Blood travels throughout out body through arteries, veins, and capillaries. Arteries are firm conduits made of several layers, able to withstand pressure. They carry blood rich in oxygen (O2) and nutrients. The heart pumps it out the aorta (our largest artery), which curves around the upper chest and heads down to the level of the belly-button. On the way, narrower arteries branch off to each arm, to the neck and head, to the kidneys, the stomach and intestine (where it picks up nutrients), the liver (which gets rid of toxins present in food, medications, & more), etc. When the aorta reaches its end, it divides into two arteries, one for each leg.
Downstream, sub-arteries constantly branch off, getting smaller and smaller, when they’re called arterioles. Finally, the vessels become so small that they’re only about the width of a single blood cell; these are called capillaries. It’s in the capillaries that O2 & nutrients ooze out to enter all our body’s cells, which need them to make the energy and proteins necessary to survive.
As cells utilize the oxygen and nutrients, they give off carbon dioxide (CO2) & wastes. These diffuse out of the cells and into the capillaries, which turn into veins. Venous blood carries these wastes; veins keep getting larger and larger en route back to the heart. The largest vein is called the Vena Cava. In contrast to arteries, veins are somewhat flimsy conduits, not so complex. The hearts pumping pressure has phased away by the time arteries reach the capillaries, so blood in veins has to eke its way along. It gets boosted by constant muscle movement.
When venous blood reaches the heart, it gets pumped into the lungs to give off its stale CO2, and pick up fresh oxygen, brought back to the heart to get pumped, and we start over. The wastes from nutrients get excreted when arteries carry them into the kidneys.
Trivia – which artery carries carbon dioxide, which vein carries oxygen? The Pulmonary Artery leaving the right side of the heart to enter the lungs, is called an “artery” because it leaves the heart, but the blood still has CO2 from the veins. The Pulmonary Vein is a “vein” because it heads back to the heart, but it carries the O2 just picked up in the lungs. See diagram of Heart.Anatomy.