21.1. The upper respiratory system

The upper respiratory system includes the nose, nasal cavity, mouth, pharynx, and larynx. This chapter deals with the structure and respiratory role of the nose, mouth, and nasal cavity. The pharynx is reviewed in the chapter on swallowing and the larynx in the chapter on voice.

The roles of the mouth

The mouth is of primary importance to both digestion and respiration. Vertebrates other than mammals lack a palate and the air that enters the nostrils travels through the mouth before reaching the larynx. Mammals also use the mouth for respiration. While humans tend to breath through the nose at rest, most air is transported through the open mouth when running. Dogs breath through the mouth (panting) to cool the body in hot weather or after exercise.

The evolution of the palate in mammals separated the nasal cavity from the oral cavity from the face to the pharynx. This allowed the nasal cavity to become dedicated to respiration and promoted the evolution of elaborate olfaction in mammals.

The Buccal Pump

Fish exchange gases by pulling oxygen-rich water through their mouths and pumping it over their gills. The gills push the oxygen-poor water out through openings in the sides of the pharynx. Some fish, like sharks and lampreys, possess multiple gill openings. However, bony fish have a single gill opening on each side. This opening is hidden beneath a protective bony cover called an operculum.

Respiration in fishes is greatly enhanced by the ability to force a flow of water by the gills (ventilation). Fishes evolved the buccal pump, a mechanism in which muscular contractions at the mouth produce ventilation. In a cyclic patter, the mouth is opened and the gill openings are closed. The oral cavity is then expanded by lowering the floor of the mouth and/or expanding the operculum laterally. This causes an inflow of water through the mouth. The mouth is then closed and the gill openings are opened. The volume of the oral cavity is reduced by elevation of the floor of the mouth and/or contraction of the opercular muscles. This forces water in the oral cavity to pass by the gills and leave through the gill openings.

Some cartilaginous fishes have diminished or abandoned buccal pumping as a method of ventilation. These animals swim constantly and they can produce flow by their gills by simply opening the mouth as they swim. This forces them, however, to never stop swimming, or to switch to buccal pumping when they stop.

Buccal pumping is also used by amphibians as their only method of ventilation. These animals lack a diaphragm and they have very short ribs that do not form a rib cage. They ventilate by moving the floor of the mouth in a rhythmic manner that is externally apparent.

Fishes and amphibians employ two methods of buccal pumping, defined by the number of movements of the floor of the mouth needed to complete both inspiration and expiration.

Four-stroke buccal pumping is used by some basal ray-finned fishes and amphibians. First, the glottis (opening to the lungs) is closed, and the nostrils are opened. The floor of the mouth is then depressed (lowered), drawing air in. The nostrils are then closed, the glottis opened, and the floor of mouth raised, forcing the air into the lungs for gas exchange. To deflate the lungs, the process is reversed.

Two-stroke buccal pumping is found amphibians. It has simpler mechanics but involves some mixing of used and fresh air. The floor of the mouth is lowered, drawing air from both the outside and lungs into the buccal cavity. When the floor of the mouth is raised, the air is pushed out and into the lungs; the amount of mixing is generally small, about 20%.

With the exception of some lizards, amniotes have abandoned buccal pumping as a ventilation mechanism and adopted aspiration breathing instead. The organization of the musculature of the floor of the mouth, which drives the buccal pump in amphibians, was maintained in amniotes.

Nose

All vertebrates have nasal cavities and the nostrils form the entrance into the respiratory system. A protruding nose is a specialization mostly observed in mammals and it is highly variable in size and structure. In cetaceans, the nose has been reduced to the nostrils, which have migrated to the top of the head and allow the animal to breathe while mostly submerged. Other mammals have evolved muscular noses that can be stirred to direct the sense of olfaction. Aquatic species have muscular control over the closure of the nostrils, to prevent flooding of the nasal cavity during dives. An extreme development is observed in the elephant, in which the nose (trunk) is used not only for breathing and smelling, but also to manipulate objects and transport water.

The rhinarium (or nose pad) is the naked skin surface surrounding the external openings of the nostrils in most mammals. It is typically crenellated (wrinkled or embossed) but its structure and role is variable among mammals according to ecological niche. In aquatic species, development of lobes next to the nostrils allow them to be closed for diving. In mammals that dig or root with their noses, the rhinarium often develops into a resilient pad, with the nostrils off to the side or below, and capable of closing for exclusion of dust. Examples include the common wombat, marsupial mole, and members of the Chrysochloridae. In the elephants it has become a tactile organ. In the walrus, it is covered with stiff bristles to protect it during foraging for shellfish. In many other species, the role of the rhinarium remains to be elucidated. The philtrum (or medial cleft) is a vertical groove in the middle of the upper lip, extending up to the nose. In most mammals, the philtrum is a narrow groove that may carry dissolved odorants from the rhinarium to the vomeronasal organ via ducts inside the mouth. For humans and most primates, the philtrum survives only as a vestigial medial depression between the nose and upper lip.

The human nose has a pair of nostrils containing nasal hairs that remove larger particles or insects from the inspired air. It is structured by the nasal bone, the maxilla and six cartilages: the cartilage of the septum, greater alar cartilage, lateral nasal cartilage, lesser alar cartilages, vomeronasal cartilage and the accessory nasal cartilages. Several skeletal muscles provide some limited movement to the nose and flaring of the nostrils.

Development of the nose

The facial prominences are five swellings that appear in the fourth week of development and derive from the first and second pharyngeal arches. They are basically made of mesenchyme that derives from the neural crest. The frontonasal prominence originates the nose, the medial part of the upper lip and the primary hard palate. The maxillary prominences (one on each side) form the secondary hard palate, and the sides of the upper jaw and lip. The mandibular prominences (one on each side) form the lower jaw.

The frontonasal prominence is ventral to the forebrain. It is derived from neural crest cells, which have an ectodermal origin. These neural crest cells migrate from the ectoderm as the forebrain closes, invading the space that will form the frontonasal prominence. The maxillary and mandibular prominences are derived from the first arch. The maxillary prominence is initially located superior/lateral to the stomodeum (primitive mouth) while the mandibular prominence is located inferior to it and will fuse early on.

The frontonasal prominence originates a medial nasal prominence and two lateral nasal prominenses. As the maxillary prominences grow, they fuse with the medial nasal prominences. This establishes the bridge of the nose and the intermaxillary segment, which is the region of the medial nasal prominence located inferior to the bridge of the nose and superior to the mandibular prominence. The intermaxillary segment yields the portion of the upper lip containing the philtrum, the upper jaw with 4 incisors, and the primary palate. The medial prominence fuses with the maxillary prominence, giving rise to a smooth upper lip while fusing the primary and secondary palate. Meanwhile, the lateral nasal prominence gives rise to the alae of the nose and fuses with the maxillary prominence, forming the nasolacrimal duct. This duct is formed when the ectoderm thickens into a cord and sinks into the underlying mesenchyme.

Development of the nasal cavity

The separation of the medial from the lateral nasal prominences forms two nasal pits. The nasal pits deepen and develop the nasal sacs in the sixth week. These new structures grow dorsocaudally in front of the forming brain. In the beginning, the nasal sacs are separated from the oral cavity by the oronasal membrane. This membrane disappears in the seventh week leaving a connection between the nasal cavities and the oral cavity, called the primitive choanae. Later, when the development of the secondary palate occurs, the choanae is displaced to the junction of the nasal cavity and the pharynx. The nasal septum grows as a descending growth from the merged nasal prominences and fuses with the palatine process between the ninth and eleventh week. Finally, the lateral walls and the superior, middle and inferior conchae develop in each nasal cavity.

Nasal cavity

The nasal cavities in mammals are exceptionally large among vertebrates, typically occupying up to half the length of the skull. In some groups, however, including primates, bats, and cetaceans, the cavity has been secondarily reduced, and these animals consequently have a relatively poor sense of smell. The nasal cavity of mammals has been enlarged, in part, by the development of a palate that separates the entire upper surface of the original oral cavity The palate therefore forms a new roof to the mouth. The cavity also extends into neighbouring skull bones, forming additional air cavities known as paranasal sinuses.

The enlarged nasal cavity contains complex turbinates (or conchae) which are convoluted structures of thin bone or cartilage. They are lined with mucous membranes that can perform two functions: 1) They can improve the sense of smell by increasing the area available to absorb airborne chemicals; 2) They can warm and moisten inhaled air, then extract heat and moisture from exhaled air to prevent desiccation of the lungs.

Turbinates are divided in two functional types. Olfactory turbinates are found in all living tetrapods, whereas respiratory turbinates have been found in mammals, birds and possibly some reptiles. Dogs and other canids possess well-developed nasal turbinates. These turbinates allow for heat exchange between small arteries and veins on their maxilloturbinate (turbinates positioned on maxilla bone) surfaces in a counter-current heat-exchange system. Dogs are capable of prolonged chases, in contrast to the ambush predation of cats, and these complex turbinates play an important role in enabling this (cats only possess a much smaller and less-developed set of nasal turbinates). This same complex turbinate structure help conserve water in arid environments. The water conservation and thermoregulatory capabilities of these well-developed turbinates in dogs may facilitate their survival in the harsh Arctic environment.

Olfactory turbinates that are involved in sensing smell rather than preventing desiccation. While the maxilloturbinates are located in the path of airflow to collect moisture, sensory turbinates are positioned farther back and above the nasal passage, away from the flow of air. These are commonly called ethmoturbinates due to their proximity to the ethmoid bone.

Vomeronasal (Jacobson's) organs

Many animals, including most mammals and reptiles, but not humans, have two distinct and segregated olfactory systems: a main olfactory system, which detects volatile stimuli, and an accessory olfactory system, which detects fluid-phase stimuli. Behavioral evidence suggests that these fluid-phase stimuli often function as pheromones, although pheromones can also be detected by the main olfactory system. In the accessory olfactory system, stimuli are detected by the vomeronasal organ, located in the vomer, between the nose and the mouth. Snakes use it to smell prey, sticking their tongue out and touching it to the organ. Some mammals make a facial expression called flehmen to direct stimuli to this organ.

The sensory receptors of the accessory olfactory system are located in the vomeronasal organ. As in the main olfactory system, the axons of these sensory neurons project from the vomeronasal organ to the accessory olfactory bulb, which in the mouse is located on the dorsal-posterior portion of the main olfactory bulb. Unlike in the main olfactory system, the axons that leave the accessory olfactory bulb do not project to the brain's cortex but rather to targets in the amygdala and bed nucleus of the stria terminalis, and from there to the hypothalamus, where they may influence aggression and mating behavior.

The vomeronasal organ of mammals is generally similar to that of reptiles. In most species, it is located in the floor of the nasal cavity, and opens into the mouth via two nasopalatine ducts running through the palate, but it opens directly into the nose in many rodents. It is, however, lost in bats, and in many primates, including humans.

Human Nasal Cavity

In humans, the nasal cavity is separated into left and right sections by the nasal septum. The nasal septum is formed anteriorly by a portion of the septal cartilage (the flexible portion you can touch with your fingers) and posteriorly by the perpendicular plate of the ethmoid bone (a cranial bone located just posterior to the nasal bones) and the thin vomer bones (whose name refers to its plough shape).

Each lateral wall of the nasal cavity has three bony projections, called the superior, middle, and inferior nasal conchae. The inferior conchae are separate bones, whereas the superior and middle conchae are portions of the ethmoid bone. Conchae serve to increase the surface area of the nasal cavity and to disrupt the flow of air as it enters the nose, causing air to bounce along the epithelium, where it is cleaned and warmed. The conchae and meatuses also conserve water and prevent dehydration of the nasal epithelium by trapping water during exhalation.

The floor of the nasal cavity is composed of the palate. The hard palate at the anterior region of the nasal cavity is composed of bone. The soft palate at the posterior portion of the nasal cavity consists of muscle tissue. Air exits the nasal cavities via the internal nares and moves into the pharynx. The internal nares are the narrowing that marks the passage from the nasal cavity to the nasopharynx.

Several bones that help form the walls of the nasal cavity have air-containing spaces called the paranasal sinuses, which serve to warm and humidify incoming air. Sinuses are lined with a mucosa. Each paranasal sinus is named for its associated bone: frontal sinus, maxillary sinus, sphenoidal sinus, and ethmoidal sinus. The sinuses produce mucus and lighten the weight of the skull.

The nares and anterior portion of the nasal cavities are lined with mucous membranes, containing sebaceous glands and hair follicles that serve to prevent the passage of large debris, such as dirt, through the nasal cavity. An olfactory epithelium used to detect odors is found deeper in the nasal cavity.

The conchae, meatuses, and paranasal sinuses are lined by respiratory epithelium composed of pseudostratified ciliated columnar epithelium. The epithelium contains goblet cells, one of the specialized, columnar epithelial cells that produce mucus to trap debris. The cilia of the respiratory epithelium help remove the mucus and debris from the nasal cavity with a constant beating motion, sweeping materials towards the throat to be swallowed. Interestingly, cold air slows the movement of the cilia, resulting in accumulation of mucus that may in turn lead to a runny nose during cold weather. This moist epithelium functions to warm and humidify incoming air. Capillaries located just beneath the nasal epithelium warm the air by convection. Serous and mucus-producing cells also secrete the lysozyme enzyme and proteins called defensins, which have antibacterial properties. Immune cells that patrol the connective tissue deep to the respiratory epithelium provide additional protection.

Summary

The mouth is an important air passage in mammals during accelerated respiration. It is also a key component of the buccal pump in fishes and amphibians. The nose is cartilaginous and muscular and can be stirred to direct the olfaction. The vomeronasal organ analyses non-volatile odorants collected from the environment. The nose develops from the frontonasal and the medial nasal prominences of the embryo. The respiratory turbinates in the nasal cavity humidify, warm up and clean the inspired air. They also cool down and remove humidity from the expired air. The olfactory turbinates contain the olfactory epithelium. The upper respiratory system is lined with pseudostratified ciliated columnar eiphelium, except for the mouth, oropharynx and laryngopharynx with are lined with stratified squamous epithelium.

Key terms

Mouth, thermoregulation, panting, buccal pump, operculum, ventilation, two-stroke pumping, four-stroke pumping, nose, nostrils, rhinarium, crenellation, philtrum, vomeronasal organ, septal cartilage, greater alar cartilages, lateral nasal cartilage, lesser alar cartilage, vomeronasal cartilage, accessory cartilages, frontonasal prominence, medial nasal prominence, lateral nasal prominence, intermaxillary segment, nasal pits, turbinate, Jacobson’s organ, perpendicular plate, nasal septum, superior nasal conchae, middle nasal conchae, inferior nasal conchae, external nares, internal nares, paranasal sinuses, respiratory epithelium, pseudostratified ciliated columnar epithelium.

Figure credits

Figure 1 by OpenStax College – Anatomy and Physiology. Https://cnx.org/contents/FPtK1zmh@8.79:t2sgkCQ-@9/Organs-and-Structures-of-the-R. F

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Figure 8 by Original uploader - his Shi Tzu's nose. - Transferred from en.wikipedia by SreeBot, CC BY-SA 2.5, https://commons.wikimedia.org/w/index.php?curid=17275464

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Figure 10 by Henry Vandyke Carter - Henry Gray (1918) Anatomy of the Human Body (See "Book" section below)Bartleby.com: Gray's Anatomy, Plate 852, Public Domain, https://commons.wikimedia.org/w/index.php?curid=566786

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Figure 15 by OpenStax College – Anatomy and Physiology. https://cnx.org/contents/FPtK1zmh@8.79:t2sgkCQ-@9/Organs-and-Structures-of-the-R. F. Fig. 3.

Figure 16 by OpenStax College – Anatomy and Physiology. https://cnx.org/contents/FPtK1zmh@8.79:t2sgkCQ-@9/Organs-and-Structures-of-the-R. F. Fig. 4.