Veins: Anatomy and Function (2023)

Overview

What are 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.

What type of blood do veins carry?

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.

What are venules?

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.

Function

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.

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.

Anatomy

What do veins look like?

Your veins make up an extensive network of blood vessels that wind their way through your entire body. Together, your veins and other blood vessels form a major part of your circulatory system. Your veins connect with venules and capillaries in many places. When mapped out in a drawing, your upper body circulatory system resembles the complex wires and circuits inside a computer. Your lower body circulatory system resembles an upside-down tree with two large branches (one on each leg) and many small twigs on each branch.

What color are veins?

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.

What are veins made of?

Each vein is made of three layers of tissues and fibers:

• The tunica adventitia (outer layer) gives structure and shape to your vein.
• The tunica media (middle layer) contains smooth muscle cells that allow your vein to get wider or narrower as blood passes through.
• The tunica intima (inner layer) has a lining of smooth endothelial cells, allowing blood to move easily through your vein.

Veins and arteries share this general structure. However, veins are different from arteries because they sometimes also contain one-way valves that keep blood flowing in the right direction. These valves are especially important in your legs, where they help blood move up toward your heart. If these valves get damaged, blood can leak backward and cause varicose veins or other problems.

Veins are also different than arteries when it comes to the thickness of their walls. Veins have thinner and less muscular walls. This is because veins have a lower level of pressure than arteries. So, their walls don’t need to be as thick to handle the pressure.

What are the different types of veins?

You have three types of veins that help your circulatory system function.

Deep veins

These veins can be found in your muscles and along your bones. Your deep veins do the important work of moving your oxygen-poor blood back to your heart. In your legs, your deep veins hold about 90% of the blood that travels back to your heart. Your deep veins contain one-way valves that keep your blood moving in the right direction.

Superficial veins

Your superficial veins are generally smaller than your deep veins. Like deep veins, they contain valves. Unlike deep veins, they’re not surrounded by muscle. Instead, your superficial veins can be found just underneath your skin. So, you can easily see them.

Your superficial veins carry blood from your outer tissues near the surface of your skin to your deep veins (via the perforating veins). But this blood moves more slowly since it’s not being directly squeezed into motion by surrounding muscles.

The largest vein in your body is a superficial vein called the great saphenous vein. It runs all the way from your ankle to your thigh in each leg.

Perforating veins

These veins are sometimes called connecting veins or perforator veins. They are short veins that carry blood from your superficial veins to your deep veins. Perforating veins contain valves that close when your calf muscles compress so that blood doesn’t flow backward from your deep veins to your superficial veins.

What makes blood flow in the veins?

Your veins need an external force to help push your blood in the right direction. One such force is your own breathing. As your lungs expand and your diaphragm moves, they create a suction force that helps your veins push oxygen-poor blood toward your heart. Another force is your body’s muscle movement, especially in your legs. In fact, your leg muscles play a vital role in helping your blood defy gravity and move upward from your feet and legs back to your heart. For this reason, the muscles in your calves are called your “second heart.”

The “second heart”

You might not realize that your lower leg muscles act as a powerful pump that squeezes the deep veins in your lower legs. This “second heart,” also called your peripheral heart, springs into action each time you take a step. When you place your foot onto the ground, your body weight squeezes the deep veins in the bottom of your foot. As a result, those veins push any blood that’s inside up toward your calf.

Then, when you lift your heel, your calf muscles squeeze the deep veins in your calf. Your blood keeps moving up toward your thighs and beyond. This incredible system allows the blood in your feet and lower legs to defy gravity and make its way back up to your heart.

Unlike your heart in your chest, your second heart only starts pumping when your legs move. And its pumping pace adjusts to however fast your legs are moving. So, if you’re running, your calf muscles will squeeze your veins more quickly than if you’re walking. No matter the pace, your second heart allows your blood to keep flowing and complete its circuits through your body. As a result, your organs and tissues continue to receive oxygen and nutrients to function at their best.

Conditions and Disorders

What are the common conditions and disorders that affect veins?

There are several venous diseases that prevent your veins from working as they should. Some common problems include:

• Superficial thrombophlebitis. This is when a clot forms just under your skin. Usually, the clot doesn’t travel to your lungs. But there’s still a risk of that happening if the clot makes its way into your deep veins.
• Deep vein thrombosis (DVT). This serious condition happens when blood clots (called thrombi) form in your deep veins. Usually, the clot forms in your legs or pelvis. The clot can break free from your vein and travel to your lungs, causing a life-threatening pulmonary embolism.
• Varicose veins. These swollen, bulging veins are sometimes harmless but can lead to serious problems like blood clots.
• Chronic venous insufficiency. When the one-way valves in your legs are damaged, they can’t effectively pump blood to your heart. DVT often causes this condition.

What are the common signs and symptoms of vein problems?

The signs and symptoms depend on your specific condition. They generally include:

• Swelling (edema) in your legs, ankles or feet, especially after standing a while.
• Pain or tenderness.
• Achy, tired or throbbing legs.
• Leathery-looking skin on your legs.
• Flaking or itchy skin on your legs or feet.

If you have any of these symptoms, or if you notice any purple or bulging veins that weren’t there before, call your healthcare provider. Many vein problems are treatable if caught early. And it’s especially important to diagnose DVT right away before it leads to a pulmonary embolism.

What tests are used to check vein health?

Your provider will talk with you about your medical history and perform a physical exam. Your provider may want to run a doppler ultrasound test and draw blood to help diagnose DVT or other conditions.

What treatments are available for vein problems?

Treatments for vein conditions and disorders usually aim to reduce your risk of blood clots, get rid of a clot that already exists and alleviate symptoms.

• Blood thinners (anticoagulant therapies) are commonly used to treat DVT and prevent pulmonary embolism. Examples include warfarin, rivaroxaban and apixaban. Your provider will check your blood regularly to see how well the medicine is working and adjust your dose. Your dose will be lowered whenever possible to reduce the risk of bleeding, which is the most common side effect.
• Clot-dissolving drugs are not used often. They’re most effective when used within 48 hours of clot formation.
• Surgical procedures are rare. However, the insertion of a vena cava filter can be a good option for people who can’t take anticoagulants.

There’s a lot you can do at home to treat and prevent vein problems. Your healthcare provider may recommend you wear compression socks to help your blood flow better in your legs. Different types of socks apply different amounts of pressure. Your provider will guide you in choosing the right pair. Compression therapy is actually one of the oldest treatments for chronic venous insufficiency — it’s been used for over 2000 years. You just have to make sure you get the right fit so that the socks help rather than hurt your legs.

Care

What can I do to take care of my veins?

There are many ways to take care of your veins. If you’ve been diagnosed with a vein problem or have risk factors, it’s important to do the following:

• Avoid sitting or lying down for too long without moving. If you sit for much of the day, be sure to get up and walk around for a couple of minutes every hour. As you sit, lift your lower legs up, and flex your ankles. The more you can move your lower legs, the more your muscles can squeeze your veins and pump blood up toward your heart.
• Practice good foot hygiene to prevent infection. This includes keeping your feet clean and dry. You can also use moisturizer to prevent your skin from cracking or bleeding.
• Talk with your provider to decide if anticoagulants are right for you. Also, be sure to tell your provider about any changes in symptoms or how you feel.

Even if you don’t have vein problems, you can make simple lifestyle choices each day to keep your veins healthy:

• Get up and move around as much as possible throughout the day to keep your blood flowing.
• Go for walks (aim for 30-minute walks, at least five days a week).
• Eat a heart-healthy diet to help maintain a healthy weight.
• Take breaks to stretch and walk around to break up long car rides or plane flights.
• If you develop any symptoms of vein problems, even if they seem small, call your provider. Catching problems early can help avoid serious problems down the road.

It’s often easy to forget that even when you’re at rest, your heart and blood vessels are working hard. Your circulatory system keeps moving so you can keep moving. That’s why it’s important to do whatever you can to keep your blood flowing smoothly, lap after lap.

A note from Cleveland Clinic

Your veins are a vital part of your circulatory system. Together with your heart, arteries and capillaries, your veins work every day to keep your blood moving throughout your body. If you’re taking a long car ride or plane flight, remember to get up and move around as much as possible. Even lifting your lower legs and flexing your ankles can help the veins in your legs move blood up to your heart. If you think you might have problems with your veins, call your healthcare provider right away to discuss your symptoms. Many vein problems are treatable, especially if caught early.