Historically, why were the lungs considered sterile?

The perspective that the lungs were once thought to be sterile primarily arises from the understanding of the body's defense mechanisms, which include the continuous stream of mucus and phagocytic action. The respiratory tract has specific anatomical and physiological features designed to trap and eliminate potential pathogens.

Mucus produced by the respiratory epithelium serves to trap inhaled particles and microorganisms, preventing them from reaching the deeper parts of the lungs. This mucus is then moved upwards by cilia towards the throat, where it can be swallowed or coughed out, effectively clearing the airways of contaminants. Additionally, immune cells, such as macrophages, are present in the lung tissue and alveoli and play a critical role in engulfing and destroying any pathogens that may have bypassed the mucus barrier.

This combination of physical and immune defenses contributes to the notion of the lungs being a relatively sterile environment under normal healthy conditions. It is important to recognize that while the lungs are not entirely devoid of microbes, the defense mechanisms are highly efficient at preventing colonization by potentially harmful microorganisms in a healthy individual.