A healthy person at sea level breathes room air (FiO2 0.21). The PaCO2 is 40 mmHg, the respiratory quotient is 0.8 and water vapour pressure is 47 mmHg. What is the alveolar PO2?
Answer and explanation
Correct answer: A. About 100 mmHg
- Step 1: PAO2 = FiO2 x (Patm - PH2O) - PaCO2 / R. Step 2. 0.21 x (760 - 47) = 150; 40 / 0.8 = 50. Step 3. 150 - 50 = 100 mmHg. Answer: about 100 mmHg. Why not the others: 150 forgets to subtract the CO2 term, and 60 and 40 would imply severe hypoxia.
Practise more Cardiovascular, Respiratory and Renal Physiology.
Free questions with an explanation for every one, and full timed USMLE Step 1 mock tests.
More Cardiovascular, Respiratory and Renal Physiology questions
- Ventricular pressure is rising rapidly while ventricular volume stays constant, just before the aortic valve opens. Which phase of the cardi…
- A patient has an end-diastolic volume of 120 mL and an end-systolic volume of 50 mL. What is the ejection fraction?
- Which part of the systemic circulation has the greatest total cross-sectional area and therefore the slowest blood velocity?
- Which change shifts the oxygen-hemoglobin dissociation curve to the right, so that hemoglobin releases oxygen more easily to the tissues?
- A patient has a GFR of 120 mL/min and an effective renal plasma flow of 600 mL/min (from PAH clearance). What is the filtration fraction?
- Which segment of the nephron reabsorbs about two thirds of the filtered sodium and water and essentially all of the filtered glucose and ami…