The heart has four chambers, two atria and two ventricles. The atria are smaller chambers, with thin walls, while the ventricles are bigger chambers, with much thicker and stronger walls .
The atria are located on the anterior end of the heart, with one atrium on each side. The right atrium receives de-oxygenated 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.
Entrance to the atria is not controlled by valves; instead, the atria fill passively with blood. In normal physiologic states, the output of the heart is pulsatile, and the venous inflow to the heart is continuous and non-pulsatile. But without functioning atria, venous flow becomes pulsatile, and the overall circulation rate decreases significantly.
Atria have four essential characteristics that cause them to promote continuous venous flow:
- There are no atrial inlet valves to interrupt blood flow during atrial systole.
- 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 blood continues to flow uninterrupted from the veins right through the atria into the ventricles.
- The atrial contractions are slight, preventing significant back pressure that would impede venous flow.
- The relaxation of the atria is coordinated to begin before the start of ventricular contraction.
The ventricles are located on the posterior end of the heart, one on each side. The interventricular septum separates the left and right ventricles from each other. The right ventricle receives deoxygenated blood from the right atria and pumps it through the pulmonary vein and into pulmonary circulation. The left ventricle receives oxygenated blood from the left atria and pumps it through the aorta into systemic circulation. 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. During ventricular systole, the ventricles contract, pumping blood through the semi-lunar valves into systemic circulation. During diastole, the ventricles relax and fill with blood again.