Breathing is an active process and supplies your body with the oxygen it needs and rids it of carbon dioxide so it can stay in balance. The more oxygen your body needs, the harder your lungs work and the more breathless you can become. When you stop being active your breathing should return to normal within a few minutes and the breathlessness should improve.
This balance depends on the how much energy your muscles and internal organs need at any one time. The oxygen you breathe in gets in to your blood through the tiny alveoli in your lungs. Carbon dioxide is a waste product of the body after the energy has been used and this is breathed out. For more information please see: How the lungs work on this website.
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Running for the bus, your oxygen needs are diverted to your muscles causing you to feel breathless.
Your brain needs a high percentage of oxygen to function if you are doing a lot of concentrated activity.
Eating a large meal causes an increase in blood flow to the stomach, this is achieved by diversion of blood from the muscles and by increasing the volume of blood pumped by the heart.
Even at rest we need a certain amount of oxygen in the blood. If you are cold or shivery you need more oxygen to generate heat.
