You’ll frequently hear the term “lung compliance” used when measuring the
functionality of the lungs. Compliance is the ability of the lungs to be distended
and is expressed as a change in volume per unit change in pressure. That is, a
measurement of how well the lungs can stretch when filling with air.
There are two factors that affect compliance. These are the connective tis-
sue that consists of collagen and elastin and surface tension in the alveoli,
which is controlled by surfactant. Surfactant is a substance that lowers surface
tension in the alveoli, thereby preventing interstitial fluid from entering
the alveoli.
Compliance is increased in patients who have chronic obstructive pulmonary
disease (COPD). Compliance is decreased with patients who have restrictive
pulmonary disease. A decrease in compliance results in a decreased lung vol-
ume. That is, the lungs become stiff requiring more-than-normal pressure to
expand the lungs. This is typically caused by an increase in connective tissue or
an increase in surface tension in the alveoli.
CONTROLLING RESPIRATION
Respiration is controlled by three factors that sense the need for the body’s
increased or decreased requirement for oxygen. These are the concentration
of oxygen (O 2 ), carbon dioxide (CO 2 ), and hydrogen (H+) ion concentration
in the blood.
Throughout the body chemoreceptors sense the concentration of oxygen, car-
bon, and carbon dioxide and then send a message to the central chemoreceptors
located in the medulla near the respiratory center of the brain and through cere-
brospinal fluid to respond to changes.
When an increase in carbon dioxide is detected and there is an increase in
hydrogen ions, the message goes out to increase ventilation. Hydrogen ions are
measured using the pH scale. The pH of normal blood is between 7.35 and 7.45.
A pH lower than 7.35 means the blood is acidic and a pH higher than 7.45 means
pH is alkaline. The chemoreceptors respond to an increase in CO 2 and a decrease
in pH by increasing ventilation. If the CO 2 level remains elevated, the stimulus
to increase ventilation is lost.
There are chemoreceptors located in carotid arteries and aortic arteries that
monitor changes in oxygen pressure (PO 2 ) levels in the arteries. These are called
peripheral chemoreceptors. Once the oxygen pressure falls below <60 mmHg,
the peripheral chemoreceptors send a message to the respiratory center in the
medulla to increase ventilation.
CHAPTER 14 Respiratory Diseases^255