How the Respiratory System Works
The goal of the respiratory system is to exchange gases between your body’s cells and the atmosphere. Oxygen goes in and carbon dioxide, a by-product of your cells’ metabolic actions, comes out. This happens during inhalation, the process in which air enters your body from the atmosphere and travels to the lungs. Conversely, exhalation is the process in which air leaves your lungs and enters the atmosphere.
The first step of inhalation is the contraction of your diaphragm and other muscles to allow air to enter your body. The coupling of the lungs to the chest wall (inside the rib cage) creates a vacuum that pulls in air from the environment to refill your lungs. The air that enters from the atmosphere travels through your nasal and oral cavities, down your pharynx to your larynx, through your trachea and into your bronchi. As the air travels down these pathways, the cilia and mucous lining clean and filter the air to remove particles and anything harmful. From the bronchi, the air enters the bronchioles and alveolar ducts, until it reaches your alveoli. The alveoli are the site of gas exchange.
After inhaling, the alveoli contain a high concentration of oxygen and a low concentration of carbon dioxide. To enter your red blood cells, oxygen diffuses from an area of high oxygen concentration in the alveoli to an area of low oxygen concentration in the red blood cells.
Once in your red blood cells, the oxygen binds to the chemical hemoglobin. Hemoglobin carries oxygen to all of your tissues and picks up your cells’ unwanted carbon dioxide. By the process of diffusion, the oxygen moves from your oxygen-rich red blood cells to your oxygen-depleted tissue cells, while carbon dioxide travels from your body’s tissue cells, into your red blood cells, and then out again as bicarbonate in the plasma.
At the alveoli, carbon dioxide diffuses out of your red blood cells and into your alveoli for removal from the body. At the same time, oxygen diffuses out of the alveoli and into your red blood cells, and the process begins again.
To exhale, your diaphragm relaxes, causing the elastic lung tissue to relax and force air out of your body. The air is released from your alveoli, up your bronchial tree, through your larynx and pharynx, and out your nasal and oral cavities. This entire process is called the respiratory cycle.
The process of breathing is controlled by the respiratory center located in your brain, and it is regulated by your autonomic nervous system that controls unconscious body functions. This means that you do not have to think about breathing, it just happens. It is possible to momentarily stop breathing if you make a conscious effort to do so, but your brain will override such actions when the amount of carbon dioxide in your blood increases and you need air again.