Upper Respiratory Tract

∞ generated and posted on 2020.03.23 ∞

The upper respiratory tract is found starting with the nasal nares and progressing inward to approximately through the pharynx.

The upper respiratory tract consists of the nose, nasal cavity including sinuses, pharynx, and, depending on the reference, potentially the larynx, which includes the epiglottis and vocal cords. Considered also on this page is the distinction between external respiration and internal respiration.

This page contains the following terms: Lungs, Pleura, Pleural cavity, Breathing, External respiration, Internal respiration, Upper respiratory tract, Nose, Nasal cavity, Sinuses, Pharynx, Lower respiratory tract, Larynx, Epiglottis, Vocal cords


Animal gas exchange organs of breathing.
A number of different respiratory organs are displayed by animals. These include ones in which gasses are exchanged with surrounding water versus those in which gasses are exchanged with surrounding air. In addition, there are animals in which gas exchange is effected across substantial areas of the surfaces of their body's, e.g., various amphibians but also numerous small invertebrates such as worms, or instead via numerous invaginations into an animal's body (as seen with insects). All of these are approaches to what is known as external respiration and primarily are employed to bring oxygen (O2) into the body while releasing carbon dioxide (CO2) from the body.

An alternative means of external respiration and associated gas exchange with the air is via lungs, which are internal organs into which air is pulled or through which air flows. Lungs possess moist mucous membranes over which gas exchange occurs, notably the diffusion of oxygen into body tissues, particularly blood, and the diffusion of carbon dioxide out of body tissues. Our lungs are located within what is known as the pleural cavity, which is located within the thoracic cavity.

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

This video walks us through the functioning of the lungs with the actual discussion of the respiratory system and lung functioning starting at 1:12 and proceeding through about 6:15.

The above video is pretty nuts. Not quite as entertaining as it might have been, but still well worth watching.


Lung-enclosing serous membranes that serve to reduce friction between lungs and associated body cavity walls.
The pleura, as surrounds the pleural cavity, plays a role that is similar to that of the pericardium within the pericardial cavity, that is, to lubricate the interface between the lungs and other tissues. The pleural cavity, however, plays an additional, key physiological role, and that is to resist forming a vacuum or instead to resist allowing fluids to leak into the pleural cavity.

Instead, the pulling of the pleura outward is propagated directly to the lungs. This occurs through the serous fluid known as pleural fluid and is a consequence of surface tension between pleural fluid and the surrounding membrane. This results in expansion of the visceral pleura, which creates a negative pressure within the lumen of lungs into which air is pulled during inspiration.

An inflammation of the pleura is called pleurisy. Equivalently, inflammation of the pericardium is called pericarditis.

Links to terms of possible interest: Diaphragm, Parietal pleura, Pleural cavity, Pleural fluid, Visceral pleura

The above video starts out with an overview of pleura but considers numerous additional issues important to lung functioning; see also starting at 5:36 for discussion of breathing including the role played by pleura in this process.

The above video provides a fairly detailed introduction to the pleural cavity and what can go wrong with pleura and the pleural cavity in terms injury and disease.

Pleural cavity

Volume within animals containing the lungs.
The pleural cavity is found within the thoracic cavity which in turn is found within the ventral cavity. The pleural cavity is demarcated by the serous membranes known as pleura. The pleura (plural: pleurae) in turn provide lubrication to the exterior of the lungs, allowing lung expansion without friction against the thoracic cavity wall (i.e., the chest wall).

Though a single membrane (per individual lung), the pleura is folded upon itself forming an outer layer in contact with the chest wall (known as the parietal pleura) and an inner layer that is in contact with the lungs (the visceral pleura). Pleural fluid is secreted into the pleural cavity continuously and also is continuously absorbed, retaining a relatively constant as well as relatively small volume (few ml) of pleural fluid within the pleural cavity.

Links to terms of possible interest: Chest wall, Diaphragm, Intercostal muscles, Parietal pleura, Pleural cavity, Pleural sac, Rib cage, Sternum, Thoracic vertebrae, Visceral pleura

The above video combines all three of the following videos into one.

The above video provides a nice introduction to both the pleural cavity and pleural fluid.

The above video provides a nice introduction to pleural fluid.

The above video provides a somewhat complex introduction to the pathological condition of pleural effusion, which is too much pleural fluid within the pleural cavity.


Exchange by animals of gasses with those in the air as mediated by active flow of air over specialized body tissues.
Breathing involves alternating inhalation and exhalation – a.k.a., inspiration and expiration – which has the effect of drawing air into the lungs and then somewhat expelling that air from the lungs. The goal with breathing is to bring oxygen-rich air into the lungs (inhalation) and then to remove oxygen-depleted (and carbon dioxide-enriched) air from the lungs (exhalation).

In mammals, breathing involves the action of a combination of contraction of intercostal muscles and of the diaphragm. This contraction has the effect of increasing the volume of the thoracic cavity, which in turn results in a negative air pressure within the lungs (as the lungs expand as well), a pressure that is lower than that found in the surrounding air. This then leads to a pulling of air into the lungs to relieve the pressure differential.

Exhalation is a consequence of a reversal of these processes, with a relaxation of both the intercostal muscles and diaphragm along with a contraction of elastic connective tissue. Together this results in a reduction in the volume of the thoracic cavity. Consequent reduction in lung volume gives rise to a greater air pressure within the lungs versus outside the body, and thus exhalation.

Links to terms of possible interest: Abdominal muscles, Chest cavity, Chest wall, Diaphragm, Exhalation, Expiration, External intercostal muscles, Inhalation, Inspiration, Internal intercostal muscles, Lungs, Pleural cavity, Ribs, Sternum, Thoracic cavity

The above video provides an excellent overview of breathing.

The above video provides great visuals of the processes of inhalation and exhalation.

The above video is a bit off the wall look at what breathing is all about.

The above video is another look at breathing.

Discussion of elastic recoil, as involved in exhalation, begins at 1:45.

External respiration

Uptake of oxygen from and release of carbon dioxide to the environment by an organism.
External respiration is the intended consequence of breathing. It occurs specifically across what are known as alveolar membranes, which is where gasses breathed into the lungs are exchanged with gasses dissolved in the blood.

Oxygen thus moves from outside of the body to deeply inside of the lungs and then diffuses into capillaries associated with the lungs (and specifically those capillaries associated with microscopic lung structures known as alveoli).

Meanwhile, carbon dioxide that is dissolved in the blood moves in the opposite direction into the alveoli and ultimately out of the lungs and out of the body. These processes explicitly represent external respiration.

Links to terms of possible interest: Alveolar capillary, Alveolus, External respiration, Internal respiration, Pulmonary capillary, Pulmonary circulation, Respiratory membrane, Systemic capillary, Systemic circulation

This video provides a nice overview of the anatomy of breathing though not of the inhalation and exhalation process itself.

Internal respiration

Uptake of oxygen and release of carbon dioxide within the body.
The mitochondria within our cells take up oxygen – as the final electron acceptor for what is known as cellular respiration – and give off carbon dioxide, which is the oxidized form of the carbon compounds we are "burning" to generate ATP. This oxygen comes from blood as it passes through capillaries and at the same time the released carbon dioxide moves into the blood as the blood moves, also through capillaries. And that's it, that is, that's internal respiration.

Cellular respiration thus both uses and gives off gasses. Those gasses either are obtained or gotten rid of in the course of internal respiration. And the blood, now modified in terms of what gasses are being carried, then returns to a higher oxygen, lower carbon dioxide state via subsequent external respiration.

Thus, as an overview distinguishing cellular, internal, and external respiration, with one leading to the next: Cellular respiration (particularly in terms of generation of CO2) → Internal respirationExternal respiration (releasing CO2 and taking up O2) → Internal respirationCellular respiration (particularly in terms of use of O2) → Cellular respiration (particularly in terms of generation of CO2) → Internal respiration, and so on.

Links to terms of possible interest: Carbon dioxide, CO2, Cellular respiration, Internal respiration, O2, Oxygen

This video contrasts external respiration and internal respiration, starting with an overview of external respiration; note that when oxygen is described as travelling as a gas out of the blood to the tissues it is as a dissolved gas rather than as bubbles (the latter, i.e., as the concept of "travelling as a gas" might otherwise bring to mind).

Upper respiratory tract

Somewhat external air handling anatomy including the external nose, the associated nasal cavity, and the pharynx.
The upper respiratory tract includes those aspects of the respiratory system which are closer to the external environment, that is, less buried or not buried so deeply within the body. These are the morphologies that first contact inflowing air in the course of inhalation and which are the last to contact outflowing gasses during exhalation. This particularly is the respiratory anatomy associated with the head and a fair amount of the neck.

Another way of distinguishing the upper respiratory tract particularly from the lower respiratory tract is that infections of the latter, i.e., the lower respiratory tract, often are more serious than those of the former (the upper respiratory tract).

Links to terms of possible interest: Auditory tube, Epiglottis, Esophagus, Hard palate, Larynx, Nasal cavity, Nasopharynx, Nostril, Oral cavity, Pharynx, Sinus, Tongue, Tonsils, Trachea, Upper respiratory tract, Uvula

The above video provides a nice overview of the anatomy of the upper respiratory tract.

The above video provides a different perspective on the anatomy of the upper respiratory tract.


Anatomy associated with the warming, filtering, and moistening of air in the course of inhalation.
The nose can be differentiated into an external nose, which is what we recognized as "the nose" and which contains the nostrils (also known as nares), and various internal structures that are associated with the nose, particularly the nasal cavity but also the sinuses. It is hairs within the nose that perform the filtering role and the presence of mucous membranes within the noise that provides the moistening action.

Links to terms of possible interest: Alar facial groove, Columella, Domes, External nose, Glabella, Infratip break, Nasal bridge, Naslon, Nasolabial angle, Nose, Nostril, Nostril sill, Philtrum, Supra alar crease, Supratip break, Tear trough

The above video provides an overview of the upper respiratory tract, with quite a bit of emphasis on the nose and surrounding anatomy (caution: at times a bit disgusting).

Nasal cavity

The internal volume found behind the external nose.
The nasal cavity represents the internal portion of the nose. The nasal cavity is lined with epithelial tissue, and indeed mucous membrane. Found further towards the lungs are cilia, which push mucus out of the nasal cavity, in this case towards the mouth where it can be swallowed. This mucus carries trapped particles towards the pharynx, that is, towards the throat. The mucus also supplies moisture to the incoming air, and extensive vasculature in the vicinity of the epithelial tissue also provides warmth which serves to heat the incoming air.

Links to terms of possible interest: Bulla ethmoidalis, Frontonasal duct, Hiatus semilunar, Infundibulum, Nasal cavity, Nasolacrimal duct, Maxillary sinus, Nasal sinuses, Spheno-ethmoidal recess, Sphenoidal sinus, Superior meatus


Volumes that are found within aspects of bodies, particularly air-filled spaces within bones.
Sinuses found within skull and facial bones (i.e., paranasal sinuses) help to lighten our skulls (a very important consideration given the excessive weight of our brains!) and also contribute to our vocalization, providing resonance. These sinuses are lined with mucous membrane and, of course, can become inflamed, leading to sinus headaches. Note that various sinuses exist through the body as "sinus" is simply a term that describes open volumes within the body.

Links to terms of possible interest: Ethmoid sinus, Frontal sinus, Inferior concha, Inferior meatus, Maxillary sinus, Middle concha, Middle meatus, Nasal cavity, Nasal sinuses, Paranasal sinuses, Pharynx, Sphenoid sinus, Superior concha, Superior meatus

The above video provides a fascinating look at the superficial anatomy of the various sinuses found within the human skull.

The above video begins with nice visuals of the sinuses but then emphasizes sinusitis, which is fine, though most of the video is devoted to that pathology.


The back of the mouth and nose, also known as the throat.
Food as well as air, both coming and going, pass through the pharynx either after or before passing through the mouth. This results, particularly in humans, in a potential conflict, as movement of food or water into the trachea (which leads to the lungs) is neither a good idea nor pleasant.

As a consequence, we have what is known as the epiglottis, which is found at the entrance to the trachea, within the pharynx (atop a structure known as the glottis), which folds to close this entrance during swallowing. The pharynx opens also to esophagus.

Links to terms of possible interest: Laryngopharynx, Nasopharynx, Oropharynx, Pharynx

The above video walks us through the anatomy as well as some histology of the pharynx using a half-head model.

Lower respiratory tract

Larynx, trachea, bronchi, bronchioles, and alveoli.
The lower respiratory tract, as found particularly within the chest, is substantially more internal within the body than the upper respiratory tract. The lower respiratory tract consists of various increasingly branching tubes (bronchi and then bronchioles) that ultimately terminate in gas-exchange structures known as alveoli

Infections of the lower respiratory tract tend to be more serious than those of the upper respiratory tract (e.g., sniffles or a sore throat versus a deep, hacking cough and/or pneumonia). The reason that such infections can be more serious than those of the upper respiratory tract is that the lower respiratory tract is where gas exchange takes place, and infections or other diseases of the lower respiratory tract can directly interfere with this gas exchange. Upper respiratory tract infections can be unpleasant whereas lower respiratory tract infections can kill you. .

Links to terms of possible interest: Alveoli, Bronchus, Diaphragm, Larynx, Lung, Pharynx, Pleura, Throat, Trachea, Voice box, Windpipe

The above video has a lot of interesting material on the subject of lower respiratory tract infections, but also a fair amount of material that I can't easily verify, e.g., as pertaining to dietary recommendations (though the dietary advice given is otherwise mostly sound).


Post-pharynx cartilaginous connector to the trachea that includes the epiglottis, glottis, and vocal cords.
The larynx can be viewed as a series of specialized structures that serve as the upper portion of the trachea, or windpipe as the trachea otherwise is known.

If something "goes down the wrong pipe" while swallowing, it is the larynx that the fluid or food has entered. Since the lower respiratory tract is exclusively involved in breathing, versus the upper respiratory tract which is also involved especially with eating and drinking, it is not a good thing at all for materials other than air to enter the larynx. To reduce the likelihood of this happening, the upper-most portion of the larynx consists of what is known as the epiglottis.

Links to terms of possible interest: Epiglottis, Esophagus, Glottis, Larynx, Subglottis, Supraglottis, Tongue, Trachea, Vocal cord

The above video is on the lower respiratory tract but with emphasis on the larynx.


Flap responsible for sealing the larynx during swallowing so that passage of food or water into the trachea may be avoided.
The uppermost portion of the larynx as well as the uppermost portion of the lower respiratory tract is the epiglottis. During swallowing the epiglottis folds over to seal against the glottis, therefore closing off the lower respiratory tract from the upper respiratory tract. The epiglottis thus protects the transition point between the dual-use pharynx and the air-only larynx, trachea, and the rest of the lower respiratory tract.

Links to terms of possible interest: Epiglottis, Esophageal sphincter, Esophagus, Glottis, Larynx, Pharynx, Stomach, Trachea

The above video does a nice job of introducing us to why we need an epiglottis.

See the epiglottis in action (in a model) here: 2:36.

Use the above video to orient yourself as to the location of the epiglottis as viewed looking into the mouth; that's it just above the tongue.

Vocal cords

Connective tissue associated with the larynx that serves to impart vibrations on air especially during exhalation.
This connective tissue is found in association with the opening into the larynx, the same location that the epiglottis folds over during swallowing. The connective tissue can be stretched by associated muscles, with greater stretching associated with higher pitch. The voice box is found within a cartilaginous cage that we call the Adam's apple (as so called particularly in males) and which also defines more or less the location of our larynx versus our pharynx/throat.

Links to terms of possible interest: Cartilage, Epiglottis, Glottis, Trachea, Vocal cords, Windpipe

The above video is a short, cute look at the larynx.

Steven Tyler's vocal cords, and their rehabilitation.