The blood moves along two closed systems of blood vessels connected to the heart – the small and large circles of blood circulation. The circulation of blood in a large circle of blood circulation occurs in about 20 seconds, in a small circle – 5 times faster.
Small circle of blood circulation
Small (pulmonary) circle of blood circulation provides blood oxygen saturation. It starts from the right ventricle of the heart. From the right ventricle, blood is pushed into the pulmonary artery. Through this vessel, venous blood flows from the heart, containing little oxygen.
The pulmonary artery is divided into two arteries: the left and right pulmonary arteries. In the lungs, the arteries branch into vessels of ever-smaller diameter up to the capillaries. Venous blood passes through the capillaries of the lungs, gives off carbon dioxide there and is saturated with oxygen, that is, it becomes arterial. The capillaries then merge into larger vessels that form the pulmonary veins. Through the pulmonary veins, arterial blood first enters the left atrium, and then into the left ventricle. From the left ventricle, it returns to the systemic circulation.
A large circle of blood circulation
The systemic circulation begins from the left ventricle with the largest artery – the aorta. Large ascending arteries (carrying blood to the head and upper limbs) and descending arteries (carrying blood to all organs of the body, including the heart itself) depart from it.
Arteries gradually branch out, forming a network of capillaries in organs and tissues, in which substances are exchanged between blood and tissues. By giving oxygen and nutrients to the tissues, the blood takes up metabolic products and carbon dioxide and becomes venous.
Venous blood returns to the heart through two large veins: from the head and arms through the superior vena cava, and from the lower body through the inferior vena cava. Both veins flow into the right atrium.