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).
Breathing
The primary purpose of breathing is to bring atmospheric air (in small doses) into the alveoli where gas exchange with the gases in the blood takes place. The equilibration of the partial pressures of the gases in the alveolar blood and the alveolar air occurs by diffusion. At the end of each exhalation the adult human lungs still contain 2,500–3,000 mL of air, their functional residual capacity or FRC. With each breath (inhalation) only as little as about 350 mL of warm, moistened atmospheric is added, and well mixed, with the FRC. Consequently, the gas composition of the FRC changes very little during the breathing cycle. Since the pulmonary capillary blood equilibrates with this virtually unchanging mixture of air in the lungs (which has a substantially different composition from that of the ambient air), the partial pressures of the arterial blood gases also do not change with each breath. The tissues are therefore not exposed to swings in oxygen and carbon dioxide tensions in the blood during the breathing cycle, and the peripheral and central chemoreceptors do not need to "choose" the point in the breathing cycle at which the blood gases need to be measured, and responded to. Thus the homeostatic control of the breathing rate simply depends on the partial pressures of oxygen and carbon dioxide in the arterial blood. This then also maintains the constancy of the pH of the blood.