We honor the legacy of Professor Philip H. Quanjer, respiratory physiologist and teacher extraordinaire, and also the founder of spirXpert and lead author of the spirometric reference equations published by the Global Lung Function Initiative (GLI-2012). On this website, Dr. Quanjer previously discussed the most relevant aspects of the respiratory system, within the framework of lung growth and aging. The following pages summarize his presentation, now updated by other contributing authors.
The respiratory system is part of an integrated network that sustains homeostasis, wherein internal physiologic responses are calibrated to maintain an individual’s functional capacity, whether it be to meet the demands imposed by a change in the environment or level of physical activity. The respiratory system shares unique properties with the cardiovascular system. In particular, the respiratory and cardiovascular systems both use a pump to move gas and blood, and both control the pump's output by modulating volume (tidal and stroke volume) and frequency (respiratory rate and heart rate), respectively. The noted modulation is affected by the length-force relationship of the respiratory and cardiac muscles, and by feedback signals that originate within the lung and heart. Importantly, the energy expenditure of the pump is also efficient, in part due to operating within a low resistance system. To illustrate, the bronchial tree and vasculature branch extensively, such that the overall resistance to gas and blood flow is low and further reduced within smaller airways and vessels, respectively.
As shown in the accompanying Figure, the milieu interne (internal physiologic equilibrium) is highly dynamic, being tightly regulated by an integrated network of multiple organ systems that, in turn, sustain homeostasis in response to a variable mileu externe (external environment). The respiratory component of homeostasis evolves substantially across the lifespan, differing in physiologic capacity in an infant weighing 3 kg as compared with a young adult of 80 kg and, thereafter, through progressive aging. The spirXpert website describes this evolutionary process, focusing on key developmental patterns that characterize lung growth and aging.
Figure. Internal physiologic equilibrium (homeostasis)