What goes on inside your body when you pedal a bike or take a stroll? These activities set off complicated physical processes that affect nearly every organ system. When you exercise several times a week or more, your body adapts so you’re able to do so more efficiently. Knowing about this process will help you understand why physical activity has so many benefits.
Energy to Burn
Like all machinery, your muscles must have fuel. This fuel comes from the food you eat and your body’s reserves of fat and glucose. The catch is that nutrients from food cannot be turned directly into usable energy for the trillions of cells in your body. Each cell has one primary source of energy: a molecule called Adenosine Triphosphate (ATP). Your body’s ability to create ATP is critical because it determines your capacity for physical exertion. And the reverse is also true: your physical conditioning influences how well you can generate ATP.
Body Needs to Extract that Energy
The food you eat contains energy stored in a variety of forms — proteins, fats, and carbohydrates. Your body needs to extract that energy and capture it in the form of ATP. To do this, your stomach and small intestine break the food into millions of tiny molecules, which enter the bloodstream and find their way to every cell in the body (see the figure). There, in small cell structures called mitochondria, the food molecules undergo a series of chemical reactions that ultimately lead to the creation of ATP.
Your body stores only a small amount of ATP, but makes it as quickly as it’s needed. When demand increases — such as when you are exercising — your body must churn out more. To do this, it taps into glucose stored in the muscle and liver and fats from various places in the body. These substances make their way through the bloodstream to the muscles.
Aerobic processes produce
Stored glucose (also known as glycogen) and fat can be broken down for ATP production in two ways: aerobic (requiring oxygen) and anaerobic (requiring no oxygen). Aerobic processes produce more ATP, but grind to a halt without oxygen. When your body is working so hard that it is unable to deliver enough oxygen to support aerobic metabolism of food for fuel, it switches to anaerobic production of ATP, which creates a byproduct known as lactic acid. The lactic acid enters the bloodstream, creating an acid imbalance. To compensate, your breathing speeds up to take in more oxygen and your heart beats faster to move that oxygen to your muscles.
But you can’t sustain anaerobic activity. Your lungs and heart reach their maximum work efforts, and your body can only neutralize the resulting acid imbalance for a short time. The lactic acid generated from the anaerobic process also leaves muscles feeling fatigued. Eventually, you need to slow down. By doing so, you are able to take in enough oxygen that once again you can rely primarily on aerobic production of ATP. Lactic acid production stops, the muscles start to recover, and your body restores normal acid balance.
Regular Exercise Conditions the Lungs
Your level of fitness determines how swiftly this happens. Regular exercise conditions the lungs, heart, and blood vessels, enabling them to deliver oxygen to muscle cells more quickly and efficiently. Walking up a hill with a fitter friend illustrates this nicely. While you’re still huffing and puffing, your friend isn’t struggling to catch her breath. When you engage in physical activity, your body doesn’t rely solely on one process or the other; both are used to generate ATP, but one more so than the other. Because of this distinction, exercise is classified into two broad categories — aerobic and anaerobic — depending on which process is predominantly used for ATP production. If the intensity of exercise is such that your lungs and heart are able to supply oxygen for energy production, then the activity is almost exclusively aerobic. But if intensity rises so that demand for oxygen outstrips supply, then the activity becomes anaerobic. Walking, jogging, cycling, or swimming at an even pace are aerobic activities. Activities in which your body tends to go anaerobic more quickly include wind sprints and weight lifting.
Food, oxygen, and energy
Once the food you eat is digested in your stomach, its components are absorbed into your bloodstream and delivered to cells throughout the body. Oxygen from your lungs also travels to your cells, where tiny structures called mitochondria use it to convert the food nutrients into a chemical called adenosine triphosphate (ATP), which provides energy for everything from walking to thinking.
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