Vein

A vein is a special kind of blood vessel that helps move blood through the bodies of humans and many animals. It is part of the circulatory system, which carries blood all around the body.

Unlike arteries, which carry fresh blood away from the heart, most veins carry used blood back to the heart.

Veins come in different sizes — large, medium, and small. The tiniest ones are called venules. Because their walls are thinner, veins can hold a lot of blood.

Veins have special one-way gates called valves. These valves stop blood from flowing backward. In our legs, muscles help by squeezing the veins when we move, pushing the blood back up to the heart. This helps keep blood moving smoothly through our veins.

Structure

Veins come in three sizes: large, medium, and small. The smallest veins are called venules. Veins, like arteries, have three layers in their walls. The outer layer is made of connective tissue, the middle layer has smooth muscle, and the inner layer is a thin lining called endothelium. Unlike arteries, veins have thinner middle layers because they do not need to handle high pressure. Many veins also have tiny valves that help blood flow in one direction only.

Veins can be found in different places in each person's body. Veins close to the skin's surface often look blue because of how light scatters in the skin and how our eyes see color, not because the blood inside them is blue. The blood in veins is actually dark red.

Venous system

See also: List of veins of the human body

Veins of the human body

Main veins of the systemic circulation

Veins of the circulatory system

The venous system is the network of veins that brings blood back to the heart. Most veins carry blood that does not have oxygen from the organs and tissues. In the lungs, special veins called pulmonary veins carry blood that has oxygen back to the heart. Almost 70% of the blood in the body is found in the veins, with most of it in the smaller veins and tiny vessels called venules. The largest veins, the superior and inferior vena cavae, collect blood from all the body’s veins and empty it into the heart’s right atrium. Veins have thin walls and larger openings, which lets them hold more blood and balance pressure changes. The venous system is usually not the same on both sides of the body, and the exact places where veins are found can differ from person to person.

Veins range in size from tiny post-capillary venules to large veins. Their walls change thickness depending on where they are in the body—for example, veins in the legs have thicker walls than those in the arms. Blood enters the venous system from tiny blood vessels called capillaries, where it changes from arterial blood to venous blood.

Large arteries, such as the thoracic aorta, subclavian, femoral, and popliteal arteries, often lie close to a single vein that drains the same area. Some arteries are paired with two veins called venae comitantes, or satellite veins, which run on either side of the artery. When a nerve is also nearby, the group is called a neurovascular bundle. This closeness helps the veins move blood back to the heart.

Venules

The smallest veins begin where two or more capillaries join to form a post-capillary venule. These tiny vessels are part of the microcirculation. As venules grow larger, they become muscular venules and eventually feed into small veins.

Small, medium, and large veins

Small veins merge to form medium-sized veins, which then merge to form large veins. These large veins include the internal jugular, renal veins, and the venae cavae, which carry blood directly into the heart. The superior vena cava carries blood from the arms, head, and chest to the heart’s right atrium, while the inferior vena cava carries blood from the legs and abdomen.

Deep, superficial, and perforator veins

The venous system has three main parts: deep veins, superficial veins, and perforator veins. Superficial veins are closer to the skin’s surface and do not have corresponding arteries. Deep veins are deeper inside the body and have matching arteries. Perforator veins connect the superficial veins to the deep veins. These are most often found in the lower legs and feet. Superficial veins include very small spider veins and reticular or feeder veins.

Venous plexuses

There are places where veins group together in networks. One example is the Batson venous plexus, which runs through the spine and connects thoracic and pelvic veins.

Venous valves

Blood flows back to the heart in the deep veins, helped by one-way valves that prevent backflow. These valves also stop the veins from stretching too wide. A venous valve has two flaps and is made from a fold in the vein’s inner lining. The flaps are strengthened with special fibers. When blood tries to flow backward, the valve flaps close to keep it moving forward. Most venous valves are found in small veins.

The deep veins in the lower leg include the common femoral vein, femoral vein, deep femoral vein, popliteal vein, tibial veins, and fibular veins.

In the superficial veins, there are valves along the thigh part of the great saphenous vein. Valves divide the blood into segments to help it move forward to the heart. They are helped by muscles that squeeze the veins when they contract. There are more valves in the lower leg because of gravity, and the number of valves decreases as the veins move up toward the hip. There are no valves in the veins of the chest or abdomen.

There is a valve where the inferior vena cava meets the heart, called the valve of the inferior vena cava or Eustachian valve. This valve is a leftover from early development and usually does not do much in adults.

Circulatory routes

There are some special pathways that blood takes to reach certain parts of the body. These include the coronary circulation, which supplies the heart; the cerebral circulation, which supplies the brain; the bronchial circulation, which supplies the lungs; and the renal circulation, which supplies the kidneys.

Coronary circulation

In the coronary circulation, veins called cardiac veins drain deoxygenated blood from the heart muscle into the right atrium of the heart. Most of this blood flows through the coronary sinus.

Bronchial circulation

In the bronchial circulation, bronchial veins drain blood from the main airways into the azygos vein and then into the right atrium.

Cerebral circulation

In the cerebral circulation, which supplies the brain, the blood drainage can be split into two parts: superficial and deep. The superficial part includes dural venous sinuses, which are located on the brain’s surface. The deep part includes veins inside the brain’s deeper structures.

Portal venous systems

A portal venous system is a group of veins that connect two capillary beds. In humans, there are two: the hepatic portal system and the hypophyseal portal system.

Anastomoses

An anastomosis is a connection between two structures, such as blood vessels. In the circulatory system, an arteriovenous anastomosis connects an artery directly to a vein, skipping the capillaries.

Vascular shunt

A vascular shunt is a path that lets blood bypass the capillaries. This happens when small muscles called precapillary sphincters close, allowing blood to flow from a metarteriole into a collecting venule without passing through the capillaries.

Other

A communicating vein connects two parts of the same system. For example, the Giacomini vein connects the small saphenous vein with the great saphenous vein. Peripheral veins carry blood from the limbs and hands and feet.

Microanatomy

Veins have three main layers. The outer layer is called the tunica externa or tunica adventitia. It is made of thick connective tissue. The middle layer is called the tunica media. It has smooth muscle cells, elastic fibers, and collagen. It helps control the size of the vein and blood pressure. The inner layer is called the tunica intima. It is lined with very thin cells and supports them with delicate connective tissue. This layer also helps make important substances that affect the muscles around it.

Development

The development of an embryo depends on the flow of blood between the yolk sac and the embryo. This helps share nutrients before the placenta is fully formed.

By day 17, small blood vessels start to form in the yolk sac. These vessels grow and connect to make a network for blood to travel.

Around the middle of the fourth week, the heart starts beating and blood begins to circulate. The early heart has three pairs of arches for blood to flow out, and six paired veins for blood to flow back in, including the vitelline veins and umbilical veins.

Function

Main article: Circulatory system

Veins are tubes in our bodies that carry blood back to the heart. Most of this blood does not have oxygen. It travels from our organs and tissues to the right side of the heart. From there, the blood goes to the lungs to get oxygen and then returns to the left side of the heart.

Veins have thinner walls and are wider than arteries, which helps them hold more blood. This lets them adjust to different pressures in the body. Muscles and breathing help push the blood back to the heart. If we stand or sit for too long, it can be hard for blood to return, which might make us feel faint. But our body has ways to bring the blood back quickly to keep us feeling well.

Clinical significance

Further information: Vascular surgery § Management of venous diseases

Most problems with veins happen when blood flow gets blocked. This can be by a clump called a thrombus or when the vein’s valves don’t work well. Sometimes, inflammation or pressure on the vein can cause issues. Getting older makes vein problems more likely. Doctors who treat vein problems are called phlebologists. They use special surgeries to help treat these conditions.

One common problem is when the valves don’t work well. This can lead to spider veins or varicose veins. There are several ways to treat this, like using heat to close the veins or removing them through small cuts.

Another issue is when a blood clot forms in a vein, called venous thrombosis. This often happens in the deep veins of the legs. Doctors treat this with medicines to stop more clots from forming and special compression devices to help blood flow.

Sometimes, veins can get swollen due to pressure build-up, especially in the liver area. Inflammation of a vein, known as phlebitis, can also happen.

Doctors use special tools like ultrasound to see inside veins and find problems. Some of these tools can even help identify people by looking at the patterns of their veins.

History

Further information: Circulatory system § History

A long time ago, a doctor named Herophilus from Greece learned that veins and arteries are different. He saw that arteries have thicker walls but thought the pulse came from the arteries.

Another doctor, Erasistratus, saw that when arteries were cut, they bled. He thought tiny vessels between veins and arteries carried air and blood.

Later, a doctor named Galen from Rome knew that blood vessels carry blood. He thought blood from the liver gave the body growth and energy, and blood from the heart gave vitality.

A doctor named Ibn al-Nafis guessed that tiny connections exist between the lung’s blood vessels.

Finally, a doctor named William Harvey did many experiments and showed in 1628 that blood moves in a circle through the body, pumped by the heart. This changed how doctors thought about blood. Later, in 1661, Marcello Malpighi found tiny connections called capillaries that link arteries and veins.