The upper airway is the stretch of breathing passage that runs from your nostrils and mouth down to the lower border of the larynx, just above where the trachea begins. It includes the nasal cavity, the paranasal sinuses, the pharynx (throat), and the larynx (voice box). Far from being a simple pipe that funnels air into your lungs, this region heats, humidifies, and filters every breath you take, produces your voice, defends against pathogens, and orchestrates the surprisingly complex act of swallowing without letting food fall into your lungs. Each of those jobs depends on specialized anatomy that is worth understanding on its own terms.
The Nasal Cavity and What It Does to the Air You Breathe
Air enters the upper airway through the nostrils and passes into the nasal cavity, a chamber divided down the middle by the nasal septum. The walls are lined with a warm, moist mucous membrane draped over bony ridges called turbinates, which force incoming air into turbulent swirls. That turbulence is the point: it increases the contact between air and tissue so the nose can do its conditioning work. Inspired air is warmed and humidified through evaporation of water from the surface lining of the nasal passages.1PubMed. Observations on the ability of the nose to warm and humidify inspired air By the time air reaches the back of the nasal septum, computational studies show it has been heated and moistened to roughly 90% of what the lungs require.2PubMed Central. Effects of decongestion on nasal cavity air conditioning efficiency: a CFD cohort study The lungs are delicate; cold, dry air irritates them and impairs gas exchange. The nasal cavity prevents that.
Lining much of the nasal cavity and the rest of the upper airway is a layer of mucus-producing cells topped with microscopic hair-like projections called cilia. Together they form the mucociliary clearance system, one of the body’s most important front-line defenses. Mucus traps inhaled particles, bacteria, viruses, and fungi, while the cilia beat in coordinated waves to sweep that debris toward the throat, where it is swallowed and destroyed by stomach acid. When this system breaks down, the consequences are severe: conditions involving defective cilia or abnormal mucus lead to lifelong recurrent respiratory infections.3PubMed Central. Mucociliary clearance–a critical upper airway host defense mechanism and methods of assessment
The upper airway microbiota adds another layer of defense. Resident bacteria in the nasal passages and throat help exclude pathogens and prime the immune system, working alongside the physical barriers of mucus and cilia to keep infections in check.4PubMed Central. The Upper Airway Microbiota, Environmental Exposures, Inflammation, and Disease
The Hidden Role of the Paranasal Sinuses
Flanking the nasal cavity are the paranasal sinuses, air-filled pockets inside the skull bones. People tend to notice their sinuses only when they are congested and painful, but these spaces have a function that was unknown until the mid-1990s. The lining of healthy sinuses continuously generates large amounts of nitric oxide, a gas with strong antimicrobial and blood-vessel-dilating properties.5PubMed. Nitric oxide and the paranasal sinuses With every breath through the nose, a dose of that sinus-derived nitric oxide is carried into the lungs, where it dilates the tiny blood vessels surrounding the air sacs and improves oxygen uptake. Researchers have described it as an “aerocrine” hormone: produced in one location (the sinuses) and transported by airflow to its site of action (the lungs).
The practical significance was demonstrated in patients breathing through an endotracheal tube, which bypasses the nose entirely. When nasal air was reintroduced into their breathing circuit, pulmonary vascular resistance dropped and blood oxygen levels increased.6PubMed. Inhalation of nasally derived nitric oxide modulates pulmonary function in humans In other words, the simple act of breathing through your nose delivers a small but measurable pharmacological benefit to your lungs that mouth breathing does not provide.
The Pharynx and Its Three Regions
Behind the nasal cavity and mouth, the airway opens into the pharynx, a muscular tube about 12 centimeters long that runs from the base of the skull down to the top of the esophagus.7Oxford Textbook of Plastic and Reconstructive Surgery. Tumours of the nasopharynx, oropharynx, and hypopharynx The pharynx is where the respiratory and digestive tracts share real estate, and it is divided into three stacked regions based on what lies in front of each one:
- Nasopharynx: the uppermost section, sitting behind the nasal cavity. Air passes through here on its way from the nose. The adenoids (pharyngeal tonsils) live on its back wall, and the openings of the Eustachian tubes connect it to the middle ears.
- Oropharynx: the middle section, behind the mouth. This is the part of the throat you can see when you open wide and say “ahh.” It contains the palatine tonsils and is shared by both air and food.
- Laryngopharynx (hypopharynx): the lowest section, sitting behind the larynx. Here the pathway splits: air is directed forward into the larynx and trachea, while food is directed backward into the esophagus.
The walls of the pharynx are built from three overlapping constrictor muscles stacked from top to bottom, plus three longitudinal muscles that can lift and widen the tube.7Oxford Textbook of Plastic and Reconstructive Surgery. Tumours of the nasopharynx, oropharynx, and hypopharynx This muscular architecture gives the pharynx the ability to squeeze food downward during swallowing while also staying open for breathing the rest of the time. It is a demanding dual role, and the pharynx’s flexibility is both its strength and its vulnerability, as anyone who has experienced obstructive sleep apnea can attest.
The Larynx and Voice Production
At the bottom of the upper airway sits the larynx, a cartilage-reinforced structure that houses the vocal folds (commonly called vocal cords). When you breathe quietly, the vocal folds stay apart to let air pass. When you speak or sing, muscles pull them together so exhaled air vibrates them, producing sound. The pitch you hear depends on the tension, length, and stiffness of the vocal folds, which are controlled by two muscle groups working against each other: the cricothyroid muscles lengthen and tighten the folds, while the thyroarytenoid muscles shorten and thicken them. Their interplay creates a complex control system for fundamental frequency, the acoustic feature you perceive as pitch.8PubMed. Effects of cricothyroid and thyroarytenoid interaction on voice control: Muscle activity, vocal fold biomechanics, flow, and acoustics
Beyond voice, the larynx is the upper airway’s last-resort gatekeeper. Its most critical non-vocal job is sealing shut during swallowing to prevent food or liquid from entering the trachea. That function matters far more than it might seem: aspiration (food or liquid going down the wrong way) can cause pneumonia and, in vulnerable populations, death.
How Swallowing Works Without Drowning Your Lungs
Swallowing is one of the most complex coordinated movements in the human body, involving more than two dozen muscles firing in precise sequence. The upper airway must switch from an open breathing passage to a sealed, food-propelling channel and back again within about a second. Several protective mechanisms overlap to keep food out of the lungs.
The most important of these is glottal closure: the vocal folds snap together and the structures just above them (the false vocal folds) press inward, creating a tight seal over the entrance to the trachea. Animal research has shown that removing the epiglottis, the leaf-shaped flap that folds over the laryngeal opening during swallowing, did not increase aspiration. But disrupting vocal fold closure caused aspiration every single time.9PubMed. Relative contribution of various airway protective mechanisms to prevention of aspiration during swallowing The epiglottis still contributes one layer of protection, and the biomechanical forces that flip it downward are part of normal swallowing mechanics,10PubMed. Absent epiglottic inversion as seen on flexible endoscopic evaluations of swallowing (FEES) is associated with a gestalt reduction in swallowing mechanics but the vocal folds are the non-negotiable safeguard.
Sensory nerves in the larynx play a critical role in triggering these reflexes. When researchers anesthetized the internal branch of the superior laryngeal nerve in healthy volunteers, effectively numbing the larynx’s sensory input, around 43% of swallows led to material entering the larynx, and more than half of those episodes progressed to aspiration. Under normal conditions, that penetration rate was under 2%, and aspiration was never observed.11PubMed Central. Sensory regulation of swallowing and airway protection: a role for the internal superior laryngeal nerve in humans This is why conditions that damage laryngeal nerves, such as certain strokes or surgeries, carry a high risk of aspiration pneumonia.
More recently, researchers identified a class of specialized cells in the trachea and larynx called neuroendocrine cells that initiate protective reflexes by releasing signaling molecules that activate nearby sensory neurons, triggering swallowing and cough-like expiratory reflexes.12PubMed Central. Neuroendocrine cells initiate protective upper airway reflexes The upper airway’s defense against aspiration, in short, is not one mechanism but a stack of redundant systems: muscular closure, epiglottic deflection, sensory nerve reflexes, and chemical signaling cells all work together.
Smell and the Olfactory Epithelium
Tucked into the roof of the nasal cavity is a small patch of tissue called the olfactory epithelium, home to the sensory neurons responsible for your sense of smell. These are bipolar neurons, each extending one end into the mucus layer at the surface, where it sprouts 10 to 15 elongated cilia that bind odor molecules, and one end through the bone above and into the olfactory bulb of the brain.13Frontiers in Neural Circuits. Structures and functions of the normal and injured human olfactory epithelium This means the olfactory neurons are directly exposed to the environment, one of very few places in the body where neurons make contact with the outside world. That exposure is why smelling something is so immediate and unfiltered, and also why the olfactory epithelium is vulnerable to damage from viruses, pollutants, and trauma.
Smell does more than help you enjoy food. It serves as an early warning system for spoiled food, gas leaks, smoke, and toxic chemicals. It is also intertwined with taste; much of what people think of as flavor is actually olfactory input reaching the brain through the back of the throat during chewing. Loss of smell, which became widely discussed during the COVID-19 pandemic, is often an upper airway problem rooted in damage to or inflammation of this epithelium.
Nasal Breathing Versus Mouth Breathing
People debate whether it really matters whether you breathe through your nose or your mouth. The upper airway is designed so that the nose is the default breathing route, and the evidence suggests the distinction matters most during sleep. When researchers compared upper airway resistance during nasal and oral breathing in sleeping adults, nasal breathing produced significantly lower resistance, while mouth breathing nearly tripled resistance during sleep.14European Respiratory Journal. Effect of nasal or oral breathing route on upper airway resistance during sleep Higher resistance during oral breathing makes the airway more likely to narrow or collapse, which is one reason habitual mouth breathers are more prone to snoring and sleep-disordered breathing.
During waking exercise, the differences are more subtle. A study comparing nose-only and mouth-only breathing during resistance exercise found no significant differences in endurance, oxygen saturation, or perceived effort in either men or women. Men who breathed exclusively through their nose did show slightly lower heart rates, but the effect was small.15PubMed Central. Nose vs. mouth breathing– acute effect of different breathing regimens on muscular endurance The popular claim that nasal breathing dramatically improves athletic performance has limited support; where nasal breathing genuinely shines is in air conditioning, nitric oxide delivery, and keeping the airway stable during sleep.
Chronic mouth breathing can cause secondary changes beyond the airway itself. The forward head posture commonly seen in habitual mouth breathers overloads accessory neck muscles and reduces the effectiveness of the diaphragm, the main breathing muscle. Over time, this altered breathing pattern can lead to overuse of the accessory muscles, reduced diaphragmatic activity, and poor coordination between the abdominal and chest muscles.16PubMed Central. Effects of Nasal and Oral Breathing on Respiratory Muscle and Brain Function: A Review
The Upper Airway During Sleep and Obstructive Sleep Apnea
The pharynx has no rigid cartilage or bone to hold it open. It relies on muscle tone to keep from collapsing, which works fine while you are awake. During sleep, the brainstem neurons responsible for sleep regulation reduce the activity of the upper airway dilator muscles.17PubMed Central. Pharyngeal motor control and the pathogenesis of obstructive sleep apnea In most people, the remaining muscle activity is enough to keep the airway patent. But in people whose pharynx is anatomically narrow, whether from enlarged tonsils, a large tongue base, a recessed jaw, or excess fat deposits around the throat, the muscles cannot always compensate for the increased load. The result is obstructive sleep apnea, where the airway repeatedly collapses during sleep, cutting off airflow.18PubMed Central. Control of the pharyngeal musculature during wakefulness and sleep: implications in normal controls and sleep apnea
Nasal obstruction makes the problem worse. When sitting nasal resistance is high, the airway is more prone to collapse when lying down. Researchers found that patients with higher nasal resistance while seated were significantly less likely to have a measurable open airway when supine.19PubMed. Total nasal airway resistance while sitting predicts airway collapse when lying down This is part of why treating nasal obstruction, whether with decongestants, nasal strips, or surgery, can improve snoring and mild sleep apnea even though the actual collapse happens further down in the pharynx. The nose and the throat function as a connected system, not independent compartments.
Researchers use a measure called the critical closing pressure to quantify how collapsible a person’s pharynx is. Exactly how that collapsibility relates to the physical properties of pharyngeal tissue remains poorly understood. A systematic review found that the few studies examining pharyngeal compliance (how easily the tissue deforms under pressure) showed no clear statistical relationship with collapsibility. However, electrical stimulation of the genioglossus, the main tongue-protruding muscle, reduced collapsibility by increasing the airway’s cross-sectional area without changing the tissue’s inherent compliance.20PubMed Central. Anatomical determinants of upper airway collapsibility in obstructive sleep apnea: a systematic review and meta-analysis This finding is behind one of the newer treatments for sleep apnea: surgically implanted nerve stimulators that activate the tongue muscle during sleep to keep the airway open.
How the Upper Airway Changes with Age
The upper airway does not remain the same throughout life. In animal studies, older rats had significantly more collapsible upper airways than younger ones, even with intact nerve function to the tongue muscles. When the nerve controlling the genioglossus was stimulated, the age difference disappeared, suggesting the hardware can still work but its resting activity is less effective at keeping the airway open in older animals. The muscle composition of the genioglossus also shifted with age: the proportion of the fastest-contracting muscle fiber type decreased substantially.21American Physiological Society (J Appl Physiol). Aging increases upper airway collapsibility in Fischer 344 rats These changes parallel what clinicians see in humans: snoring and sleep apnea become more common with age, even in people who maintain a stable weight, and the pharyngeal muscles respond more sluggishly to respiratory challenges.
Age-related changes extend to the nasal cavity too. The mucous membranes thin, blood flow to them decreases, and their ability to warm and humidify air declines. Older adults often report a persistently dry nose, crusting, and a diminished sense of smell. These are not just nuisances; reduced mucociliary clearance in the aging nose means bacteria and viruses have an easier time establishing infections, which helps explain why respiratory infections tend to be more frequent and more serious in older populations.
Why the Infant Upper Airway Is Different
Newborns have an upper airway configuration that looks strikingly different from an adult’s. In infants, the larynx sits much higher in the neck, and the epiglottis reaches up to contact the back surface of the soft palate. This creates a nearly direct channel from the nasopharynx to the larynx, allowing the baby to breathe through the nose and swallow milk at the same time without choking.22Journal of Zoology. Mechanisms of swallowing and airway protection in infant mammals This arrangement is shared with all other infant mammals, and it explains why young babies are obligate nasal breathers: their anatomy routes air through the nose by default, and nasal obstruction from a cold can be genuinely dangerous for a newborn in a way it is not for an older child.
Over the first few years of life, the larynx descends in the neck, the epiglottis separates from the soft palate, and the pharyngeal space expands into the adult configuration. This descent enables the range of vowel sounds needed for speech, but it also creates the pharyngeal crossroads where food and air share a common space, introducing the lifelong risk of choking.
Why Humans Have an Unusual Upper Airway
The human pharynx develops into a configuration that is unique among primates. In most mammals, the epiglottis maintains contact with the soft palate, keeping the respiratory and digestive pathways largely separate. In adult humans, the larynx descends far enough that this contact is lost, creating the long shared pharyngeal space.23PubMed. Development of the supralaryngeal vocal tract in Japanese macaques: implications for the evolution of the descent of the larynx For decades, this descended larynx was assumed to be a uniquely human adaptation for speech. But that narrative got more complicated when researchers discovered that red deer and fallow deer also have descended larynxes, meaning laryngeal descent evolved independently at least twice.24PubMed Central. The descended larynx is not uniquely human
Deer do not speak, but they do vocalize. A lower larynx lengthens the vocal tract and drops the resonant frequencies of calls, making the animal sound larger than it is. Researchers have proposed that size exaggeration, rather than speech, may have been the original evolutionary pressure driving laryngeal descent in our own lineage, with the expanded vowel range required for articulate language arriving as a secondary benefit. The evolutionary path was likely not a single event but a series of changes: the hyoid bone descended relative to the palate (which macaques also show, driven simply by jaw growth), and then the laryngeal skeleton descended relative to the hyoid, a step that appears to have arisen in a common ancestor of the great apes and humans.23PubMed. Development of the supralaryngeal vocal tract in Japanese macaques: implications for the evolution of the descent of the larynx The trade-off is real: our long, flexible pharynx gives us speech but also makes us more vulnerable to choking and sleep apnea than virtually any other mammal.
What Anesthesiologists See
If you have ever had surgery under general anesthesia, your upper airway was a major concern for the anesthesiology team. General anesthesia relaxes the pharyngeal muscles completely, mimicking and worsening the same collapse risk that occurs naturally during sleep. The anesthesiologist must secure the airway, usually by passing a tube through the mouth, past the pharynx, through the vocal folds, and into the trachea. Whether this is easy or difficult depends almost entirely on upper airway anatomy: how wide the mouth opens, how large the tongue is relative to the oral cavity, how far the jaw can protrude, and how much space exists between the chin and the larynx.
Clinicians use bedside tests to predict difficult intubation. A recent prospective study compared several common airway assessment tools and found that the ratio of height to thyromental distance was the most accurate single predictor, outperforming the widely used mouth-opening classification system.25PubMed Central. Comparative Evaluation of Airway Assessment Parameters for Predicting Difficult Intubation: A Prospective Observational Study Short thyromental distance (the space from the chin to the larynx) relative to height suggests a crowded upper airway with less room to maneuver instruments. People with obstructive sleep apnea, obesity, a recessed jaw, or a short thick neck are statistically more likely to be difficult to intubate, and anesthesiologists routinely ask about snoring and sleep apnea during pre-surgical assessments precisely because those conditions signal challenging upper airway anatomy.