The circulatory system is the body's transport system, providing a mechanism for materials to be carried throughout the body. The transport medium inside the system is blood, which is restricted to tubular canals (therefore it is a closed system). The heart is the pump that circulates blood. It has four chambers: two atria and two ventricles. Blood is pumped from the atria through the spongy tissue of the ventricles into the bloodstream when the heart contracts. Then it travels back to the atria when the heart relaxes again. The circulatory system contributes to efficient movement because it provides an economical means of transporting oxygen and nutrients to all parts of the body while removing carbon dioxide and other waste products. The blood carries oxygenated blood from the lungs to all tissues including the brain. Here it delivers essential nutrients such as glucose and amino acids. It then returns to the lungs where it picks up more oxygen and travels back to the organs once again. This cycle continues without interruption unless something goes wrong with the heart or arteries. A healthy cardiovascular system is vital for efficient movement. Without it, someone would not be able to travel far before running out of breath or collapsing from lack of oxygen. However, if you suffer from cardiovascular disease or have had recent surgery, there are things you can do yourself or with help from your doctor to keep your heart healthy.