Tissue-Specific Metabolism
High-Yield Summary
- Liver: the body's metabolic buffer. Fed: stores glycogen, then makes fatty acids/VLDL. Fasting: glycogenolysis → gluconeogenesis (lactate, glycerol, amino acids) → ketogenesis in prolonged fasting.
- Skeletal muscle: uses glucose at rest (glycogen for LOCAL use only, never released to blood); shifts to fatty acid oxidation when fasting. Exercise fuel progression: creatine phosphate → anaerobic glycolysis → oxidative fatty acid metabolism.
- Cardiac muscle: highly aerobic, prefers fatty acids at baseline but flexible (glucose, lactate, ketones); readily uses ketone bodies when fasting — continuous, high-demand workload.
- Adipose tissue: fed = takes up glucose (for glycerol-3-phosphate) and fatty acids, stores triglycerides; fasting = lipolysis releases fatty acids (to liver/muscle) and glycerol (to liver for gluconeogenesis).
- Brain: glucose-first; adapts to ketone bodies only during prolonged fasting, sparing muscle protein. Red blood cells: always glucose, no mitochondria, never switch fuels.
Key Terms
- Metabolic buffer
- The liver's role: switching between storing and releasing glucose/fat to keep blood levels stable.
- Exercise fuel progression
- Creatine phosphate → anaerobic glycolysis → oxidative fatty acid metabolism, as exercise duration increases.
Fuel Use Across Tissues
| Tissue | Well-Fed Fuel / Fasting Fuel |
|---|---|
| Liver | Glycogen, then fatty acids/VLDL / Glycogenolysis → gluconeogenesis → ketogenesis |
| Skeletal muscle | Glucose (local glycogen) / Fatty acid oxidation |
| Cardiac muscle | Fatty acids (flexible) / Ketone bodies |
| Adipose tissue | Stores triglycerides / Lipolysis (fatty acids + glycerol) |
| Brain | Glucose / Glucose (early), ketone bodies (prolonged) |
| Red blood cells | Glucose / Glucose (always) |
Common MCAT Trap
- Muscle glycogen is strictly local — never confuse it with liver glycogen, which CAN raise blood glucose.
- Cardiac muscle PREFERS fatty acids even in the fed state — unlike skeletal muscle, which favors glucose at rest.
- The brain's ketone-body adaptation is prolonged-fasting only; don't apply it to early/postabsorptive fasting.
Quick Recall
What is the liver's overall metabolic role?
What is skeletal muscle's exercise fuel progression?
Which tissue prefers fatty acids even when well-fed?