591
CHAPTER FIFTEEN
Cardiovascular System
any water that gets into the alveoli back into the interstitial
space. Although the alveolar surface must be moist to allow
diffusion of oxygen and carbon dioxide, this mechanism
keeps excess water out of the alveoli, preventing them from
F
lling with fl
uid
(f
g. 15.41)
.
±luid in the interstitial space may be drawn back into the
alveolar capillaries by the somewhat higher osmotic pressure
the narrow interstitial space between the alveolar capillary
and the alveolus.
The epithelial cells of the alveoli are so tightly joined
that sodium, chloride, and potassium ions, as well as glucose
and urea, enter the interstitial space but usually fail to enter
the alveoli. This helps maintain a high osmotic pressure in
the interstitial fl uid. Consequently, osmosis rapidly moves
Superior
vena cava
LA
LV
RV
RA
Aorta
Left pulmonary artery
Pulmonary capillaries
Left pulmonary veins
Right pulmonary artery
Pulmonary capillaries
Pulmonary trunk
Right pulmonary veins
Right lung
Left lung
Inferior vena cava
FIGURE 15.40
Blood reaches the lungs through branches of the pulmonary arteries, and it returns to the heart through pulmonary veins.
(Structures are not drawn to scale.)
1
Slight net outflow
of fluid from capillary
3
Any excess water in
alveolus is drawn out by
the higher osmotic pressure
of the interstitial fluid
4
Fluid from the
interstitial space
enters lymphatic
capillary or alveolar
(blood) capillary
2
Solutes fail to enter
alveoli but contribute to
the osmotic pressure of
the interstitial fluid
Interstitial space
Blood flow
Blood flow
Lymph flow
Alveolar capillary
Alveolar air
Alveolar wall
Lymphatic capillary
Capillary wall
FIGURE 15.41
Cells of the alveolar wall are tightly
joined. The relatively high osmotic pressure of the
interstitial F
uid draws water out of them.
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