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Aerobic oxidative system

The endurance engine, and the system that runs your body at rest. Inside the mitochondria, fuel is fully oxidised to carbon dioxide and water, yielding far more ATP per molecule than the anaerobic systems — but slowly. It powers everything from sleeping to a marathon, and it is what rebuilds the fast systems between efforts.

Relative power output over time power → 0s10s30s1m2m5m+ATP–PCrGlycolyticOxidative — this page

Every all-out effort recruits all three systems at once — this shows which one dominates as the seconds tick by.

The metabolic pathway — where the energy comes from

FATS · triglycerideCARBOHYDRATEPROTEIN Fatty acids + glycerolGlucoseAmino acids Glycolysisnet +2 ATP · fast Pyruvate Lactateno O₂ → fatigue ↑ Cytosol (sarcoplasm) — no oxygen needed↓ Mitochondria — needs oxygen (O₂) β-oxidation Deamination→ into glycolysis or Krebs Acetyl-CoA Ketone bodies Krebs cyclecitric-acid cycle Electron transport chain ⚡ ATP Creatine phosphate → ATPinstant · 0–10s · skips the mill
The metabolic mill — how each fuel is broken down and fed into ATP. Lit path: the aerobic oxidative system.

Duration: ~2 minutes to hours · Intensity: Low to moderate (sustainable) · Fuel: Carbohydrate, fat, and (a little) protein — burned with oxygen · Oxygen: Required (aerobic)

How it works

  1. Carbohydrate (via pyruvate) and fat (via beta-oxidation) enter the mitochondria as acetyl-CoA.
  2. Acetyl-CoA feeds the Krebs (citric acid) cycle, which strips off high-energy electrons onto carriers NADH and FADH₂.
  3. Those electrons pass down the electron transport chain, pumping protons to drive ATP synthase — 'oxidative phosphorylation'.
  4. Oxygen is the final electron acceptor, forming water. Net yield is ~30–32 ATP per glucose (and ~100+ per fat molecule) — slow but enormous.

What it powers

Byproduct: Carbon dioxide (breathed out) and water — clean.

Recovery: This IS the recovery system: it resynthesises PCr and clears lactate. A bigger aerobic base means faster recovery between hard efforts.

Training: Trained with sustained low-to-moderate cardio (Zone 2) and higher-intensity intervals that raise VO₂max. Adaptations: more and bigger mitochondria, more capillaries, better fat oxidation.