214
UNIT 3
Organ Systems
Figure 11-12A.
Alveolus and gas exchange.
H&E,
3
1,077
An
alveolus
is the terminal unit of the respiratory system. It functions as a primary site for gas exchange. Its wall is composed of
type
I
and
II pneumocytes
.
Type I pneumocytes
are primary cells, which form the structure of alveolar walls and are in contact with the
capillary walls.
Type II pneumocytes
are septal cells, which are located in the connective tissue of the septal junction. They produce a
surfactant, which reduces surface tension of the alveolus. Gas exchange occurs between the alveolus and capillary wall in a structure
called the
blood-air barrier
. The blood-air barrier is composed of
type I pneumocytes
, the
endothelial cells
of the capillary
, and the
fused basement laminae
of these cells (Fig. 11-12B). The exchange of O
2
and CO
2
occurs by passive diffusion across the thin blood-
air barrier. The difference in O
2
and CO
2
tensions across the membrane determines the driving pressure for diffusion of the gases.
In normal conditions, at sea level, air has a high O
2
and a low CO
2,
concentration whereas blood in the pulmonary capillaries has a
low O
2
and a high CO
2
concentration. The net driving pressure will force CO
2
out of the blood into the alveolar space and O
2
into
the blood from the alveoli.
Figure 11-12B.
Blood-air barrier.
TEM,
3
27,000
Three elements separate the air in alveoli from
the blood in the underlying capillaries: (1) Air
± rst passes through the highly fl
attened
type I
pneumocyte
with its coating of
surfactant
. The
cytoplasm can be even thinner (25 nm) than
the segment highlighted in this view. (2) The
middle element is the
fused basal laminae
of
the type I cell and the underlying endothelial
cell. (3) The
endothelial cell
lining the alveolar
capillary, like the type I cell, has an extremely
thin cytoplasm, so that the total thickness of
the blood-air barrier can be as little as 0.1
μ
m.
T. Yang
Type II pneumocyte
Fused basement
laminae
Type I pneumocyte
Alveolar macrophage
(dust cell)
Alveolar septum
Alveolar septum
Alveolar septum
Capillary
Type II
Type II
pneumocyte
pneumocyte
Type II
pneumocyte
Endothelial cell
Endothelial cell
Endothelial cell
Endothelial cell
O
2
CO
2
A
B
Cytoplasm of capillary
Cytoplasm of capillary
endothelial cells
endothelial cells
2. Fused basal
laminae
3. Cytoplasm of capillary
endothelial cell
Air space
Cytoplasm of
Cytoplasm of
type I pneumocyte
type I pneumocyte
1. Cytoplasm of
type I pneumocyte
0.10
0.10
m
μ
0.10 m
μ
Erythrocyte in
Erythrocyte in
lumen of the capillary
lumen of the capillary
Erythrocyte in
lumen of the capillary
Respiratory Portion: Alveolar Ducts and Alveoli
previous page 229 Dongmei Cui -  Atlas of Histology with Functional and Clinical Correlations 2011 read online next page 231 Dongmei Cui -  Atlas of Histology with Functional and Clinical Correlations 2011 read online Home Toggle text on/off