Day 2: The Heart of the Matter

Anatomy of the Human Heart

Blood is kept moving along its circular route by the pumping action of the heart. The heart consists of four chambers. The upper two chambers are the right and left atria. The right and left atria are thin-walled sacs, which receive blood from the body and the lungs, respectively. In both atria the upper half of their inside wall is smooth and forms the sinuses of the great veins that empty into it. The lower half of the inside surfaces of the atria is very rough.

The lower two chambers of the heart are the right and left ventricles. The ventricles have thick walls made up of cardiac muscle. Cardiac muscle is also present in the walls of the atria. This specialized type of muscle tissue is found only in the heart; its fibers branch in such a way that when they all contract, they squeeze the heart chamber and force blood out of it. The inner surfaces of both ventricles are covered with ridges called trabeculae. Irregular muscle bundles called papillary muscles give rise to chords which anchor the heart valves. Both the trabeculae and the papillary muscles make the inside walls of the ventricles very rough.

In humans, the chambers of the atria are joined to their adjacent ventricles by valves. The tricuspid valve lies between the right atrium and right ventricle and the bicuspid valve lies between the left atrium and left ventricle. There are also valves between the ventricular chambers and the great arteries which they feed. The pulmonary valve lies between the right ventricle and the pulmonary artery. The aortic valve lies between the left ventricle and the aorta. The valves prevent blood from being forced back into the chamber from which it was expelled, and thus keep the blood flowing in one direction.

The right atrium and ventricle receive oxygen-poor blood from the body and send it to the lungs; they are therefore considered the pulmonary side of the heart. The left atrium and ventricle receive oxygenated blood from the lungs and pump it back into the body; they are therefore considered the systemic side of the heart.

The pacemaker of the heart (SA node) is located in the upper right atrium near the opening of the vena cava. The pacemaker sets the normal rhythmic beat of the heart by coordinating the contractions of the heart chambers. The pacemaker first sends a signal along specialized cardiac muscle fibers in the walls of both atria to make them contract simultaneously. The signals then converge on another bundle of specialized cardiac muscle fibers, the atrioventricular node (AV node) located in the wall separating the two ventricles. The AV node sends the signal on to the walls of the ventricles to make them contract simultaneously.

- Page 11 of 16 -