The process of taking in oxygen and expelling carbon dioxide is called respiration. When you breathe in, oxygen from the air enters the bloodstream. When you breathe out, waste carbon dioxide is removed.
Air enters the body through the nose and mouth and travels to the pharynx (throat). The pharynx splits into the esophagus for food, and the trachea (windpipe) for air. The trachea leads to the chest, where it divides into two branches, or bronchi, one to each lung.
The inner lining of the airways constantly produces a sticky fluid, called mucus, to trap particles of dirt. The mucus is always moving. In the nose, it is pushed by cilia toward the back of the throat, where it is swallowed. Mucus also moves up from the airways of the lungs to the throat to be swallowed.
Breathing is essential to keep us alive, because every living cell in the body needs a continual supply of oxygen. Inside each cell, oxygen combines with food molecules in a chemical reaction called oxidation, which releases energy. This energy powers every process in the human body.
The larynx is made up of two pieces of cartilage at the top of the trachea and the two flaps of tissue, called vocal cords, stretched across them. If the vocal cords are open, air passes silently through them. If drawn together, air makes the cords vibrate, creating sound. The tighter the vocal cords, the higher the pitch. The mouth and tongue alter the sound to make words.