What do veins do?
Veins have two main purposes. One purpose is to collect oxygen-poor blood throughout your body and carry it back to your heart. The other purpose is to carry oxygen-rich blood from your lungs to your heart. This is the only time veins carry oxygen-rich blood.
The purpose of each vein depends upon where it’s located within your body. Veins are organized into a complex network called the venous system.
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The venous system
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The venous system refers to your network of veins and the way your veins connect with other blood vessels and organs throughout your body. Your venous system is organized into two main parts or circuits. These are the systemic circuit and the pulmonary circuit. Each circuit relies on blood vessels (veins, arteries and capillaries) to keep blood moving.
To help understand how these circuits work, you might think of a racetrack. At a racetrack, the race cars must complete many laps around an entire course (circuit). But the cars can’t keep going without refueling and getting quick tune-ups. Similarly, your blood can’t keep flowing throughout your body without refueling (getting more oxygen) and getting rid of waste products like carbon dioxide.
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Your blood is a race champion because it finishes laps throughout your body every minute of the day on two different circuits. This can be hard to picture, but it helps to think about the systemic circuit first. This circuit weaves through your whole body including your arms and legs.
Here’s what one circuit through your body looks like. First, freshly oxygenated blood leaves your heart and enters your arteries. Your arteries branch off into smaller vessels called arterioles, and then capillaries. Once your blood is in your capillaries, it feeds your body’s tissues with oxygen and picks up waste products like carbon dioxide. At that point, your blood has lost oxygen and gained waste. So, it needs to be refueled. Your blood enters your venules before joining up with your veins. Your veins then carry your blood back to your heart where it can refuel. This oxygen-poor blood enters your heart through two large veins called your superior vena cava and inferior vena cava.
Once your blood comes back to your heart, it’s finished with the systemic circuit. Now it needs to complete the pulmonary circuit. In this circuit, your blood moves into your lungs. In your lungs, your blood refuels with oxygen and then returns to your heart through your pulmonary veins. This is the only time when your veins carry oxygen-rich blood! Your heart then pumps out this oxygen-rich blood so it can begin a new lap on the systemic circuit.
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