19. With respect to dead space what is the difference between Fowler’s
method and the Bohr equation.
Fowler’s method: measuring nitrogen increase when breathing 100% O2
VD/VT = (PACO2 – PECO2)/PACO2
Fowler’s method measures anatomical dead space. Bohr equation measures
physiological dead space. Anatomical dead space is simply the conducting
zone prior to bifurcation 17. Physiological dead space is all space in the
respiratory system that doesn’t receive gas exchange. It may include areas of
good ventilation but poor blood flow.
20. At altitude, hyperventilation blows off CO2. What effect does this have on
PAO2 (think of the alveolar gas equation)?
PAO2 = PIO2 – PACO2/R + F
According to the alveolar gas equation, PAO2 would rise.
21. What are the 2 primary mechanisms determining PAO2?
• VArate (rate of O2 entry)
• VO2 (rate of O2 removal)
22. Why does the VA/Q relation vary from the top to the bottom of the lung?
Going higher along the height of the lung, the ventilation decreases slower
than the blood flow. So we get a higher VA/Q relation at the top of the lung.
This will cause inspired air at top of lung to be more like inspired humidified air
(high PO2, low PCO2), and air at bottom of lung to be more like mixed venous
gas pressures (low PO2, high PCO2).
23. Why does hemoglobin increase the O2 content of blood?
Hemoglobin carries up to 4 O2 molecules. O2 in the blood will attach to Hb,
decreasing PO2 in blood. The decreased blood PO2 will allow more O2 to be
carried in the blood plasma (through diffusion)
25. Explain why N2O is perfusion limited.
Blood reaches maximum N20 carrying capacity in the time that the blood is in
the capillary surrounding the alveolus. More diffusion is not possible. So it is
limited by blood flow distribution (perfusion). 26. Why are breath-by-breath fluctuations of PACO2 much less than the
• Alveolar tidal volume (VT) is small in comparison to FRC (air remains in
lung during breathing at rest).
• Increase blood flow to lungs during inspiration will increase transfer of
gases so PACO2 is replenished by gas transfer from the blood
• Solubility of the gas creates a buffer to decrease fluctuations. CO2
more soluble than O2 so it has a better buffer against fluctuation
27. Describe the oxygen cascade.
• Sea level air: O2 = 0.21x760 mm Hg
• Inspired air (humidified) O2 = 150 mm Hg