Gases (air) are measured by virtue of the pressures that they exert. When gases are mixed they each contribute to a total pressure. Each gas contributes a partial pressure. Total atmospheric air pressure at sea level, at 15◦ C and zero humidity, is 760 mmHg (millimeters of mercury). At sea level partial pressure oxygen, written PO₂ is 159 mmHg (20.93), and partial pressure carbon dioxide, written PCO₂, is 0.3 mmHg (less than 0.04%).
Air is transported by means of airways. Airways branch out into smaller airways, e.g., bronchioles, up to about 22 times, ultimately ending with the alveolus, the fundamental respiratory unit. In the initial airways there is no exchange of O₂ and CO₂, an area known as the anatomical dead space. Before reaching the alveoli much of the branching is surrounded with smooth muscles, bronchial muscles, which contract and dilate, making breathing easier or more difficult, a phenomenon known as airway resistance. There are about 300 million alveoli, surrounded by about 280 billion pulmonary capillaries.

Most of the gas exchange, O₂ and CO₂, takes place in the alveolar-capillary unit. Normal inhalation, at sea level, increases alveolar PO₂ (average PO₂ in the alveoli) to about 104 mmHg.   Because the venous blood arriving in the pulmonary capillary network contains only about 40 mmHg PO₂, rapid diffusion from the alveoli takes place, resulting in an arterial PO₂ (PaO₂) of about 100 mmHg, most of which (98.5%) is transported to the tissues by hemoglobin in the red blood cells. Without pure oxygen (where PO₂ = 760 mmHg) or hyperbaric chamber pressure (where PO₂ = 600 mmHg), the O₂ dissolved in blood plasma by itself is not adequate to support life.

Carbon dioxide is transported to the lungs where it is (1) excreted into the alveoli of the lungs for discharge into the atmosphere, and (2) reallocated to the body for proper maintenance of acid-base physiology. Reallocation of CO₂ means reflexive coordination of breathing depth and rate, where arterial PCO₂ (PaCO₂), which under normal circumstances, is maintained at about 40 mmHg for normalizing blood plasma pH (about 7.4). PCO₂ in capillary venous blood, at rest, is about 46 to 48 mmHg, whereas inspired atmospheric air contains only about 0.3 mmHg PCO₂. Because pulmonary capillary PCO₂ equilibrates with alveolar PCO₂ as a result of diffusion, alveolar PCO₂ levels must also be continuously maintained at about 40 mmHg.   Thus, if alveolar PCO₂ increases, so too does arterial PCO₂, and if alveolar PCO₂ drops as a result of overbreathing, so too does arterial CO₂. Respiration is compromised when learned breathing behavior disturbs the proper regulation of CO₂ allocation.

Hypocapnia (CO₂ deficit), as a consequence of overbreathing behavior, is measured with a capnograph (or capnometer), an instrument used to measure average alveolar PCO₂. In a lung-healthy and cardiovascular-healthy person the alveolar PCO₂ is equivalent to PaCO₂. Generally, PaCO₂ levels below 35 mmHg constitute hypocapnia (CO₂ deficit): 30-35 mmHg is mild to moderate, 25-30 mmHg is serious, and 20-25 mmHg is severe hypocapnia. These instruments are used worldwide in emergency medicine, in critical care, and during surgery for gas monitoring and regulation purposes; these are medical applications. The CapnoTrainer provides for educational applications. The CapnoTrainer is a capnometer specifically designed and manufactured for evaluating, observing, and self-regulating overbreathing behavior.

Click here to learn more about external respiration.