Whether you're wide awake and getting ready for a big date or asleep during your most snooze-worthy afternoon class, you don't have to think about breathing. It's so important to life that it happens automatically. If you didn't breathe, you couldn't live.

Lungs & Respiratory System Basics

Each day we breathe about 20,000 times. All of this breathing couldn't happen without help from the respiratory system, which includes the nose, throat, voice box, windpipe, and lungs. With each breath, you take in air through your nostrils and mouth, and your lungs fill up and empty out. As air is inhaled, the mucous membranes of the nose and mouth warm and humidify the air.

Although we can't see it, the air we breathe is made up of several gases. Oxygen is the most important for keeping us alive because body cells need it for energy and growth. Without oxygen, the body's cells would die.

Carbon dioxide is the waste gas that is produced when carbon is combined with oxygen as part of the body's energy-making processes. The lungs and respiratory system allow oxygen in the air to be taken into the body, while also enabling the body to get rid of carbon dioxide in the air breathed out.

Respiration is the term for the exchange of oxygen from the environment for carbon dioxide from the body's cells. The process of taking air into the lungs is called inhalation or inspiration, and the process of breathing it out is called exhalation or expiration.

Even if the air you breathe is dirty or polluted, your respiratory system filters out foreign matter and organisms that enter through the nose and mouth. Pollutants are breathed or coughed out, destroyed by digestive juices, or eaten by macrophages, a type of blood cell that patrols the body looking for germs to destroy.

Tiny hairs called cilia (pronounced: sih-lee-uh) protect the nasal passageways and other parts of the respiratory tract, filtering out dust and other particles that enter the nose with the breathed air. As air is inhaled, the cilia move back and forth, pushing any foreign matter (like dust) either toward the nostrils, where it is blown out, or toward the pharynx, where it travels through the digestive system and out with the rest of the body's waste.

What They Do

The two openings of the airway (the nasal cavity and the mouth) meet at the pharynx (pronounced: far-inks), or throat, at the back of the nose and mouth. The pharynx is part of the digestive system as well as the respiratory system because it carries both food and air. At the bottom of the pharynx, the pathway for both food and air divides in two. One passageway is for food (the esophagus, pronounced: ih-sah-fuh-gus, which leads to the stomach) and the other for air. The epiglottis (pronounced: eh-pih-glah-tus), a small flap of tissue, covers the air-only passage when we swallow, keeping food and liquid from going into our lungs.

The larynx (pronounced: lar-inks), or voice box, is the uppermost part of the air-only passage. This short tube contains a pair of vocal cords, which vibrate to make sounds. The trachea (pronounced: tray-kee-uh), or windpipe, extends downward from the base of the larynx. It lies partly in the neck and partly in the chest cavity. The walls of the trachea are strengthened by stiff rings of cartilage to keep it open so air can flow through on its way to the lungs. The trachea is also lined with cilia, which sweep fluids and foreign particles out of the airway so that they stay out of the lungs.

At its bottom end, the trachea divides into left and right air tubes called bronchi (pronounced: brahn-ky), which connect to the lungs. Within the lungs, the bronchi branch into smaller bronchi and even smaller tubes called bronchioles (pronounced: brahn-kee-olz). Bronchioles, which are as thin as a strand of hair, end in tiny air sacs called alveoli (pronounced: al-vee-oh-lie). Each of us has hundreds of millions of alveoli in our lungs — enough to cover a tennis court if they were spread out on the ground. The alveoli are where the exchange of oxygen and carbon dioxide takes place.

With each inhalation, air fills a large portion of the millions of alveoli. In a process called diffusion (pronounced: dih-fyoo-zhun), oxygen moves from the alveoli to the blood through the capillaries (tiny blood vessels, pronounced: kah-puh-ler-eez) that line the alveolar walls. Once in the bloodstream, oxygen gets picked up by a molecule called hemoglobin (pronounced: hee-muh-glo-bun) in the red blood cells. This oxygen-rich blood then flows back to the heart, which pumps it through the arteries to oxygen-hungry tissues throughout the body.

In the tiny capillaries of the body tissues, oxygen is freed from the hemoglobin and moves into the cells. Carbon dioxide, which is produced during the process of diffusion, moves out of these cells into the capillaries, where most of it is dissolved in the plasma of the blood. Blood rich in carbon dioxide then returns to the heart via the veins. From the heart, this blood is pumped to the lungs, where carbon dioxide passes into the alveoli to be exhaled.

The lungs also contain elastic tissues that allow them to inflate and deflate without losing shape and are encased by a thin lining called the pleura (pronounced: plur-uh). This network of alveoli, bronchioles, and bronchi is known as the bronchial tree.

The chest cavity, or thorax (pronounced: thor-aks), is the airtight box that houses the bronchial tree, lungs, heart, and other structures. The top and sides of the thorax are formed by the ribs and attached muscles, and the bottom by a large muscle called the diaphragm. The chest walls form a protective cage around the lungs and other contents of the chest cavity.

The diaphragm (pronounced: dye-uh-fram), which separates the chest from the abdomen, plays a lead role in breathing. When we breathe out, the diaphragm moves upward, forcing the chest cavity to get smaller and pushing the gases in the lungs up and out of the nose and mouth.

When we breathe in, the diaphragm moves downward toward the abdomen, and the rib muscles pull the ribs upward and outward, enlarging the chest cavity and pulling air in through the nose or mouth. Air pressure in the chest cavity and lungs is reduced, and because gas flows from high pressure to low, air from the environment flows through the nose or mouth into the lungs.

As we exhale, the diaphragm moves upward and the chest wall muscles relax, causing the chest cavity to contract. Air pressure in the lungs rises, so air flows from the lungs and up and out of respiratory system through the nose or mouth.

Things That Can Go Wrong

Many factors — including genetics, pollutants and irritants, and infectious diseases — can affect the health of your lungs and respiratory system and cause respiratory problems. Problems of the respiratory system that can affect people during their teen years include:

Asthma. More than 20 million people have asthma in the United States, and it's the #1 reason that kids and teens chronically miss school. Asthma is a long-term, inflammatory lung disease that causes airways to tighten and narrow when a person with the condition comes into contact with irritants such as cigarette smoke, dust, or pet dander.

Bronchitis. Although bronchitis doesn't affect most teens, it can affect those who smoke. In bronchitis, the membranes lining the larger bronchial tubes become inflamed and an excessive amount of mucus is produced. The person with bronchitis develops a bad cough to get rid of the mucus.

Common cold. Colds are caused by over 200 different viruses that cause inflammation in the upper respiratory tract. The common cold is the most common respiratory infection. Symptoms may include a mild fever, cough, headache, runny nose, sneezing, and sore throat.

Cough. A cough is a symptom of an illness, not an illness itself. There are many different types of cough and many different causes, ranging from not-so-serious to life threatening. Some of the more common causes affecting kids and teens are the common cold, asthma, sinusitis, seasonal allergies, and pneumonia.

Cystic fibrosis (CF). CF is an inherited disease affecting the lungs. CF causes mucus in the body to be abnormally thick and sticky. The mucus can clog the airways in the lungs and make a person more likely to get bacterial infections.

Pneumonia. Pneumonia is an inflammation of the lungs, which usually occurs because of infection with a bacteria or virus. Pneumonia causes fever, inflammation of lung tissue, and makes breathing difficult because the lungs have to work harder to transfer oxygen into the bloodstream and remove carbon dioxide from the blood. Common causes of pneumonia are influenza and infection with the bacterium Streptococcus pneumoniae.

Although some respiratory diseases like asthma or cystic fibrosis can't be prevented, you can prevent many chronic lung and respiratory illnesses by avoiding smoking, staying away from pollutants and irritants, washing your hands often to avoid infection, and getting regular medical checkups.

Reviewed by: Yamini Durani, MD
Date reviewed: October 2012