Carbaminohemoglobin (or Carbaminohaemoglobin, also known as carbhaemoglobin and carbohaemoglobin) is a compound of hemoglobin and carbon dioxide, and is one of the forms in which carbon dioxide exists in the blood. 10% of carbon dioxide is carried in blood this way (85% carried in blood as bicarbonate [hydrogen carbonate], 5% carried as free CO2, in solution).
When carbon dioxide binds to hemoglobin, carbaminohemoglobin is formed, lowering haemoglobin's affinity for oxygen via the Bohr effect. In the absence of oxygen, unbound hemoglobin molecules have a greater chance of becoming carbaminohaemoglobin. (The Haldane Effect relates to the increased affinity of de-oxygenated hemoglobin for H+: offloading of oxygen to the tissues thus results in increased affinity of the hemoglobin for carbon dioxide, and H+, which the body wants to get rid of, which can then be transported to the lung for removal). Carbaminohemoglobin has a distinctive blue color that may contribute to the dark red color of deoxygenated venous blood (compared to bright, saturated red of oxygenated arterial blood), blue color of veins and the purplish or bluish color of tissues in hypoxia.
Hemoglobin can bind to four molecules of carbon dioxide. The carbon dioxide molecules form an amide linkage to the four terminal-amine groups of the four protein chains in the deoxy form of the molecule. Thus, one hemoglobin molecule can transport four carbon dioxide molecules back to the lungs, where they are released when the molecule changes back to the oxyhemoglobin form.