The human circulatory system consists of blood, blood vessels that carry blood away from the heart, around the body, and back to the heart, and the heart itself, which acts as a central pump. The systemic circuit supplies blood to the whole body, the coronary circuit supplies blood to the heart, and the pulmonary circuit supplies blood flow between the heart and lungs.
Blood travels from the right atrium to the right ventricle of the heart through the tricuspid valve, then from the right ventricle to the pulmonary artery through the pulmonary valve. Pulmonary veins then carry the blood to the left atrium of the heart, from which it is carried to the left ventricle through the mitral valve. Finally, the left ventricle pumps blood to the aorta (the largest artery in the body) through the aortic valve.
Arteries, which carry blood away from the heart, split and get progressively smaller, becoming arterioles and eventually a series of capillaries, the sites of gas exchange. Capillaries converge to become larger venules, and eventually merge into veins, which bring blood back to the heart.
Humans have a double circulatory system, in which blood travels through the heart twice via the pulmonary and systemic circuits. First, the heart receives deoxygenated blood in its right side and then pumps it to the nearby pulmonary circuit, the capillaries that are involved in gas exchange. Here, oxygen enters the blood, while carbon dioxide leaves the blood.
The blood is then returned for its second trip through the heart. After passing through the left of side of the heart, blood is pumped out to the entire body through the systemic circuit, all the way to the toes, the fingers, the head, and everywhere in between. The strength required for this task makes the left side of the heart much thicker than the right side, giving the heart a lopsided, non-symmetrical appearance.
The right side of the heart receives deoxygenated blood. So, how does the heart get oxygenated blood? There is a network of arteries and veins surrounding the heart tissue in a crown-like manner that are together known as the coronary circuit. These bring oxygenated blood to the heart and carry deoxygenated blood away.
The walls of the blood vessels reflect their function. Capillary walls are very thin to facilitate gas exchange, whereas arteries and veins are thicker and more complex. Arteries are the thickest blood vessels and are very strong, with two additional layers of tissue (compared to capillaries) to accommodate the high pressure of blood pumped from the heart. Dilation and contraction of the muscle tissue of the arteries are regulated by hormones, directing blood to or from specific areas. Blood returns to the heart at a much lower pressure, so veins have walls that are much thinner than those of arteries.
