Respiratory System


The respiratory system consists of organs that deliver oxygen to the circulatory system for transport to all the cells of the body. The respiratory system also assists in the removal of carbon dioxide (CO2), thus preventing a deadly buildup of this waste product in the body.


The respiratory system consists of the upper and lower respiratory tracts, extending from the nose to the lungs.

The upper respiratory tract encompasses the:

The lower respiratory tract includes the:

These organs all work together to provide air to and from the lungs. The lungs then operate in conjunction with the circulatory system to deliver oxygen and remove carbon dioxide.

Breathing is an unconscious process carried out on a continual basis and is necessary for survival. Under normal conditions, a person takes 12–20 breaths per minute, although newborns breathe at a faster rate, approximately 30–50 breaths per minute. The breathing rate set by the respiratory center can be altered by conscious control, for example, by holding the breath. This alteration occurs when the part of the brain involved in thinking, the cerebral cortex, sends signals to the diaphragm and rib muscles to momentarily ignore the signals from the respiratory center. If a person holds his or her breath too long, carbon dioxide accumulates in the blood, which then causes the blood to become more acidic. The increased acidity interferes with the action of enzymes, which are specialized proteins that coordinate all biochemical reactions in the body. To prevent too much acid from building up in the blood, special receptors located in the brainstem and in the blood vessels of the neck called chemoreceptors monitor the acid level in the blood. These chemoreceptors send nervous signals to the respiratory center when acid levels are too high, which overrides the signals from the cerebral cortex, forcing a person to exhale and then resume breathing. The blood acid level is brought back to normal levels by exhalation, which expels the carbon dioxide. Irreversible damage to tissues occurs, followed by the failure of all body systems, and ultimately, death if the respiratory system's tasks are interrupted for more than a few minutes.

Nasal passages

The primary function of the respiratory system is to deliver oxygen to the circulatory system, while removing carbon dioxide.

Filtering air through the nasal passage prevents airborne bacteria, viruses, smog, dust particles, and other potentially disease-causing substances from entering the lungs or the bronchioles. Just inside the nostrils are coarse hairs that assist in trapping airborne particles as they are inhaled. The particles then drop down onto the mucous membranes in the lining of the nasal passages. The particles are then propelled out of the nose or downward to the pharynx by the wave of mucus created by the cilia. From the pharynx, mucus is swallowed and travels to the stomach where subsequently the particles are destroyed by stomach acid. If there are more particles in the nasal passages than the cilia can cope with, a reflex will trigger a sneeze. The sneeze, designed to flush out the polluted air, is due to particles building up on the mucus and irritating the membrane below it.

Pharynx Larynx

Air passes from the pharynx to the larynx, which is approximately 2 inches (5 cm) long and situated near the middle of the neck. The larynx is comprised of several layers of cartilage, a tough and flexible tissue.

In addition to transporting air to the trachea, the larynx serves other functions:

A thin, leaf-like flap of tissue called the epiglottis prevents food and fluids from entering the larynx from the pharynx. The epiglottis is held in a vertical position, like an open trap door when a person is breathing. When swallowing, a reflex forces the larynx and the epiglottis to move toward each other. This reflex diverts food and fluids to the esophagus. The swallowing reflex may not work if one eats or drinks too rapidly or laughs while swallowing. Food or fluid enters the larynx and a coughing reflex is initiated to clear the obstruction. This situation may cause life-threatening choking if coughing does not clear the larynx of the obstruction.

Trachea, bronchi, and bronchioles

Air is passed from the larynx into the trachea, the largest airway in the respiratory system. The trachea is a tube located just below the larynx, approximately 5–6 inches (12–15 cm) long. Fifteen to 20 C-shaped rings of cartilage form the trachea. Air passes freely at all times because the trachea is held open by the sturdy cartilage rings. The open part of the C-shaped cartilage rings is situated at the back of the trachea with the ends connected by muscle tissue. The trachea branches into two tubes at its base, located just below where the neck meets the trunk of the body. These two tubes are called the left and right bronchi, and they deliver air to the left and right lungs, respectively. The bronchi branch into smaller tubes called bronchioles within the lungs. The trachea, bronchi, and the first few bronchioles are lined with mucous membranes and ciliated cells; thus, they contribute to the cleansing action of the respiratory system by moving mucus upward to the pharynx.

Alveoli and lungs

The bronchioles divide many more times in the lungs into an upside-down tree-like structure with progressively smaller branches. Tiny air sacs called alveoli are at the ends of the branches. Some of the bronchioles are no larger than 0.5 mm (0.02 inches) in diameter. The alveoli comprise most of the lung tissue, with about 150 million alveoli per lung, and resemble bunches of grapes. The alveoli send oxygen to the circulatory system while removing carbon dioxide. Alveoli have thin elastic walls, thus allowing air to flow into them when they expand; they collapse when the air is exhaled. Alveoli are arranged in clusters, and each cluster is surrounded by a dense network of capillaries. The walls of the capillaries are very thin; thus, the air in the wall of the alveoli is very near to the blood in the capillaries (only about 0.1–0.2 microns).

Oxygen diffuses from the alveoli to the capillaries because the concentration of oxygen is much higher in the alveoli than in the capillaries. From the capillaries, the oxygen flows into larger vessels and is then carried to the heart where it is pumped to the rest of the body. The forces of exhalation cause the carbon dioxide to go back up through the respiratory passages and out of the body. Numerous macrophages are interspersed among the alveoli. Macrophages are large white blood cells that remove foreign substances from the alveoli that have not been previously filtered out. The presence of the macrophages ensures that the alveoli are protected from infection; they are the last line of defense of the respiratory system.

Carbon dioxide is a waste product that is dumped into the bloodstream from the cells. It flows throughout the body in the bloodstream, then to the heart, and finally to the alveolar capillaries, so it can be expelled.

The lungs are the largest organ in the respiratory system and resemble large pink sponges. The left lung is slightly smaller than the right lung because it shares space with the heart, which is also located in the left side of the chest. Each lung is divided into lobes, with two in the left lung and three in the right. A slippery membrane called the pleura covers the lungs and lines the inside of the chest wall. It helps the lungs move smoothly during each breath. Normally, the two lubricated layers of the pleura have very little space between them. They glide smoothly over each other when the lungs expand and contract.

The diaphragm is the most important muscle involved in respiration. It lies just under the lungs and is a muscle shaped like a large dome. The sternum (or breastbone), ribs, and spine protect the lungs and the other organs in the chest. Twelve pairs of ribs curve around the chest and are joined to the vertebrae of the spine. The intercostal muscles are also important for respiration. They lie between the ribs and assist in breathing by helping to move the rib cage.


The main function of the respiratory system is the delivery of oxygen and removal of carbon dioxide. To achieve this purpose, the nervous system controls the flow of air in and out of the lungs, while maintaining a regular rate and pattern of breathing. Regulation is controlled by the respiratory center, a cluster of nerve cells in the brainstem. These cells simultaneously send signals to the muscles involved in inhalation: the diaphragm and rib muscles. The diaphragm flattens out when stimulated by a nervous impulse. The thoracic or chest cavity contains the lungs. The volume of the cavity expands with the downward movement of the diaphragm, thus expanding the lungs. The rib muscles also contract when stimulated, which pulls the rib cage up and out, at the same time expanding the thoracic cavity. This movement reduces pressure in the chest. When the volume is increased in the thoracic cavity, air rushes into the lungs to equalize the pressure. This nervous stimulation is quick, and when it is over, the diaphragm and rib muscles relax and a person exhales.

Working in conjunction with the circulatory system, the oxygen-rich blood travels from the lungs through the pulmonary veins into the left side of the heart. From there, blood is pumped to the rest of the body. Blood that is oxygen-depleted, but carbon dioxide-rich, returns to the right side of the heart through two large veins called the superior and inferior venae cavae. This blood is then pumped through the pulmonary artery to the lungs, where oxygen is picked up and carbon dioxide is released. This process is repeated continually under normal circumstances.

In addition to normal respiration, the respiratory system assists in the regulation of acid–base balance in the body, a critical process for normal cellular function. It also protects the body against toxic substances inhaled as well as against disease-causing organisms in the air. The respiratory system also assists in detecting smell using the olfactory receptors located in the nasal passages. Furthermore, it aids in producing sounds for speech.

Common problems

The diseases and disorders of the respiratory system can affect any part of the respiratory tract and may range from mild to life-threatening conditions such as:

Some of the most common symptoms of respiratory disorders are a cough, shortness of breath, chest pain, wheezing, cyanosis (bluish discoloration), finger clubbing, stridor (a crowing sound when breathing), hemoptysis (coughing up of blood), and respiratory failure. These symptoms do not necessarily signify a respiratory problem, but can be a sign of another problem. For example, chest pain may be due to a heart or a gastrointestinal problem.

Cystic fibrosis is a genetic disease that causes excessive mucus production and clogs the airways.

Acidosis is a condition resulting from higher than normal acid levels in the body fluids. It is not a disease but may be an indicator of disease. Respiratory acidosis is due to the lungs failing to remove carbon dioxide, therefore reducing the pH in the body. Several conditions such as chest injury, blockage of the upper air passages, and severe lung disease may result in respiratory acidosis. Blockage of the air passages may be due to bronchitis, asthma, or airway obstruction resulting in mild or severe acidosis. Regular, consistent retention of carbon dioxide in the lungs is referred to as chronic respiratory acidosis. This disorder results in only mild acidosis because of an increased bicarbonate (alkali) production by the kidneys.

Alkalosis is a condition resulting from a higher than normal level of base or alkali in the body fluids. Respiratory alkalosis results from decreased carbon dioxide levels caused by conditions such as hyperventilation (a faster breathing rate), anxiety, and fever. The pH becomes elevated in the body. Hyperventilation causes the body to lose excess carbon dioxide in expired air and can be triggered by altitude or a disease that reduces the amount of oxygen in the blood. Symptoms of respiratory alkalosis may include dizziness, lightheadedness, and numbing of the hands and feet. Treatments include breathing into a paper bag or a mask that induces rebreathing of carbon dioxide.


Most people who exercise know firsthand the effect that exercise has on their respiratory rate. Exercise activity necessitates interchange with the respiratory system and includes many beneficial physiological responses.

As individuals take part in calisthenics, aerobics, or other exercise activities, their metabolism increases with movement of the body, that is, the muscular, respiratory, and the circulatory system. When they work harder and increase motion, their respiration increases both in rate and depth, and their patterns of breathing may change as well. Increasing the respiratory rate expands the lungs during inhalation and exhalation of oxygen and carbon dioxide. This lung expansion necessitates active movement of the muscles of the diaphragm (expansion and contraction) as lung capacity (tidal volume) increases. Increasing body movement and breathing clears the respiratory tract and stimulates the lungs, while the cardiovascular and circulatory system circulate oxygen-rich blood throughout the body. In addition, the body temperature increases, further necessitating good respiratory function to help keep the temperature in balance.

Some exercises and physical activities that enhance the respiratory system include:

All of these exercises promote cardiovascular fitness, including improved heart function and increased heart, lung, and muscle endurance.

A dangerous condition in which the blood and body tissues are less alkaline (or more acidic) than normal.
Excessive alkalinity of the blood and body tissue.
The two tubes at the base of the trachea. Called the left and right bronchi, they extend from the trachea to deliver air to the left and right lungs, respectively. The bronchi branch into smaller tubes called bronchioles within the lungs.
Small passageways for air and carbon dioxide. The bronchioles are no larger than 0.5mm (0.02 in.) in diameter and divide many times in the lungs to form a tree-like structure; they have progressively smaller branches and tiny air sacs called alveoli at the ends.
Tiny blood vessels. In the respiratory system, they lie beneath the mucous membrane, near the surface of the nasal passages.
Carbon dioxide (CO2)—
A gaseous waste product that passes into the bloodstream from the cells. A by-product of respiration, it is released upon exhalation of air from the body.
Each epithelial cell is fringed with thousands of these tiny fingerlike extensions of the cells.
A muscle shaped like a large dome located just under the lungs.
A thin, leaflike flap of tissue that prevents food and fluids from entering the larynx from the pharynx.
A thick, moist fluid that coats epithelial cells and cilia.
The negative logarithm of H+ (hydrogen) concentration. Acid-base balance can be defined as homeostasis (equilibrium) of the body fluids at a normal arterial blood pH ranging between 7.37 and 7.43.
Thoracic cavity—
Also called the chest cavity. The portion of the body located between the neck and the diaphragm. It is enclosed by the ribs, the vertebral column, and the sternum. It is separated from the abdominal cavity by the diaphragm.

Yoga is also a popular activity that enhances the respiratory system, as participants pay particular attention to their breathing during yoga sessions. Bikram yoga is a style of hatha yoga developed by Bikram Choudhury and includes a specific series of 26 breathing exercises and asanas (body postures) designed to help move oxygenated blood throughout the entire body. Iyengar yoga also utilizes specific breathing exercises and patterns. Individuals with diminished lung function due to age-related changes, smoking, or other factors generally find the breathing activity associated with yoga to be beneficial.

Pilates is a low-impact form of exercise that requires deep breathing exercises while progressing from one movement to the next. Many individuals find the controlled, balanced, rhythmic flow of this exercise very appealing.

When weightlifting, individuals should inhale prior to moving the weight, and exhale when exerting force while moving the weight. They should take care to avoid holding their breath, as doing so prevents lung expansion and much needed oxygen from getting to muscle groups. Breath-holding may also cause an unsafe increase in blood pressure.

As always, it is important to consult with a doctor before beginning an exercise program.

A good exercise routine should start off slowly and progress moderately and at a safe degree of intensity, keeping in mind that fitness levels will change over time. Paying attention to breathing rate and levels of exertion before, during, and after exercising can help individuals decide when and how to progress to the next level of intensity, duration, and type of exercise. The American Heart Association maintains that the “talk test” is useful to consider when exercising. That is, if exercisers can talk and hold a moderate conversation while exercising, they are working at a safe and sustainable level of activity. If, however, they cannot hold a conversation or have breathing difficulty, they are working too hard. A professional trainer may be of help when putting together a fitness schedule and activity routine.

The American Heart Association recommends 30 minutes of exercise five times a week in conjunction with a heart-healthy diet. A regular exercise routine and nutritious diet will also help in weight management. Reading food labels is important to examine not only calorie, fat, carbohydrate, and protein content, but also to determine serving size in relation to these numbers. Dieticians may be of assistance in understanding these food labels.

  • What are the indications that I may have a problem with my respiratory system?
  • What diagnostic tests are needed for a thorough assessment?
  • What kind of fitness program should I follow?
  • What tests or evaluation techniques can you perform to see if my fitness and nutritional choices promote a healthy condition?
  • What treatment options do you recommend for me?
  • What physical or health limitations do you foresee?
  • What measures can be taken to prevent respiratory system problems?
  • How can my quality of life be improved?
  • What symptoms are important enough that I should seek immediate treatment?

A balanced, systematic, and routine workout offers individuals numerous fitness benefits including:

Respiratory conditions, such as allergies, asthma, and bronchitis, increase mucus and inflammation in the respiratory tract. Certain foods have been found to help reduce symptoms caused by these breathing ailments. Foods high in antioxidants, such as many fruits and vegetables, minimize free radicals in the body. Free radicals cause internal “rust” and inflammation to the cells, and foods high in free radicals, such as green leafy vegetables, citrus fruits, and berries, help decrease lung inflammation. Warm (not hot) fluids, such as light soups and teas, may help loosen mucus and soothe the respiratory tract. Milk or cream–based products should be avoided as they thicken mucus, making the respiratory tract more difficult to clear.

Making good nutritional choices and developing a healthy exercise regimen is beneficial to the respiratory system and the entire body as a whole. Individuals should make small, incremental changes in the look and feel of their bodies. Results will not occur overnight, so paying attention to one's level of energy, changes in physique, and the fit of clothing can spur exercisers on, rather than looking at numbers on a scale. Other rewards may be seen in better blood pressure readings and blood lab values during visits to the doctor. A strong, effective respiratory system will support an individual's wellness efforts. A well-balanced fitness and nutrition regimen can improve not only the way people look and feel, but can also have a positive impact on the health and quality of their lives.



Miller, Mark D., and Stephen R. Thompson, eds. DeLee and Drez's Orthopaedic Sports Medicine. 4th ed. Philadelphia, PA: Saunders, 2015.

Murray, John Frederick, et al. Murray & Nadel's Textbook of Respiratory Medicine. 6th ed. Philadelphia, PA: Elsevier Saunders, 2016.

Safran, Marc R., et al. Instructions for Sports Medicine Patients. Philadelphia, PA: Elsevier Saunders, 2012.

Seidenberg, Peter H., and Anthony I. Beutler, eds. Sports Medicine Resource Manual. Philadelphia, PA: Elsevier Saunders, 2008.


Güell Rous, et al. “Pulmonary Rehabilitation.” Archives of Bronchology 50, no. 8 (August 2014): 332–44.

Koubaa, A., et al. “Effect of Low-Intensity Continuous Training on Lung Function and Cardiorespiratory Fitness in Both Cigarette and Hookah Smokers.” African Health Sciences 15, no. 4 (December 1, 2015): 1170–81.

McKenzie Donald. “Respiratory Physiology: Adaptations to High-Level Exercise.” British Journal of Sports Medicine 46, no. 6 (May 2012): 381–4.


American Heart Association, 7272 Greenville Ave., Dallas, TX, 75231, (800) 242-8721, .

American Medical Association, 330 N. Wabash Ave., Ste. 39300, Chicago, IL, 60611 -5885, (800) 621-8335, .

National Heart, Lung, and Blood Institute, PO Box 30105, Bethesda, MD, 20824-0105, (301) 592-8573, Fax: (240) 629-3246,, .

National Institutes of Health (NIH), 9000 Rockville Pike, Bethesda, MD, 20892, (301) 496-4000, .

US National Library of Medicine, 8600 Rockville Pike, Bethesda, MD, 20894, (888) 346-3656, .

Crystal Heather Kaczkowski, MSc
Revised by Laura Jean Cataldo, RN, EdD

  This information is not a tool for self-diagnosis or a substitute for professional care.