Natural selection is regarded as a theory because:

I have seen this question
b.it cannot be proved absolutely. 

Scientific skepticism always allows for unforeseen possibilities, no matter how remote.

1. when the tempature rises,your sweat glands and then springs into action making perspiration. some sweat evaporates with your skin taling heat with it.2.some of the blood pumped by the heart travels by your lungs to get rid of carbon dioxide and pick up oxygen.3.saliva produces by, you drinking water which saliva is 98% of. when mucus builds up in your body it also creates saliva as well as, antibacterial compunds.4.when the core body tempature drops, the shivering reflex is triggered to maintain homeostasis. for example: when you get out the pool you start to shiver because it is cold.