The circulatory system is made up of the heart and blood vessels. The heart pumps oxygen-rich blood through vessels called arteries. Veins carry blood from various parts of the body back to the heart. Arteries and veins are connected by tiny capillaries (KAP'ih-lair"eez).
The heart's right side receives dark bluish blood from the superior and inferior vena cava. (The superior vena cava is the large vein that brings blood back from the upper part of the body. The inferior vena cava is the vein that brings blood from the lower body.) The heart's right side pumps this blood to the lungs. There, waste gas (carbon dioxide) is removed and oxygen is picked up. The bright red oxygenated blood returns to the heart's left side. Then it's pumped out into a large artery called the aorta to be distributed by smaller arteries to all parts of the body.