Beachey: Respiratory Care Anatomy and Physiology, 2nd Edition

Answers to Concept Questions

Chapter 8: Oxygen Equilibrium and Transport

8-1	Use the following equations:
  PO2  0.003 = dissolved oxygen in mL/dL
  PO2  0.003 = 1.8 mL/dL
  PO2 = 1.8/0.003
  PO2 = 600 mm Hg

8-2	Use the following equation: CaO2 = Hb  1.34  Sat% + (PaO2  0.003). Sat% is assumed to be equal to 1.0 because the PaO2 is 550 mm Hg. 
CaO2 = 10  1.34  1 + (550  0.003)
CaO2 = 13.4 + 1.65 = 15.05 mL/dL
Compare this with a normal CaO2 of approximately 20.1 mL/dL when Hb equals 15 g/dL.

8-3	The answer is “greater than.” The oxygen-hemoglobin equilibrium curve is quite steep in the 40– to 60–mm Hg range, whereas it is almost flat above 100 mm Hg. A PO2 change from 40 to 60 mm Hg represents a 15% hemoglobin saturation or oxygen content gain (from 75% to 90%). A change in PO2 from 100 to 600 mm Hg represents an oxygen saturation and content gain of only 2.5% (from 97.5% to 100%). The additional dissolved oxygen generated by a PO2 change from 100 to 600 mm Hg is only 1.5 mL/dL (500 mm Hg  0.003), not enough to outweigh the oxygen content gain accompanying a 40– to 60–mm Hg PO2 change.

8-4	Consider the effect of acidosis and alkalosis on hemoglobin affinity for oxygen; acidosis decreases hemoglobin’s affinity for oxygen, whereas alkalosis increases it. Therefore at a given PO2 of 40 mm Hg, alkalotic blood contains more oxygen than does acidotic blood.

8-5	As blood flow slows, it spends more time in the capillaries and loses more oxygen to the tissues. This lowers the venous blood oxygen content, increasing the arterial-venous oxygen content difference.

8-6	Cyanosis is usually apparent when the average amount of desaturated hemoglobin in the capillaries is 5 g/dL. Normal arterial blood is 2.5% desaturated (100% minus 97.5%). This person’s venous blood is 50% desaturated (100% minus 50%). At a hemoglobin concentration of 15 g/dL, arterial blood contains 0.375 g/dL desaturated hemoglobin (0.025  15 g/dL). Venous blood contains 7.5 g/dL desaturated hemoglobin (0.50  15 g/dL). An average of these two desaturations is (0.375 + 7.5)/2 = 3.938 g/dL desaturated hemoglobin in the capillaries. Cyanosis is probably not present in this instance.