The cardiovascular system consists of the heart and blood vessels. The heart has four chambers - two atria and two ventricles. It is surrounded by layers including the pericardium, myocardium, and endocardium. There are four valves in the heart that allow blood flow in only one direction. Blood vessels include arteries, which carry blood away from the heart; veins, which carry blood back to the heart; and capillaries, where gas exchange occurs. The circulatory system can be divided into systemic circulation and pulmonary circulation. Cholesterol is carried through the bloodstream by lipoproteins including LDL, VLDL, HDL, and chylomicrons.