Blood

Vein Function – What Are The Functions Of The Veins?

Veins are the blood vessels which help in returning the blood back to the heart from the various parts of the body. In systemic circulation, the oxygenated blood is pumped into the muscles and the organs of the body through the arteries by the left ventricle. The nutrients and gases are exchanged at the capillaries. Then the blood enters the veins filled with cellular waste and carbon dioxide. This deoxygenated blood is then taken to the right atrium by the veins.

The veins are blood vessels which carry the deoxygenated blood back to heart from the body tissues in our circulatory system. The word “vein” has been coined from the Latin word ‘vena’. But there are some exceptions in the functioning of some veins – the pulmonary veins and the umbilical veins. These two exceptional veins carry the oxygenated blood. Veins are different from arteries in both their structure and function. This articles also explains about the classification of veins and about venous diseases and vein function.

Vein Functions:

Valve Position

  • Vein’s lumens generally collapse when they do not get completely filled with the blood.
  • Vein’s outermost layer is very thick and is made up of connective tissues that are called tunica externa or tunica adventitia.
  • The smooth muscle bands are deeper and thin, called the tunica media.
  • The thin muscle bands do not interfere as the veins do not undergo contraction or relaxation very often.
  • The inside of the vein, called tunica intima, is lined with the help of endothelial cells. The veins are mostly one-sided.
  • They have flaps called venous valves that can prevent the back flow of the blood.
  • These valves also restricts the pooling of the blood in the lower extremities due to the gravitational force.
  • The in-foldings of tunica intima makes up the venous valves.
  • The exact location of veins may vary from person to person.

Cycle of Blood Circulation

  • The right atrium transfers blood to the right ventricle.
  • The right ventricle pumps the blood through pulmonary arteries into the lungs.
  • In pulmonary circulation, the oxygenated blood is returned to the left atrium from the lungs through pulmonary veins.
  • Then, blood is emptied by the left atrium emptied into left ventricle. This completes one cycle of blood circulation.

Veins And The Vein Systems

  • The vein in the lower limb which is most superficially placed is the greater saphenous vein.
  • These veins usually hide inside the thigh along its own fascial compartment leaving the compartment at the knee.
  • This is how it named has been coined as ‘ Safina’ meaning hidden.
  • This was described by Avicenna, the Persian physician, for the first time.
  • Incompetence of the saphenous vein can lead to varicose veins and this usually occurs in the lower limbs.
  • The relatively oxygenated blood is carried by the pulmonary veins to the heart from the lungs and the relatively deoxygenated blood is carried by the superior vene cava and the inferior vena cava.
  • When a series of venules or veins connect two capillaries directly, a portal venous system is formed.
  • Example of such a system is hypophyseal portal system and hepatic portal vein.

Carrying Blood to the Heart

  • The skeletal-muscle pump helps in maintaining the blood pressure of the venous system at a very low measure.
  • This is how the blood is returned to the heart.
  • If any failure occurs in the skeletal-muscular pump, it can lead to the individual becoming unconscious.
  • Sometimes when we stand for longer periods of time, blood gets pooled in the legs.
  • As a result, the blood pressure becomes too high to be returned to the heart.
  • Hypovolaemic shock and neurogenic shock can also be another reason for fainting.
  • In such cases, smooth muscles that tend to surround the veins may become extremely slack.
  • Veins get filled with a lot of blood eventually resulting in restricting the blood from reaching the brain. This can lead to unconsciousness.

Portal Veins

  • As we know, most veins carry the blood to the heart from various other parts of the body with some exceptions like the pulmonary vein. There is another exception- the Portal veins.
  • These play a major role in carrying blood between each capillary bed.
  • For instance, hepatic portal vein plays a major role in carrying blood to the capillary beds that are present in the liver and from the capillary beds to the digestive tract.
  • This blood is again drained inside gastrointestinal tract as well as inside the spleen.
  • From here hepatic veins carry this blood back to the heart.
  • Damage to this hepatic portal vein may result in causing serious danger especially to the mammals.
  • If blood clotting takes place inside hepatic portal vein, then it can result in causing portal hypertension further resulting in reduced blood fluid flow to the liver.

Color

  • The color of the vein is determined by the color of the venous blood.
  • The low oxygen content of the blood makes it appear dark red.
  • The subcutaneous fat permits wavelengths of only highly energetic blue light to penetrate through the skin, into the dark veins and then reflect back.
  • But it absorbs the low frequency light.
  • This is why the veins appear blue. This phenomenon is opposite of the theory of Rayleigh scattering (the reason the sky is blue in color).

Classification Of Veins

Superficial Veins And Deep Veins

  • Superficial Veins: These veins are located very close to the body surface and they are superficially placed. These veins may not have any corresponding arteries.
  • Deep Veins: These veins are placed deep inside the body, hence the name ‘deep veins’. These deep veins also have their corresponding arteries.

Systemic Veins and Pulmonary Veins

  • Pulmonary Veins: These are set of veins that carry oxygenated blood from lungs to heart, are known as pulmonary veins.
  • Systemic Veins: The systemic veins help in draining blood from body tissues and carrying this deoxygenated blood back to the heart.

Phlebology

The medical discipline involving the diagnosis and treating the disorders. This mainly involves venous origin is known as Phlebology. Physical examination and medical history and, laboratory evaluations and various imaging techniques are the basis of diagnosis. A Phlebology specialist is generally termed as a ‘Phlebologist’ and the related image is a ‘Phlebography’.

Venous Diseases

‘Venous insufficiency’ is a disorder that occurs in the venous system. It is found most commonly as spider veins and varicose veins. The type of treatment administered is dependent on the patterns or types of veins and the preference of the physician. Treatments include ambulatory phlebectomy, compression, foam, lasers, sclerotherapy, radiofrequency ablation or vein stripping. The postphlebitic syndrome is considered as ‘venous insufficiency’ that eventually develops into ‘deep vein thrombosis’. Read to know about them in detail.

  • Deep vein thrombosis: This condition arises when a blood clot is formed inside a deep vein. This often leads to chronic venous insufficiency and pulmonary embolism.
  • Thrombophlebitis: This condition is considered as an inflammatory condition and is generally caused by blood clots in the veins.

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