Flow in the Lungs - Biofluidmechanics: An Introduction to Fluid Mechanics Macrocirculation and Microcirculation - page 257

FIGURE 9.3 Smooth muscle cells The anatomical

structure of a single alveolar lobule.
Each lobule is surrounded by a
capillary network to efficiently ex-
change gas with the alveolar space.
Bronchioles are wrapped with
smooth muscle cells to change the
diameter of the bronchiole and
increase or decrease the resistance
of air flow into the alveolar sac.
This figure also shows an expanded
section of an alveolar sac, which is
composed of epithelial cells and
the surrounding capillaries. Adapted
from Martini and Nath (2009).

Respiratory bronchiole

Venule

Arteriole Alveolar

duct

Capillaries

Alveolus

Alveolar macrophage

Alveolar epithelial cell

Capillary composed of
endothelial cells

FIGURE 9.4 Red blood cell The respiratory

membrane, which is composed of
an alveolar epithelial cell, a capil-
lary endothelial cell, and their
fused basal lamina layer. In gen-
eral, a red blood cell would be
within one cell diameter of the
endothelial cell. As discussed in
the text, this boundary can be
modeled as a single permeable
membrane or a composite perme-
able membrane (with different
diffusion permeabilities). Adapted
from Guyton and Hall (2000).

Endothelial cell
nucleus

Endothelium

Alveolar epithelium

Fused basal lamina

capillary ( Figure 9.4 ). The first layer of the respiratory boundary is the epithelial cells that
line the alveoli. The second layer is termed the fused basal laminae layer which anchors
the alveolar epithelial cells to the capillary endothelial cells. The basal laminae are the
extracellular matrix of each cell (epithelial and endothelial) and are primarily composed of
elastin and collagen fibers within the respiratory boundary. The third layer is the single
endothelial cell that forms the blood capillary wall. Because oxygen and carbon dioxide
are uncharged and lipophilic, they easily and rapidly diffuse across the respiratory bound-
ary. At the arterial end of the blood capillary, there is a large concentration gradient for
oxygen to diffuse into the blood and a small concentration gradient for carbon dioxide to
diffuse out of the blood. Under normal conditions, at the venous end of the blood capil-
lary, both the oxygen and carbon dioxide concentrations between the alveoli and the blood