1. Exchange of oxygen and carbon dioxide between blood and muscle cells
2. Gases move from high to low partial pressure
3. At rest, high pp of oxygen in blood, low pp of oxygen in muscle cells, oxygen diffuses into muscles
4. At rest, high pp of carbon dioxide in muscle cells, low pp of carbon dioxide in blood, carbon dioxide diffuses into bloodstream
5. During exercise, high pp of oxygen in blood, lower pp of oxygen in muscle cells, oxygen diffuses into muscles down steeper gradient
6. During exercise, higher pp of carbon dioxide in muscle cells, low pp of carbon dioxide in blood, carbon dioxide diffuses into bloodstream down steeper gradient