Human respiratory and cardiovascular systems, illustration. The trachea (windpipe) leads from the larynx (voice box, in neck) to the lungs where it splits into the two main bronchi. The bronchi further divide within the lungs, terminating in alveoli (air sacs). Alveoli are the site of gaseous exchange, where carbon dioxide is removed from the blood and oxygen added. The heart (centre) is a hollow muscle that pumps blood around the body. Arteries, which carry oxygenated blood around the body, are red. Veins, which carry deoxygenated blood back to the heart and lungs, are blue. The large artery down centre is the aorta, the large vein is the vena cava.