| Date | May 2010 | Marks available | 1 | Reference code | 10M.3.HL.TZ1.13 |
| Level | Higher level | Paper | Paper 3 | Time zone | Time zone 1 |
| Command term | Suggest | Question number | 13 | Adapted from | N/A |
Question
The human body suffers significant physiological changes at extreme altitudes. Extensive scientific information has been obtained from medical research expeditions to Mount Everest (8848 m above sea level). The figure below shows the relationship between the partial pressures of oxygen (Po2) and carbon dioxide (Pco2) in the alveoli as altitude increases from sea level (at top right) to the summit of Mt. Everest (at bottom left).
The table below shows the data from the field study on the alveolar gas and arterial blood values for a climber at sea level and on the summit of Mt. Everest.
Outline the changes in the partial pressures of carbon dioxide and oxygen as altitude increases.
Predict, with a reason, how the ventilation rate will change as a climber ascends from sea level to the summit of Mt. Everest.
Calculate the percentage change in the arterial partial pressure of carbon dioxide (Pco2) at the summit compared with that at sea level.
Suggest a reason for the low arterial partial pressure of carbon dioxide at the summit.
State one adaptation of people who live permanently in high altitude areas.
Markscheme
both Po2 and Pco2 fall with increasing altitude;
above certain altitude there is little change in alveolar Po2 / Po2 remains close to 37 mm Hg over a wide range of altitudes;
Pco2 changes over the entire range of altitudes;
the Po2 is always higher than Pco2;
the rate of ventilation would increase;
expelling large quantity of CO2 / causing fall in blood CO2/Pco2;
rise in blood pH hampers ventilation/inhibits chemoreceptors;
(Allow answers in the range 81–81.5 %)
low partial pressure/level of carbon dioxide in the air;
hyperventilation/high rate of ventilation;
high lung capacity;
larger tidal volumes;
high proportion of hemoglobin / high red blood cell count;
hemoglobin with higher affinity for oxygen;
Examiners report
Many were able to see that both PO2 and PCO2 fell with increasing altitude but only the best candidates could correctly identify another change.
This was a very difficult section and many did not refer to the table provided when answering this question. Most of the candidates tended to refer to oxygen rather than carbon dioxide.
Many were able to calculate the percentage change correctly in (i). Those that didn‟t usually had 18.7% rather than 81.3%.
In (ii) many were able to get a mark for answering that the reason was due to the low partial pressure of carbon dioxide in the air.
The most common correct adaptation given was that those who live permanently at higher altitude had a larger lung capacity.
