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There are two atria in the human heart – the left atrium receives blood from the pulmonary circulation, and the right atrium receives blood from the venae cavae of the systemic circulation. During the cardiac cycle , the atria receive blood while relaxed in diastole , then contract in systole to move blood to the ventricles.
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Contraction of the atria follows depolarization, represented by the P wave of the ECG. As the atrial muscles contract from the superior portion of the atria toward the atrioventricular septum, pressure rises within the atria and blood is pumped into the ventricles through the open atrioventricular (tricuspid, and mitral or bicuspid) valves.
The rhythmic sequence (or sinus rhythm) of this signaling across the heart is coordinated by two groups of specialized cells, the sinoatrial (SA) node, which is situated in the upper wall of the right atrium, and the atrioventricular (AV) node located in the lower wall of the right heart between the atrium and ventricle.
This electrical impulse is propagated throughout the right atrium, and through Bachmann's bundle to the left atrium, stimulating the myocardium of the atria to contract. The conduction of the electrical impulses throughout the atria is seen on the ECG as the P wave. [5] [7]
The foramen secundum (from Latin 'second opening') is formed from small perforations that develop in the septum primum.The septum primum is a septum that grows down between the single primitive atrium of the developing heart to separate it into left and right atria.
Cardiac muscle forms both the atria and the ventricles of the heart. Although this muscle tissue is very similar between cardiac chambers, some differences exist. The myocardium found in the ventricles is thick to allow forceful contractions, while the myocardium in the atria is much thinner.
At the beginning of the cardiac cycle the atria, and the ventricles are synchronously approaching and retreating from relaxation and dilation, or diastole. The atria are filling with separate blood volumes returning to the right atrium (from the vena cavae), and to the left atrium (from the lungs). After chamber and back pressures equalize, the ...