why does tidal volume increase during exercise
Tidal volume is the volume of air you breathe in a single breath. Exercise causes an increase in tidal volume because your requirements for oxygen go up. This increase is mediated in different ways depending on when it occurs during your exercise. An increase in tidal volume is necessary to effectively meet your body's increased oxygen requirements, as an increase in your rate of respiration alone is not sufficient. According to Michael G. Levitzky in his "Pulmonary Physiology," in a normal adult, tidal volume is approximately 500 ml of air per breath. However, changes in air requirements, such as those that occur during exercise, and changes in your lungs' ability to expand and contract, such as those that occur with certain neuromuscular and respiratory diseases, can change tidal volume. Your lungs' job is to meet your body's demand for oxygen, which is required for the cells in your body to produce enough energy to function. Your lungs do this by bringing in fresh air with every breath; this amount is measured in part by the tidal volume. During exercise, your body's production of carbon dioxide goes up. Increasing tidal volume is one way for your lungs to accommodate the exhalation of this increased carbon dioxide load. According to Kim Barrett and colleagues in their "Review of Medical Physiology," the drivers of increased tidal volume, and to a lesser extent of increased breathing rate which also helps your lungs to exhale carbon dioxide, are divided into those that function immediately upon beginning exercise, and those that function after you've been exercising for a while.
Immediate increases are likely mediated by reflexes involving your brainstem and motor cortex, in what is likely a reflex loop between these structures and your respiratory muscles. Later increases in tidal volume are likely mediated by chemical receptors in your circulation which recognize that your body's metabolism has increased, and respond by instructing your lungs to emit more carbon dioxide. The chemicals involved in this increase include oxygen, carbon dioxide, lactic acid, arachadonic acid and bradykinin, and the receptors involved are located on many organs throughout your body. Your lungs are made up of tissues that can exchange carbon dioxide for oxygen, and also of tissues that cannot. The air included in a tidal volume measurement interacts with both of these parts -- so it follows that not all of the air in the tidal volume is exchanging carbon dioxide for oxygen. The component of your tidal volume that is not involved in carbon dioxide/oxygen exchange is known as the "dead space" volume. In order to move more air into your lungs during exercise, you have two options: increase the rate at which you're breathing, or increase your tidal volume. Increasing only the rate will increase the proportion of air that you breathe that belongs to the dead space volume; hence, an increase in tidal volume is necessary to facilitate effective gas exchange.
When you are exercising aerobically, your muscles consume more oxygen and produce more carbon dioxide than they do at rest.
These gases are exchanged with the atmosphere when you breathe. This means that during an aerobic exercise session, you must breathe more and when this overload is encountered on a regular basis, the way you breathe is altered. Minute ventilation is the volume of air you breathe in a minute. Minute ventilation is measured in liters and can be calculated by multiplying tidal volume by breathing rate. Tidal volume is the volume of air that you inhale with each breath and breathing rate is the number of breaths you take each minute. During normal quiet breathing at rest, the typical minute ventilation of 6 liters is achieved by a tidal volume of 0. 5 liters and a breathing rate of 12 breaths per minute. Minute ventilation increases during exercise because both tidal volume and breathing rate increase. At relatively low exercise intensities, tidal volume and breathing rate increase proportionally. However, at higher relative intensities, tidal volume reaches a plateau and further increases in minute ventilation depend exclusively upon increasing breathing rate. During strenuous exercise, healthy young adults typically breathe 35 to 45 times each minute and some elite endurance athletes breathe as many as 60 to 70 times. Minute ventilation values of 200 liters have been observed in athletes during maximal bicycle exercise.
Oxygen is transported to your muscles and carbon dioxide is transported away from your muscles by your blood. The actual exchange of these gases with the air you breathe occurs in your lungs. Blood is circulated through your lungs by your cardiovascular system. Generally speaking, in most people, the capacity of your lungs far exceeds the capacity of your cardiovascular system. For example, as explained by Illinois State UniversityБs Dale Brown in БExercise and Sport Science,Б a four- to five-fold increase in breathing rate and a five- to seven-fold increase in tidal volume during exercise compared to rest provide the potential to elevate minute ventilation to 20 to 30 times the resting value. This far exceeds the increase in the amount of blood you can circulate through your lungs during exercise, which is typically only five to seven times the resting value. This means that your cardiovascular system is the limiting factor and there will be no overload on your lungs that forces them to increase their capacity over time due to regular aerobic exercise. Regular aerobic exercise might not alter the maximum amount of air you can breathe, but it does improve the way you move the air that you do require. At any submaximal work rate, you will ventilate less and also establish a given minute ventilation with a greater tidal volume and reduced breathing rate. This is important because it makes breathing easier and reduces fatigue in your ventilatory muscles.
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