alleryStart learning →By the time blood returns to the heart, what has its oxygen partial pressure become?
AAbout 100 mm Hg
BAbout 104 mm Hg
CAbout 20 mm Hg
DAbout 40 mm Hg
Answer & Solution
Correct answer: D. About 40 mm Hg
1. Blood leaves the systemic capillaries after unloading oxygen into the tissues.
2. By the time it returns to the heart its oxygen partial pressure has fallen to about 40 mm Hg.
3. Its carbon dioxide partial pressure has risen to about 45 mm Hg over the same journey.
4. The 100 and 104 mm Hg figures belong to oxygenated blood and alveolar air, so they describe the other end of the circuit.
_Source: OpenStax Anatomy and Physiology (CC BY 4.0), Ch 22 "The Respiratory System", section 22.4 Gas Exchange_
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