Hemoglobin is the oxygen transporter in the red blood cells of all invertebrates, and is produced in the bone marrow. As the red blood cell travels through the vessels, it transports oxygen from the lungs towards the brain and other active organs, exchanges it for carbon dioxide, then circles back to the lungs gills to repeat the same cycle. Hemoglobin is a protein containing four active iron-containing units, which can each bind one molecule of oxygen. Hemoglobin is essential to life as we know it.
Different species, also in different stages of development, may have different globulin structures in their red blood cells, allowing them to collect oxygen in alternative circumstances. Globulin structures of a fetus can thus steal oxygen from their mothers as it has no lungs to breathe. The binding of oxygen changes the structure of the hemoglobin molecule, making the red blood cells scarlet red when oxygenated, and dark red without bound oxygen.
Problems with hemoglobin structure account for many pathologies, like anemia (decreased binding of oxygen), sickle-cell disease and thalassemia, which can have serious consequences. The sugar content bound to hemoglobin is also used as a marker for glycemia in diabetes type 2 patients; if glucosylated hemoglobin levels are too high, it is a sign of poor glucose control or glucose intolerance.
Hemoglobin also has an affinity for binding gases other than oxygen. For example, carbon monoxide resulting from cigarette smoke, exhausts or defective heating will reduce transported oxygen levels, and high concentrations can result in asphyxiation and carbon monoxide poisoning.