In vertebrates, hemoglobin is found in red blood cells. Another oxygen binding protein found in vertebrates is myoglobin, which is found in skeletal muscles. A newly discovered protein called neuroglobin may serve the same function in our brains. The binding of oxygen to hemoglobin is typically represented using oxygen dissociation curves. It is important to appreciate that hemoglobin is a general term, referring to a family of proteins.
Different species have different hemoglobins, with different properties.
Even within a single species, different hemoglobins are often present at different times of life. Also, the properties of a single form of hemoglobin vary in response to changing tissue temperature and pH. The waste product of cellular respiration, carbon dioxide, moves from the tissues to the lungs via the systemic circulation. Most of the carbon dioxide that leaves the tissues is converted to bicarbonate ion inside the red blood cells. An enzyme found in red blood cells, carbonic anhydrase, facilitates this conversion. Smaller amounts of carbon dioxide travel to the lungs as dissolved gas or bound to hemoglobin.