- Venous blood
In the
circulatory system , venous blood isblood returning to theheart (invein s). With one exception (thepulmonary vein ) this blood is deoxygen ated and high incarbon dioxide , having released oxygen and absorbed CO2 in the tissues. It is also typicallywarm er than arterial blood, has a lowerpH , has lower concentrations ofglucose and other nutrients, and has higher concentrations ofurea and other waste products.Venous blood can be obtained by
venipuncture (also called phlebotomy), or in small quantities byfingerprick . Most medical laboratory tests are conducted on venous blood, with the exception of arterialblood gas es.Color
Veneous blood is dark red, not blue as it is often depicted in many medical diagrams.
Veins often look blue when seen through the skin, but this is due to
Rayleigh scattering – venous blood itself 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. This is due to the difference in color between
deoxyhemoglobin andoxyhemoglobin ; the red color ultimately originates from theiron atom inheme . If blood is drawn for a medical test, 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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