Question

Figure 12.1 shows two blood oxygen dissociation curves. ‘A’ represents the oxygen partial pressure in normal alveoli, ‘H’ the lowered alveolar oxygen pressure in hypoxic lungs due to high altitude or pulmonary disease and ‘V’ the mixed systemic venous oxygen pressure in the person suffering from hypoxia. In this diagram: 

a. If (i) is a normal person’s curve, then (ii) is the hypoxic person’s curve, rather than vice versa. 

b. The blood in curve (i) has a higher red cell level of 2,3-diphosphoglycerate (2,3-DPG). 

c. The O₂ saturation of blood leaving the hypoxic lungs is lower with curve (ii) than with curve (i). 

d. The oxygen extracted by the tissues equals oxygen uptake in the lungs for both curves in both people, other things being equal. 

e. The curve labelled (i) is more suitable for fetal conditions than the curve labelled (ii).

Answer 1

a. True The hypoxic person’s curve is displaced to the right. 

b. False 2,3-DPG shifts the curve to the right. 

c. True This is apparent from the graph and is a disadvantage of the increased 2,3-DPG; notice that at this point the curve is shifted downwards as well as to the right. 

d. True Neither normal people nor hypoxic people can run up an oxygen debt at rest, so in both cases pulmonary uptake must balance tissue delivery; in the case of the hypoxic person, pulmonary uptake corresponds to XY which equals YZ which corresponds to tissue delivery. 

e. True Fetal blood is shifted even further to the left and is able to take up O₂ at the low P_O2 levels found in the maternal sinusoids