Describe the changes in inspired and alveolar oxygen partial pressure with increasing altitude. (20% of marks) Outline the respiratory physiological responses to altitude. (80% of marks)

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College Answer

Any description of the changes in inspired and alveolar PO2 with altitude required the
description of, and an understanding of, the equations for calculation of PiO2
(PiO2=0.21(BMP-47)mmHg or 0.21(BMP-6.3)kPa) and PAO2 (PAO2 =PiO2 – PaCO2/R). It
was important to mention alteration in atmospheric pressure (BMP) with altitude and that
saturated vapour pressure (SVP) is constant for the same body temperature.
A good answer to the second part of the question (respiratory physiological responses to
altitude) was one that had some structure, eg responses divided into acute and chronic,
respiratory, renal, haematological, cardiovascular and CNS categories, and relevant detail for
each category. Candidates were expected to at least mention, but not restrict themselves to the
following: acute responses, eg carotid/aortic body chemoreceptor stimulation
hyperventilation, proportional to reduced BMP and hypoxia, limitations to hypocarbia (due to
alterations in CSF pH) etc: physiological responses to early and chronic acclimatisation (eg
renal HCO3-excretion, respiratory centre CO2 response curve resets to left, altered peripheral
chemoreceptor sensitivity, 2,3, DPG levels and related right. Shift in HbO2 curve, alterations
in pulmonary diffusing capacity due to increased alveolar surface area and pulmonary blood
volume, etc).
It is important that current and future candidates use the examination reports to guide their
learning. Thus it is essential that candidates not just read the feedback provided to each exam
question but also refer to the reference(s) supplied below and other relevant sources they find
to be useful.
Syllabus – B1k, 2i
Reference – Nunn’s Respiratory Physiology, Ch 17 and 5.

Power and Kam 432-4.