The word “respiration” is often used to describe breathing, but it also describes the process by which cells convert sugar to energy. The respiratory system is the combination of external respiration, breathing, and cellular respiration.
When you inhale, the air around you comes in through your throat, the pharynx, and then passes through the larynx and trachea and enters your lungs through the bronchi. In your lungs, the air branches into smaller and smaller tubes, called bronchioles. Eventually, the air reaches alveoli at the end of the bronchioles. Here, the respiratory system is intertwined with the cardiovascular system. Capillaries surround the alveoli – the oxygen is transported from the air into the blood, and carbon dioxide is transported from your blood to the alveoli. When you exhale, you are exhaling the carbon dioxide and the non-oxygen parts of the air you inhaled. Every minute, an average adult inhales and exhales 12-15 times!
Once the oxygen is in your blood, the cardiovascular system pumps the blood through the heart and to the cells around your body. When the blood goes through capillaries near cells that need oxygen, the oxygen unbinds the blood and enters the cells. Combining the oxygen with sugar allows the cells to break down the sugars to create energy to use for many cellular processes.
Your blood exchanges the oxygen for carbon dioxide waste products from the cells and travels back to the lungs through the heart. There, the cycle starts again.