At the same time, your heart beats faster in order to drive a larger amount of blood through the lungs.

If you run too far, or wrestle too hard, your heart and your lungs both go faster and faster, until finally they reach a point when they cannot go any quicker, and the poisonous waste substances are formed in your muscles faster than they can possibly be burned up, even by the quickest breathing and the hardest pumping of your heart.

Then you begin to get "out of breath"; and if you were compelled--in order to save your life, for instance--to keep on running, or fighting, you would at last be suffocated by your own waste and dirt, and fall exhausted, or unconscious.
On the other hand, by carefully training your muscles and your heart and your lungs by exercises of various sorts in the open air, beginning with easy ones and going on to harder and longer ones, you can "improve your wind," so that your heart will be able to pump more blood through the lungs per minute, and your lungs will be able to expand themselves more fully and more rapidly without fatigue.
If you can recall having had a fever of any sort, even a slight one, such as comes with a sore throat or a bad cold, you may remember that you breathed faster and that your heart beat faster, and yet you were not doing any work with your muscles.

The cause, however, is the same; namely, the amount of waste that is being produced in the body--in this case, by the poisons (toxins) of the germs that cause the fever.

The more waste that is formed in the body, the more effort the heart and lungs will make to try to get rid of it.
The Ribs.