Lymph nodes blackened by soot particles can often be seen here and the substance of the lung itself may be blackened by soot in city dwellers or heavy smokers. Each lobe of the lung is further divided into a pyramidal bronchopulmonary segments . Bronchopulmonary segments have the apex of the pyramid in the hilum whence they receive a tertiary bronchus, and appropriate blood vessels. The 10 segments of the right lung and eight of the left are virtually self contained units not in communication with other parts of the lung. This is of obvious use in surgery when appropriate knowledge will allow a practically bloodless excision of a diseased segment.
Gaseous exchange relies on simple diffusion.
In order to provide sufficient oxygen and to get rid of sufficient carbon dioxide there must be a large surface area for gaseous exchange a very short diffusion path between alveolar air and blood concentration gradients for oxygen and carbon dioxide between alveolar air and blood. The surface available in an adult is around 140m2 in an adult, around the area of a singles tennis court. The presence of this tennis court, separated from the outside air by a very narrow barrier imposes demands on the respiratory tract. At the alveolar surface it must be at body temperature varies from very dry to very humid. At the alveolar surface it must be saturated with water vapour contains dust and debris.