Oxygen travels 60,000 miles to reach one cell. Here's the whole relay.

Day 13 closes the cardiovascular and respiratory arc. The heart pumps, the vessels route, the blood carries, the lungs fill — but none of that matters until oxygen actually crosses into a cell and CO₂ gets out. Today is about the crossing.

The alveolus wall and the capillary wall together measure 0.2 µm — thinner than a soap bubble. Gases move across it by diffusion, driven purely by partial pressure gradients: no pumps, no energy spent.

In your alveoli, O₂ sits at ~104 mmHg. In the venous blood arriving from your tissues, it's only ~40 mmHg. That gap pushes O₂ across. CO₂ runs the reverse: ~45 mmHg in blood, ~40 mmHg in the alveoli, so it diffuses out. Every breath resets both gradients.

The same swap happens again at the tissue end: cells consume O₂ (dropping local pO₂ to ~20 mmHg) and produce CO₂ — so the gradient keeps running the exchange passively. Your 300 million alveoli give this system 70 m² of membrane to work with.

Carbon monoxide is deadly for one mechanical reason. CO binds hemoglobin 250 times more tightly than O₂ does. With CO occupying the binding sites, blood still circulates, lungs still work, but no O₂ gets picked up. The gradient across the alveolar membrane can be perfect — there's simply nothing left to carry the oxygen.

Every warm-up you've ever done was really about partial pressure. Cold, resting muscle uses little O₂, so local pO₂ stays relatively high and the gradient driving O₂ off hemoglobin is shallow. As you warm up, metabolic demand rises, local pO₂ drops, and hemoglobin releases O₂ more readily — the Bohr effect in action. A 10-minute warm-up measurably improves how much O₂ active muscle can extract from the same volume of blood.

Your breathing drive comes from chemoreceptors in the medulla detecting CO₂ concentration, not from an O₂ sensor. You could breathe pure nitrogen and feel no urge to breathe — until CO₂ built up. That's why hyperventilating before diving is dangerous: it flushes CO₂ without raising O₂, silencing the alarm while the oxygen clock is still ticking.

Days 9–13 form a complete circuit: the heart pumps, vessels distribute, blood carries, lungs renew the cargo, and gas exchange hands oxygen off at the cellular door. Tomorrow the series enters the digestive and endocrine arc — Day 14 opens with the stomach.