Empirical Cycling Podcast Watts Doc #21: VO2max and The Most Interesting Protein In The World
Jul 3, 2020
They trace how hemoglobin’s structure makes it the central oxygen carrier and explain cooperative O2 binding without getting molecular insights. They cover lung design and why oxygen exchange is so fast. They compare hemoglobin mass versus concentration for performance and why plasma volume changes matter. They review a classic detraining and plasma-reinfusion study that raises questions about the heart’s role in VO2max.
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Fractal Lung Design Creates Massive Exchange Area
- Lungs maximize gas exchange by fractal branching into millions of tiny alveoli, yielding ~50 m² surface area with only ~200 mL of pulmonary blood at any moment.
- That huge surface area plus thin alveolar-capillary walls lets hemoglobin saturate to ~98–99% almost instantly during transit through the lungs.
Hemoglobin Cooperativity Drives Rapid Loading
- Hemoglobin is tetrameric with four heme sites and shows positive cooperativity: binding of O2 to one site shifts Hb from T (low affinity) to R (high affinity).
- Two O2 bindings can flip the whole molecule's conformation, explaining the sigmoidal O2 dissociation curve and rapid loading in lungs.
Local Chemistry Tunes Hemoglobin Oxygen Delivery
- Local factors shift Hb oxygen affinity: lower pH, higher CO2, 2,3-BPG and temperature all right-shift the curve to promote O2 unloading in active muscle.
- Carbonic anhydrase in RBCs rapidly converts CO2↔bicarbonate+H+, increasing intracellular acidity to help Hb release O2 and then reverses in lungs to expel CO2.