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Alveoli diagram.png
Diagram of the alveoli with both cross-section and external view.
System Respiratory system
MeSH D055745
TA A06.5.02.026
TH H3.
FMA 7410 76497, 7410
Anatomical terminology

The bronchioles or bronchioli are the smaller branches of the bronchial passageways in the respiratory tract that deliver air to the smaller terminal bronchioles in the conducting zone, and even smaller respiratory bronchioles in the respiratory zone. The bronchioles no longer contain the cartilage, that is found in the bronchi, or glands in their submucosa.

Bronchus airway in the respiratory tracti

A bronchus is a passage of airway in the respiratory system that conducts air into the lungs. The first bronchi to branch from the trachea are the right main bronchus and the left main bronchus. These are the widest and enter the lungs at each hilum, where they branch into narrower secondary bronchi or lobar bronchi, and these branch into narrower tertiary bronchi or segmental bronchi. Further divisions of the segmental bronchi are known as 4th order, 5th order, and 6th order segmental bronchi, or grouped together as subsegmental bronchi. The bronchi when too narrow to be supported by cartilage are known as bronchioles. No gas exchange takes place in the bronchi.

Respiratory tract Organs involved in transmission of air to and from the point where gases diffuse into tissue

In humans, the respiratory tract is the part of the anatomy of the respiratory system involved with the process of respiration. Air is breathed in through the nose or the mouth. In the nasal cavity, a layer of mucous membrane acts as a filter and traps pollutants and other harmful substances found in the air. Next, air moves into the pharynx, a passage that contains the intersection between the oesophagus and the larynx. The opening of the larynx has a special flap of cartilage, the epiglottis, that opens to allow air to pass through but closes to prevent food from moving into the airway.


The submucosa is a thin layer of tissue in various organs of the gastrointestinal, respiratory, and genitourinary tracts. It is the layer of dense irregular connective tissue that supports the mucosa and joins it to the muscular layer, the bulk of overlying smooth muscle.



A lobule of the lung showing r. b respiratory bronchiole; al. d alveolar duct; a. s alveolar sac; 'a' alveolus Gray975.png
A lobule of the lung showing r. b respiratory bronchiole; al. d alveolar duct; a. s alveolar sac; 'a' alveolus

A pulmonary lobule is the portion of the lung ventilated by one bronchiole. Bronchioles are approximately 1 mm or less in diameter and their walls consist of ciliated cuboidal epithelium and a layer of smooth muscle. Bronchioles divide into even smaller bronchioles, called terminal, which are 0.5 mm or less in diameter. Terminal bronchioles in turn divide into smaller respiratory bronchioles which divide into alveolar ducts. Terminal bronchioles mark the end of the conducting division of air flow in the respiratory system while respiratory bronchioles are the beginning of the respiratory division where gas exchange takes place.

Lung Essential respiration organ in many air-breathing animals

The lungs are the primary organs of the respiratory system in humans and many other animals including a few fish and some snails. In mammals and most other vertebrates, two lungs are located near the backbone on either side of the heart. Their function in the respiratory system is to extract oxygen from the atmosphere and transfer it into the bloodstream, and to release carbon dioxide from the bloodstream into the atmosphere, in a process of gas exchange. Respiration is driven by different muscular systems in different species. Mammals, reptiles and birds use their different muscles to support and foster breathing. In early tetrapods, air was driven into the lungs by the pharyngeal muscles via buccal pumping, a mechanism still seen in amphibians. In humans, the main muscle of respiration that drives breathing is the diaphragm. The lungs also provide airflow that makes vocal sounds including human speech possible.

Epithelium Type of animal tissue and human

Epithelium is one of the four basic types of animal tissue, along with connective tissue, muscle tissue and nervous tissue. Epithelial tissues line the outer surfaces of organs and blood vessels throughout the body, as well as the inner surfaces of cavities in many internal organs. An example is the epidermis, the outermost layer of the skin.

Smooth muscle

Smooth muscle is an involuntary non-striated muscle. It is divided into two subgroups; the single-unit (unitary) and multiunit smooth muscle. Within single-unit cells, the whole bundle or sheet contracts as a syncytium.

The diameter of the bronchioles plays an important role in air flow. The bronchioles change diameter to either increase or reduce air flow. An increase in diameter is called bronchodilation and is stimulated by either epinephrine or sympathetic nerves to increase air flow. A decrease in diameter is called bronchoconstriction and is stimulated by histamine, parasympathetic nerves, cold air, chemical irritants and other factors to decrease air flow.

Bronchoconstriction Constriction of the airways in the lungs

Bronchoconstriction is the constriction of the airways in the lungs due to the tightening of surrounding smooth muscle, with consequent coughing, wheezing, and shortness of breath.

Histamine chemical compound

Histamine is an organic nitrogenous compound involved in local immune responses, as well as regulating physiological function in the gut and acting as a neurotransmitter for the brain, spinal cord, and uterus. Histamine is involved in the inflammatory response and has a central role as a mediator of itching. As part of an immune response to foreign pathogens, histamine is produced by basophils and by mast cells found in nearby connective tissues. Histamine increases the permeability of the capillaries to white blood cells and some proteins, to allow them to engage pathogens in the infected tissues. It consists of an imidazole ring attached to an ethylamine chain; under physiological conditions, the amino group of the side-chain is protonated.

Parasympathetic nervous system one of the three divisions of the autonomic nervous system, the others being the sympathetic nervous system and enteric nervous system.

The parasympathetic nervous system (PSNS) is one of the two divisions of the autonomic nervous system, the other being the sympathetic nervous system. The autonomic nervous system is responsible for regulating the body's unconscious actions. The parasympathetic system is responsible for stimulation of "rest-and-digest" or "feed and breed" activities that occur when the body is at rest, especially after eating, including sexual arousal, salivation, lacrimation (tears), urination, digestion and defecation. Its action is described as being complementary to that of the sympathetic nervous system, which is responsible for stimulating activities associated with the fight-or-flight response.


Lungs showing bronchi and bronchioles Illu bronchi lungs.jpg
Lungs showing bronchi and bronchioles

The primary bronchi, in each lung, which are the left and right bronchus, give rise to secondary bronchi known as lobar bronchi. These in turn give rise to tertiary bronchi (tertiary meaning "third"), known as segmental bronchi. The segmentary bronchi subdivide into fourth order, fifth order and sixth order segmental bronchi before dividing into the bronchioles. These are histologically distinct from the bronchi in that their walls do not have hyaline cartilage and they have club cells in their epithelial lining. The epithelium starts as a simple ciliated columnar epithelium and changes to simple ciliated cuboidal epithelium as the bronchioles decreases in size. The diameter of the bronchioles is often said to be less than 1 mm, though this value can range from 5 mm to 0.3 mm. As stated, these bronchioles do not have hyaline cartilage to maintain their patency. Instead, they rely on elastic fibers attached to the surrounding lung tissue for support. The inner lining (lamina propria) of these bronchioles is thin with no glands present, and is surrounded by a layer of smooth muscle. As the bronchioles get smaller they divide into terminal bronchioles. These bronchioles mark the end of the conducting zone, which covers the first division through the sixteenth division of the respiratory tract. Alveoli only become present when the conducting zone changes to the respiratory zone, from the sixteenth through the twenty-third division of the tract.

Hyaline cartilage Type of cartilage found on many joint surfaces

Hyaline cartilage is the glass-like (hyaline) but translucent cartilage found on many joint surfaces. It is also most commonly found in the ribs, nose, larynx, and trachea. Hyaline cartilage is pearl-grey in color, with a firm consistency and has a considerable amount of collagen. It contains no nerves or blood vessels, and its structure is relatively simple.

Club cells, also known as bronchiolar exocrine cells, and originally known as Clara cells, are dome-shaped cells with short microvilli, found in the small airways (bronchioles) of the lungs.

Simple columnar epithelium tissue type

A simple columnar epithelium is a columnar epithelium that is uni-layered. In humans, a simple columnar epithelium lines most organs of the digestive tract including the stomach, small intestine, and large intestine. Simple columnar epithelia line the uterus.

Terminal bronchioles

The terminal bronchiole is the most distal segment of the conducting zone. It branches off the lesser bronchioles. Each of the terminal bronchioles divides to form respiratory bronchioles which contain a small number of alveoli. Terminal bronchioles are lined with simple cuboidal epithelium containing club cells. Terminal bronchioles contain a limited number of ciliated cells and no goblet cells. Club cells are non-ciliated, rounded protein-secreting cells. Their secretions are a non-sticky, proteinaceous compound to maintain the airway in the smallest bronchioles. The secretion, called surfactant, reduces surface tension, allowing for bronchioles to expand during inspiration and keeping the bronchioles from collapsing during expiration. Club cells, a stem cell of the respiratory system, produce enzymes that detoxify substances dissolved in the respiratory fluid.

Goblet cell Goblet cell

Goblet cells are simple columnar epithelial cells that secrete gel-forming mucins, like mucin MUC5AC. The goblet cells mainly use the merocrine method of secretion, secreting vesicles into a duct, but may use apocrine methods, budding off their secretions, when under stress. The term goblet refers to the cell's goblet-like shape. The apical portion is shaped like a cup, as it is distended by abundant mucus laden granules; its basal portion lacks these granules and is shaped like a stem.

Surfactant Substance that lowers the surface tension between a liquid and another material

Surfactants are compounds that lower the surface tension between two liquids, between a gas and a liquid, or between a liquid and a solid. Surfactants may act as detergents, wetting agents, emulsifiers, foaming agents, and dispersants.

Surface tension Tendency of a liquid surface to shrink to reduce surface area

Surface tension is the tendency of fluid surfaces to shrink into the minimum surface area possible. Surface tension allows insects, usually denser than water, to float and slide on a water surface.

Respiratory bronchioles

The respiratory bronchioles are the narrowest airways of the lungs, one fiftieth of an inch across. [1] The bronchi divide many times before evolving into the bronchioles. The bronchioles deliver air to the exchange surfaces of the lungs. [2] They are interrupted by alveoli which are thin walled evaginations. Alveolar ducts are distal continuations of the respiratory bronchioles.

Pulmonary alveolus Hollow cavity found in the lungs

A pulmonary alveolus is a hollow cup-shaped cavity found in the lung parenchyma, and is the basic unit of ventilation. Lung alveoli are the ends of the respiratory tree, branching from either alveolar sacs or alveolar ducts, which like alveoli are both sites of gas exchange with the blood as well. Alveoli are particular to mammalian lungs. Different structures are involved in gas exchange in other vertebrates. The alveolar membrane is the gas exchange surface. Carbon dioxide rich blood is pumped from the rest of the body into the capillaries that surround the alveoli where, through diffusion, carbon dioxide is released and oxygen is absorbed.

Clinical significance

Bronchospasm, a potentially life-threatening situation, occurs when the smooth muscular tissue of the bronchioles constricts, severely narrowing their diameter. The most common cause of this is asthma. Bronchospasm is commonly treated by oxygen therapy and bronchodilators such as albuterol.

Diseases of the bronchioles include asthma, bronchiolitis obliterans, respiratory syncytial virus infections, and influenza.


The medical condition of inflammation of the bronchioles is termed bronchiolitis.

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Related Research Articles

Respiratory system A biological system of specific organs and structures for gas exchange in animals and plants

The respiratory system is a biological system consisting of specific organs and structures used for gas exchange in animals and plants. The anatomy and physiology that make this happen varies greatly, depending on the size of the organism, the environment in which it lives and its evolutionary history. In land animals the respiratory surface is internalized as linings of the lungs. Gas exchange in the lungs occurs in millions of small air sacs called alveoli in mammals and reptiles, but atria in birds. These microscopic air sacs have a very rich blood supply, thus bringing the air into close contact with the blood. These air sacs communicate with the external environment via a system of airways, or hollow tubes, of which the largest is the trachea, which branches in the middle of the chest into the two main bronchi. These enter the lungs where they branch into progressively narrower secondary and tertiary bronchi that branch into numerous smaller tubes, the bronchioles. In birds the bronchioles are termed parabronchi. It is the bronchioles, or parabronchi that generally open into the microscopic alveoli in mammals and atria in birds. Air has to be pumped from the environment into the alveoli or atria by the process of breathing which involves the muscles of respiration.

Trachea cartilaginous tube that connects the pharynx and larynx to the lungs

The trachea, colloquially called the windpipe, is a cartilaginous tube that connects the pharynx and larynx to the lungs, allowing the passage of air, and so is present in almost all air-breathing animals with lungs. The trachea extends from the larynx and branches into the two primary bronchi. At the top of the trachea the cricoid cartilage attaches it to the larynx. This is the only complete tracheal ring, the others being incomplete rings of reinforcing cartilage. The trachealis muscle joins the ends of the rings and these are joined vertically by bands of fibrous connective tissue – the annular ligaments of trachea. The epiglottis closes the opening to the larynx during swallowing.

Gas exchange is the physical process by which gases move passively by diffusion across a surface. For example, this surface might be the air/water interface of a water body, the surface of a gas bubble in a liquid, a gas-permeable membrane, or a biological membrane that forms the boundary between an organism and its extracellular environment.

Pulmonary artery artery in the pulmonary circulation that carries deoxygenated blood from the heart to the lungs

A pulmonary artery is an artery in the pulmonary circulation that carries deoxygenated blood from the right side of the heart to the lungs. The largest pulmonary artery is the main pulmonary artery or pulmonary trunk from the heart, and the smallest ones are the arterioles, which lead to the capillaries that surround the pulmonary alveoli.

Alveolar duct

Alveolar ducts are tiny ducts that connect the respiratory bronchioles to alveolar sacs, each of which contains a collection of alveoli. They are tiny end ducts of the branching airways that fill the lungs. Each lung holds approximately 1.5 to 2 million of them. The tubules divide into two or three alveolar sacs at the distal end. They are formed from the confluence openings of several alveoli. Distal terminations of alveolar ducts are atria which then end in alveolar sacs.

Cryptogenic organizing pneumonia Inflammation of the bronchioles (bronchiolitis) and surrounding tissue in the lungs

Cryptogenic organizing pneumonia (COP), also known as bronchiolitis obliterans organizing pneumonia (BOOP), is an inflammation of the bronchioles. It should not be confused with bronchiolitis obliterans, a form of non-infectious pneumonia.

Alveolar lung disease disease that affect the alveoli of the lungs

Alveolar lung diseases, are a group of diseases that mainly affect the alveoli of the lungs.

Respiratory epithelium

Respiratory epithelium is a type of ciliated columnar epithelium found lining most of the respiratory tract, where it serves to moisten and protect the airways. It is not present in the larynx and pharynx. It also functions as a barrier to potential pathogens and foreign particles, preventing infection and tissue injury by the action of mucociliary clearance.

Acinus considered by some (but not all) sources to be synonymous with Alveolus

An acinus refers to any cluster of cells that resembles a many-lobed "berry", such as a raspberry. The berry-shaped termination of an exocrine gland, where the secretion is produced, is acinar in form, as is the alveolar sac containing multiple alveoli in the lungs.

Simple cuboidal epithelium

Simple cuboidal epithelium is a type of epithelium that consists of a single layer of cuboidal (cube-like) cells. These cuboidal cells have large, spherical and central nuclei.

Lung bud

The lung bud sometimes referred to as the respiratory bud forms from the respiratory diverticulum, an embryological endodermal structure that develops into the respiratory tract organs such as the larynx, trachea, bronchi and lungs. It arises from part of the laryngotracheal tube.

Respiratory system of the horse

In biology, the respiratory system of the horse is the means by which a horse circulates air around its internal organs.

The canals of Lambert are accessory connections in the lungs between some bronchioles and their adjacent alveoli. Their diameter is up to 200 µm.Along with the Pores of Kohn they facilitate collateral movement of gases within the smallest parts of the lungs. They are poorly formed in children. However, while the Pores of Kohn connect alveoli to adjacent alveoli, the Canals of Lambert connect terminal bronchioles to alveoli.


  1. Merck Manual of Medical Information, home edition, copyright 1997, first printing of Pocket Books 1999, Pocket Books, a division of Simon and Schuster Inc
  2. Human Anatomy by Frederic Martini sixth edition, page 643

Further reading