I was once asked to delineate in 45 minutes to first year medical students aspects of lung growth and aging. The following pages summarize the gist of the presentation.
The lung forms part of a complex and carefully controlled organism. It helps in maintaining conditions which allow the organism to function properly, and receives input signals from various sources to that end. The illustration on the right depicts this schematically.
The respiratory system and the circulatory system share many properties. They both use a pump to move gas and blood, respectively, and are capable of controlling the pump's stroke volume and frequency. The length-force relationship of muscles, and signals from within heart or lung, and from other organs, play a role in determing the pump's output. It is important that the energy expenditure of each pump is minimal. To that end the bronchial tree and vascular system branch extenmsively, so that the overall resistance to flow is low and can yet be tweaked in small vessels to accommodate the needs of local tissues.
Thus the volume, the pressure and the composition of the 'milieu interne' - the internal environment - are tightly controlled by close interaction between kidneys, gastrointestinal system, central nervous system and other organs. Homeostasis must be warranted at every stage of life. That includes the period of growth of an individual of 3 kg to an adult of 80 kg, as well as during the ensuing process of aging.
We will now analyze the order in this evolutionary process, but first review what you will learn in the process