The Cardiovascular system (CVS) or circulatory system is a closed system composed of two main elements, the heart (central pump) and the blood vessels (distributing channels).
During the cardiac cycle the heart functions as two pumps in series with each other. The distributing system of blood vessels starts with aorta through which the left ventricular outflow is diverted to the arteries, arterioles and capillaries. The capillary blood reaches the veins through the venules and the large veins (superior and inferior vena cavae) return the blood to the right atrial chamber of the heart. This circuit forms the 'major or systemic circulation.
The lesser or pulmonary circulation is composed of the blood flowing from the right atrium to the right ventricle which pumps to the lungs through the pulmonary trunk and pulmonary arteries. While flowing through the blood vessels of the lungs, blood is re-oxygenated before being carried to the left atrium by the pulmonary veins.