Page 118 - Biomedical Engineering and Design Handbook Volume 1, Fundamentals

CHAPTER 4

RESPIRATORY MECHANICS

AND GAS EXCHANGE

James B. Grotberg
University of Michigan, Ann Arbor, Michigan

4.1 ANATOMY 95 4.6 AIRWAY FLOW, DYNAMICS,
4.2 MECHANICS OF BREATHING 97 AND STABILITY 105
4.3 VENTILATION 98 REFERENCES 106
4.4 ELASTICITY 101 BIBLIOGRAPHY 108
4.5 VENTILATION, PERFUSION,
AND LIMITS 103

4.1 ANATOMY

4.1.1 General
As shown in Fig. 4.1, the pulmonary system consists of two lungs; each lung is conical in shape, with
an inferior border (or base) that is concave as it overlies the diaphragm and abdominal structures and
a superior border (or apex) that is convex and extends above the first rib. The anterior, lateral, and
posterior lung surfaces are adjacent to the rib cage, while the medial surface is adjacent to the
mediastinum, which contains the heart, great vessels, and esophagus. All of the lung surface is covered
by the visceral pleural membrane, while the inside of the chest wall, mediastinum, and diaphragm
are covered by the parietal pleural membrane. The two pleural membranes are separated by a thin
liquid film (~20 to 40 mm thick) called the pleural fluid which occupies the pleural space, Fig. 4.1.
This fluid lubricates the sliding motion between the lung and chest wall, and its pressure distribution
determines lung inflation and deflation. Because the air-filled lung is surrounded by the pleural
liquid, it experiences a buoyancy force that contributes to the overall force, including lung weight and
fluid pressures. 17

4.1.2 Airway Divisions
The right main bronchus branches from the end of the trachea to the right lung, while the left main
bronchus branches to the left. This division of a “parent” airway into two “daughter” airways is
called a bifurcation. Each level of branching is called a generation and is given an integer value
n, according to the Weibel symmetric model 35 shown in Table 4.1. The trachea is designated as
n = 0, the main bronchi, n = 1, etc. For a symmetrically bifurcating geometry, then, there would
n
be 2 airways at each generation level. Generations 0 ≤ n ≤ 16 constitute the conducting zone

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