How Exercise Works

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What Happens When You Exercise

So imagine that you start running. Here’s what happens:

  • The muscle cells burn off the ATP they have floating around in about 3 seconds.
  • The phosphagen system kicks in and supplies energy for 8 to 10 seconds. This would be the major energy system used by the muscles of a 100-meter sprinter or weight lifter, where rapid acceleration, short-duration exercise occurs.
  • If exercise continues longer, then the glycogen-lactic acid system kicks in. This would be true for short-distance exercises such as a 200- or 400-meter dash or 100-meter swim.
  • Finally, if exercise continues, then aerobic respiration takes over. This would occur in endurance events such as an 800-meter dash, marathon run, rowing, cross-country skiing and distance skating.

When you start to look closely at how the human body works, it is truly an amazing machine!

If you are going to be exercising for more than a couple of minutes, your body needs to get oxygen to the muscles or the muscles will stop working. Just how much oxygen your muscles will use depends on two processes: getting blood to the muscles and extracting oxygen from the blood into the muscle tissue. Your working muscles can take oxygen out of the blood three times as well as your resting muscles. Your body has several ways to increase the flow of oxygen-rich blood to working muscle:

  • increased local blood flow to the working muscle
  • diversion of blood flow from nonessential organs to the working muscle
  • increased flow of blood from the heart (cardiac output)
  • increased rate and depth of breathing
  • increased unloading of oxygen from hemoglobin in working muscle

These mechanisms can increase the blood flow to your working muscle by almost five times. That means that the amount of oxygen available to the working muscle can be increased by almost 15 times!

Exercise and Increased Blood Flow

Making the Pipe Bigger

As you exercise, the blood vessels in your muscles dilate and the blood flow is greater, just as more water flows through a fire hose than through a garden hose. Your body has an interesting way of making those vessels expand. As ATP gets used up in working muscle, the muscle produces several metabolic byproducts (such as adenosine, hydrogen ions and carbon dioxide). These byproducts leave the muscle cells and cause the capillaries (small, thin-walled blood vessels) within the muscle to expand or dilate (vasodilation). The increased blood flow delivers more oxygenated blood to the working muscle.

Exercising the Heart and Lungs

Making the Heart Pump Harder

Your heart, also a muscle, gets a workout during exercise, too, and its job is to get more blood out to the body’s hard-working muscles. The heart’s blood flow increases by about four or five times from that of its resting state. Your body does this by increasing the rate of your heartbeat and the amount of blood that comes through the heart and goes out to the rest of the body. The rate of blood pumped by the heart (cardiac output) is a product of the rate at which the heart beats (heart rate) and the volume of blood that the heart ejects with each beat (stroke volume). In a resting heart, the cardiac output is about 5 liters a minute (0.07 L x 70 beats/min = 4.9 L/min). As you begin to exercise, sympathetic nerves stimulate the heart to beat with more force and faster; the heart rate can increase about threefold. Also, the sympathetic nerve stimulation to the veins causes them to constrict. This, along with more blood being returned from the working muscles, increases the amount of blood returned to the heart (venous return). The increased venous return helps to increase the stroke volume by about 30 to 40 percent. When the heart is pumping at full force, the cardiac output is about 20-25 liters per minute.

Breathing Faster and Deeper

So far, we have talked about getting more blood to working muscle. Your lungs and the rest of your respiratory system need to provide more oxygen for the blood, too. The rate and depth of your breathing will increase because of these events:

  • Sympathetic nerves stimulate the respiratory muscles to increase the rate of breathing.
  • Metabolic byproducts from muscles (lactic acid, hydrogen ions, carbon dioxide) in the blood stimulate the respiratory centers in the brainstem, which, in turn, further stimulates the respiratory muscles.
  • Slightly higher blood pressure, caused by the increased force of each heartbeat and by the elevated cardiac output, opens blood flow to more air sacs (alveoli) in the lungs. This increases the ventilation and allows more oxygen to enter the blood.

As the lungs absorb more oxygen and the blood flow to the muscles increases, your muscles have more oxygen.

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