The physics of human breathing: flow, timing, volume, and pressure parameters for normal, on-demand, and ventilator respiration

Abstract

Normal breathing for healthy humans is taken for granted; it occurs without conscious effort using ambient (1-atmosphere) pressure with 21% oxygen (O₂) concentration. The body automatically adjusts for stress, exercise, altitude, and mild disease by increasing the volume and frequency of breathing. Longer term adaptations for exercise and altitude include increases in red blood cell counts and higher concentrations of capillaries in muscle tissue. When more challenging external environmental conditions or pulmonary illnesses exceed the capability for these adaptations, the human system requires technology to maintain sufficient ventilation to preserve life. On the environmental side there are two conditions to be addressed: toxicity of the surrounding atmosphere and changes in external pressure and O₂concentration. On the medical side, mechanisms for assisting breathing include O₂supplementation at ambient pressure, positive pressure/flow without additional O₂, or a combination of both. This overview describes the various technologies applied to maintaining a safe breathing environment. Topics for environmental intervention include filter-based and flowing air-supply masks for toxic environments (occupational and laboratory protection), and on-demand gas supply systems for firefighters, self-contained underwater breathing apparatus divers, and altitude (high performance aircraft, spacecraft) applications. The topics for medical intervention include nasal cannula, continuous positive airway pressure, and medical ventilators. The primary purpose of this article is to provide a basic understanding of normal human breathing and the adaptation of breathing in different environments using available technologies.

Keywords: CPAP; SCUBA; firefighter; jet pilot; nasal cannula; on demand mask; ventilator.

Figure 1.

Conceptual diagram of O₂ transport from the face to the brain as adapted from NASA PBA study. Despite adequate input air, there are potential losses from various physiological factors including bronchoconstriction, pulmonary inflammation, and impaired circulation (NASA PBA Report 2020).

Figure 2:

Examples of pulmonary function testing in the laboratory and in the field drawn from US EPA and NASA studies.

Figure 3:

NASA photos of space mission environments with the individual space suit, the “shirtsleeve” environment inside the International Space Station, and an exterior shot of the loneliness of the self-contained human ISS habitat.

Figure 4:

Examples of common protective masks used for safe breathing in various settings. (from US Government archives).

Figure 5:

Examples of respiratory protection/supplementation for scuba, firefighting, aircraft maintenance and jet pilots drawn from the authors’ field studies.

Figure 6:

Examples of hospital ventilators that either supplement breathing (left panel), or fully control breathing (right panel) (from https://commons.wikimedia.org/).