Gross Anatomy of the Lungs

Gross Anatomy of the Lungs

An accurate and complete model of the branching pattern of the human

bronchial tree remains elusive, though several different models have

been described (Phillips et al, 1994). The most useful and widely

accepted approach remains that of Weibel (Weibel, 1963; Weibel, 1991),

who numbered successive generations of air passages from the trachea

(generation 0) down to alveolar sacs (generation 23). This “regular

dichotomy” model assumes that each bronchus regularly divides into two

approximately equal-size daughter bronchi; it is unlikely to be tru in

practice where bronchus length is variable, pairs of daughter bronchi

are often unequal in size, and trifurcations may be demonstrated.

However, as a rough approximation it may be assumed that the number of

passages in each generation is double that in the previous generation,

and the number of air passages in each generation is approximately

indicated by the number 2 raised to the power of the generation

number. This formula indicates one trachea, two main bronchi, four

lobar bronchi, 16 segmental bronchi and so forth.

1.1.1

Trachea (generation 0)

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The adult trachea has a mean diameter of 1.8cm and length of 11cm. It

is supported by U-shaped cartilages that are joined posteriorly by

smooth muscle bands. The part of the trachea in the neck is not

subject to intrathoracic pressure changes, but it is very vulnerable

to pressures arising in the neck due, for example, to tumours or

haematoma formation after surgery. An external pressure of the order

of 4kPa is sufficient to occlude the trachea. In the chest, the

tr...

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...atory bronchiole and includes the respiratory

bronchioles, alveolar ducts and alveolar sacs distal to a single

terminal bronchiole. This represents generations 15-23 above, but in

practice the number of generations in a single acinus is quite

variable, being between 6 and 12 divisions beyond the terminal

bronchiole (Weibel, 1991). A human being contains about 30,000 acini

(Haefeli-Bleuer & Weibel, 1988), each with a diameter of about 3.5mm

and containing about 10,000 alveoli (Weibel, 1991). A single

pulmonary acinus is probably the equivalent of the alveolus when it is

considered from a functional standpoint, as gas movement in the acinus

is by diffusion rather than by tidal ventilation. The path length

between the start of the acinus and the most distal alveolus therefore

becomes crucial and is between 5 and 12mm.