Venous blood is deoxygenated blood which travels from the peripheral vessels, through the venous system into the right atrium of the heart. Deoxygenated blood is then pumped by the right ventricle to the lungs via the pulmonary artery which is divided in two branches, left and right to the left and right lungs respectively. Blood is oxygenated in the lungs and returns to the left atrium through the pulmonary veins.
Venous blood is typically colder than arterial blood, and has a lower oxygen content and pH. It also has lower concentrations of glucose and other nutrients, and has higher concentrations of urea and other waste products. The difference in the oxygen content of arterial blood and venous blood is known as the arteriovenous oxygen difference.
Most medical laboratory test are conducted on venous blood, with the exception of arterial blood gas tests. Venous blood is obtained for lab work by venipuncture (also called phlebotomy), or by finger prick for small quantities.
Human blood is red, ranging from bright red when oxygenated to dark red when not. It owes its color to hemoglobin, to which oxygen binds. Deoxygenated blood is darker due to the difference in color between deoxyhemoglobin and oxyhemoglobin. There exists a popular misconception that deoxygenated blood is blue and that blood only becomes red when it comes into contact with oxygen. Blood is never blue, but veins appear blue because light is diffused by skin. Moreover, the blood inside is dark red and exhibits poor light reflection. Blood also changes color due to the amount of oxygen in it.
The blue appearance of surface veins is caused mostly by the scattering of blue light away from the outside of venous tissue if the vein is at 0.5 mm deep or more. Veins and arteries appear similar when skin is removed and are seen directly.
Venous blood is mainly used for blood transfusion. Commonly only components of the blood, such as red blood cells, white blood cells, plasma, clotting factors, and platelets are used. It is one of the three sources of stem cells, which are extracted through apheresis.
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