Blood
About 98.5% of the oxygen in a sample of arterial blood in a healthy human breathing air at sea-level pressure is chemically combined with the hemoglobin. About 1.5% is physically dissolved in the other blood liquids and not connected to hemoglobin. The hemoglobin molecule is the primary transporter of oxygen in mammals and many other species (for exceptions, see below). Hemoglobin has an oxygen binding capacity between 1.36 and 1.40 ml O per gram hemoglobin, which increases the total blood oxygen capacity seventyfold, compared to if oxygen solely were carried by its solubility of 0.03 ml O per liter blood per mm Hg partial pressure of oxygen (about 100 mm Hg in arteries).
Allotropes of oxygen
The common allotrope of elemental oxygen on Earth, , is generally known as oxygen, but may be called "dioxygen", "diatomic oxygen", "molecular oxygen", or "oxygen gas" to distinguish it from the element itself and from the triatomic allotrope "ozone," . As a major component (about 21% by volume) of Earth's atmosphere, elemental oxygen is most commonly encountered in the diatomic form. Aerobic organisms utilize atmospheric dioxygen as the terminal oxidant in cellular respiration. The ground state of dioxygen is known as triplet oxygen, O, because it has two unpaired electrons. The first excited state, singlet oxygen, ""O has no unpaired electrons and is metastable.