The Circulatory System

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by the four pulmonary veins, which enter the left auricle. When this
chamber contracts, blood is forced into the left ventricle and then by
ventricular contraction into the aorta. The bicuspid, or mitral valve
prevents the blood from flowing back into the auricle, and the semilunar
valves at the beginning of the aorta stop it from flowing back into the
ventricle. Similar valves are present in the pulmonary artery. The aorta
divides into a number of main branches, which in turn divide into smaller
ones until the entire body is supplied by an elaborately branching series
of blood vessels. The smallest arteries divide into a fine network of still
more minute vessels, the capillaries, which have extremely thin walls; thus,
the blood is enabled to come into close relation with the fluids and
tissues of the body. In the capillaries, the blood performs three functions
then it releases its oxygen to the tissues, it furnishes to the body cells
the nutrients and other essential substances ...

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valve tissue. Blood from the coronary capillary circulation
enters several small veins, which then enter directly into the right
auricle without first passing into the vena cava. The action of the heart
consists of successive alternate contraction and relaxation of the muscular
walls of the auricles and ventricles. During the period of relaxation, the
blood flows from the veins into the two auricles, gradually distending them.
At the end of this period, the auricles are completely dilated then their
muscular walls contract, forcing almost the entire contents through the
auriculoventricular openings into the ventricles. This action is sudden and
occurs almost simultaneously in both ...

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