Page 234 - Industrial Ventilation Design Guidebook

5.2 HUMAN RESPIRATORY TRACT PHYSIOLOGY I 9 5

5.2 HUMAN RESPIRATORY TRACT PHYSIOLOGY

5.2.1 Introduction
Industrial environments expose individuals to a plethora of airborne chem-
ical compounds in the form of vapors, aerosols, or biphasic mixtures of
both. These atmospheric contaminants primarily interface with two body
surfaces: the respiratory tract and the skin. Between these two routes of
systemic exposure to airborne chemicals (inhalation and transdermal ab-
sorption) the respiratory tract has the larger surface area and a much
greater percentage of this surface exposed to the ambient environment. Or-
dinary work clothing generally restricts skin exposures to the arms, neck,
and head, and special protective clothing ensembles further limit or totally
eliminate skin exposures, but breathing exposes much of the airway to
contaminants.
Inhaling potentially noxious airborne mixtures exposes respiratory tis-
sue and the supporting vasculature to disease and injury. In addition, other
organs can be injured due to transepithelial transport along the airway to
the bloodstream and subsequent bulk transport throughout the body. Conse-
quently, understanding the relationship between industrial ventilation and
human health requires knowledge of how the respiratory tract interacts with
the surrounding environment. It is the goal of this chapter to lay the ground-
work for understanding how the human airway deals with potential air-
borne threats.

5.2.2 Anatomical Overview
The human respiratory tract serves to deliver oxygen to the bloodstream and
remove carbon dioxide. It accomplishes this by utilizing two large air bags
(lungs) with extremely large internal surface areas to transport these gases be-
tween the pulmonary airstream and capillaries. The lungs are situated inside a
semirigid bony structure (rib cage), which is joined together by intercostal
muscles and supported from below by a large sheet of muscle tissue (dia-
phragm). These structures serve to physically protect the lungs and generate
the forces required for inspiration and exhalation. Immediately surrounding
the lungs are bags (pleura), which transfer force generated by the diaphragm
and intercostal muscles to the lungs and are penetrated by numerous blood
vessels.
The respiratory tract can be theoretically subdivided into distinct func-
tional regions (Fig. 5.14). Dividing the respiratory tract into conducting and
respiratory airways is perhaps the simplest division. Framed in this way, the
respiratory tract consists of two airway regions: a series of tubes (nasal and
oral cavities, pharynx, larynx, trachea, bronchi, and nonalveolated bronchi-
oles) leading to a terminal region of essentially bag-like structures (respiratory
bronchioles, alveoli), where gas is exchanged between the airway lumen and
the surrounding capillaries.
A slightly more detailed airway organization suggested by the ICRP
1
Task Group on Lung Dynamics divides the airway into five regions: nasal