> After all, what's the point of circulation, if there's no oxygen going in?
Short answer: There's already a bit of oxygen in your lungs.
Long answer: You can think of the airways in your lungs as a binary tree. Generations 17-23 are the respiratory unit, where gas exchange takes place; generations 0-16 are the conducting zone, which move air to the respiratory unit, but don't factor into gas exchange. An average lung with a volume of about 5 L has a respiratory unit with a volume of ~2.75 L, leaving a conducting zone volume of 2.25 L of air yet to be breathed. At 21% atmospheric O2, you've got about 0.47 L O2 in that 2.25 L of air, not including extra, yet-to-be-used O2 in your blood. Don't forget that Reinhold Messner summited Mount Everest breathing atmospheric air and with only about 53 ppm O2 (~1/3 sea level atmospheric O2 of 160 ppm); hemoglobin only needs about 80 ppm O2 to fully saturate. At 53 ppm, Reinhold was probably kicking it with ~80% hemoglobin O2 saturation.
How?
Your body has compensation mechanisms to deal with lower than ideal O2. The Bohr effect (Christian, not Neils), causes hemoglobin to loose affinity for oxygen in high CO2 environments, resulting in higher O2 delivery to metabolizing tissues. O2 diffuses from your lungs passively to your blood in 250 msec and your blood goes through your lungs (when your heart is working) for about 750 msec, so your lungs are seriously overspec'ed for the amount of O2 you need at rest (or unconscious) and are about the surface area of a tennis court!
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> After all, what's the point of circulation, if there's no oxygen going in?
Short answer: There's already a bit of oxygen in your lungs.
Long answer: You can think of the airways in your lungs as a binary tree. Generations 17-23 are the respiratory unit, where gas exchange takes place; generations 0-16 are the conducting zone, which move air to the respiratory unit, but don't factor into gas exchange. An average lung with a volume of about 5 L has a respiratory unit with a volume of ~2.75 L, leaving a conducting zone volume of 2.25 L of air yet to be breathed. At 21% atmospheric O2, you've got about 0.47 L O2 in that 2.25 L of air, not including extra, yet-to-be-used O2 in your blood. Don't forget that Reinhold Messner summited Mount Everest breathing atmospheric air and with only about 53 ppm O2 (~1/3 sea level atmospheric O2 of 160 ppm); hemoglobin only needs about 80 ppm O2 to fully saturate. At 53 ppm, Reinhold was probably kicking it with ~80% hemoglobin O2 saturation.
How?
Your body has compensation mechanisms to deal with lower than ideal O2. The Bohr effect (Christian, not Neils), causes hemoglobin to loose affinity for oxygen in high CO2 environments, resulting in higher O2 delivery to metabolizing tissues. O2 diffuses from your lungs passively to your blood in 250 msec and your blood goes through your lungs (when your heart is working) for about 750 msec, so your lungs are seriously overspec'ed for the amount of O2 you need at rest (or unconscious) and are about the surface area of a tennis court!