Respiration: ventilation

Ventilation is the process by which air moves into and out of the lungs and is made available for gas exchange. Weibel's description divided the lungs into a conductive zone and a respiratory zone of 23 generations of dichotomously branching airways. Gas flow within the proximal airways is in the form of bulk movement and via diffusion in the distal airways. Resistance to gas flow in the airways is determined by a number of factors. Airway radius is the most important factor influencing resistance and gas flow changes from being turbulent proximally to laminar distally. Inspiration is an active process. The diaphragm is the main muscle of inspiration. Expiration is normally a passive process during quiet breathing but requires energy expenditure during certain actions (e.g. coughing). Ventilation is not evenly distributed throughout the lungs and distribution is related to the compliance of alveoli in different areas. Under normal circumstances, in the standing position, basal alveoli are the most compliant and are therefore preferentially ventilated. Ventilation occurs automatically in a continuous rhythmic pattern without conscious effort. It is controlled by neural and chemical inputs and is concerned with the homeostasis of oxygen and carbon dioxide and acid-base balance.