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The atria do not have inlet valves, but are separated from the ventricles by valves. The atria facilitate circulation by allowing uninterrupted blood flow into the heart during ventricular systole.
The right and left ventricles are the bottom chambers of the heart. They have thicker walls than the atria, and they create higher blood pressures to pump blood out of the heart into circulation to the body and the lungs.
The right ventricle pumps deoxygenated blood into pulmonary circulation. The left ventricle pumps oxygenated blood into systemic circulation.
Systole describes ventricular contraction, when blood is pumped from the heart into circulation.
Diastole describes ventricular relaxation, when blood moves from the atria to the ventricles, and the ventricles fill in preparation for systole.
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.
The 2 ventricles at the anterior end of the heart pump blood into the 2 atria at the posterior end, The 2 atria at the posterior end of the heart pump blood into the 2 ventricles at the anterior end, The 2 atria at the anterior end of the heart pump blood into the 2 ventricles at the posterior end, or The 2 ventricles at the posterior end of the heart pump blood into the 2 atria at the anterior end
Source: Boundless. “Chambers of the Heart.” Boundless Anatomy and Physiology. Boundless, 28 Jul. 2015. Retrieved 26 Nov. 2015 from https://www.boundless.com/physiology/textbooks/boundless-anatomy-and-physiology-textbook/the-cardiovascular-system-18/the-heart-172/chambers-of-the-heart-865-681/