Heme is the iron-containing prosthetic group at the core of hemoglobin, the protein that transports oxygen in red blood cells. Each hemoglobin molecule contains four heme units, and each heme unit holds one iron atom in its center. It is this iron that binds a single molecule of oxygen reversibly — picking it up in the oxygen-rich environment of the lungs and releasing it in the oxygen-poor environment of working muscle tissue.
The iron within heme must be in the ferrous (Fe²⁺) oxidation state to bind oxygen effectively. When iron is oxidized to the ferric (Fe³⁺) state, the resulting compound — methemoglobin — cannot carry oxygen. This oxidative conversion is a toxicological concern in horses exposed to certain plants or drugs that generate oxidative stress; severe methemoglobinemia causes visible cyanosis and exercise intolerance. The color of oxygenated blood (bright red) compared to deoxygenated blood (dark red) reflects the optical properties of heme in its oxygen-bound versus unbound states.
Heme is also the active component of myoglobin, the oxygen-storage protein found in muscle tissue. Horses with a high proportion of oxidative muscle fibers — such as endurance-adapted animals — have denser myoglobin concentrations in their muscles, giving the tissue a deeper red color. Heme degradation products include bilirubin, which is processed by the liver; elevated serum bilirubin in a horse can indicate red blood cell destruction or liver disease that disrupts bilirubin clearance. Assessment of pulse and membrane color as oxygen-saturation proxy and mucous membrane color provides indirect evidence of hemoglobin-oxygen saturation at the clinical level.