Your Heart
When exercising in a cold environment the muscle will be cold and oxygen and blood will not flow smoothly during exercise. It is important to dress warmly and warm-up before exercising to prevent injury.
Your heart may need to work harder when your body requires more oxygen. For instance, if you are exercising, your heart rate increases to keep up with the muscles' oxygen requirements.
Overload in PE is the principle of training that states we must work our body systems harder than usual in order for them to be improved.
(My guess is that) When you're running your muscles have to work harder, so the muscles in your heart start to work faster & pumping more blood. But you're breathing isn't exactly a muscle so you start to breathe harder trying to catch up with the rest of your body.
Body systems work together to keep the body healthy and the body systems are interdependent (they rely on each other).
When you are active, your respiratory and cardiovascular systems work harder to deliver oxygen and nutrients to your muscles. Your body temperature increases as metabolism rises. When you rest, these systems slow down, allowing your body to repair and recover from the physical stress of activity.
when you stop exercising you start to get muscle break down, this is called atrophy. This is why if you do not work out for a long time then start again you will find it harder than you thought.
Cold. Your body has to work harder to warm you up, therefore you burn more calories
all systems
Try exercising to allow your body work normally.
That is the organismal level. Many organisms have several organ systems.
All the systems of the body work together; there are no isolated systems.