Page 203 - 84 human physiolofy part-1

The Respiratory System

Inspiration

Inspiration is initiated by contraction of the diaphragm and in some cases the intercostals muscles
when they receive nervous impulses. During normal quiet breathing, the phrenic nerves stimulate the
diaphragm to contract and move downward into the abdomen. This downward movement of the
diaphragm enlarges the thorax. When necessary, the intercostal muscles also increase the thorax by
contacting and drawing the ribs upward and outward.

The active increase of the thorax changes the stability set up in a resting lung. As the thoracic wall
moves away from lung which increases the space between the thoracic wall and lung and decreases the
pressure in the intrapleural cavity. This decrease in pressure causes the pressure in the alveoli to
become greater than the elastic recoil that is inherent in lung tissue. Thus, when contraction of the
diaphragm and the intercostal muscles actively increase the size of the thorax, the lungs are passively
forced to expand. This expansion increases the size of the alveoli which decreases pressure in the
alveoli. Pressure within the alveoli is now lower than atmospheric pressure which allows air to move
into the lungs through the structures discussed above.

Expiration

During quiet breathing, expiration is normally a passive process and does not require muscles to
work. When the lungs are stretched and expanded, stretchy receptors within the alveoli send inhibitory
nerve impulses to the medulla oblongata, causing it to stop sending signals to the rib cage and
diaphragm to relax and rise. This elastic recoil causes the lungs and chest cavity to shrink and increase
the air pressure within the lungs. This increased positive air pressure pushes the air out of the lungs.
Expiration happens as the diaphragm relaxes. Although the respiratory system is primarily under
involuntary control, and regulated by the medulla oblongata, we have some voluntary control over it
also. This is due to the higher brain function of the cerebral cortex.

When under physical or emotional stress, more frequent and deep breathing is needed, and both
inspiration and expiration will work as active processes. Additional muscles in the rib cage forcefully
contract and push more air out of the lungs. (This cannot occur during rest.) In addition to deeper
breathing, when coughing or sneezing we exhale forcibly. Our abdominal muscles will contract
suddenly (when there is an urge to cough or sneeze), raising the abdominal pressure. The rapid increase
in pressure pushes the relaxed diaphragm up against the pleural cavity. This causes air to be forced out
of the lungs.

Another function of the respiratory system is to sing and to speak. Our exert of conscious control
over our breathing is what allows us to speak and sing.

Lung Compliance

Lung Compliance is the magnitude of the change in lung volume produced by a change in
pulmonary pressure. Compliance can be considered the opposite of stiffness. A low lung compliance
would mean that the lungs would need a greater than average change in intrapleural pressure to change
the volume of the lungs. A high lung compliance would indicate that little pressure difference in
intrapleural pressure is needed to change the volume of the lungs. More energy is required to breathe
normally in a person with low lung compliance. Persons with low lung compliance due to disease

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