The Heart

Generalities.

  The heart is a muscle that contracts rhythmically, and for ever, since the early months of our conception to our death. In a lifetime, it will beat about 3 billion times. Its function is to pump blood to all organs of our body, and it pumps about 5 liters of blood per minute.

    The heart has four chambers: two atria and two ventricles. At the beginning of the beat cycle, the auricles contract, forcing blood into the ventricles. Then, the ventricles contract to propel the blood to the lung (right ventricle) and to the entire body (left ventricle). These two circulations, pulmonary and peripheral, are serially linked and represent a closed circuit. The non-oxygenated blood enters the right atrium through the superior and inferior vena cava. The atria contract and blood is pumped into the right ventricle. Then, the ventricle contracts, pushing blood into the pulmonary artery to the lungs where it will be oxygenated. The blood returns to the heart, via the pulmonary vein, into the left atrium. It then passes into the left ventricle, and will be finally propelled into the aorta to be distributed to all tissues.

    All this, of course, happens in less than a second, or about to one second if we consider the whole cycle, including the relaxation phase. The contraction phase of the heart is called the systole whereas the relaxation phase is called diastole. This succession of systole and diastole occurs automatically. Indeed, the heart beats by itself, all it needs to beat, it is a supply of well oxygenated blood, and enough nutrients. By the way, the blood flow into the heart muscle itself is called the coronary circulation.

     The automatist of the heart falls into two main structures: the sinoatrial node (the pacemaker), the bundle of His and the Purkinje fibers which innervate the ventricles.

Longitudinal section of the heart.

Transverse section of the heart.

Cardiac Muscle.

    The heart muscle is arranged like a mesh, a network of muscle fibers.

    These muscle fibers are physically and electrically connected to each other by intercalated discs. The apposition is so tight that the electrical resistance of a disc is 400 times less than the resistance of the membrane. In addition, these muscle fibers behave somewhat like neurons and can generate an electrical current that propagates from fiber to fiber.

The Conduction Network of the Heart.

    The conduction network of the heart is composed of specialized muscle fibers. These fibers are very small compared to the heart muscle fibers, and they specialized in the generation and conduction of electricity. Then, when the electrical signal reaches the muscle fibers, it triggers their contraction.

The Cardiac Cycle.

    The cardiac cycle starts at the sinus node which is a small, crescent-shaped, mass of 1 cm by 3 mm. These cells constitute the heart pacemaker. They have the ability to depolarize spontaneously (generating an electric current), in a rhythmic fashion. Each second, an electrical impulse is generated at the sinoatrial node, and spreads to both atria, which will contract as a result of this electric current. On the electrocardiogram, contraction of the atria appears as the "P" wave. During this time, electric current runs along the internodes network until it reach the atrioventricular node. At this level, the current is first delayed to allow termination of the contraction-relaxation cycle of the atria, and then it is retransmitted, via the two branches of the bundle of Purkinje, to the ventricles and provokes their contractions. On the electrocardiogram, the contraction of the ventricles appears as the "QRS" wave. Finally, after contraction, the cells re-polarized and return to rest, producing the wave "T" on the electrocardiogram.