Heart. Coloured 3D CT (computed tomography) scan of the normal heart of a 46-year-old patient as viewed from above. The left and right pulmonary arteries branch from the top of the heart from where they carry deoxygenated blood to the lungs. The arteries branch into progressively smaller vessels. The smallest vessels wrap around tiny air sacs (alveoli), which are the site of gaseous exchange. Oxygen enters the blood and carbon dioxide leaves. The oxygenated blood then travels through the pulmonary veins (not seen) back to the heart where it is pumped around the body. This heart is now healthy but was previously treated for arrhythmia (irregular heartbeats).