Lower Respiratory Tract

∞ generated and posted on 2020.03.23 ∞

The lower respiratory tract begins approximately at the trachea and continues inward to the alveolar sacs.

Beginning either at or below the larynx, the lower respiratory tract consists of the trachea, bronchi, bronchioles, and alveoli, the latter making alveolar sacs and which are lined with mucous membrane that secretes pulmonary surfactant. Important as well to external respiration are the thoracic diaphragm, intercostal muscles, pulmonary arteries, and pulmonary veins, etc.

This page contains the following terms: Trachea, Bronchi, Bronchioles, Alveoli, Alveolar sacs, Surfactant, Pulmonary arteries, Pulmonary veins, Intercostal muscles, Diaphragm, Tidal volume, Vital capacity, Respiratory center Cystic fibrosis, Emphysema, Pneumonia, Rhinitis, Tuberculosis


Cartilage reinforced, collapse-resistant passageway from larynx to lungs.
The trachea, along with the larynx, connects the pharynx with the lungs. It exists by necessity as a collapse-resistant structure because during inhalation the interior of the trachea possesses a lower pressure than the outside air. That is, the same negative pressure that gives rise to inhalation of air also would result in the collapse of the trachea were it not for the cartilage reinforcement that prevents this collapse.

This contrasts with the esophagus, down which swallowing occurs, which is both not subject to negative pressure in the same way that the trachea is and needs to display some flexibility towards the swallowing of relatively large boluses of food. (The trachea does display flexibility where it contacts the esophagus, however, which aids as well in the movement of large boluses of food down the esophagus.)

As with the nasal cavity as well as the lungs, the trachea also possess epithelium upon which cilia are present. As also with the nasal cavity, these cilia are responsible for the movement of mucus along with any trapped particles (including trapped microorganisms) out of the lower respiratory tract and into the pharynx, where the mucus can then be swallowed. This movement is referred as the mucociliary escalator, and this helps to keep the lungs a relatively microorganism- or foreign-material free environment.

Links to terms of possible interest: Esophagus, Hyoid bone, Larynx, Lumen, Mucosa, Nasopharynx, Primary bronchi, Secondary bronchi, Thyroid gland, Trachea, Tracheal cartilage, Trachealis muscle,

The above video introduces us to the trachea as well as bronchi; there also is discussion of what the mucociliary escalator is at about at 2:15; reference to the bronchi begins in earnest at 3:46.

The above video walks through the anatomy of the trachea, using a model.

The above video shows a living trachea from the inside; I found that it was helpful to slow the speed of the video while viewing it.


Large tubes through which air passes within lungs.
These are the tubes that branch off from the single tube of the trachea and then plunge as well as branch further into the lungs. As with the trachea, lungs, and nasal cavity, the bronchi too are lined with epithelial tissue that consists of ciliated mucous membrane. Like the trachea, all bronchi are supported by cartilage that prevents the bronchi from collapsing in the course of inhalation. The bronchi then further branch into smaller tubes called bronchiole.

There are two primary bronchi, one for each lung (that is, left and right). Each branches into a number of secondary bronchi (three on the right and two on the left) which in turn branch into a total of approximately 10 tertiary bronchi in each lung (usually 10 on the right and, due to fusion during development, 8 on the left). Each tertiary bronchus is associated with a single bronchopulmonary segment which can be thought of as the macroscopic functional units of the lung (contrasting alveoli which are microscopic functional units.

Links to terms of possible interest: Alveoli, Inferior lobe of left lung, Inferior lobe of right lung, Lungs, Middle lobe of right lung, Primary bronchus, Secondary bronchus, Tertiary bronchus, Superior lobe of left lung, Superior lobe of right lung, Trachea

The anatomical features of bronchi are discussed in the above video as based upon a model.

This video is just a repeat of a video found above, with reference to the bronchi beginning in earnest at 3:46.


Smaller tubes through which air passes within the lungs.
Unlike the bronchi, which are specific well-defined anatomical structures, the bronchioles (or bronchioli; bronchiolus or bronchiole as the singular) are less consistently located structures (as going from person to person) that multiply branch into ever smaller structures, and which ultimately terminate as the smallest of these tubes into closed sacks known as alveoli.

The bronchioles branch from the tertiary bronchi and can be distinguished into more conductive versus more respiratory structures (respiratory bronchioles), with terminal bronchioles found in between. Like bronchi, the bronchioles branch into finer, more numerous tubes, ultimately terminating in alveoli.

Links to terms of possible interest: Alveolar sac, Alveoli, Bronchiole, Capillary network, Pulmonary arteriole, Pulmonary venule, Respiratory bronchiole, Terminal bronchiole

The above video provides a densely packed though nonetheless low visual resolution overview of the microanatomy of the lungs, the bronchioles and alveoli.


Microscopic pouches within lungs where the majority of gas exchange occurs.
Alveoli, or alveolus as the singular, are cup shaped containers of what are known as the alveolar membrane, alveolar-capillary barrier, or blood-air barrier. This is the membrane within the lungs over which gas exchange explicitly occurs, with oxygen following its concentration gradient into the blood and carbon dioxide following its concentration gradient out of the blood.

To assure the maintenance of these concentration gradients, we breathe. We breathe, that is, to exchange the oxygen-depleted air that is found after some time in the alveoli (measured in seconds) with fresh air derived from outside of the body. For air to reach the alveoli, however, it must first pass through the pharynx, larynx, trachea, bronchi, and bronchiole (as well as the nose or, potentially also, the mouth). Note that alveoli are typically found in 'grape-like' clusters known as alveolar sacs.

Links to terms of possible interest: Alveolar sac, Alveolar duct, Alveoli, Bronchiole, Capillary network, Intralobular bronchiole, Pulmonary arteriole, Pulmonary venule, Respiratory bronchiole, Terminal bronchiole

The above video has great graphics and presents the information at just the right level for the beginning student of lung function and anatomy.

The above video does a nice job of introducing alveolar function though suffers from very low visual resolution.

Alveolar sacs

Collections of microscopic pouches responsible for majority of gas exchange within lungs.
The alveolar membrane is actually continuous over multiple alveoli that together make up the alveolar sacs. Indeed, the alveoli can be viewed simply as a means by which the body increases the surface area associated with the alveolar sacs – by providing ridges – but also perhaps serves as a means by which the potential for the alveolar sacs to collapse upon themselves is modestly reduced.

Links to terms of possible interest: Alveolar sac, Alveolar duct, Alveolar pores, Alveoli, Bronchiole, Capillary network, Intralobular bronchiole, Pulmonary arteriole, Pulmonary venule, Respiratory bronchiole, Terminal bronchiole

The above video walks through the respiratory system (over very little time) and mentions alveolar sacs at 0:16; it has very low visual resolution, however.


Molecule that can simultaneously dissolve in both water and lipids, potentially resulting in suspension of the latter in the former.
The role of surfactant within the lungs (pulmonary surfactant) is to prevent the collapse of alveolar sacs, which then are difficult to re-inflate. The surfactant prevents the alveolar membrane from literally sticking to itself in the course of exhalation, that is, it makes the membrane less sticky. This works quite well and in the absence of surfactant, as is seen in prematurely born infants, the alveolar sacs are indeed prone to collapse.

Note, by the way, how similar the words "surface" and "surfactant" are. It is surfactant that prevents the inner surfaces of alveoli from sticking to themselves that would result, were sticking to occur, in alveolar collapse. With surfactant, adjacent surfaces can't (get it?) as easily stick to one another.

Links to terms of possible interest: Alveolar fluid, Alveolar wall, Alveolus, Expiration, Inspiration, Pulmonary surfactant, Surfactant

The above video provides very much a chemist's introduction to the concepts of surface tension and surfactant; you will note however the complete absence of a negative-surfactant control in the presented experiment/demonstration!

Discussion of pulmonary surfactant begins at 3:50.

The above video doesn't quite get to the meat of why water will collapse alveoli in the absence of surfactant, but nonetheless does discuss surfactant within the context of alveoli.

Pulmonary arteries

Blood vessels that supply unoxygenated blood to the lungs.
The pulmonary arteries branch into arterioles which give rise to the capillaries that are associated with alveolar membrane. That is, these are the capillaries which receive oxygen from the alveolar sacs as well as give off carbon dioxide to the alveolar sacs. The pulmonary arteries begin with the pulmonary trunk, and then branch into the left and right pulmonary arteries. The pulmonary valve, one of the heart's two semilunar valves, is found at the base of the pulmonary trunk.

Links to terms of possible interest: Pulmonary arteries, Pulmonary circulation, Pulmonary veins

Pulmonary veins

Blood vessels that supply oxygenated blood to the heart.
The pulmonary veins arise from venules which form from capillaries that are associated with the alveolar membrane. They carry freshly oxygenated blood and return that blood to the left atrium within a total of four veins, two to the left (returning blood from the left lung) and two to the right (returning blood from the right lung).

Links to terms of possible interest: Aorta, Atrioventricular valve, Brachiocephalic artery, Common carotid artery, Coronary arteries, Heart, Inferior vena cava, Left atrium, Left ventricle, Mitral valve, Pulmonary arteries, Pulmonary veins, Right atrium, Right ventricle, Semilunar valves, Septum, Subclavian artery, Superior vena cava, Tricuspid valve

The above video provides a look at blood flow through the heart, with mention as well as illustration of the pulmonary trunk (the largest of the pulmonary arteries) along with the largest of the pulmonary veins.

Intercostal muscles

Means by which the rib cage is moved upward to effect inhalation.
The intercostal muscles (specifically the external intercostal muscles) work in conjunction with the diaphragm to effect inhalation. They are found in between the individual ribs and their contraction has the effect of moving the ribs and therefore the rib cage upward. Breathe in deeply and you will note that your chest will rise. This effect is not a consequence of your lungs filling with air but instead what is in part causing your lungs to fill with air.

The internal intercostal muscles, by contrast, are involved in forced exhalation. Thus, due to your intercostal muscles, you are able to both expand your thoracic cavity, and thereby your pleural cavity, so as to breathe in more fully than can be accomplished using your diaphragm alone, and you are able to breathe out other than passively by forcibly decreasing the volume of your thoracic cavity and thereby your pleural cavity.

Links to terms of possible interest: Diaphragm, Exhalation, Inhalation, Intercostal muscles, Rib cage

It was difficult to find a video that focused on the intercostal muscles, was relatively short, and presented the consequence of the contraction of these muscles in a compelling manner; the above video was the best that I was able to find though I'm not terribly happy with its depiction of rib movement.

The above video describes in the internal intercostal muscles, which rather than being involved in effecting inhalation instead are involved in effecting forced exhalation.


Means by which the thoracic cavity expands towards the abdominal cavity to effect inhalation.
Upon contraction the diaphragm, or thoracic diaphragm, moves downward (in the inferior, that is, posterior direction). It accomplishes this feat because in the relaxed state the diaphragm is curved upward into the thoracic cavity. Thus, by contracting, the curve is straightened and as a consequence the volume of the thoracic cavity is expanded at the expense of the volume of the abdominal cavity.

The diaphragm additionally serves to divide the thoracic cavity from the abdominal cavity. Note that the esophagus, descending aorta, and inferior vena cava all pass through the diaphragm.

Links to terms of possible interest: Aorta, Central tendon of diaphragm, Diaphragm, Esophageal hiatus, Esophagus, Psoas major, Quadratus lumborum, Sternum, Vena cava, Vertebrae

The above video is a fairly amazing view and discussion of the action of the thoracic diaphragm in breathing.

Tidal volume

Amount of air entering the lungs per each inhalation or exiting the lungs per each exhalation under normal unforced conditions.
The tidal volume is simply the amount of air (gas) that enters and then exits the lungs during normal breathing. It is the amount of air that flows into and out of you while you are resting and otherwise not thinking about breathing. Note that this tidal volume is highly relevant when artificially ventilating a patient since excessive ventilation can result in lung injury while insufficient tidal volume can result in inadequate ventilation.

Links to terms of possible interest: Expiratory reserve volume, Inspiratory capacity, Inspiratory reserve volume, Lung volume, Maximum voluntary expiration, Residual volume, Tidal volume, Total lung capacity, Vital capacity

The above video provides a broad discussion of what can be measured in terms of lung capacity and how these measurements can be accomplished.

Vital capacity

Volume of air that is the difference between fully inflated lungs via inhalation and fully deflated lungs via exhalation.
Vital capacity is basically how much gas passes into your lungs if you start with lungs following exhalation of as much as you are capable of and then breathe in until you can't possibly breathe in further. Alternatively, it is the amount of air that you can exhale if you start with maximally filled lungs and then exhale until you are no longer able anatomically to further exhale.

Vital capacity can decline with lung diseases that impair breathing, so measurement of vital capacity can be diagnostic.

Links to terms of possible interest: Expiratory reserve volume, Functional residual capacity, Functional residual volume, Inspiratory capacity, Inspiratory reserve volume, Lung volume, Maximum voluntary expiration, Residual volume, Tidal volume, Total lung capacity, Total lung volume, Vital capacity

The above video provides a relatively short overview of the various measures of lung capacity, in tabular form.

Respiratory center

Cells found within the medulla oblongata of the brain stem that control the pace of breathing.
Basically an action potential is generated within the respiratory center every couple of seconds, which results in an unconsciously controlled inhalation and then exhalation, though obviously breathing can be under conscious control as well. In addition, the pace and depth of breathing can change in response to demand, with exercise resulting in greater levels of gas exchange within the lungs (external respiration), which can be met via deeper breathing as well as more frequent breathing.

These increases in the extent of breathing are controlled by blood levels of especially carbon dioxide (CO2) and hydrogen ions (H+; i.e., acidity), where the latter is generated also by the presence of carbon dioxide in the blood, though levels of O2 as well can impact breathing rate and depth. Note that in addition to the medulla oblongata, cells also are found in the pons which have an impact on breathing.

Links to terms of possible interest: Apneustic center, Diaphragm, Dorsal respiratory group, Intercostal muscles, Medulla, Pneumotaxic center, Pons, Pontine respiratory group, Respiratory center, Ventral respiratory group

The above video very quickly provides an overview of the various locations within the brain that have an impact on breathing.

Cystic fibrosis

Genetic disease that is associated with an accumulation of thick mucus in the lungs.
Cystic fibrosis results in changes in function of other body organs including hepatic (liver), intestinal, and pancreatic, and also gives rise to skin that tastes salty. The disease is a consequence of a defect in a membrane protein that is responsible for transporting the anion chloride

This defect is inherited as a recessive mutation. Thus, cystic fibrosis typically appears from the mating of two otherwise not affected carriers (heterozygotes) whereas the affected individual has received two copies of the defective gene (i.e., allele; these "two-copy" individuals are described instead as homozygotes).

Treatment involves efforts to reduce the impact of mucus buildup in the lungs on breathing ability, though other organs can require treatment as well.

The above video is a fast introduction to the molecular basis of cystic fibrosis, but can be a little difficult to fully appreciate towards the end.

The above video provides a nice overview of cystic fibrosis pathophysiology but exactly halfway through it repeats itself without sound!?!

The above video is a fairly high-level overview of cystic fibrosis pathology and genetics but is relative short.

The above video is a fairly high-level overview of cystic fibrosis pathology and genetics.


Breakdown and reduced functioning of lung tissue as occurs in association with chronic obstructive pulmonary disease.
Emphysema is a result of chronic damage to lungs that results in a fragility to alveoli and consequent degeneration. It is typically seen in association with chronic bronchitis, which together with emphysema make up chronic obstructive pulmonary disease (COPD). Together these result in difficulties in breathing.

There is no cure for emphysema, or for COPD more generally, though medical management is possible. Prevention involves an avoidance of lung damaging circumstances or materials, particularly though not exclusively an avoidance of smoking.

The above video walks through the symptoms and causes of chronic obstructive pulmonary disease.

As advertised, the above video provides a "Really short video on the basics of emphysema."


Inflammation of the alveoli.
Pneumonia is typically caused by infection by microorganisms and also typically results in the accumulation of fluid within the lungs, particularly the alveoli. This can result is breathing difficulties and, if sufficiently severe, in death. Treatment is of the underlying infection, if that is possible, such as via the application of antibiotics.

The above video provides a quick overview of what pneumonia is all about.

The above video provides a reasonably comprehensive overview of what pneumonia is all about; note that they get their definition of bronchiole a bit wrong, however.


Inflammation of the nose resulting for example in a stuffy, runny nose.
Rhinitis can be caused by allergic reactions (allergies), viral infections, or bacterial infections, with only the latter potentially treatable using antibiotics. Rhinitis associated with viral infections is often accompanied by a common cold. Viral or bacterial infections occasionally can progress to pneumonia, a far more serious infection.

The above is a patient education video on allergic rhinitis; unfortunately, the sound quality isn't great.


Often fatal, difficult to cure, bacterial infection particularly of the lungs.
Tuberculosis (TB) is caused by the bacterium Mycobacterium tuberculosis. These bacteria can be aerosolized in the course of coughing, and upon being breathed in can initiate infections.

Infections are slow to develop and can be difficult to cure using antibiotics, even given infection by fully antibiotic-sensitive bacteria. In addition, there are numerous examples of antibiotic-resistant tuberculosis that are extremely difficult and in some cases in fact impossible to cure.

Each year approximately 1 to 2 million people die of tuberculosis, worldwide, and approximately 10 million people become newly infected, with most of these infections and deaths occurring in developing countries.

The above video provides a pretty good overview of what tuberculosis is and how it spreads, etc.

The above video is a very short but very well done video on COVID-19 dating from fairly early in the pandemic.

The above video is a longer and also well done video on COVID-19, also dating from fairly early in the pandemic. It's perspective is on the medical treatment of COVID-19.