The heart is a muscular pump responsible for distributing blood throughout the body. A little larger than a fist, the heart is located behind and protected by the sternum and ribs.
A sac or membrane called the pericardium surrounds the heart. A layer of fluid in the pericardium allows the heart to move easily within the body.
As with any muscle, the heart is nourished by oxygenated blood. A network of vessels surrounds the heart, feeding the muscle. Large arteries connect the heart to the body and the lungs, delivering de-oxygenated blood from the body into the heart. Blood is pumped through the right side of the heart into the lungs, and back to the left side of the heart which pumps oxygenated through the body.
One-way valves control the flow of the blood. Healthy valves form a tight seal, preventing blood from flowing back into the heart or lungs.
Internally, the heart is divided into four chambers. The top two smaller chambers are called atria, while the bottom two larger chambers are known as ventricles. The septum is a wall of muscle that divides the left and right sides of the heart. (Right and left are labeled from the perspective of the heart as it rests in the body.) De-oxygenated blood flows from the body into the right atrium. The tricuspid valve then controls the flow from the right atrium into the right ventricle. When the right ventricle beats or contracts, the blood flows through the pulmonary valve and into the lungs where it becomes oxygenated. The oxygenated blood returns to the heart, pooling in the left atrium. The mitral valve allows the blood to pass from the left atrium to the left ventricle.
Once the left ventricle–the most powerful of the four chambers–contracts, the blood travels through the aortic valve, into the aortic arch, and is distributed throughout the body through an elaborate network of blood vessels.
Heart beats are controlled by an internal electrical or conduction system. The center of this system is a natural pacemaker known as the SA (sinoatrial) node, located in the upper right atrium. Electrical impulses from this node travel through the atria and ventricles, telling them when to contract. A secondary node located in the septum is known as the AV (atrialventrical) node. The AV node serves as a relay station, slowing down the signal, giving the atria the chance to fully contract, before passing the signal on to the ventricles.
Diagnosing Heart Disease
A variety of diagnostic tools exist to aid in envisioning an individual heart. Listening to the heart through a stethoscope (auscultation) can help physicians determine whether the heart is beating normally, whether blood is flowing correctly through the valves, or if the valves themselves are damaged.
EKGs show the functioning of the heart's electrical system. Twelve electrodes are carefully placed on the skin to pick up the electrical pattern of the blood as it flows from the heart and through the body. Changes in the heart's electrical signature on an EKG may indicate a previous heart attack or other cardiac condition.
In addition, a variety of imaging tools help visualize the heart's anatomy. Typically the first line of diagnostic imaging, an x ray reveals the heart's physical structures.
CAT or CT scans can help with the detection of coronary artery disease by highlighting areas of increased density within the heart. The plaque or calcifications that form with coronary artery disease are quite dense and therefore visible on a CAT scan.
Digital echocardiography relies upon ultrasound technology to image the heart and is used to diagnose a range of types of heart disease. Sound waves are transmitted to the heart from a device called a transducer, which is applied to the skin. Once the waves reach the heart, they bounce back across the skin and are stored in a computer.
Doppler ultrasound, which depicts blood flow, is another adaptation of ultrasound testing. It is used to diagnose the location and degree of valvular blockages and leaks, as well as the presence of heart muscle (pericardial) disease.
MRI has traditionally been used to assess anatomy and heart muscle function, as well as to identify the presence of any scar tissue in the musculature. In particular, MRI scanning can help distinguish between muscle that is impaired but can be improved through medical therapies, surgery, or angioplasty, and muscle that has turned permanently into scar tissue.
MRA is specifically designed to detect coronary artery disease, identifying plaque in the arterial system and determining degrees of narrowing. It can also highlight so-called vulnerable plaques that may not noticeably limit blood flow, but have the potential to suddenly rupture and unexpectedly close an artery.