759
CHAPTER NINETEEN
Respiratory System
19.6
ALVEOLAR GAS EXCHANGES
The tubelike parts of the respiratory system move air in and
out of the air passages. The alveoli are the sites of the vital
process of gas exchange between the air and the blood.
Alveoli
Alveoli are microscopic air sacs clustered at the distal ends of
the F nest respiratory tubes—the alveolar ducts. Each alveolus
is a tiny space within a thin wall that separates it from adja-
cent alveoli. Tiny openings, called
alveolar pores,
in the walls
of some alveoli may permit air to pass from one alveolus to
another
(f g. 19.32)
. The pores provide alternate air pathways
if the passages in some parts of the lung become obstructed.
Phagocytic cells called
alveolar macrophages
are in
alveoli and in the pores. These macrophages phagocytize
PRACTICE
34
Which chemical factors aF
ect breathing?
35
Describe the in±
ation re±
ex.
36
How does hyperventilation decrease respiratory rate?
FIGURE 19.31
In the process of inspiration, motor impulses travel from the respiratory center to the diaphragm and external intercostal muscles,
which contract and cause the lungs to expand. This expansion stimulates stretch receptors in the lungs to send inhibiting impulses to the respiratory
center, preventing overin±
ation.
Sometimes a person who is emotionally upset may hyperventilate,
become dizzy, and lose consciousness. This is due to a lowered CO
2
concentration followed by a rise in pH (respiratory alkalosis) and a
localized vasoconstriction of cerebral arterioles, decreasing blood
flow to nearby brain cells. Hampered oxygen supply to the brain
causes fainting. A person should never hyperventilate to help hold
the breath while swimming, because the person may lose conscious-
ness under water and drown.
Respiratory center
Motor pathways
Spinal cord
External intercostal
muscles
Intercostal nerve
Rib
Diaphragm
Sensory pathway
Vagus nerve
Phrenic nerve
Stretch receptors
Lung
TABLE
19.6
|
Factors Af
ecting Breathing
Factors
Receptors Stimulated
Response
Ef
ect
Stretch of tissues
Stretch receptors in visceral pleura, bronchioles, and alveoli
Inhibits inspiration
Prevents overin±
ation of lungs
during forceful breathing
Low plasma P
O
2
Chemoreceptors in carotid and aortic bodies
Increases alveolar ventilation
Increases plasma P
O
2
High plasma P
CO
2
Chemosensitive areas of the respiratory center
Increases alveolar ventilation
Decreases plasma P
CO
2
High cerebrospinal ±
uid
hydrogen ion concentration
Chemosensitive areas of the respiratory center
Increases alveolar ventilation
Decreases plasma P
CO
2
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