Your body cannot store enough oxygen for more than a minute of intense exercise. Oxygen is continually consumed to produce energy. Only increased breathing can meet the demands of increased activity, as explained by exercise physiologists at Eastern Kentucky University. Your brain balances your levels of blood oxygen, O2, and carbon dioxide, CO2, the end-product of oxygen use, to match your breathing to your exercise demands. This is automatic and requires no conscious control.
Regulating Factors
Many factors control your breathing rate during exercise. The amount of CO2 in your blood effects your breathing rate because your medulla, part of your brain stem, is very sensitive to CO2. The amount of CO2 that your body makes depends on the amount of O2 it uses during exercise. Exercise increases O2 use, CO2 production and breathing rate to match it. Your carotid artery, where you can feel your pulse in your neck, senses blood O2 levels. Increased exercise lowers O2 and signals your brain to increase your breathing rate. Even your emotions can raise your breathing rate. The excitement of a race or your response to an emergency can increase your breathing rate in anticipation of greater oxygen needs of flight or fight.
Lung Volumes
Raising your breathing rate is only one way your lungs increase O2 uptake and CO2 clearance. If your breaths are too shallow, even a very high rate will accomplish little. The larger each breath is, the fewer breaths you will need to exchange enough O2 and CO2. During exercise your breathing rate will depend, in part, on how well you have trained and strengthened your breathing muscles to fully expand your lungs on each breath. Therefore, a well-trained athlete may breathe somewhat more slowly and deeply than an untrained person at the same exercise levels.
Minute Ventilation
The product of breathing rate and the volume of each breath is called your minute ventilation. For instance, 20 breaths per minute at 1 liter per breath yields a minute ventilation of 20 liters per minute If your exercise requires the O2 in 20 L of air every minute, your body can also achieve that with a higher rate and a lower volume per breath. You re-breathe some stale air every time you inhale, so shallow breaths contain a lower proportion of fresh air. The minute volume of fresh air you need will regulate your breathing to a proportionately higher rate for shallow breathing. Fourteen breaths per minute at 1/2 L per breath, or 7 LPM, is normal at rest. Hard exercise can result in 40 breaths per minute at 1 L per breath, or 40 LPM, or even more.
Maximum Breathing Rate
Your breathing rate may not be the highest rate you can achieve. If you breathe as fast as possible you may exceed 120 breaths per minute, but with less than 1/2 second for each inhalation and exhalation, you will have too little time to move enough fresh air in each breath. No standard number can be stated as the normal, maximum or ideal breathing rate during exercise.
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