<P> In mammals, the protein makes up about 96% of the red blood cells' dry content (by weight), and around 35% of the total content (including water). Hemoglobin has an oxygen - binding capacity of 1.34 mL O per gram, which increases the total blood oxygen capacity seventy-fold compared to dissolved oxygen in blood . The mammalian hemoglobin molecule can bind (carry) up to four oxygen molecules . </P> <P> Hemoglobin is involved in the transport of other gases: It carries some of the body's respiratory carbon dioxide (about 20--25% of the total) as carbaminohemoglobin, in which CO is bound to the globin protein . The molecule also carries the important regulatory molecule nitric oxide bound to a globin protein thiol group, releasing it at the same time as oxygen . </P> <P> Hemoglobin is also found outside red blood cells and their progenitor lines . Other cells that contain hemoglobin include the A9 dopaminergic neurons in the substantia nigra, macrophages, alveolar cells, and mesangial cells in the kidney . In these tissues, hemoglobin has a non-oxygen - carrying function as an antioxidant and a regulator of iron metabolism . </P> <P> Hemoglobin and hemoglobin - like molecules are also found in many invertebrates, fungi, and plants . In these organisms, hemoglobins may carry oxygen, or they may act to transport and regulate other small molecules and ions such as carbon dioxide, nitric oxide, hydrogen sulfide and sulfide . A variant of the molecule, called leghemoglobin, is used to scavenge oxygen away from anaerobic systems, such as the nitrogen - fixing nodules of leguminous plants, before the oxygen can poison (deactivate) the system . </P>

Factors which will increase the oxygen carrying capacity of hemoglobin include