Appearance
8.1 — The Airway
Air entering your nose is conditioned before it reaches your lungs: warmed to within a degree of body temperature, humidified to 100 percent saturation, and filtered of essentially every particle above 10 micrometres. This happens in about a quarter of a second, over a distance of a few centimetres, and it is why breathing freezing dry air on a mountain does not destroy your lungs.
The airway also has a second job, which is to keep everything except air out of it — and Chapter 3.6 explained why that is harder than it should be.
The nose
Three shelves of bone — the conchae — project into each nasal cavity, covered in richly vascular mucous membrane. They break the airflow into thin sheets and force it into contact with the warm wet surface.
The result is impressive conditioning. Air at −10 °C and 20 percent humidity arrives at the back of the throat at about 32 to 34 °C and near-full saturation.
This is why mouth-breathing feels different and dries the throat, and why a person who cannot breathe through the nose — from a deviated septum, polyps, or chronic congestion — sleeps badly and wakes with a dry sore throat.
Filtering is done by hairs at the entrance for large particles, and by the mucus blanket for smaller ones.
And nasal resistance is a real number. The nose accounts for around half of total airway resistance in quiet breathing, which is why you switch to mouth breathing during exercise — the resistance becomes limiting.
The nose also alternates. The nasal cycle swings congestion from one side to the other every few hours, so one nostril is always more open. Most people only notice it when they have a cold.
Smell happens at the roof of each cavity, where olfactory receptors sit (Chapter 11.13). This is why food tastes flat with a blocked nose — most of "taste" is smell.
Epistaxis — nosebleed. Around 90 percent come from a plexus of vessels on the front of the septum, called Little's area, where several arteries meet just under thin mucosa.
The correct first aid follows from that location. Sit up and lean forward — leaning back sends blood down the throat, causing vomiting and hiding how much is being lost. Pinch the soft part of the nose, not the bony bridge, because the bleeding point is in the soft part. Hold continuously for 10 to 15 minutes without checking, because releasing to look disrupts the forming clot.
The pharynx and the larynx
The pharynx is the shared passage for air and food (Chapter 6.3), running from the back of the nose to the larynx.
The tonsils and adenoids form a ring of lymphoid tissue around its entrance — a sensible position for the immune system's first checkpoint, sampling everything entering the body (Chapter 13.1). They are largest in childhood and shrink after puberty, which is why tonsillitis is a childhood disease.
Enlarged adenoids and tonsils are the leading cause of obstructive sleep apnoea in children, and it presents quite differently from the adult version: not sleepiness but hyperactivity, poor concentration, bedwetting and poor growth. Removing them frequently transforms a child's behaviour and school performance, which is a satisfying outcome from a simple operation.
The larynx does three things: it protects the airway, it produces voice, and it closes to allow the pressure needed for coughing, lifting and straining.
The thyroid cartilage is the "Adam's apple", larger in men because testosterone enlarges the larynx at puberty — which is also why the voice deepens: a longer vocal cord vibrates more slowly, exactly like a longer guitar string.
The cricoid cartilage below it is the only complete ring in the entire airway — everything below is C-shaped with a soft back wall.
That completeness has two practical consequences. It is the narrowest point of the adult airway, so it is where an inhaled object most often lodges. And it can be pressed backward to compress the oesophagus behind it — cricoid pressure, used during emergency anaesthesia to reduce the risk of stomach contents entering the lungs, though its effectiveness is now debated.
The vocal cords are two folds of tissue stretched across the airway. Air passing between them makes them vibrate, and changing their tension and length changes pitch. The sound produced at the cords is a buzz — it becomes speech only after being shaped by the throat, mouth, tongue and lips.
Both cords are supplied by the recurrent laryngeal nerve, with its absurd detour (Chapter 3.6). A hoarse voice that persists for more than three weeks needs investigation, because it can be the first sign of a laryngeal cancer or of a lung cancer pressing on that nerve where it loops under the aortic arch.
The trachea and bronchi
The trachea is about 10 to 12 centimetres long and 2 centimetres wide, held open by 15 to 20 C-shaped cartilage rings.
The gap at the back is deliberate. It is filled with muscle and lies against the oesophagus, so a swallowed bolus can bulge into that space. A complete ring would make swallowing large mouthfuls impossible.
The trachea divides at the carina, at the level of the sternal angle (Chapter 5.4).
The right main bronchus is wider, shorter and more vertical than the left, because the heart displaces the left one.
So an inhaled object goes right, most of the time. This is one of the highest-yield anatomical facts in clinical medicine: an inhaled peanut, tooth, or piece of food most commonly lodges in the right lower lobe, and an endotracheal tube inserted too far goes down the right main bronchus, ventilating only the right lung. Unequal chest movement and absent breath sounds on the left after intubation means the tube is too deep, and the fix is to withdraw it a couple of centimetres.
The branching pattern. Trachea → two main bronchi → lobar bronchi (three right, two left) → segmental bronchi → smaller bronchi → bronchioles → terminal bronchioles → respiratory bronchioles → alveolar ducts → alveoli.
About 23 generations of branching, and the branching is roughly dichotomous — each tube splits into two.
Cartilage disappears and muscle takes over as you descend. Bronchi have cartilage; bronchioles have none, and instead have a ring of smooth muscle.
This is exactly why asthma affects the bronchioles. They have no cartilage to hold them open and a muscular wall that can contract, so they can narrow dramatically — and because resistance depends on the fourth power of radius, modest narrowing has a large effect (Chapter 7.5).
And it is why bronchodilators work. Salbutamol relaxes that smooth muscle within minutes (Chapter 22.10).
The mucociliary escalator lines the whole conducting airway: goblet cells produce mucus, and ciliated cells beat it upward at about 1 centimetre per minute (Chapter 4.2). You swallow about 100 ml of respiratory mucus a day without noticing.
Cigarette smoke paralyses cilia within minutes and destroys them with chronic exposure, and it increases mucus production. So the escalator stops and the load increases — which is exactly why smokers cough, and why the cough is worst first thing in the morning after a night of accumulation with no coughing.
Encouragingly, ciliary function begins recovering within days to weeks of stopping, and the cough often temporarily worsens as the escalator restarts and clears the backlog. People frequently interpret that as harm and resume smoking, so it is worth knowing in advance that it is a sign of recovery.
Dead space
Not all inhaled air reaches the alveoli. The air filling the conducting airways at the end of a breath never gets far enough to exchange gas.
Anatomical dead space is about 150 ml in an adult — roughly 2 ml per kilogram.
With a tidal volume of 500 ml, only 350 ml of each breath is fresh air reaching the alveoli.
This has a consequence for breathing pattern that is genuinely important and not obvious. Compare two patterns delivering the same minute volume:
Deep and slow: 500 ml × 12 breaths = 6,000 ml/min. Alveolar ventilation = (500 − 150) × 12 = 4,200 ml/min.
Shallow and fast: 250 ml × 24 breaths = 6,000 ml/min. Alveolar ventilation = (250 − 150) × 24 = 2,400 ml/min.
Same total air moved; almost half the useful ventilation. Because the dead space is fixed, it takes a larger proportional bite out of a small breath.
This is why shallow rapid breathing is inefficient and why it is a warning sign. A patient with rib fractures breathing shallowly (Chapter 5.4), an exhausted asthmatic, or a person with abdominal pain all suffer from it. And it is why "take slow deep breaths" is genuinely useful advice rather than a platitude.
Physiological dead space adds alveoli that are ventilated but not perfused — normally negligible, but greatly increased by a pulmonary embolus, where a whole region receives air and no blood.
Airway emergencies, briefly
Choking — Chapter 23.2 has the full procedure. Anatomically, a complete obstruction above the cords means no air moves and no sound is produced, which is why silence is more alarming than coughing. A person who can cough forcefully should be encouraged to cough — their own cough generates far higher pressures than any manoeuvre.
Anaphylaxis — swelling of the tongue, throat and larynx can close the airway within minutes. Chapter 23.5.
Croup — a viral infection causing swelling just below the cords in young children, producing a characteristic barking cough and a harsh noise on inspiration. The reason it is so much worse in children is geometric: a young child's airway is only about 4 millimetres across, and 1 millimetre of circumferential swelling reduces the cross-sectional area by about 75 percent. The same swelling in an adult is barely noticed. Steroids work extremely well and have transformed what used to be a frightening illness.
Epiglottitis — bacterial infection of the epiglottis, which swells and can obstruct completely. Once a feared paediatric emergency, it has become rare where Hib vaccination is routine — one of the clearest vaccine success stories, and a reminder of how much of the frightening paediatrics of fifty years ago has simply been abolished.
When the airway must be secured, the options run from a simple manoeuvre (head tilt, chin lift) through airway adjuncts to a tube passed through the cords, and finally to a surgical opening through the cricothyroid membrane — a thin gap between two cartilages, easily felt, with no major structures in front of it. That anatomical accessibility is why it is the emergency route of last resort.
What the next page fixes
The airway conducts. Chapter 8.2 covers where the work actually happens: the lungs, the pleura that lets them slide against the chest wall, and the alveoli — 300 million air sacs providing a gas exchange surface roughly the size of a tennis court, separated from your blood by a barrier half a micrometre thick.