 The circulatory system works to transport oxygen and nutrients to cells in your body while removing waste products such as carbon dioxide at the same time. Hemoglobin, which is the protein present in red blood cells, serves as the main transporter, much like a school bus, with oxygen and carbon dioxide being able to hop on or hop off at the correct destinations. This system is actually a double circulatory system made of two circuits, the pulmonary circuit and the systemic circuit. In the pulmonary circuit, deoxygenated blood is pumped from your heart to your lungs to become oxygenated. In the systemic circuit, this oxygenated blood that is returned to the heart is pumped to the rest of your body. Unlike other muscles in your body, your heart never tires and works very hard to ensure that blood reaches all parts from your head to your toes. Blood is pumped away through the aorta and then through the arteries. Arteries have very thick muscular walls so to withstand the pressure of very fast flowing blood. These arteries branch into smaller arterioles, which then branch into smaller capillaries that surround all cells. The movement of blood slows down significantly at the capillaries for two reasons. The capillaries are much, much smaller and slower movement allows for gas exchange. Here, oxygen is offloaded onto cells so blood becomes deoxygenated and carbon dioxide and other waste products are picked up. Deoxygenated blood then travels away from the capillaries into the venules and then into the veins. Unlike arteries, veins have much thinner walls and valves that prevent deoxygenated blood from flowing backwards. This deoxygenated blood doesn't travel straight to the lungs to become oxygenated. It must first travel to the heart so it can be pumped into the lungs, ready for another round.