8.8 - Response to Exercise

The body's response to exercise

• During exercise, muscles work harder, so they need more energy.
• Muscles get this energy from respiration. Some muscles contract much more frequently than usual, so they respire faster.
• This means they need a larger supply of oxygen and glucose, and produce more carbon dioxide that must be removed.

How the body meets the demand

Several changes happen at once to increase the supply of oxygen and glucose and remove waste carbon dioxide:

• Breathing rate and depth increase - More air is drawn in and out of the lungs, so more oxygen passes into the blood and more carbon dioxide is breathed out.
• Heart rate increases - The heart pumps a greater volume of blood each minute (its cardiac output rises), so oxygenated blood carrying oxygen and glucose reaches the working muscles faster and carbon dioxide is carried away more quickly.
• Blood flow is directed towards the active muscles.
• Regular exercise over time also strengthens the heart muscle itself.

Gentle versus vigorous exercise

• During gentle exercise, the increased oxygen supply is enough to keep muscles respiring aerobically.
• During very vigorous exercise, the lungs, heart, and blood cannot deliver oxygen quickly enough. The muscles begin to respire anaerobically as well, producing lactic acid.
• The glucose is only partly broken down (its oxidation is incomplete), so the lactic acid builds up in the muscles. This creates an oxygen debt - extra oxygen that the body has to take in after exercise to remove the lactic acid.
• Over a long period of hard activity, muscles become fatigued - they grow tired and stop contracting efficiently.