The human respiratory system is a series of organs responsible for taking in oxygen and expelling carbon dioxide. The primary organs of the respiratory system are lungs, which carry out this exchange of gases as we breathe.

Red blood cells collect the oxygen from the lungs and carry it to the parts of the body where it is needed, according to the American Lung Association. During the process, the red blood cells collect the carbon dioxide and transport it back to the lungs, where it leaves the body when we exhale.

The human body needs oxygen to sustain itself. A decrease in oxygen is known as hypoxia and a complete lack of oxygen is known as anoxia and, according to MedLine Plus. These conditions can be fatal; after about four minutes without oxygen, brain cells begin dying, according to NYU Langone Medical Center, which can lead to brain damage and ultimately death.

In humans, the average rate of breathing is dependent upon age. A newborn's normal breathing rate is about 40 times each minute and may slow to 20 to 40 times per minute when the baby is sleeping, according to the Children's Hospital of Philadelphia. 

For adults, the average resting respiratory rate for adults is 12 to 16 breaths per minute, according to Johns Hopkins Medicine. Physical exertion also has an effect on respiratory rate, and healthy adults can average 45 breaths per minute during strenuous exercise.

Parts of the respiratory system

As we breathe, oxygen enters the nose or mouth and passes the sinuses, which are hollow spaces in the skull. Sinuses help regulate the temperature and humidity of the air we breathe. 

The trachea, also called the windpipe, filters the air that is inhaled, according to the American Lung Association. It branches into the bronchi, which are two tubes that carry air into each lung. The bronchial tubes are lined with tiny hairs called cilia. Cilia move back and forth, carrying mucus up and out. Mucus, a sticky fluid, collects dust, germs and other matter that has invaded the lungs. We expel mucus when we sneeze, cough, spit or swallow.

The bronchial tubes lead to the lobes of the lungs. The right lung has three lobes; the left lung has two, according to the American Lung Association. The left lung is smaller to allow room for the heart, according to the National Heart, Lung and Blood Institute (NHLBI). Lobes are filled with small, spongy sacs called alveoli, and this is where the exchange of oxygen and carbon dioxide occurs. 

The alveolar walls are extremely thin (about 0.2 micrometers). These walls are composed of a single layer of tissues called epithelial cells and tiny blood vessels called pulmonary capillaries.

Blood passes through the capillaries. The pulmonary artery carries blood containing carbon dioxide to the air sacs, where the gas moves from the blood to the air, according to the NHLBI. Oxygenated blood goes to the heart through the pulmonary vein, and the heart pumps it throughout the body.

The diaphragm, a dome-shaped muscle at the bottom of the lungs, controls breathing and separates the chest cavity from the abdominal cavity, the American Lung Association noted. When a breath it taken, it flattens out and pulls forward, making more space for the lungs. During exhalation, the diaphragm expands and forces air out.

Diseases of the respiratory system

Diseases and conditions of the respiratory system fall into two categories:  Viruses such as influenza, bacterial pneumonia and the new enterovirus respiratory virus that has been diagnosed in children; and chronic diseases, such as asthma and chronic obstructive pulmonary disease (COPD). According to Dr. Neal Chaisson, who practices pulmonary medicine at the Cleveland Clinic, there is not much that can be done for viral infections but to let them run their course. "Antibiotics are not effective in treating viruses and the best thing to do is just rest," he said. 

COPD is the intersection of three related conditions — chronic bronchitis, chronic asthma and emphysema, Chaisson told Live Science. It is a progressive disease that makes it increasingly difficult for sufferers to breath.

Asthma is a chronic inflammation of the lung airways that causes coughing, wheezing, chest tightness or shortness of breath, according to Tonya Winders, president of the Allergy & Asthma Network. These signs and symptoms may be worse when a person is exposed to their triggers, which can include air pollution, tobacco smoke, factory fumes, cleaning solvents, infections, pollens, foods, cold air, exercise, chemicals and medications. 

Lung cancer is often associated with smoking, but the disease can affect non-smokers as well. Every year, about 16,000 to 24,000 Americans die of lung cancer, even though they have never smoked, according to the American Cancer Society. Like all cancers, lung cancer is caused by the uncontrolled growth of abnormal cells.

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Diagnosing and treating respiratory ailments

Pulmonologists, a subspecialty of internal medicine, treat the respiratory system, including the lungs, according to the American College of Physicians. Because of the critical nature of the respiratory system, pulmonologists work in hospitals as well as in private practice. A pulmonologist must first be certified by the American Board of Internal Medicine and then obtain additional training in the subspecialty.

Common diagnostic tools for diagnosing respiratory disease include chest X-rays and a pulmonary function test (PFT), according to Merck Manuals. A PFT measures how well the lungs take in and release air and how well they circulate oxygen. 

A doctor may also perform a bronchoscopy by inserting a tube with a light and camera into the airways — the trachea and the bronchial tubes — to examine for bleeding, tumors, inflammation or other abnormalities, according to the NHLBI. A similar procedure is a thoracoscopy, in which a doctor uses an optical device to examine the surfaces of the lungs. 

A physician may order a PFT as part of a routine exam — especially for smokers, according to the University of Pittsburgh Medical Center. A PFT may also be ordered to test lung function before surgery or to help diagnose lung conditions or diseases.

For most healthy individuals, the most common respiratory ailment they may face is an infection, according to Dr. Matthew Exline, a pulmonologist and critical care expert at The Ohio State University Wexner Medical Center. A cough is the first symptom, possibly accompanied by a fever.

"However, cough can be a sign of chronic respiratory conditions such as asthma, chronic bronchitis or emphysema," he said. "In chronic lung disease, most respiratory diseases present with shortness of breath, initially with exertion, such as walking a significant distance or climbing several flights of stairs."

The most certain way to diagnose asthma is with a lung function test, a medical history and a physical exam, according to Winders. "However, it's hard to do lung function tests in children younger than 5 years. Thus, doctors must rely on children's medical histories, signs and symptoms, and physical exams to make a diagnosis."

For COPD, many patients benefit from respiratory rehabilitation, according to Dr. Brian Carlin, assistant professor of medicine at Drexel University College of Medicine. "It is much like cardiac rehabilitation for heart patients, and can provide education, exercise and training to reduce the number of respiratory incidents."