Blood travels through two types of passages. Oxygen-rich blood travels from your heart to the rest of your body through arteries (arteries move blood away from your heart). Oxygen-depleted blood makes the return trip back to your heart in veins. Together, arteries and veins are called blood vessels, and their winding tubes stretch tens of thousands of miles. Arteries are more than just passive tubes. Healthy arteries are strong, flexible, and lined with muscle. As your heart pumps blood, your arteries expand and contract rhythmically to help move that blood along. Veins are more passive—they transport blood steadily rather than pumping it in time with each heartbeat. Tiny valves prevent blood from seeping backward, and normal muscular activity helps it flow forward (which is why getting up and walking around helps your circulation). Varicose veins occur when the valves in the veins weaken and blood becomes trapped. The condition is mostly harmless, but sometimes uncomfortable.
To better understand how your arteries and veins work, it helps to take a trip through them yourself. You can start at the pump that keeps the traffic flowing—your heart.
1. Inject yourself into the right side of your heart (which is on the left side of the front-facing picture on this page). The blood that arrives here is the dark, purplish kind that has little oxygen left in it. With a forceful contraction, your heart squeezes itself together and propels your blood toward your lungs.
2. As your blood moves through the tiny blood vessels that line your lungs, it releases carbon dioxide and picks up a full cargo of oxygen. New blood keeps arriving, driving the oxygen-filled blood back to your heart.
3. When your blood reaches your heart a second time, it enters the left side (on the right side of the diagram). Your heart contracts again, pushing this oxygen-rich blood out through the aorta, which is the largest artery in your body.
4. As blood flows through your arteries, it moves through smaller and smaller passageways, eventually flowing into a microscopic network of capillaries, which are the tiniest blood vessels in your body. Nutrients and oxygen pass through the walls of these capillaries into neighboring cells.
5. Having completed its nutrient and oxygen delivery, your blood picks up waste products (carbon dioxide and so on) and is ready to return to the heart. Eventually, the capillaries join back together to form tiny veins. These tiny veins combine to create larger veins, and they eventually unite in the massive vena cava veins that lead back to the right side of your heart, and back to step 1.
Your blood makes this journey continuously, travelling from your heart to your lungs, back to your heart, through your body, and then back to your heart once more. Despite the winding route, each drop of blood makes the full circuit in less than 60 seconds.
Contrary to what you might expect, your veins don’t reclaim all the fluid your arteries release. Instead, a small amount lingers in your cells, and is collected into another system of tiny passageways, collectively called the lymph system. The lymph system is primarily known as a part of your immune system. To fight infection, it filters lymph fluid through small, bean-shaped lymph nodes that are scattered throughout your body. The lymph nodes store heavy reserves of white blood cells that destroy any invaders.
Source of Information : Oreilly - Your Body Missing Manual