Blood Flows round and round through blood vessels. This is called circulation. From the heart's left side blood passes into the aorta (main artery), then into other arteries. Each artery branches 15 or 20 times, becoming smaller and smaller. The smallest arteries lead into a network of minute capillaries. The thin walls of these capillaries allow oxygen and nutrients to diffuse through them from the blood into the body tissues. In due course, the capillaries join other capillaries, eventually forming veins. These take blood back to the heart's right side. This side pumps it to the lungs to collect more oxygen, and from there it returns to the heart's left side.