Heart Health Article

Definition

The heart is a muscular organ of the cardiovascular system that contracts to cause movement of the blood throughout the body.

Description

The heart is approximately fist-sized and located in the chest between the two lungs and behind the ribs and breastbone (sternum). It rests at a slight tilt from vertical, which makes it appear to be on the left side of the body. The walls of the heart are made up of three layers of tissue: epicardium, myocardium, and endocardium. The epicardium is a thin layer on the outer surface of the heart. The myocardium is the muscular layer, made up of cardiac muscle that contracts to do the work of the heart moving the blood. The endocardium is the smooth inner lining of the heart.

The entire structure of the heart is enclosed in a fibrous sac called the pericardium. A small amount of liquid is normally found in the space between the heart and the pericardium, which helps reduce the friction between the epicardial and pericardial membranes.

The heart is divided by a central wall (or septum) into its right and left sides. Each of these sides contains a smaller, upper chamber known as an atrium, and a lower, larger chamber known as a ventricle. The atria and ventricles are separated by a valve made of flaps of tissue that prevent blood flow in the wrong direction. The valve on the left side of the heart is the mitral (or bicupsid) valve, which has two flaps. The right atria and ventricle are separated by the tricupsid valve, which has three flaps.

There are five great vessels branching off from the heart that are responsible for carrying blood into or out of the organ. These five vessels are the aorta, the pulmonary artery and vein, and the superior and inferior venae cavae. The aorta is the main artery, carrying oxygenated blood from the heart out into the body. The pulmonary artery carries blood away from the heart to the lungs, and the pulmonary vein carries blood from the lungs to the heart. The superior and inferior venae cavae carry deoxygenated blood from the upper and lower parts of the body back to the heart.

Unidirectional valves separate two of the great vessels from the chambers of the heart. The pulmonic (or pulmonary valve) separates the right ventricle and the pulmonary artery. The aorta and left ventricle are separated by the aortic valve.

The coronary arteries are two vessels that divide off the aorta and branch out over the entire surface of the heart. These vessels bring oxygenated blood to the heart tissue itself.

Function

The heart functions as a strong, four-chambered muscular pump. It can move more than five quarts of blood through the body each minute, the equivalent of about 2,000 gallons per day. At a typical heart rate of 72 beats per minute, the heart contracts on average 100,000 times per day. This adds up to more than 2.5 billion beats in a 70-year lifetime.

One key to the functioning of the heart is the unique characteristics of its muscular tissue. Cardiac muscle differs from other muscles of the body in that its normal function is a rhythmic contraction, which is the basis for the tissue's ability to respond to the electrical impulses that govern the beating of the heart. The natural pacemaker of the heart, the sinoatrial (SA) node, is located in the right atrium. Cardiac muscle cells that naturally contract at the fastest rate when compared to the other cells of the heart surround this cluster of nerve cells. This area of the heart therefore has the ability to initiate the contraction by sending wavelike electrical signals throughout the organ.

First, the electrical signal causes the two atria to contract, when sends the blood from those chambers into the two ventricles. Then the signal passes down through a group of nerve cells known as the atrioventricular (AV) node. This nerve cluster is located near the center of the heart. The travel through this area slows down the signal so that it reaches the ventricles after the atria have finished their contraction. Then the ventricles contract, moving the blood out of the heart, and the cycle starts again. The heart's electrical activity can be measured using electrocardiography.

The physical functions of the full heartbeat is known as the cardiac cycle. The cycle can be divided into two phases: diastole and systole. Diastole occurs when the heart relaxes and the myocardial fibers lengthen. As the heart dilates, the cavities fill with blood. Diastole of the atria occurs slightly before the diastole of the ventricles.

Systole happens when the part of the heart is in contraction and the myocardial fibers shorten. Again, systole of the atria precedes systolic phase of the ventricles. Systole of the ventricles cause blood to surge out of the heart and into the aorta and pulmonary artery.

Over time, the cardiac cycle occurs as follows. It begins with the diastole of the atrium, where both the left and right atria relax and fill with blood. The right atria fills with deoxygenated blood from the superior and inferior venae cavae. The pulmonary artery fills the left atria with newly oxygenated blood from the lungs. The SA node signals the beginning of systole and the atrium contract, sending blood through the tricuspid and mitral valves into the right and left ventricles, respectively. During ventricle filling, the valves of the great vessels are closed so blood already pumped out of the heart does not leak back.

The electrical signal has now reached the ventricles and they contract, sending the deoxygenated blood of the right ventricle into the pulmonary artery to the lungs and the oxygenated blood of left ventricle into the aorta to the body. During contraction, the tricupsid and mitral valves close to prevent flow back into the atrium. This cycle is repeated continuously.