Structure and Function of the Normal Lung
General Considerations
The main function of the lungs is (rapid) gas exchange. This is accomplished by a well-coordinated interaction
of the lungs with the central nervous system, the diaphragm and chest wall musculature, and the circulatory
system.
Gas exchange occurs in the alveolus where the thin laminar blood flow and inspired air are separated only by a
thin tissue layer. Gas exchange takes 0.25 seconds or 1/3 of the total transit time of a red cell. The entire blood
volume of the body passes through the lungs each minute in the resting state, that is 5 litres per minute. The total
surface area of the lung is about 80 meters square, equivalent to the size of a tennis court.
Only about 10% of the lung is occupied by solid tissue, whereas the remainder is filled with air and blood.
Supporting structures of the lung must be delicate to allow gas exchange, yet strong enough to maintain
architectural integrity, that is sustain alveolar structure. The functional structure of the lung can be divided into
(1) the conducting airways (dead air space), and (2) the gas exchange portions. The two plumbing systems are:
airways for ventilation, and the circulatory system for perfusion. Both are under low pressure. Total lung weight
is about 300-400 gms.
Structure of the Gas Exchange Portion
Respiratory Bronchiole
This is the first bronchiole along which alveoli appear. There are 2-5 "generations" of respiratory bronchioles.
Since these bronchioles are lined by cuboidal epithelium and have muscular walls, they function primarily as
conducting tubes, and probably account for minimal gas exchange.
Alveolar Duct
Alveolar ducts are completely lined by alveoli, have no muscle in their walls and are covered by squamous
epithelium.
Alveolar Sacs and Alveolus
Alveolar sacs represent the termini of alveolar ducts and are completely lined by alveoli. The alveolar sacs and
alveoli are lined by squamous epithelium.
Histology of the Airways
General Considerations
The conducting airways are compliant tubes lined by respiratory mucosa and containing variable amounts of
muscle and/or cartilage in their wall. Airways are for conducting air and are for clearance and filtering of foreign
particles that are in the approximately 10,000 liters of inspired air per day. Bronchi are distinguished from
bronchioles primarily by the presence of cartilage in their walls.
Epithelium
Pseudostratified ciliated columnar cells and mucous (goblet) cells are the two major components of the
epithelium. Cilated cells predominate in number. Both derive from basal cells. Cilia beat at 1,000 to 1,500 cycles
per minute resulting in movement of the mucus blanket at 0.5-1 mm/min in small airways and 5-20 mm/min in
the trachea and main bronchi. All material which accumulates in the lungs is removed in about 24 hours. Goblet
cells which release mucus granules into the bronchial lumen can increase in number dramatically following acute
bronchial injury. The mucus blanket moistens inspired air, prevents drying of the walls, and traps particulate
matter.