I. RESPIRATORY SYSTEM
For every part of your body to stay active and functionable, they need the nutrients from the food you eat and the oxygen from the air you breathe. As oxygen is breathed in, carbon dioxide is released. The respiratory system in our body helps deliver oxygen to the red blood cells and remove the waste product, carbon dioxide.
The respiration process begins when the air penetrates through your nostrils. Dust from the air is filtered out by the thick hair in your nostrils. Further down in the nasal cavity and sinus, bacteria in the air are mostly removed by special hairs and special secretions. (Whenever you catch a cold, the membranes of the sinus cavities swell up to prevent infection. The nasal canals are then blocked, and you will find it hard to breathe.) Air then draws through the pharynx and travels down the trachea. The epiglottis (located at the top of the trachea) remains open when you are breathing.

Your trachea is not just as simple as a pipe. It is composed of cartilaginous rings that are covered with cilia (tiny hairs which help filter particulates). These hairs push dust back to the top of the trachea. The bottom of this big windpipe divides into two bronchi that go to your lungs respectively. Your lungs are spongy and elastic organs that are protected by moist membranes. The bronchi are similar to trees - they extend into smaller and smaller branches; at the very end of each branch, there is a moist tiny air sac called the alveolus. In your lungs, there are 300,000,000 air sacs!  Being surrounded by a network of tiny blood vessels, these alveoli (plural), with membranes as thin as a single layer of cells, are where the oxygen and carbon dioxide are exchanged.