What Happens When We Exercise – Part 1

If you are reading this blog, then there’s a pretty good chance that you try and practice some sort of exercise on a regular basis. Have you ever stopped to think about what exactly is happening in your body while you are running, biking, lifting, rowing, swimming, climbing or even doing yoga? I’m going to take these next few posts and give a brief description of what your body does while you exercise.


The body’s cardiovascular system consists of the heart, blood and the blood vessels (arteries, capillaries and veins) that carry it through the body. Blood consists of plasma, red blood cells, white blood cells, and platelets. It is circulated by the heart through the body, carrying oxygen and nutrients to and waste materials away from all body tissues.

As the blood flows through the body, it gives up oxygen to the body tissues before it returns to the heart. A protein called hemoglobin (found in red blood cells) binds oxygen and allows it to be transported to your muscles and organs. After going back through the heart, the blood is then pumped to the lungs, where their capillaries replenish the oxygen. The oxygen-rich blood heads from the lungs back to the heart, where the process repeats. The video below does a pretty good job of explaining this process:

The terms aerobic (cardio) and anaerobic are often used to describe types of exercise. These terms come from there either being the presence (aerobic) or absence (anaerobic) of oxygen to meet energy demands of the muscles during exercise. As you exercise, muscle glycogen is broken down to produce glucose, which then reacts with oxygen to produce energy. When you begin to work out more vigorously, there is a temporary shortage of oxygen being delivered to the working muscles. During harder (anaerobic) exercise, you are breathing heavier and your body’s demand for oxygen exceeds the oxygen supply available. Anaerobic exercise is not dependent on oxygen from the air that you are breathing into your lungs. If there is a shortage of oxygen in the blood, carbohydrate is used as energy and is consumed more rapidly by the muscle.

The fitter you are, the longer you can fuel your body with the aerobic system before the anaerobic system needs to take over. You can improve your aerobic efficiency by doing specific workouts designed to raise the point at which your body switches over to anaerobic. If you have a heart rate monitor, you can do a test to roughly determine when your body moves from aerobic to anaerobic exercise. If you want a more accurate test, it can be done in a lab where blood samples are taken as you slowly ramp up exercise intensity. As you become more fit, you will be able exercise at a greater intensity while keeping your heart rate low and staying in an aerobic state.

The heart responds to exercise by increasing its size (after all, it is a muscle) and thus increasing the force at which is contracts. With more forceful contractions, the heart is able to pump blood more efficiently to your working muscles. The amount of blood that the heart pumps into the arteries over a given period of time is often a calculation that is used to measure how strong your heart is. As your heart gets stronger through exercise, your overall heart health improves in a number of ways. A lower heart rate (number of times your heart pumps per minute) both during exercise, recovery and even while resting are all indicators of a stronger heart.

In Part 2 of this discussion, I’ll explain what happens to your muscles as you exercise.

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