Page 245 - Industrial Ventilation Design Guidebook

206 CHAPTER 5 PHYSIOLOGICAL AND TOXICOLOGICAL CONSIDERATIONS
(ciliated epithelium, serous, and goblet cells) are supplied with nutrients,
oxygen, water, and heat via these submucosal anastomoses.
Respiratory bronchioles and alveoli are supplied with deoxygenated
blood from the right ventricle of the heart by the pulmonary arteries. Five lo-
bar arterial branches follow the bronchi, and subsequent bronchopulmonary
arterial branches run adjacent to smaller airways to the level of the respiratory
bronchioles. Dense coiled capillary networks beyond this point distribute
deoxygenated blood to capillaries and return oxygenated blood to the venules
arising from the respiratory bronchiolar, alveolar, and alveolar duct capillary
beds. Pulmonary capillaries directly attach to lung connective tissue, reducing
diffusive resistance to gas exchange.
Vessels Unking bronchial arteries directly with pulmonary alveolar mi-
crovessels are commonly found in neonates but apparently decrease in fre-
quency with age. There is also evidence of direct communication between
bronchial arteries and pulmonary veins. Venous blood originating from ex-
trapulmonary airways (proximal to approximately generation 3 bronchi)
drains into the right atrium via the azygos and hemiazygos veins. Intrapul-
monary bronchial venous flow, returning blood to the heart from bronchi
distal to the third generation, drains into the pulmonary circulation, which
subsequently drains into the left atrium either directly or via the pulmo-
nary vein.
Airway surfaces, like skin, are continually exposed to the ambient envi-
ronment. In contrast to skin submucosal vessels, however, which shed excess
heat by vasodilating when heated and conserve heat by vasoconstricting when
chilled, it is unclear how the airway vasculature responds to temperature ex-
tremes. Inspiring cold air poses two challenges to conducting airway tissues:
the risk of tissue injury should inadequate heat reach the airway surface and
excessive body heat loss due to increasing the radial temperature gradient. Va-
sodilation would protect airway tissue but increase heat loss, while vasocon-
striction would produce the opposite effect.
Nasal vasculature may offer some insight into this question, though re-
search to date has been equivocal. Nasal turbinate vessels can be classified as
either capacitance vessels or resistive vessels. Capacitance vessels appear to va-
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sodilate in response to infection while resistance vessels appear to respond to
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cold stimuli by vasoconstriction. Buccal vascular structures also respond to
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thermal stimuli but appear to respond principally to cutaneous stimuli. How
pharyngeal and tracheobronchial submucosal vessels react to thermal stimuli
is not known, though cold-induced asthma is believed to result from broncho-
spasms caused by susceptible bronchial smooth muscle responding to expo-
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sure to cold dry air. ' This asthmatic response suggests an inadequate
vascular response to surface cooling.

5.2.3 Ventilation Patterns

5.2.3.1 Breathing Mechanics
Breathing consists of the cyclic action of the lungs to inspire and expire
atmospheric gases. Inspiration occurs when the diaphragm and intercostal
muscles contract, generating a negative pressure in the pleura surrounding