The heart is the complex pump of the circulatory system, pumping blood throughout the body for the purposes of tissue oxygenation and gas exchange. The heart has four chambers through which blood flows: two sets of each type of chamber (atria and ventricles), one per side, each with distinct functions. The left side of the heart deals with systemic circulation while the right side of the heart deals with pulmonary circulation. 

The Atria

The atria are chambers in which blood enters the heart. They are located on the anterior end of the heart, with one atrium on each side. The right atrium receives deoxygenated blood from systemic circulation through the superior vena cava and inferior venae cavae. The left atrium receives oxygenated blood from pulmonary circulation through the left and right pulmonary veins.

Blood passively flows into the atria without passing through valves. The atria relax and dilate (expand) while they fill with blood in a process called atrial diastole. The atria and ventricles are separated by the mitral and tricuspid valves. The atria undergo atrial systole, a brief contraction of the atria that ejects blood from the atria through the valves and into the ventricles. The chordae tendinae are elastic tendons that attach to the valve from the ventricles and relax during atrial systole and ventricular diastole, but contract and close off the valve during ventricular systole.

One of the defining characteristics of the atria is that they do not impede venous flow into the heart. Atria have four essential characteristics that cause them to promote continuous venous flow:

1. There are no atrial inlet valves to interrupt blood flow during atrial systole. The venous blood entering the heart has a very low pressure compared to arterial blood, and valves would require venous blood pressure to build up over a long period of time to enter the atria.
2. The atrial systole contractions are incomplete and do not block flow from the veins through the atria into the ventricles. During atrial systole, blood not only empties from the atria to the ventricles, but continues to flow uninterrupted from the veins right through the atria into the ventricles.
3. The atrial contractions are slight, preventing significant back pressure that would impede venous flow.
4. The relaxation of the atria is coordinated to begin before the start of ventricular contraction, which also helps prevent the heart from beating too slowly.

Ventricles

The ventricles are located on the posterior end of the heart beneath their corresponding atrium. The right ventricle receives deoxygenated blood from the right atria and pumps it through the pulmonary vein and into pulmonary circulation, which goes into the lungs for gas exchange. The left ventricle receives oxygenated blood from the left atria and pumps it through the aorta into systemic circulation to supply the tissues of the body with oxygen. 

The walls of the ventricles are thicker and stronger than those of the atria. The physiologic load on the ventricles, which pump blood throughout the body and lungs, is much greater than the pressure generated by the atria to fill the ventricles. Further, the left ventricle has thicker walls than the right because it pumps blood throughout the body, while the right ventricle pumps only to the lungs, which is a much smaller volume of blood. 

During ventricular diastole, the ventricles relax and fill with blood. During ventricular systole, the ventricles contract, pumping blood through the semi-lunar valves into systemic circulation.

Structure of the heart

Structure diagram of a coronal section of the human heart from an anterior view. The two larger chambers are the ventricles.

This diagram of the heart's structure indicates the brachiocephalic artery, superior vena cava, right and left pulmonary arteries and veins, right and left atrium, septum, atrioventricular (tricuspid valve), chordae tendinae, right and left ventricle, inferior vena cava, semilunar valves, atrioventricular (mitral valve), aorta, left common carotid artery, and left subclavian artery.