Vein

Veins are tubes that carry blood back to your heart.

Most of your blood stays in these tubes. They come in three sizes. Some are large and some are very small.

Tiny flaps inside them work like doors. These doors only open one way. This stops the blood from flowing backward. This is helpful when you are standing up.

These tubes are often close to your skin. They can even look blue to your eyes!

Veins are blood vessels that carry blood back to the heart.

Most veins carry blood that has used up its oxygen. However, some veins in the lungs carry fresh oxygen to the heart.

Veins come in three sizes. They can be large, medium, or small. The smallest ones are called venules.

Because veins have thin walls, they can hold a lot of blood. In fact, nearly 70% of your blood is in your veins at any time.

To keep blood moving, veins have special parts called valves. These work like one-way doors. They stop blood from flowing backward due to gravity. In your legs, your muscles help too. When muscles squeeze, they push the blood upward.

Veins are often close to your skin. Even though the blood is dark red, veins can look blue. This happens because of how light hits your skin. The largest veins are called the venae cavae. They empty blood into the right side of the heart.

Veins are special blood vessels that carry blood back toward the heart. Most of them carry blood that has used up its oxygen from the body's tissues. However, there are two important exceptions to this rule. In the lungs, the pulmonary veins carry oxygen-rich blood back to the heart. In a growing baby, fetal circulation also uses veins to move oxygenated blood.

These vessels are vital for keeping the whole body working. Without them, the blood could not return to start its journey again.

Veins work by using a specific structure to hold and move blood. They have three layers called tunicae that form their walls. The outer layer is the tunica externa, which is made of connective tissue. The middle layer is the tunica media, which contains smooth muscle. The inner layer is a thin lining called the tunica intima.

Because their walls are thinner than arteries, veins can expand easily. This allows them to act as capacitance vessels that hold a lot of blood. In fact, nearly 70% of your total blood volume stays in your veins at any time.

To keep blood moving in one direction, veins use tiny one-way doors called valves. These valves prevent blood from flowing backward due to gravity or low pressure.

In your legs, your muscles help this process happen. When your leg muscles contract, they squeeze the deep veins. This is called a muscle pump, and it pushes the blood upward.

This teamwork between valves and muscles is very important for moving blood against gravity.

Veins come in many different sizes throughout your body. The smallest ones are called post-capillary venules, which are microscopic.

These grow into larger venules, then small veins, and finally medium or large veins. The two largest veins in the body are the superior and inferior vena cava. The superior vena cava brings blood from your head and arms. The inferior vena cava brings blood from your legs and abdomen.

The inferior vena cava is the larger of these two main paths.

You might notice veins near the surface of your skin. Even though the blood inside is actually dark red, these veins often look blue. This happens because of how light scatters when it hits your skin.

Veins are also often found right next to arteries. This close placement helps your body share heat between the vessels. It is a clever way for your body to stay at a steady temperature. This connection helps keep your whole system balanced and healthy.

Veins are critical blood vessels within the circulatory system of humans and most other animals. Their primary function is to carry blood back toward the heart. While most veins transport deoxygenated blood from body tissues, there are two notable exceptions. In the pulmonary circulation, pulmonary veins carry oxygenated blood from the lungs to the heart. Similarly, in fetal circulation, veins are used to transport oxygenated blood.

Because they can expand to hold large amounts of blood, veins are often called capacitance vessels.

The structure of a vein is defined by three concentric layers known as tunicae. The outermost layer is the tunica externa, or adventitia, which consists of thick connective tissue. The middle layer is the tunica media, made of bands of smooth muscle. The innermost layer is the tunica intima, a thin lining of endothelium.

Compared to arteries, veins have a thinner tunica media and a wider internal diameter, called a lumen. This wider lumen and thinner wall allow veins to accommodate significant pressure changes. In fact, nearly 70% of the total blood volume in the human body is held within the veins at any given time.

Veins vary significantly in size and complexity. The smallest vessels are post-capillary venules, which are microscopic and measure between 10 and 30 micrometres in diameter. These lack a smooth muscle layer and are instead supported by cells called pericytes. As they grow to 50 micrometres, they become muscular venules. These eventually merge into small veins, which then feed into medium and large veins.

The largest veins are the superior and inferior vena cava. The superior vena cava drains blood from the head, arms, and chest. The inferior vena cava, which is the larger of the two, drains blood from the legs and abdomen.

To ensure blood moves in only one direction, many veins contain bicuspid valves. These one-way valves are formed by infoldings of the tunica intima. They prevent blood from flowing backward, or regurgitating, due to gravity or low pressure.

When blood attempts to reverse direction, the pressure fills small pockets called valvular sinuses. This action forces the valve leaflets together to seal the vessel. Approximately 95% of these valves are found in small veins measuring less than 300 micrometres. In the lower limbs, these valves work alongside skeletal muscle pumps. When leg muscles contract, they exert pressure on intramuscular veins, driving blood upward toward the heart.

The venous system is organized into three main compartments: deep veins, superficial veins, and perforator veins. Deep veins are located further inside the body and have corresponding arteries. Superficial veins are closer to the skin's surface and do not have matching arteries. Perforator veins act as bridges, draining blood from the superficial veins into the deep veins.

In the legs, the walls of the veins are much thicker than those in the arms to handle different physical demands. While the main veins stay in relatively constant positions, the exact location of other veins varies between different individuals.

Veins often travel in close proximity to arteries. Some arteries are accompanied by a pair of veins called venae comitantes, or satellite veins, which run on either side of the artery. When an artery, vein, and nerve are all enclosed in a single sheath, it is called a neurovascular bundle. This close arrangement serves two important purposes. First, the pulsations of the artery help assist venous blood return. Second, it allows for a counterflow exchange where heat is transferred from the large arteries to the veins. This process helps the body preserve its normal internal heat.

Understanding the venous system also requires looking at specialized networks. There are several venous plexuses, which are groups of veins combined into networks. One example is the Batson venous plexus, which runs through the inner vertebral column and connects thoracic and pelvic veins. Because these specific veins lack valves, they are noted for their role in the metastasis of certain cancers. The venous system also supports specific circulatory routes, such as the coronary circulation for the heart and the renal circulation for the kidneys. These complex pathways ensure that every part of the body is efficiently serviced by the blood flow.