Beachey: Respiratory Care Anatomy and Physiology, 2nd Edition

Answers to Concept Questions

Chapter 7: Gas Diffusion

7-1	A reduced alveolar capillary membrane area is an example of true diffusion impairment. Reduced diffusion surface area limits the amount of oxygen that can diffuse from the lungs into the blood each minute. A partially obstructed airway is a ventilation or bulk flow problem, not a diffusion problem in the sense that diffusion mechanics are impaired.

7-2	When the respiratory exchange ratio (R) equals 1.0, the amount of carbon dioxide entering the alveoli exactly matches the amount of oxygen leaving the alveoli.

7-3	PAO2 = FIO2(PB  47)  (PaCO2  1.2)
  PAO2 = 0.4(760  47)  (35  1.2)
  PAO2 = 285.2  42
  PAO2 = 243.2 mm Hg

7-4	Oxygen diffusion normally increases with exercise in healthy individuals because (1) increased cardiac output and blood pressure force open (recruit) pulmonary capillaries that had been collapsed under resting conditions, which increases the surface area available for diffusion; (2) although blood capillary transit time decreases in exercise, oxygen equilibrium with alveolar gas still occurs; thus the more rapid blood flow of exercise means that more blood enters and leaves the capillary each minute, which takes up more oxygen from the alveolar gas each minute.

7-5	Normally, oxygen equilibrium between alveolar gas and blood occurs well before the blood leaves the pulmonary capillary. As the blood flow rate increases, more deoxygenated blood enters the capillary each minute. This means that capillary blood takes up more oxygen from the alveoli each minute than it would if it were flowing more slowly.

7-6	Carbon dioxide diffuses across the alveolar-capillary membrane approximately 20 times more rapidly than oxygen because carbon dioxide is much more soluble than oxygen; this outweighs the fact that oxygen is the lighter molecule.

7-7	Both pulmonary embolism and completely obstructed airways represent a decrease in the gas-exchange surface area. Either blocked blood flow or blocked airflow removes the effective gas-exchange surface area.

7-8	The DLCO test detects an oxygen diffusion defect before the PaO2 test. Even when membranes are significantly thickened, equilibrium between alveolar gas and capillary blood PO2 usually occurs, although blood may flow the entire length of the 
capillary before this happens. Such equilibrium never occurs for CO pressures. Thus even when DLCO is 50% reduced, PaO2 may still be normal.