Chapter 6
Answers to Review Questions
Pages 192–193
Fill in the Blank
1. alveoli
2. less, inspiration, greater, expiration
3. greater, less
4. oxyhemoglobin, deoxyhemoglobin
5. ventilatory breakpoint
Multiple Choice
1. E
2. A
3. D
4. D
5. E
True/False
1. False
2. True
3. False
4. False
5. True
Short Answer
1. The millions of saclike alveolar clusters located at the ends of the respiratory bronchioles
provide tremendous surface area (approximately the size of a tennis court) for gas exchange.
Pulmonary diffusion is further aided by a thin respiratory membrane composed of the
membrane of the alveolar cells and the cells making up the wall of the capillary. So, the
respiratory membrane through which gases must pass to move between the blood and the air
within the alveoli is only two cells thick . Moreover, each alveolus is wrapped in capillaries,
which aids in increasing the surface area available for pulmonary diffusion.
2. Air moves into and out of the lungs because of pressure differences between the atmosphere
and the lungs created by changes in the volume of the thoracic cavity. Lung volume is
changed as the volume of the thoracic cavity changes; the lungs adhere to the inner surface of
the thoracic cavity and diaphragm because of the intrapleural pressure created by a fluid in
the pleural cavity.
3. During exercise, pulmonary ventilation increases up to 20 times from rest. As pulmonary
ventilation increases during exercise, one starts to breathing through the mouth. Compared
with the nasal passages alone, incorporating breathing through the mouth provides a larger
diameter conduit for airflow. The diameter or cross-sectional area of the airway is the most
important factor affecting resistance to airflow. So, with a larger-diameter conduit for airflow
resistance to flow decreases and it is easier to inspire and expire.
4. Blood flow through the pulmonary circulation and through the systemic circulation are
equivalent; however, blood pressure within the pulmonary circulation is very low compared
with the systemic circulation. The low pulmonary circulating blood pressures are caused by
lower vascular resistance within the pulmonary circulation, which helps to protect the thin
respiratory membrane from damage from high blood pressure.
Critical Thinking
1. Chemoreceptors are located within the medulla, carotid arteries, and the aortic arch and
respond to chemical changes. Central chemoreceptors are located within the medulla and are
especially sensitive to changes in acidity of the cerebral spinal fluid. The major cause of
increased acidity within the cerebral spinal fluid is increased carbon dioxide concentration,
which quickly increases the hydrogen ion concentration because of the bicarbonate reaction.
The peripheral chemoreceptors of both the aortic and carotid bodies are sensitive to changes
in blood acidity and partial pressure of carbon dioxide; however, only the carotid bodies are
sensitive to changes in the partial pressure of oxygen. The location of the aortic bodies allows
monitoring of blood that has just returned from the lungs and should be fully oxygenated.
The location of the carotid bodies allows monitoring of the blood and oxygen supply to the
brain. This is important because the brain is the organ of the body most sensitive to a lack of
oxygen; without a sufficient oxygen supply to the brain, fainting will occur within a matter of
seconds. If a chemoreceptor senses increased partial pressure of carbon dioxide, decreased
partial pressure of oxygen, or increased acidity, as will occur during exercise, an increase in
pulmonary ventilation will be stimulated. Increased pulmonary ventilation increases the
amount of carbon dioxide leaving the blood and being expired as well as the amount of
oxygen diffusing into the blood from the interior of the alveoli. Decreases in these same
measures would stimulate a decrease in pulmonary ventilation. Changes in pulmonary
ventilation caused by the chemoreceptors match the body’s needs to expire carbon dioxide
produced by metabolism and oxygen needed for aerobic metabolism.
2. At sea level PO2 is 159.1 mm Hg and PCO2 is 0.2 mm Hg. However, within the alveoli, PO2
decreases to 105 mm Hg and PCO2 increases to 40 mm Hg. These changes are caused by
several factors. One function of the nasal passages and pulmonary conduits is to humidify the
inspired air; at the alveoli the relative humidity is 100%. This would tend to decrease the
partial pressure of all gases except water vapor because, according to Dalton’s law, the total
pressure of a gas mixture is equivalent to the sum of all the pressures of all the gases that
compose the mixture. So, if the partial pressure of water vapor increases, the partial pressure
of all other gases decrease. Additionally, PO2 is also decreased because of the mixing of
atmospheric air with a high PO2, with air left within the lungs after exhalation that has a
lower percentage of oxygen because oxygen is diffusing from the air within the alveoli into
the blood. These two factors decrease PO2 to 105 mm Hg in the alveoli. Air within the alveoli
has a high PCO2 because carbon dioxide is diffusing into the air in the alveoli from the blood.
The net effect of humidifying the air and mixing inspired air with the air left in the alveoli
after exhalation in an increase in PCO2 compared to atmospheric air, resulting in a PCO2 of 40
mm Hg.