 Hemoglobin is a tetramer with four oxygen binding sites . The binding of carbon monoxide at one of these sites increases the oxygen affinity of the remaining three sites , which causes the hemoglobin molecule to retain oxygen that would otherwise be delivered to the tissue . This situation is described as carbon monoxide shifting the oxygen dissociation curve to the left . Because of the increased affinity between hemoglobin and oxygen during carbon monoxide poisoning , little oxygen will actually be released in the tissues . This causes hypoxic tissue injury . Hemoglobin acquires a bright red color when converted into carboxyhemoglobin , so poisoned cadavers and even commercial meats treated with carbon monoxide acquire an unnatural reddish hue . 
