The heart is the most important organ in the body. Its importance comes from the fact that is it supplies the other organs of the body with oxygen and nutrients through pumping the blood into the circulation. The heart is a blood pump that is composed of four chambers.
These chambers are divided into two atria and two ventricles. The left atrium and left ventricle are separated from the right atrium and right ventricle by a continuous wall. While the atria and ventrilces are separated by the atrioventricular valves.
The valve between the right atrium and right ventricle is called the tricuspid valve while the valve between the left atrium and the left ventricle is called the bicuspid valve.
The atria have thin walls because the blood pressure inside them is very low, in contrast to the ventricles which have thick walls in comparison with the atria.
This is so due to the increased pressure of blood inside them. The left ventricle has higher blood pressure than the right ventricle.
The pumping process of the blood is divided into a systole and a diastole. Systole refers to the contraction of the atria and ventricles while diastole refers to their relaxation.
During atrial systole or contraction the ventricles are relaxed, while during atrial diastole the ventricles are contracted.
The left ventricle pumps the blood into the general circulation through the aortic artery, while the right ventricle pumps the blood into the pulmonary circulation through the pulmonary artery. The coronary artery is a branch of the aortic artery that supplies the muscle of the heart with oxygen and nutrients.
Occlusion of this artery leads to heart attack which can be treated by dilating the artery using medications such as calcium channel blockers.
Between the right ventricle and the pulmonary artery there is a semilunar valve that prevents the blood from backflowing into the ventricle. analogously, there is also a semilunar valve between the left ventricle and the aortic valve that prevents the blood from backflowing into the left ventricle.
The contraction and relaxation of the atria and ventricles is an automatic process that happens spontaneously. The contraction of the atrium occurs at the sinus node or alternatively sinuatrial node. The contraction of the atrium begins with a depolarization of the muscle cells of the atrium.
This depolarization intitiates an action potential that allows the atrium to contract. The action potential occurs after sodium ion chennels open which allows for the sodium ions to flow to the inside of the muscle cells.
Thus initiating its contraction. After the depolarization of the muscle cells a process of repolarization occurs in which potassium ion channels open which allow the potassium ions to flow outside the cell. Thus leading to muscle relaxation by restoring the resting membrane potential to its normal value.
By this the muscle contraction of the atrium ends. By a similar process for the ventricles, a depolarization process occurs of the muscle fibers of the ventricle followed by a repolarizaton which allows the ventricle to contract and relax. This process is initiated at the atrioventricular node of the ventricle.
This process of muscle contraction and relaxation of the atria and ventricles can be depicted by a diagram that is called electrocardiogram or ECG. The depolarization process of the atria is manifested on this diagram by a P-wave while the depolarization of the ventricles is manifested by a QRS-wave.
The ventricular repolarization is manifested by a T-wave. The repolarization process of the atria is overlapped by the QRS-wave and thus usually is not seen.
Cardiomegaly or enlargement of the heart occurs often due to chronic high blood pressure. Especially true is left ventricular hypertrophy. Epinephrine and norepinephrine that are secreted by the adrenal gland stimulates through the sympathetic nervous system the contraction of the heart and thus increasing its pumping rate.
Changes in electrolytes balance in the extracellular space such as occurs in hyperkalemia and kypokalemia can change the rythem of heart contraction.