When the respiratory system is mentioned, people generally think of breathing, but breathing is only one of the activities of the respiratory system. The body cells need a continuous supply of oxygen for the metabolic processes that are necessary to maintain life. The respiratory system works with the circulatory system to provide this oxygen and to remove the waste products of metabolism. It also helps to regulate pH of the blood.

Respiration is the sequence of events that results in the exchange of oxygen and carbon dioxide between the atmosphere and the body cells. Every 3 to 5 seconds, nerve impulses stimulate the breathing process, or ventilation, which moves air through a series of passages into and out of the lungs. After this, there is an exchange of gases between the lungs and the blood. This is called external respiration. The blood transports the gases to and from the tissue cells. The exchange of gases between the blood and tissue cells is internal respiration. Finally, the cells utilize the oxygen for their specific activities. This is cellular metabolism, or cellular respiration. Together these activities constitute respiration


Ventilation, or breathing, is the movement of air through the conducting passages between the atmosphere and the lungs. The air moves through the passages because of pressure gradients that are produced by contraction of the diaphragm and thoracic muscles.

Pulmonary ventilation

Pulmonary ventilation is commonly referred to as breathing. It is the process of air flowing into the lungs during inspiration (inhalation) and out of the lungs during expiration (exhalation). Air flows because of pressure differences between the atmosphere and the gases inside the lungs.

Air, like other gases, flows from a region with higher pressure to a region with lower pressure. Muscular breathing movements and recoil of elastic tissues create the changes in pressure that result in ventilation. Pulmonary ventilation involves three different pressures:

    • Atmospheric pressure
    • Intraalveolar (intrapulmonary) pressure
    • Intrapleural pressure

Atmospheric pressure is the pressure of the air outside the body. Intraalveolar pressure is the pressure inside the alveoli of the lungs. Intrapleural pressure is the pressure within the pleural cavity. These three pressures are responsible for pulmonary ventilation.


Inspiration (inhalation) is the process of taking air into the lungs. It is the active phase of ventilation because it is the result of muscle contraction. During inspiration, the diaphragm contracts and the thoracic cavity increases in volume. This decreases the intraalveolar pressure so that air flows into the lungs. Inspiration draws air into the lungs.


Expiration (exhalation) is the process of letting air out of the lungs during the breathing cycle. During expiration, the relaxation of the diaphragm and elastic recoil of tissue decreases the thoracic volume and increases the intraalveolar pressure. Expiration pushes air out of the lungs.

Under normal conditions, the average adult takes 12 to 15 breaths a minute. A breath is one complete respiratory cycle that consists of one inspiration and one expiration.
An instrument called a spirometer is used to measure the volume of air that moves into and out of the lungs, and the process of taking the measurements is called spirometry. Respiratory (pulmonary) volumes are an important aspect of pulmonary function testing because they can provide information about the physical condition of the lungs.

Respiratory capacity (pulmonary capacity) is the sum of two or more volumes.

Factors such as age, sex, body build, and physical conditioning have an influence on lung volumes and capacities. Lungs usually reach their maximumin capacity in early adulthood and decline with age after that.

The respiratory conducting passages are divided into the upper respiratory tract and the lower respiratory tract. The upper respiratory tract includes the nose, pharynx, and larynx. The lower respiratory tract consists of the trachea, bronchial tree, and lungs. These tracts open to the outside and are lined with mucous membranes. In some regions, the membrane has hairs that help filter the air. Other regions may have cilia to propel mucus.

Click a menu item listed below to learn more about a component(s) of the conducting passages.

Nose and Nasal Cavities

The framework of the nose consists of bone and cartilage. Two small nasal bones and extensions of the maxillae form the bridge of the nose, which is the bony portion. The remainder of the framework is cartilage and is the flexible portion. Connective tissue and skin cover the framework.

Air enters the nasal cavity from the outside through two openings, the nostrils, or external nares. The openings from the nasal cavity into the pharynx are the internal nares. Nose hairs at the entrance to the nose trap large inhaled particles.

Paranasal Sinuses

Paranasal sinuses are air-filled cavities in the frontal, maxilae, ethmoid, and sphenoid bones. These sinuses, which have the same names as the bones in which they are located, surround the nasal cavity and open into it. They function to reduce the weight of the skull, to produce mucus, and to influence voice quality by acting as resonating chambers.

The pharynx, commonly called the throat, is a passageway that extends from the base of the skull to the level of the sixth cervical vertebra. It serves both
the respiratory and digestive systems by receiving air from the nasal cavity and air, food, and water from the oral cavity. Inferiorly, it opens into the larynx and esophagus. The pharynx is divided into three regions according to location: the nasopharynx, the oropharynx, and the laryngopharynx (hypopharynx).

The nasopharynx is the portion of the pharynx that is posterior to the nasal cavity and extends inferiorly to the uvula. The oropharynx is the portion of the pharynx that is posterior to the oral cavity. The most inferior portion of the pharynx is the laryngopharynx that extends from the hyoid bone down to the lower margin of the larynx.

The upper part of the pharynx (throat) lets only air pass through. Lower parts permit air, foods, and fluids to pass.

The pharyngeal, palatine, and lingual tonsils are located in the pharynx. They are also called Waldereyer's Ring.

The retromolar trigone is the small area behind the wisdom teeth.


The larynx, commonly called the voice box or glottis, is the passageway for air between the pharynx above and the trachea below. It extends from the fourth to the sixth vertebral levels. The larynx is often divided into three sections: sublarynx, larynx, and supralarynx. It is formed by nine cartilages that are connected to each other by muscles and ligaments.


The larynx plays an essential role in human speech. During sound production, the vocal cords close together and vibrate as air expelled from the lungs passes between them. The false vocal cords have no role in sound production, but help close off the larynx when food is swallowed.

The thyroid cartilage is the Adam's apple. The epiglottis acts like a trap door to keep food and other particles from entering the larynx.


The trachea, commonly called the windpipe, is the main airway to the lungs. It divides into the right and left bronchi at the level of the fifth thoracic vertebra, channeling air to the right or left lung.

The hyaline cartilage in the tracheal wall provides support and keeps the trachea from collapsing. The posterior soft tissue allows for expansion of the esophagus, which is immediately posterior to the trachea.

The mucous membrane that lines the trachea is ciliated pseudostratified columnar epithelium similar to that in the nasal cavity and nasopharynx. Goblet cells produce mucus that traps airborne particles and microorganisms, and the cilia propel the mucus upward, where it is either swallowed or expelled.

Bronchi and Bronchial Tree

In the mediastinum, at the level of the fifth thoracic vertebra, the trachea divides into the right and left primary bronchi. The bronchi branch into smaller and smaller passageways until they terminate in tiny air sacs called alveoli.

The cartilage and mucous membrane of the primary bronchi are similar to that in the trachea. As the branching continues through the bronchial tree, the amount of hyaline cartilage in the walls decreases until it is absent in the smallest bronchioles. As the cartilage decreases, the amount of smooth muscle increases. The mucous membrane also undergoes a transition from ciliated pseudostratified columnar epithelium to simple cuboidal epithelium to simple squamous epithelium.

The alveolar ducts and alveoli consist primarily of simple squamous epithelium, which permits rapid diffusion of oxygen and carbon dioxide. Exchange of gases between the air in the lungs and the blood in the capillaries occurs across the walls of the alveolar ducts and alveoli.



The two lungs, which contain all the components of the bronchial tree beyond the primary bronchi, occupy most of the space in the thoracic cavity. The lungs are soft and spongy because they are mostly air spaces surrounded by the alveolar cells and elastic connective tissue. They are separated from each other by the mediastinum, which contains the heart. The only point of attachment for each lung is at the hilum, or root, on the medial side. This is where the bronchi, blood vessels, lymphatics, and nerves enter the lungs.

The right lung is shorter, broader, and has a greater volume than the left lung. It is divided into three lobes and each lobe is supplied by one of the secondary bronchi. The left lung is longer and narrower than the right lung. It has an indentation, called the cardiac notch, on its medial surface for the apex of the heart. The left lung has two lobes.

Each lung is enclosed by a double-layered serous membrane, called the pleura. The visceral pleura is firmly attached to the surface of the lung. At the hilum, the visceral pleura is continuous with the parietal pleura that lines the wall of the thorax. The small space between the visceral and parietal pleurae is the pleural cavity. It contains a thin film of serous fluid that is produced by the pleura. The fluid acts as a lubricant to reduce friction as the two layers slide against each other, and it helps to hold the two layers together as the lungs inflate and deflate.

Functions of the Respiratory System

The main role of the Respiratory System is the inhalation of fresh oxygen (O2) needed by the body's cells and the exhalation of waste carbon dioxide (CO2). It also helps maintain body temperature and eliminate excess water from the body. The Respiratory system is dependent on the proper functioning of the circulatory system as the O2 and waste CO2 are carried in the blood stream.


The flow of air from the nose to the lungs

  1. The nose is a PASSAGEWAY FOR AIR and is also a sensory organ. It warms and moistens air, and hair like processes (cilia) filter the air before it reaches the lungs.


  2. The pharynx or "throat" is a funnel shaped tube acting as a passage way for air and food. The lowest portion of the pharynx joins the esophagus (food tube).


  3. The larynx or "voice box" is lined with mucous membrane, two folds of membrane divide the larynx in two, between these is the GLOTTIS which is the narrowest part of the air passage. The glottis is protected by a lid of fibro cartilage (the EPIGLOTTIS), this closes over the glottis when you swallow.


  4. The trachea or "windpipe" is a tube composed of cartilage and lined with mucus membrane lying in front of the esophagus. The trachea diverges into the right and left bronchi.


  5. The bronchi are "C" shaped rings of cartilage lined with ciliated mucus membrane that 'sweeps' out dust particles. The bronchi branch into SECONDARY BRONCHI as they enter the lungs, these further divide into BRONCHIOLES. As the secondary bronchi and bronchioles divide the walls become thinner and more elastic, branching into minute ALVEOLI which transfer gases in the lungs.


  6. The lungs are the main organ of respiration. In the lungs millions of ALVEOLI and blood Capillaries exchange Oxygen and carbon dioxide. Each lung is housed in a separate PLEURAL CAVITY (which in turn are located in the larger THORACIC CAVITY). PLEURA are SACS of membrane that line the pleural cavity to lessen friction caused by breathing.


  7. The diaphragm is the main muscle of respiration located at the base of the thoracic cavity. The muscle contracts and flattens so that the thorax and lungs have room for incoming air causing you to INHALE, it then relaxes causing you to EXHALE.


Roots, suffixes, and prefixes

component meaning example
A- lack of anosmia = lack of the sense of smell.
PNEA- breath dyspnea = difficult or painful breathing.
BRONCH- bronchi Bronchoscope = instrument to examine the bronchi.
LARYNG- larynx laryngitis = inflammation of the larynx.
PULMO- lung pulmonary metastases = lung secondaries.
PNEUM- lung / Air pneumonia = severe infection of the lungs.
Rhino- nose rhinoplasty = cosmetic surgery, a 'nose job'.


Cancer Focus


Lung Cancer Overview
Lung cancer is one of the most common types of cancer. The lungs are a pair of cone-shaped organs situated inside the chest, they bring oxygen into the body and take out waste carbon dioxide. There is a strong link between smoking and lung cancer. There are two main categories of lung cancer; Small Cell Lung Cancer (SCLC) , and Non-Small Cell Lung Cancer (NSCLC). World-wide over 1 million people are diagnosed with lung cancer each year.
Internet Resources for Lung Cancer


Small Cell Lung Cancer
Small Cell Lung Cancer (SCLC) accounts for one quarter of all lung cancers. SCLC is also known as "oat cell" carcinoma because of the shape of the cancer cells.
Internet Resources for Small Cell Lung Cancer (SCLC)


Non-Small Cell Lung Cancer
Non-small cell lung cancer (NSCLC) accounts for about three quarters of all lung cancers. Included in the NSCLC group of cancers are a) Squamous cell, or epidermoid, cancer which arises from cells that line the airways - this is the most common type of lung cancer; b) Adenocarcinoma which arises in the mucus-producing cells that line the upper airways; and c) Large cell carcinoma.
Internet Resources for Non-Small Cell Lung Cancer (NSCLC)


Causes of Lung Cancer
It is estimated that about 85% of lung cancers are caused by smoking, it is usually found in people who smoke or who used to smoke cigarettes. Lung cancer is also associated with passive smoking and exposure to radon (a radioactive gas).
Internet Resources for Smoking


Head and Neck Cancer
Head and neck cancers are malignancies arising in the upper aerodigestive tract (this includes: lip, tongue, salivary glands, mouth, oropharynx, nasopharynx, hypopharynx, nasal cavity, and larynx). Laryngeal (voice-box) cancer is the most frequent type, accounting for about a quarter of head and neck cancers.
Internet Resources for Head and Neck Cancer


Laryngeal Cancer
Laryngeal cancer is a malignancy arising in the tissues of the larynx (voicebox). People who smoke have a higher risk of developing the disaese. Most laryngeal cancers are of squamous cell histology, these can be categorised as either keratinizing or non-keratinizing. There are a variety of other non-squamous cell laryngeal cancers.
Internet Resources for Laryngeal Cancer


Nasal, Paranasal, and Nasopharynx Cancer
Internet Resources for Nasal Cancer


Oral cavity, Lip, Salivary gland Cancer
Internet Resources for Oral cavity Cancer

Related Abbreviations and Acronyms:


ENT Ear nose throat
FEV Forced expectorant volume (a lung test)
IASLC International Association for the Study of Lung Cancer
NSCLC Non-small cell lung cancer
SCLC Small cell lung cancer
SPOHNC Support for People with Oral and Head and Neck Cancer
URTI Upper respiratory tract infection
VEF Ventricular ejection fraction (tests lung function)

More Cancer Related Abbreviations