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The skin is a fairly difficult barrier for viruses and bacteria to cross, and is our first line of defense. Tears not only lubricate the eye, but help it to continually flush out debris. The nose helps to filter and warm the air that we breathe, not to mention the sense of smell that sometimes allows us to sense danger (fire, etc.).
The throat aids in digestion by releasing enzymes and digestive fluids that protect our gastrointestinal tract. It also keeps us from swallowing an entire mini-mart burrito in one bite, which is always wise. The digestive system selects those nutrients, fats and sugars we need to survive, and transforms them into food for our bodies, while eliminating the wastes and foreign, indigestible matter (such as the burrito, again) and therefore keeping it out of our system.
skin: a barrier for bacteria and viruses
tears: keeps the eye clear of dust, fight against bacterial infection
nose: the nose hair stops unwanted particles from entering the lungs
saliva: protects from infections in the mouth
throat: has a flap that separates the esophagus and the trachea, to prevent chocking
digestive system: Provides a way for foreign substance to come in and out of the body without causing an infection
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