Your heart and circulatory all become more efficient at delivering oxygen to exercising muscles as a result of training – but what about another major organ that’s important for oxygen delivery – your lungs? Your lungs are the organ that brings oxygen in from the outside and humidifies it so it can be delivered to red blood cells at rest and during exercise. Since the other organs of oxygen-delivery adapt to exercise training, you might think that your lungs would adapt too by becoming larger in size so they can take in more oxygen. Surprisingly, this isn’t the case. Unlike the heart, the lungs don’t increase in size as a result of exercise training, and, in normal people, they aren’t the organ that limits endurance exercise.
The Effect of Exercise on Your Lungs
When you exercise, your muscle cells need more oxygen. Your lungs are the organ that brings this oxygen into the body where it can enter the bloodstream and be carried to “hungry” muscle cells. To bring in more oxygen, two things happen. The amount of air that moves in and out of the lungs, called the tidal volume, increases. In other words, you breathe deeper. At the same time, your breathing rate increases. When you’re resting quietly, you breathe around 12 times per minute and move around 6 liters of air in and out of your lungs per minute. During vigorous exercise, your breathing rate rises to as high as 50 times per minute and you move as much as 150 liters of air in and out each minute. That’s a 25 times increase!
What happens next? Once you breathe in oxygen, it moves down through your trachea, to the bronchi and bronchioles in your lungs to the alveoli, tiny sacs where gas exchange takes place. It’s at the alveoli that oxygen diffuses across the membrane of the sac to attach to red blood cells and carbon dioxide is released to the lungs. The alveoli of the lungs are where gas exchange takes place.
Despite the fact that you have to breathe faster and take in a larger volume of air with each breath, lung capacity or function changes very little as a result of exercise training. The only real adaptation that occurs is your respiratory muscles become a bit stronger so they’re less prone to fatigue. There’s no real change in lung size or lung capacity, how much air you can take in during a single inspiration, as a result of exercise training. Even years of endurance training doesn’t seem to change lung size or function.
In a healthy person, the lungs don’t limit the amount of oxygen that can be delivered to exercising muscles. They’re perfectly capable of supplying enough oxygen to meet the demands of exercise. For some elite endurance athletes, the lungs CAN be a limiting factor. In this case, the well-trained hearts of the athletes are able to pump blood so rapidly through their pulmonary system that gas exchange can’t take place quickly enough during very high-intensity exercise. For the average person, their lungs aren’t a limiting factor. It’s their heart’s ability to get blood to the working muscles fast enough, or the ability of muscle cells to extract oxygen from red blood cells that limits endurance exercise. There is some limited evidence that at extremely high exercise intensities, the respiratory muscles may become fatigued. This could limit endurance performance.
Other Adaptations That Occur in Response to Exercise Training
One adaptation that takes place at the level of the muscle with endurance exercise training is an increase in capillary density. During exercise, oxygen transported on red blood cells comes into contact with muscle cells as it flows through tiny capillaries. Oxygen flows from the capillaries to the muscle cells that need it to make energy. When there are more capillaries available, as a result of exercise training, more oxygen can reach muscle cells. Once there, it can be used by mitochondria to produce ATP, the energy source that fuels movement. Endurance training also increases the number of ATP-generating enzymes that can be used to make ATP, so more ATP can be produced.
In People with Lung Disease, It’s a Different Story
For people who have chronic lung disease like COPD or asthma, lung function can be a limiting factor. In both cases, the airways are narrower. In asthma, this narrowing reverses after an attack whereas it doesn’t with COPD. This partially blocks the airways and increases the work of breathing. This forces the respiratory muscles to work harder to move air in and out of the lungs. As a result, the respiratory muscles fatigue quickly and limit exercise performance.
The Bottom Line?
Unlike your heart, your lungs don’t adapt to endurance exercise by becoming larger or increasing their capacity to take in oxygen. Your respiratory muscles may become a little stronger and fatigue resistant. On the other hand, your lungs don’t usually limit endurance exercise unless you have asthma or another lung disease or are an elite athlete.
References:
Medscape.com. “Why Doesn’t Exercise Grow Lungs When Other Factors Do?”
Exercise Physiology: Theory and Application to Fitness and Performance. Seventh edition. Powers and Howley
Respiratory Factors Limiting Exercise. Annual Review of Physiology. Vol. 45: 439-451.
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