Venous blood
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In the circulatory system, venous blood is blood returning to the heart. It is also typically warmer than arterial blood. Venous blood can be obtained by venipuncture (also called phlebotomy), or in small quantities by fingerprick. Most medical laboratory tests are conducted on venous blood, with the exception of arterial blood gases.
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Venous blood is often depicted as blue in color in medical diagrams, and veins sometimes look blue when seen through the skin. However, venous blood is actually a dark red color (but looks purple through the opaque skin), while arterial blood is bright red. The appearance of veins as dark blue is a wavelength phenomenon of light, having to do with the reflection of blue light away from the outside of venous tissue if the vein is @ 0.02in deep or more. If blood is drawn from a vein for a medical test in a vacuum tube void of oxygen, the dark red color can be seen however, if it is exposed to oxygen in the air, it will turn bright red like arterial blood.:)
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