Human Body Veins

A vein is an elastic blood vessel that transports blood from various regions of the body to the heart. Veins are components of the cardiovascular system, which circulates blood to provide nutrients to the cells of the body. Unlike the high pressure arterial system, the venous system is a low pressure system that relies on muscle contractions to return blood to the heart. Sometimes vein problems can occur, most commonly due to either a blood clot or a vein defect.

Types Of Vein:

Pulmonary Veins:
Pulmonary hypertension is a type of high blood pressure that affects the arteries in your lungs and the right side of your heart. In one form of pulmonary hypertension, tiny arteries in your lungs, called pulmonary arterioles, and capillaries become narrowed, blocked or destroyed. This makes it harder for blood to flow through your lungs, and raises pressure within your lungs' arteries. As the pressure builds, your heart's lower right chamber (right ventricle) must work harder to pump blood through your lungs, eventually causing your heart muscle to weaken and fail.

Systemic Veins: 
The systemic circuit carries deoxygenated blood from the rest of the body back to your heart, where it then enters the pulmonary circuit for oxygen. Most veins are systemic veins.

Deep Veins: 
These are found in muscles or along bones. The tunica in time of a deep vein usually has a one-way valve to prevent blood from flowing backward. Nearby muscles also compress the deep vein to keep blood moving forward.

Superficial Veins:
These are located in the fatty layer under your skin. The tunica intima of a superficial vein can also have a one-way valve. However, without a nearby muscle for compression, they tend to move blood more slowly than deep veins do.
Connecting Veins: 
Blood from superficial veins is often directed into the deep veins through short veins called connecting veins. Valves in these veins allow blood to flow from the superficial veins to your deep veins, but not the other way.

Vein Size:

A vein can range in size from 1 millimeter to 1-1.5 centimeters in diameter. The smallest veins in the body are called venules. They receive blood from the arteries via the arterioles and capillaries. The venules branch into larger veins which eventually carry the blood to the largest veins in the body, the vena cava. Blood is then transported from the superior vena cava and inferior vena cava to the right atrium of the heart.
Vein Structure:

Veins are composed of layers of thin tissue. The vein wall consists of three layers:
  • Tunica Adventitia: The strong outer covering of arteries and veins. It is composed of connective tissue as well as collagen and elastic fibers. These fibers allow the arteries and veins to stretch to prevent over expansion due to the pressure that is exerted on the walls by blood flow.
  • Tunica Media: The middle layer of the walls of arteries and veins. It is composed of smooth muscle and elastic fibers. This layer is thicker in arteries than in veins.
  • Tunica Intima: The inner layer of arteries and veins. In arteries, this layer is composed of an elastic membrane lining and smooth endothelium (a special type of epithelial tissue) that is covered by elastic tissues. Veins do not contain the elastic membrane lining that is found in arteries. In some veins, the tunica intima layer also contains valves to keep blood flowing in a single direction.
Vein walls are thinner and more elastic than artery walls. This allows veins to hold more blood than arteries.


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