrium, which receives deoxygenated blood from the superior vena cava, which drains blood from the veins of the upper organs and arms. The right atrium also receives blood from the inferior vena cava, which drains blood from the veins of the lower organs and legs.

This deoxygenated blood then passes to the right ventricle through the right atrioventricular valve (tricuspid valve), a flap of connective tissue that opens in only one direction to prevent the backflow of blood. After it is filled, the right ventricle pumps the blood through the pulmonary arteries to the lungs for re-oxygenation.

After blood passes through the pulmonary arteries, it will have visited the lungs and picked up oxygen; it is not oxygenated. It passes through the pulmonary arteries back to the heart, entering into the left atrium. The blood passes through the bicuspid valve to the left ventricle where it is pumped out through the aorta, the major artery of the body, taking oxygenated blood to the organs and muscles of the body. Once blood is pumped out of the left ventricle and into the aorta, the aortic semilunar valve (or aortic valve) closes, preventing blood from flowing backwards into the left ventricle. This pattern of pumping is referred to as double circulation as the blood reaches the heart twice.

The heart is composed of three layers: the epicardium, the myocardium, and the endocardium. The inner wall of the heart is lined by the endocardium. The myocardium consists of the heart muscle cells that make up the middle layer and the bulk of the heart wall. The outer layer of cells is called the epicardium, the second layer of which is a membranous layered structure (the pericardium) that surrounds and protects the heart. It allows enough room for vigorous pumping, but also keeps the heart in place, reducing friction between the heart and other structures.

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Key Points

• The heart is divided into four chambers consisting of two atria and two ventricles; the atria receive blood, while the ventricles pump blood.

• The right atrium receives blood from the superior and inferior vena cavas and the coronary sinus; blood then moves to the right ventricle where it is pumped to the lungs.

• The lungs re-oxygenate the blood and send it to the left atrium.

• Blood moves from the left atrium to the left ventricle via the bicuspid valve; blood is pumped out of the left ventricle to the aorta, which sends blood to the organs and muscles of the body.

• The type of circulation seen in the human heart is known as blood circulation.

• The heart is composed of three layers: the epicardium (outer layer), the myocardium (middle layer), and the endocardium (inner layer).

Key Terms

aorta: the largest artery in the human body which carries the blood from the heart to all parts of the body except the lungs

inferior vena cava: a large vein that carries deoxygenated blood from the lower half of the body to the right atrium of the heart

superior vena cava: a large vein that carries deoxygenated blood from the upper half of the body to the right atrium of the heart

oxygenated: enriched with oxygen

deoxygenated: enriched with carbon dioxide or a reduced amount of oxygen

endocardium: the inner cells of the heart

myocardium: consists of the heart muscle cells that make up the middle layer and the bulk of the heart wall

epicardium: the outer layer of heart cells

valve: a structure in the heart that prevents the backflow of blood