From the mouth to the lungs
The diaphragm is a large muscle that pushes and pulls the lungs to draw in and push out air. When you inhale, the diaphragm moves down, expanding the lungs and creating a lower pressure for the outside air to be sucked into. Air travels down your nasal passage and throat into your trachea (windpipe).
The trachea breaks off into two bronchial tubes, or bronchi, which connect to the lungs. In the lungs, the tubes break off into smaller passages called bronchioles. Connected to the ends of the bronchioles are little air sacs called alveoli. The alveoli are surrounded by networks of blood capillaries which serve as an interface for oxygen transport. As blood passes through these capillaries, oxygen from the alveoli seeps into the blood while carbon dioxide from the blood seeps into the alveoli.
Now that the carbon dioxide waste in your blood is in your lungs, the diaphragm relaxes. The diaphragm is pushed higher, pushing against the lungs to deflate them. Upon inhalation and exhalation, air gets filtered by the hairs in your nose and air passages. These hairs, or cilia, filter out impurities in the air and trap large particles from getting into your lungs. They keep out bacteria, viruses, dust, etc.
Beyond breathing: homeostasis
The lungs play another important role besides breathing. They help maintain homeostasis, or bodily equilibrium, by bringing the air to the correct temperature and humidity level. They also help support our ability to smell (there are smell sensors in your lungs) and protect the body from harmful substances (cilia filtration).
Chronic Obstructive Pulmonary Disease (COPD)
COPD is a lung disease that harms the alveoli, air passageways, or both. The main causes are smoking and deficiency in a certain protein (alpha-1 antitrypsin). COPD results in bronchitis, emphysema, or both. Common symptoms include coughing, shortness of breath, chest tightness, excess mucus production, and wheezing. Normal lungs have many, small alveoli that are used to collect bring oxygen to blood and remove carbon dioxide. The lungs of those with COPD have fewer, larger alveoli. This is due to the destruction of alveoli walls. Bronchioles also lose their shape and clog with mucus.