Vein

Latin: vena
German: Vene

1 Definition

Veins are blood vessels that lead the blood circulating in the body back to the heart. Except for the pulmonary veins, they transport blood poor in oxygen, the so-called "venous" blood. The veins are part of the cardiovascular system, and they belong to the so-called low-pressure system, which serves as blood reservoir.

2 Classification

2.1 ...according to localization

• Superficial veins (venae superficiales): Veins that are located above the common fascia of the body. In most cases, they form extensive venous networks, eg. the dorsal venous network of the hand.
• Deep veins (venae profundae): Veins that are located underneath the common fascia of the body.
• Central veins: veins that are located close to the heart.
• Peripheral veins: veins that are located in the periphery of the body, away from the heart
• Accompanying veins (venae comitantes): smaller pairs of veins that run very close to an arterial leading vessels.

2.2 ...according to the supplied area

• Cerebral veins (venae cerebri)
• Brachial veins (venae membri superioris)
• Leg veins (venae membri superioris)

2.3 ...according to size

• Vein
• Venole

3 Anatomy

Most veins run parallel to the eponymous arteries, however, there are numerous exceptions, especially among the veins of the skin or the cerebral veins. The advantage of this collocation results from the fact that the expansion of the artery during systole causes a slight compression of the neighboring vein, which further supports the blood flow by the system of venous valves.

3.1 Important veins

• Superior vena cava
• Inferior vena cava
• Subclavian vein
• Internal jugular vein
• External jugular vein
• Femoral vein
• Median cubital vein

Moreover, there are numerous veins that are denominated as eponym according to their discoverer due to their clinical significance, eg.:

• Hunter's vein
• Cocket's vein
• Sherman's vein
• Boyd's vein

4 Histology

The histological structure of the veins greatly differs from the structure of the arteries. The venous wall contains larger quantities of collagenous fibrils alongside smooth muscles and elastic material. This reduces the elasticity of the venous tube. The middle coat is significantly thinner, and the adventitia has no exact delimitation, since its collagen fibers insert into the surroundings.

4.1 Local characteristics

The structure of the veins also differs depending on their location:

• In the periphery of the body, the veins are equipped with venous valves, which consist of two sails and prevent the blood reflux within the vessel. The veins of the legs are, due to hydrostatic pressure, more muscularized than the veins of the superior regions of the body.
• The veins close to the heart predominantly consist of connective tissue and have almost no smooth muscles, so that they cannot change their transverse section.
• Muscleless veins exist in organs that have a constant blood flow, such as the brain and the retina.
• In the adrenal medulla, you can find the so-called jugular veins that regulate the blood flow from the tissue.

5 Clinical presentation

Veins present an ideal access for the parenteral administration of liquids and drugs, or to draw blood for diagnostic purposes. Therefore, the venipuncture belongs to the most important medical techniques.

5.1 Aspect

The superficial veins of the human body are recognizable with the naked eye. The appear as blue protrusions in or above the level of the skin. Under stasis of an extremity they become more pronounced.

Superficial veins seem to be blue because the long-wave red light has a higher depth of penetration into the tissue than the short-wave blue light. Blue light is reflected, red light is absorbed - this makes the veins seem blue until a tissue depth of 1 to 2 millimeters.

5.2 Diseases

The diseases of the veins are the domain of phlebology. Important diseases of the veins include:

• Thrombophlebitis (vein inflammation)
• Vein thrombosis
• Chronic venous insufficiency (CVI)
• Varices (varicose veins)