Venous blood

Blood is oxygenated in the lungs and returns to the left atrium through the pulmonary veins.

[citation needed] Most medical laboratory tests 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.

Though veins might make it appear as such, human blood is never naturally blue.

Veins and arteries appear similar when skin is removed and are seen directly.