A closed circulatory system is a system that transports blood throughout an organism's body using a system of blood vessels. The heart pumps blood rich in oxygen through arteries throughout the body. Blood filled with waste and deprived of oxygen returns to the heart through the veins.
Blood coming from the arteries travel through smaller vessels called arterioles. The arterioles then branch into capillaries, the smallest blood vessels of the circulatory system. Capillaries bring blood to individual cells. Each cell in an organism's body lies near a capillary. Oxygen, carbon dioxide and waste products flow freely through the thin-walled capillaries to and from individual cells in the body. From the capillaries, venules form. Venules then lead into larger veins, which then bring blood back to the heart.
In a closed circulatory system, unlike an open circulatory system, blood never leaves the blood vessels. All vertebrates and some invertebrates have closed circulatory systems. Most invertebrates, however, have open circulatory systems. The arteries carry blood under high pressure from the pumping action of the heart. Blood pressure decreases as blood flows through consecutive vessels; therefore, blood pressure is lowest in the veins.
The hearts of organisms with closed circulatory systems consist of atria and ventricles. Atria receive blood from the blood vessels. Ventricles pump blood through the blood vessels to the rest of the body. Fish have one atrium and one ventricle. Amphibians such as frogs and salamanders have two atria and a single ventricle. Reptiles all have two atria, but some have a partially divided ventricle, and others, such as the crocodilians, have two separate ventricles. Bird and mammal hearts have two atria and two ventricles.