As Hemoglobin Is Depleted of Oxygen, Its Color Changes from Bright Red to Bluish Red
The change in the color of hemoglobin from bright red to the bluish red is known as cyanosis. It is observed in the mucous membranes of animals when the blood in the underlying capillaries is hypoxic. Cyanosis can result from deficient oxygen uptake in the lungs, but it can also result from reduced blood flow to the peripheral tissues. The latter can occur when animals are in cardiovascular failure.
The color change that occurs as hemoglobin loses its oxygen is the basis of oximetry. Oximeters use differences in light absorption to distinguish oxygenated and deoxygenated hemoglobin. A pulse oximeter is a clinical tool that can be attached to an ear or the lips of anesthetized animals to measure the hemoglobin saturation of arterial blood.