I don’t mean heavy activity, I know that causes us to get out of breath. I mean, what physically takes place in the body that causes us to get out of breath from heavy physical movement and activity?
Under normal circumstances, our bodies work aerobically; glucose is broken down in the presence of oxygen to release energy and the waste products are water and carbon dioxide.
If there isn’t enough oxygen (i.e. the level of activity is such that the lungs and circulatory system cannot supply sufficient oxygen quickly enough to the muscles, the muscles continue to ‘burn’ glucose, but differently, and the waste product is lactic acid; when the activity stops, the lactic acid must be broken down and this consumes more oxygen - this is called the ‘oxygen debt’ and is why it takes a while to recover from strenuous activity.