Discuss the significance of the oxygen dissociation curves for adult hemoglobin and fetal hemoglobin.

a. oxyhemoglobin forms when partial pressure of oxygen is high
OR
oxyhemoglobin dissociates/breaks apart when partial pressure of oxygen is low ✔

b. respiring tissues have low partial pressure of oxygen ✔

c. sketch/statement of S-shaped «oxygen dissociation» curve ✔

d. axes of graph labelled correctly as percentage oxygen saturation of hemoglobin on y-axis AND partial pressure of oxygen on x-axis ✔ Both needed. Do not accept reverse axes.

e. «small» decrease in oxygen partial pressure over steep part of curve results in dissociation of oxyhemoglobin/oxygen release to tissues ✔

f. fetal hemoglobin is structurally different from adult/maternal hemoglobin ✔

g. fetal dissociation curve to left of adult dissociation curve ✔

h. fetal hemoglobin has greater affinity for oxygen than adult/maternal blood ✔

i. fetus obtains its oxygen from mother’s blood «at placenta» ✔

j. at any given partial pressure of oxygen fetus will take up oxygen from mother
OR
fetal hemoglobin always more saturated with oxygen than maternal blood ✔

Some of these points may be present in annotated diagrams.

Most candidates generally provided good, and many complete, thorough explanations including well annotated diagrams, but there were also answers lacking sufficient details or containing diverse inaccuracies or incorrect terminology (e.g. concentration instead of partial pressure). Some candidates ventured sometimes in long explanations of hemoglobin structure or gas exchange at lung level in adults, or description of Bohr shift, all irrelevant for this question which differed slightly from questions in past papers on similar topics.