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 is like a network of tiny roads that carry blood all around the body. Unlike arteries, which carry fresh, oxygen-rich blood away from the heart, most veins carry used, oxygen-poor blood back to the heart for cleaning and reoxygenating.

Veins come in different sizes — large, medium, and small. The tiniest ones are called venules, and they help in the very small blood movements near the body's tissues. Because their walls are thinner and they have wider spaces inside, veins can hold a lot of blood. In fact, about 70% of all the blood in the body is stored in the veins at any given time.

To make sure blood flows in the right direction, veins have special one-way gates called valves. These valves stop blood from flowing backward. In our legs, muscles also help by squeezing the veins when we move, pushing the blood back up to the heart. This teamwork between valves and muscles keeps 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 that make up 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. In the body, 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 and also helps transfer heat from arteries to veins to keep the body warm.

Venules

The smallest veins begin where two or more capillaries join to form a post-capillary venule. These tiny vessels, between 10 and 30 micrometres wide, are part of the microcirculation. They are supported by special cells called pericytes. 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. The inferior vena cava is the larger of the two and runs close to the spine.

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. These veins do not have valves, which can sometimes help cancers spread.

A subcutaneous venous plexus is a continuous network with a high flow of blood supplied by small connections between arteries and veins. This high flow helps transfer heat to the vein walls.

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 and covered with cells that line the blood vessels. The cells on the valve flaps are arranged in different directions to help the valve open and close properly. When blood tries to flow backward, the valve flaps close to keep it moving forward. Most venous valves are found in small veins less than 300 micrometres wide.

The deep veins in the lower leg include the common femoral vein, femoral vein, deep femoral vein, popliteal vein, tibial veins, and fibular veins. The common femoral vein has a valve above the saphenofemoral junction called the suprasaphenic valve. The femoral vein often has three valves, with one commonly found just below where the deep femoral vein joins. The deep femoral vein and its connecting veins also have valves. The popliteal veins have one to three valves, the posterior tibial veins have between 8 and 19 valves, and the anterior tibial veins have between 8 and 11 valves.

In the superficial veins, there are between one and seven valves along the thigh part of the great saphenous vein, two to six below the knee, and one to four in the small veins of the foot. A special valve at the end of the great saphenous vein, called the terminal valve, stops blood from flowing back into the femoral vein. Another valve just below where smaller veins join, called the preterminal valve, stops blood from flowing back into the great saphenous vein. When the great saphenous vein does not work well, it can cause varicose veins.

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. Valves in the connecting veins close when leg muscles contract to stop blood from flowing back into the superficial veins. 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, but it can sometimes cause problems if it remains active.

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, but some goes directly to the right atrium.

Bronchial circulation

In the bronchial circulation, bronchial veins drain blood from the main airways into the azygos vein and then into the right atrium. Some blood from inside the lungs drains into the pulmonary veins and goes into the left atrium. This blood was not oxygenated because it did not pass through capillaries.

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. These deep veins join to form the vein of Galen, which then connects to the superficial system.

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. Sometimes, irregular connections called arteriovenous malformations can occur, often from a tangle of capillaries. A special type of anastomosis called a glomus body helps transfer heat in the fingers and toes.

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, called the tunica externa or tunica adventitia, is made of thick connective tissue. The middle layer, called the tunica media, has smooth muscle cells, elastic fibers, and collagen. It helps control the size of the vein and blood pressure. The inner layer, called the tunica intima, is lined with very thin cells and supports them with delicate connective tissue. This layer also helps produce 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 special tubes in our bodies that carry blood back to the heart. Most of this blood does not have oxygen anymore. 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 to start the journey again.

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, either 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 too. Getting older makes it more likely to have vein problems. Doctors who specialize in treating vein problems are called phlebologists, and they use special surgeries to help treat these conditions.

One common vein problem is when the valves don’t work well, leading 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. If a piece of the clot breaks off, it can travel to the lungs and cause serious problems. 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. This can cause visible veins in places like the esophagus. Inflammation of a vein, known as phlebitis, can also happen and may come with a blood clot.

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

Long ago, a doctor named Herophilus from Greece figured out that veins and arteries are different. He noticed that arteries have thicker walls but thought the pulse came from the arteries themselves. Another doctor, Erasistratus, saw that when arteries were cut, they bled. He thought that tiny vessels between veins and arteries let air and blood mix.

Later, a doctor named Galen from Rome knew that blood vessels carry blood. He thought that blood from the liver gave the body growth and energy, while blood from the heart gave vitality. He believed the heart did not pump blood but instead sucked it in.

A doctor named Ibn al-Nafis suggested that tiny connections exist between the lung’s blood vessels, which was a smart guess long before these connections were discovered.

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