When you exercise, your muscle tissue breaks down nutrients to generate energy. This metabolic process consumes oxygen and produces carbon dioxide as a waste product. Your circulatory system responds to this change by both pumping carbon dioxide away from your muscle tissue to prevent accumulation and by pumping oxygen to your muscle tissue to support increased oxygen consumption.
Background
During physical activity, your muscle tissues perform a metabolic process called oxygen-dependent, or aerobic, cellular respiration, which transforms energy stored in molecules such as amino acids into usable biochemical energy. This process creates an imbalance in your blood by depleting oxygen and producing carbon dioxide. To correct this your body responds by increasing blood flow, which carries inspired oxygen from your lungs to your muscle tissue and carries away excess carbon dioxide from your muscle tissue to your lungs so you can breathe it out.
Oxygen Consumption
When you exercise, your muscles demand oxygen to sustain energy production. Oxygen in air you inhale travels through your lungs and binds to protein in your blood, which circulates it throughout your body. Therefore, when chemoreceptors in your blood vessels detect a decrease in oxygen supply, they signal your brain to increase oxygenated blood flow from your lungs to your muscles. Your body responds by increasing your heart rate, which pumps blood carrying oxygen from your lungs to your working muscles more quickly.
Carbon Dioxide Production
When performing aerobic cellular respiration during exercise, your muscle tissues release carbon dioxide into your blood as a waste product. Carbon dioxide increases blood acidity and lowers blood pH, which must remain within a narrow range for vital metabolic functions to take place. Chemoreceptors in your blood vessels and brain detect this decrease in pH and your body responds by increasing your heart rate and breathing rate, which increases blood flow to your lungs so you can expire the accumulating carbon dioxide. This increase in blood circulation rate also increases the flow of oxygenated blood from your lungs to muscle tissue, and in this way the carbon dioxide produced by exercising indirectly helps pump more oxygen to your muscles.
Risks of Insufficient Oxygen
When exercising vigorously, your muscle tissues consume oxygen very rapidly to sustain energy production. Though your body may increase your heart rate, blood pressure and breathing rate to satiate this increased demand for oxygen, you may still struggle to maintain oxygen supply to your working muscle tissue. This can lead to symptoms such as weakness, fatigue or breathlessness. Therefore, before beginning a rigorous exercise regimen, consult with your health care practitioner to ensure that your routine is suitable for your fitness level.
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