In humans, blood circulates within a closed system of blood vessels. The system is said to be closed because arteries and veins are connected with each other through small vessels. This requires the action of a pump, which is provided by the heart. The heart is composed of four chambers, right atrium and ventricle, and the left atrium and ventricle. The right and left atria receive blood from the venous system and the left atrium and ventricles pumps blood into the arterial system.
Atrioventricular valves separate the atria and ventricles (mitral valve on the left and tricuspid valve on the right). The right atrium and ventricle is separated from the left atrium and ventricle by a septum. Deoxygenated blood returns from the body via the great veins, superior and inferior vena cava, to the right atrium and then passes through the tricuspid valve into the right ventricle. From the right ventricle, blood is pumped through the pulmonary valve into the pulmonary artery—the only artery in the body that carries deoxygenated blood—and into the pulmonary capillaries in the lung. In the lungs, carbon dioxide is removed from the blood and the blood is oxygenated.
Blood returns to the left side of the heart via the pulmonary veins—the only vein in the body that carries oxygenated blood—and into the left atrium. The blood that returns to the left atrium by way of pulmonary veins is, therefore, enriched with oxygen and partially depleted of carbon dioxide. The path of blood from the heart (right ventricle), through the lungs, and back to the heart (left atrium) completes one circuit—the pulmonary circulation. Oxygen-rich blood in the left atrium enters the left ventricle (LV) via the mitral valve and is pumped out of the LV into the systemic circulation via the aorta. The arterial branches from the aorta supply oxygen-rich blood to all the organ systems and thus the systemic circulation.
Like all organs, the heart is made of tissue that requires a steady supply of oxygen and nutrients. Although its chambers are full of blood, the heart does not receive nourishment from this blood. The heart receives its own supply of blood from a network of arteries, called the coronary arteries. There are two major coronary arteries (left and right) that provide blood supply to the heart and both these arteries originate from the beginning (root) of the aorta, immediately above the aortic valve.
The left and right coronary arteries originate at the base of the aorta from openings called the coronary ostia, located behind the aortic valve leaflets. These two major vessels provide blood flow to different regions of the heart and because their branches lie on the surface of the heart they are sometimes called the epicardial coronary vessels. The left and right coronary arteries further branch into arterioles, and the arterioles branch into numerous capillaries that ...