Biology | Answers to Review Questions

Author Dr. George B. Johnson, Washington University
Author Dr. Peter H. Raven, Missouri Botanical Gardens & Washington University
Contributor Dr. Susan Singer, Carleton College
Contributor Dr. Jonathan Losos, Washington University

Answers to Review Questions

Chapter 53 (p. 1070)

1. Nitrogen is the most common gas in dry air, occupying 78.09% by volume. Oxygen occupies 20.95%, and carbon dioxide 0.03%.

2. Gases can only diffuse at a specific rate through a thin enough area; this serves to put severe limitations on the sizes organisms can attain if they are respiring entirely through diffusion. As an organism increases in size, its surface-to-volume ratio decreases, and those areas more distantly removed from the surface will receive less or no oxygen than regions near the surface.

3. In fish gills, the direction of blood flow is opposite the flow of the passing water, countercurrent flow (180^o difference). As a result, there is maximal difference between the partial pressure of gases at any point in the gill. At the beginning the blood is lowest in oxygen, the water has already had much of its oxygen removed; at the end the blood is highest in oxygen and the water is freshest and has its highest concentration of oxygen.

4. Amphibians exhibit positive pressure respiration; they literally force air into their lungs. All other vertebrates use negative pressure respiration; they expand their lungs (by movement of rib cage and diaphragm) and suck air inside.

5. In birds, airflow is one-way. Air is drawn in through the trachea to the posterior air sacs, the lung, the anterior air sacs and out the trachea. There is no dead air volume as in mammals. Birds also exhibit a cross-current flow (90^o difference) of blood versus air in the parabronchi.

6. The lungs are connected at the junction of the lung and the bronchus and supported by the water tension of the interpleural fluid between the two pleural membranes.

7. Respiration is a primitive function carried out by specific areas in the brainstem. Chemoreceptors in the circulatory system are sensitive to CO₂ levels in the blood. When they sense an increase in CO₂, they will tell the brainstem to accelerate breathing.

8. Eight percent of CO₂ dissolves in the blood plasma, twenty percent binds to hemoglobin, and the rest diffuses into the cytoplasm of red blood cells. Increased absorption of CO₂ is achieved in red blood cells as CO₂ is catalyzed by carbonic anhydrase to dissociate into bicarbonate and H⁺. CO₂ is unloaded at the lungs because lower CO₂ levels cause a reverse reaction to occur; at low CO₂ levels, hemoglobin has a greater affinity for O₂ than CO₂, causing the cells to give up the CO₂ for new O₂.