Gas Exchange Under Altered Gravitational Stress

Abstract

Efficient gas exchange in the lung depends on the matching of ventilation and perfusion. However, the human lung is a readily deformable structure and as a result gravitational stresses generate gradients in both ventilation and perfusion. Nevertheless, the lung is capable of withstanding considerable change in the applied gravitational load before pulmonary gas exchange becomes impaired. The postural changes that are part of the everyday existence for most bipedal species are well tolerated, as is the removal of gravity (weightlessness). Increases in the applied gravitational load result only in a large impairment in pulmonary gas exchange above approximately three times that on the ground, at which point the matching of ventilation to perfusion is so impaired that efficient gas exchange is no longer possible. Much of the tolerance of the lung to alterations in gravitation stress comes from the fact that ventilation and perfusion are inextricably coupled. Deformations in the lung that alter ventilation necessarily alter perfusion, thus maintaining a degree of matching and minimizing the disruption in ventilation to perfusion ratio and thus gas exchange.

Oh G: The x, y and z of human physiological responses to acceleration

“…Limited measurements during simulated suborbital profiles also indicated a degree of arterial oxygen desaturation (Blue et al, 2012(Blue et al, , 2014) that is consistent with the hypoxaemia demonstrated during previous and subsequent work studying various magnitudes and durations of +Gx (Alexander et al, 1966;Ax et al, 2013;Glaister, 1970;Nolan et al, 1963;Pollock, Jolley, et al, 2021;Prisk, 2011;Rohdin et al, 2003).…”

“…Limited measurements during simulated suborbital profiles also indicated a degree of arterial oxygen desaturation (Blue et al., 2012, 2014) that is consistent with the hypoxaemia demonstrated during previous and subsequent work studying various magnitudes and durations of +Gx (Alexander et al., 1966; Ax et al., 2013; Glaister, 1970; Nolan et al., 1963; Pollock, Jolley, et al., 2021; Prisk, 2011; Rohdin et al., 2003). Any in‐flight hypoxaemia is likely to be exacerbated by the use of mildly hypoxic airline‐style cabin pressurization, which is anticipated on some suborbital vehicles (Pollock, Jolley, et al., 2021).…”

“…Research investigating the physiology of +Gx has tended to focus on the respiratory system. Pulmonary physiology is well known to be gravity dependent, although the precise role of gravity and its interaction with other factors is not completely understood (Glenny, 2008; Hughes & West, 2008; Prisk, 2011). Regional ventilation and blood flow, which are normally greater in the dependent region of the lung, become more inhomogeneous as increasing Gx is applied, resulting in ventilation–perfusion mismatching and hypoxaemia.…”

Lung Structure and the Intrinsic Challenges of Gas Exchange

“…Figure 70 shows an example of anatomical lesions causing ${\stackrel{\cdot }{V}}_A/\stackrel{\cdot }{Q}$ mismatch. For detailed discussions on this topic, see separate reviews (117,287,333,360,418). …”

Total Lung and Lobar Quantitative Assessment Based on Paired Inspiratory–Expiratory Chest CT in Healthy Adults: Correlation with Pulmonary Ventilatory Function

“…The pulmonary ventilation and perfusion are gravitational dependent, the upper lobes are more well ventilated and less perfused, whereas the lower lobes are better perfused and less ventilated in the upright position [28,29]. The CT scans in our study were performed in the supine position, which can balance the influence of gravity on upper and lower lung field.…”