Page 243 - Industrial Ventilation Design Guidebook

204 CHAPTER 5 PHYSIOLOGICAL AND TOXICOLOGICAL CONSIDERATIONS

FIGURE 5.17 Gas exchange between alveolar and capillary compartments.

5.2.2.4 Airway Vasculature
Pulmonary gas exchange is intimately connected to cardiovascular
function. Deoxygenated blood from the right ventricle of the heart passes
through the pulmonary artery to the lungs, exchanges carbon dioxide and
oxygen across the alveolar wall, and returns to the left atrium of the heart
via the pulmonary veins. The heart propels this oxygenated blood from the
left ventricle through the aorta and hence throughout the body via a high-
pressure system of thick-walled vessels known as arteries branching out
from the aorta. Further branching gradually reduces the cross-section of
arteries until, at a diameter of approximately 30 jxm, they are termed arte-
rioles. Total vascular surface area increases as arterioles continue to
branch and diminish in diameter until they terminate at a capillary bed or
connect directly with venuoles in an anastomosis, a dense network of in-
terconnected vessels.
Arteriole wall smooth muscle controls vascular diameter and regulates
blood flow by modulating the pressure drop along the length of the vessel.
Enlisting groups of arterioles regulates local or regional vascular resistance
by modulating capillary flow in response to temperature changes or other
stimuli. Active control of arteriole wall smooth muscle tone due to a variety
of internal and external stimuli also regulates blood flow through anasto-
moses and consequently peripheral blood volume and pressure. Anastomoses
control peripheral blood flow by allowing a portion of total blood flow to
bypass capillary beds.