Peripheral Blood Flow Through the Veins
Mechanisms for Venous Return to the Heart from the Arms and Legs
Oct 10, 2009
Veins are blood vessels that return blood to the heart. In the arms and legs, venous blood is nutrient-poor and deoxygenated. The blood pressure in these peripheral veins is much lower when compared with the arteries, so with each beat of the heart, blood flow is significant within the arteries, but hardly affected within the veins. There are other mechanisms, however, that work together to move venous blood in the correct direction.
Venous Valves
The veins of the arms and legs contain valves that prevent the blood from moving away from the heart. These unidirectional valves open when blood flows toward the heart. The backward movement of blood causes the valves to close.
People with varicose veins, or venous insufficiency, most likely have valves that are damaged and have ceased working properly. This allows the blood to move in the wrong direction and to place more pressure on other valves and the veins.
Muscle Contraction
When the arm and leg muscles are contracted, such as with walking or lifting an object, the contracted muscles put pressure on the vein. This squeezes blood through the vein like a hand squeezes toothpaste through a tube. Since valves keep the blood from flowing backward, the muscle contraction pushes the blood toward the heart.
Breathing and Blood Flow
Respiration changes the pressure within the thoracic and abdominal cavities. This pressure change helps move blood toward the heart. During inhalation, the diaphragm contracts resulting in lower pressure in the thoracic cavity and higher pressure in the abdominal cavity. This pulls the blood into the veins of the thoracic cavity toward the heart.
During a venous Doppler ultrasound, deep breaths may be used to move blood through the legs. The patient is asked to take in a breath and hold it, then to forcefully push the air out. Just after exhalation, the ultrasound will usually indicate a surge of blood moving faster through the veins.
Vasoconstriction in Veins
The veins act as a reservoir where a considerable amount of blood is stored. When extra blood is needed, such as during sudden blood loss, the veins will constrict to push more blood towards the heart. This helps to prevent a drop in blood pressure.
Vasoconstriction can also result from trauma to the vein. It is not uncommon to see vasoconstriction during a laser vein treatment called endoluminal laser ablation surgery of the greater saphenous vein. This may begin at the site of incision, or as a guide wire is being pushed through the vein.
While there are several mechanisms for returning venous blood to the heart, it does not move as readily as arterial blood. Slowly-moving venous blood may be a risk factor for deep venous thrombosis (DVT). Exercise uses the muscle contraction mechanism to facilitate venous blood flow and is often prescribed for those who are at risk for DVT.
Reference
Shier, D., Butler, J., Lewis, Ricki. Hole's Human Anatomy & Physiology. New York: McGraw-Hill, 2002.
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