ANATOMY: UNDERSTANDING YOUR VEINS AND THE VENOUS SYSTEM 

Did you know that nearly 75 percent of your blood is in your veins, and that your arteries are different from your veins?

Veins are blood vessels located throughout your body that collect oxygen-poor blood and return it to your heart. Veins are part of your circulatory system. They work together with other blood vessels and your heart to keep your blood moving. Veins hold most of the blood in your body. In fact, nearly 75% of your blood is in your veins. Veins in your legs fight gravity to push blood up toward your heart. 

The major difference between arteries and veins is the type of blood they carry. While arteries carry oxygen-rich blood, veins carry oxygen-poor blood. Your pulmonary veins are an exception to this rule. These four veins, located between your heart and lungs, carry oxygen-rich blood from your lungs back to your heart. From there, your heart pumps the oxygen-rich blood back throughout your body.

Your venules are very small blood vessels that connect your capillaries with your veins throughout your body. Your venules have the important function of moving blood that contains waste and lacks oxygen from your capillaries to your veins. From there, your blood can make its way back to your heart. Your venules are wider than your capillaries but narrower than your veins. Venules vary in size, but even the widest venule is about 16 times smaller than your typical vein.

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. 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.

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.

Many people think veins are blue because they look blue through our skin. But that’s just a trick that our eyes play on us. Your veins are actually full of dark red blood — darker than the blood in your arteries, which is cherry red. The blood in your veins is darker because it lacks oxygen. Your veins look blue because of the way light rays get absorbed into your skin. Blood is always red both in your veins and arteries. Common problems with veins include chronic venous insufficiency, deep vein thrombosis and varicose veins.

Source:

https://my.clevelandclinic.org/health/body/23360-veins