FRANCIS LEROY & CHRIS LIPPENS, BIOCOSMOS / SCIENCE PHOTO LIBRARY FRANCIS LEROY & CHRIS LIPPENS, BIOCOSMOS / SCIENCE PHOTO LIBRARY
Contraction of heart chambers. Animation showing the main phases of contraction of the heart and the movement of blood (schematically represented as small spheres) into and out of the heart chambers (atria and ventricles). The colours indicate deoxygenated (blue) and oxygenated (red) blood. The atria are the upper heart chambers that receive blood arriving at the heart. They then pass the blood to the ventricles (the lower heart chambers) which pump the blood away from the heart. The valves (flaps) prevent the backflow of blood as it moves into and out of these chambers. The phases of heart contraction are diastole and systole. During diastole, the atria and ventricles are relaxed and the atria fill with blood from the veins. During systole, contraction of the atria precedes, by a fraction of a second, that of the ventricles. This expels the blood from the atria into the ventricles and completes the filling of the latter. The muscles of the ventricles then undergo a powerful contraction, pumping blood into the arteries. The sequence of blood flow is deoxygenated blood (blue) entering the right atrium (centre left) from the vena cava, then passing into the right ventricle (lower left) where it is pumped into the pulmonary arteries (upper right and upper left) to the lungs. It returns to the heart as oxygenated blood (red) via the pulmonary veins (centre left and centre right) and enters the left atrium (centre right). It then passes into the left ventricle (lower right) and is pumped round the body through the aortic arch (upper centre).
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