The respiratory system is oxygen inhaled into the body and oxygen exhaled from the body. The oxygen is carried throughout all the body systems by blood through the blood vessels. The blood is then pumped by the heart. When a person is resting the blood pumps 5 liters of blood, when the person is exercising the heart pumps 7 times the amount of blood.
The amount of air you breathe depends on how active you are. When you start to move around and exercise the muscles in your body send messages to your brain telling you that they need more oxygen. Your brain will then send signals to your muscles that control breathing so they can relax and you take fewer breaths and you can get less tired. Oxygen will be taken from your lungs and carried to the muscles that you are using for exercising.
To get more active and be fit the muscles in the body will need to produce energy.
The muscles will need oxygen to produce some of this energy. If there is too little oxygen the muscles will produce little energy. This can lead up to some problems though; your body can build up a chemical called lactic acid and can make your muscles ache. Athletes can train so that their lungs and muscles can become more efficient and it will take longer for lactic acid to build up in their bodies. People with diseases with their lungs cannot provide enough oxygen for their muscles so they cannot even walk a long distance.