Anatomy · Head & Neck

The Nose and Sinuses: Where the Bleeding Starts and the Infection Hides

Every breath you take is warmed to body temperature, saturated with water and stripped of its dust in the few centimetres between your nostril and your throat — a piece of air-conditioning so good that the air reaching your lungs is conditioned within a fraction of a degree, whatever the weather outside. The organ that does it is folded, scrolled and hollowed into a shape of astonishing intricacy, and hidden inside its walls are four air-filled caves that lighten the skull and give a voice its resonance. But the same anatomy that makes the nose so good at its job explains its two commonest troubles: a single patch of skin-thin mucosa on the septum where almost every nosebleed begins, and a set of drainage holes placed so awkwardly that a simple cold blocks them and turns into sinusitis. Learn where each sinus empties and you can predict, from a single blocked nostril, exactly where the infection will spread.

⏱ 14 min read🎯 Linked lesson: The nose and paranasal sinuses· Updated 2026-07-19

THE SCENE

A child comes to the emergency department with a nosebleed that will not stop. The parents are frightened; there is blood down the front of a school shirt and on both hands. The nurse who takes them in does not reach first for cautery, or for a pack, or for a drug. She sits the child forward, tells him to breathe through his mouth, and pinches the soft lower part of his nose — not the bony bridge, the fleshy tip — firmly, between finger and thumb, and holds it there without letting go while she counts out ten full minutes on the clock. The parents think she is doing nothing. In fact she is doing everything: she is pressing the two halves of the septum together over a coin-sized patch of mucosa where four arteries meet and where, in nine out of ten nosebleeds, the bleeding vessel lies. Ten minutes of unbroken pressure is long enough for a clot to form and hold. The bleeding stops. No instrument was used, because none was needed — she simply knew, to the millimetre, where the blood was coming from.

The external nose and the nasal cavity

Bone above, cartilage below — and a partition down the middle that is rarely quite straight. The EXTERNAL NOSE has a bony upper third — the two nasal bones and the frontal processes of the maxillae — and a cartilaginous lower two thirds that give the tip and the nostrils their mobility. Behind it lies the NASAL CAVITY, a tall narrow space divided into two by the NASAL SEPTUM. The septum is cartilage in front (the SEPTAL CARTILAGE) and bone behind, formed above by the PERPENDICULAR PLATE OF THE ETHMOID and below by the VOMER; a small crest of the maxilla and palatine bones completes its floor. It is almost never dead straight — a DEVIATED SEPTUM, congenital or from an old injury, is so common as to be normal, and matters only when it obstructs airflow or blocks a sinus opening. The ROOF of the cavity is the delicate, perforated CRIBRIFORM PLATE of the ethmoid, through which the olfactory nerves pass — the plate whose fractures are described among the foramina of the skull base. The FLOOR is the HARD PALATE — the palatine processes of the maxillae and the horizontal plates of the palatine bones — so that the roof of the mouth is literally the floor of the nose. Each cavity opens anteriorly at a NOSTRIL (naris) and posteriorly, into the nasopharynx, through a CHOANA.

The lateral wall: three scrolls and the gutters beneath them

The medial wall of each cavity — the septum — is smooth; all the interesting anatomy is on the LATERAL WALL. Projecting from it are three (occasionally four) curved shelves of bone covered in mucosa, the CONCHAE or TURBINATES: SUPERIOR, MIDDLE and INFERIOR, stacked one above another. The superior and middle conchae are parts of the ethmoid bone; the INFERIOR CONCHA is a separate bone in its own right. Their job is aerodynamic: they break the incoming air into turbulent eddies and force it across a huge, warm, wet mucosal surface, so that the air is heated, humidified and filtered before it reaches the larynx below. Beneath the overhang of each concha is a space — a MEATUS — named for the concha above it: the SUPERIOR, MIDDLE and INFERIOR MEATUS. These gutters are where things drain, and above the superior concha, tucked against the roof, is a small recess called the SPHENOETHMOIDAL RECESS. The whole arrangement — scrolls creating turbulence, gutters collecting drainage — is the reason a decongestant that shrinks the mucosa over the conchae can open a nose that felt completely blocked a moment before.

THE ANALOGY

Think of the lateral wall as the side of a multi-storey car park, with three sloping ramps (the conchae) projecting from it and a gutter running along under each ramp (the meatus). Rainwater from every level has to be piped down and emptied into one of these gutters. The engineers who built it made one careless decision: the drain from the biggest chamber of all — the maxillary sinus — was set not at the bottom of that chamber but high up on its inner wall, so its water has to climb before it can escape. On a dry day it manages. But let the pipes swell in the damp, and that one badly placed drain blocks first and floods its chamber, while all the others, drained from below, stay clear. That single design error is why, of the four paranasal sinuses, the maxillary is the one that floods — gets infected — again and again.

What drains where — the openings into the meatuses

Memorise this list and you can predict the spread of any sinus infection. Under each concha, a specific set of channels opens, and the pattern is worth learning by heart. The INFERIOR MEATUS receives just one thing — the NASOLACRIMAL DUCT — which is why tears that do not evaporate drain down into the nose, and why a hard cry makes the nose run; the same duct carries topical eye drops into the nose, gathered with the rest of the drainage apparatus in the orbit and its contents. The MIDDLE MEATUS is the busy one: into it drain the FRONTAL sinus (via the frontonasal duct), the MAXILLARY sinus (through a curved slit, the semilunar hiatus), and the ANTERIOR and MIDDLE ETHMOIDAL air cells (the middle cells raising a bulge, the ethmoidal bulla). These openings cluster at a small, functionally critical crossroads called the OSTIOMEATAL COMPLEX — block it and the frontal, maxillary and anterior ethmoidal sinuses obstruct together, which is exactly why it is the target of ENDOSCOPIC SINUS SURGERY. Note the maxillary sinus's ostium sits HIGH on its medial wall, so it must drain against gravity — the anatomical reason it is the sinus most often infected. The SUPERIOR MEATUS receives the POSTERIOR ETHMOIDAL cells. And the SPHENOETHMOIDAL RECESS, above everything, receives the SPHENOID sinus.

💡 CLINICAL PEARL

There is one clean rule that unlocks the whole list. ALL of the sinuses drain into the MIDDLE MEATUS EXCEPT the ethmoids, which split, and the sphenoid — and the exceptions follow a logic. Say it as: "everything drains into the middle meatus except the posterior ethmoidal cells (superior meatus) and the sphenoid (sphenoethmoidal recess); and the one non-sinus opening, the nasolacrimal duct, is the lone tenant of the inferior meatus." Anterior structures drain to the anterior/middle meatus; posterior structures drain to the posterior/superior recesses — front to front, back to back. Once that pattern is fixed, the clinical spread of infection reads off it directly: a blocked ostiomeatal complex threatens the frontal, maxillary and anterior ethmoidal sinuses as a group, because they share the same crossroads, and clearing that one crossroads is what an ENT surgeon is really doing when the operation is described, loosely, as "draining the sinuses".

✅ Key points

The nasal cavity is divided by the SEPTUM — septal cartilage in front, perpendicular plate of the ethmoid + vomer behind — with a roof of cribriform plate, a floor of hard palate, and a lateral wall bearing three (sometimes four) conchae.
The conchae (superior + middle = ethmoid; inferior = a separate bone) create turbulence to warm, humidify and filter air; the gutter beneath each is a MEATUS.
INFERIOR MEATUS: nasolacrimal duct only (tears → nose). MIDDLE MEATUS: frontal sinus, maxillary sinus (semilunar hiatus), anterior + middle ethmoidal cells — all at the OSTIOMEATAL COMPLEX.
SUPERIOR MEATUS: posterior ethmoidal cells. SPHENOETHMOIDAL RECESS: the sphenoid sinus. Front drains forward, back drains backward.
The maxillary sinus ostium sits HIGH on its medial wall, so it drains against gravity — the anatomical reason it is the sinus most often infected.

The four sinuses and their dangerous neighbours

Air-filled caves that lighten the skull — and each one shares a wall with something it can wreck. The PARANASAL SINUSES — FRONTAL, ETHMOIDAL, MAXILLARY and SPHENOID — are air-filled outgrowths of the nasal cavity, lined by the same respiratory mucosa, that lighten the skull, add resonance to the voice and buffer the face. Each is named for the bone it hollows out, and each has a relation that turns a local problem into a serious one. The MAXILLARY sinus, the largest, has a floor that lies directly against the roots of the upper PREMOLAR and MOLAR teeth — so a dental abscess can erupt upward into the sinus, and extracting an upper molar can push a root into it or leave an OROANTRAL FISTULA, a permanent hole between mouth and sinus, a link explored in the mouth, tongue and teeth. The ETHMOIDAL air cells are separated from the ORBIT by a wafer of bone literally named the LAMINA PAPYRACEA ("the paper-thin plate") — so ethmoid sinusitis is the commonest cause of ORBITAL CELLULITIS in children, the infection crossing that paper wall to threaten the eye. The SPHENOID sinus sits directly below the PITUITARY GLAND — which is why a pituitary tumour is reached by the TRANS-SPHENOIDAL route, up the nose and through the sinus roof — and immediately beside the CAVERNOUS SINUS and the internal carotid artery, so its surgery is done a millimetre from disaster. Only the FRONTAL sinus, above the orbit, is comparatively forgiving — though its infection can still track back into the skull.

The blood supply — Little's area and Woodruff's plexus

Two anastomoses account for almost every nosebleed a doctor ever sees. The nasal mucosa is astonishingly vascular — that is how it warms the air — and its blood comes from BOTH carotid systems at once, which is precisely what makes it bleed so readily. From the INTERNAL carotid, via the ophthalmic artery, come the ANTERIOR and POSTERIOR ETHMOIDAL arteries to the roof. From the EXTERNAL carotid, via the maxillary artery, come the SPHENOPALATINE artery (the main artery of the nose, entering through the sphenopalatine foramen) and the GREATER PALATINE artery; and via the facial artery comes the SUPERIOR LABIAL artery to the front of the septum. These vessels — from two different arterial trees, mapped in the vessels of the head and neck — meet in a single patch on the ANTERIOR-INFERIOR SEPTUM called LITTLE'S AREA, where their anastomosis is known as KIESSELBACH'S PLEXUS. Because it is superficial, exposed to dry air and easily picked or knocked, this patch is the source of roughly 90% of nosebleeds — ANTERIOR EPISTAXIS, typically in children and young adults, usually visible and controllable by simple pressure. Far back, on the lateral wall behind the inferior meatus, a second anastomosis, WOODRUFF'S PLEXUS, is the source of POSTERIOR EPISTAXIS: bleeding largely from the sphenopalatine artery, heavier, harder to see, harder to control, and commoner in the elderly and the hypertensive — the nosebleed that needs a posterior pack or arterial ligation rather than a pinch.

Nerve supply and the sense of smell

General sensation of the nasal cavity is trigeminal: the roof and upper septum from the ANTERIOR ETHMOIDAL nerve (a branch of the ophthalmic division, V1), and most of the rest from branches of the maxillary division (V2) — the two divisions set out in the trigeminal nerve. This is why an irritant, or a bright light, provokes the SNEEZE reflex, and why probing the nose is so unpleasant. But the nose also carries a SPECIAL sense that belongs to it alone: SMELL. High in the roof, over the superior concha and the adjacent septum, lies a small patch of OLFACTORY EPITHELIUM. Its receptor neurons send fine unmyelinated fibres — the fibres of the OLFACTORY NERVE, cranial nerve I — straight up through the perforations of the CRIBRIFORM PLATE to synapse in the OLFACTORY BULB lying on the plate inside the skull. This intimate arrangement is the anatomical basis of two classic injuries. A blow to the head that shears the delicate olfactory fibres as they cross the plate causes ANOSMIA — loss of smell, and with it most of the sense of taste, since flavour is largely olfactory. And a fracture OF the cribriform plate can tear the meninges with it, opening a leak of cerebrospinal fluid down the nose — CSF RHINORRHOEA — and a direct route for bacteria to ascend into the cranial cavity, causing meningitis. A clear, salty drip from one nostril after a head injury is that leak until proven otherwise.

◆ A cheek that aches when she bends forward

A woman has had a head cold for a week, and now, instead of clearing, it has changed. She has a dull, throbbing ache over her right cheek and upper teeth, a blocked right nostril with thick discharge, and — the giveaway — the pain sharpens when she bends forward to tie her shoes or leans over the sink. That last feature is pure anatomy: her MAXILLARY sinus, its ostium set high on its medial wall, cannot drain the pus pooling in its floor, and bending forward swings that fluid against the inflamed lining and raises the pressure. This is acute MAXILLARY SINUSITIS, the commonest sinusitis precisely because of that badly placed drain. The location of each sinus predicts the location of its pain: frontal sinusitis aches over the forehead and brow, ethmoid sinusitis between and behind the eyes, and sphenoid sinusitis deep in the centre of the head or the vertex. Because her upper molar roots sit in the sinus floor, she may also feel that her teeth hurt — and, conversely, a neglected upper molar abscess can be the thing that seeds the sinus in the first place. Most acute sinusitis is viral and settles; the anatomy simply explains why the maxillary is the one that so often does not.

✅ Key points

Four PARANASAL SINUSES (frontal, ethmoidal, maxillary, sphenoid) lighten the skull, add vocal resonance and are lined by respiratory mucosa.
Dangerous relations: MAXILLARY floor ↔ upper molar/premolar roots (dental infection, oroantral fistula); ETHMOID ↔ orbit through the LAMINA PAPYRACEA (orbital cellulitis); SPHENOID ↔ pituitary above (trans-sphenoidal surgery) + cavernous sinus and carotid beside it.
Blood supply comes from BOTH carotids: anterior/posterior ethmoidal (internal carotid → ophthalmic); sphenopalatine + greater palatine (external carotid → maxillary); superior labial (facial).
LITTLE'S AREA (Kiesselbach's plexus) on the anterior-inferior septum = ~90% of nosebleeds (ANTERIOR epistaxis, controllable by pressure). WOODRUFF'S PLEXUS posteriorly = posterior epistaxis (heavier, elderly/hypertensive).
Sensation: V1 (anterior ethmoidal) + V2. Smell: OLFACTORY nerve (CN I) from the roof through the CRIBRIFORM PLATE to the olfactory bulb — a plate fracture causes ANOSMIA and CSF RHINORRHOEA.

⚠️ Common mistakes

Telling a patient to pinch the bony bridge of the nose for a nosebleed. The bleeding is almost always from Little's area on the anterior SEPTUM — you must pinch the SOFT lower part, over the cartilage, to compress that patch. Pinching the bony bridge does nothing.
Assuming the maxillary sinus drains well because it is the lowest sinus. Its ostium is HIGH on the medial wall, so it drains against gravity — which is exactly why it obstructs and becomes infected more than any other sinus, not less.
Forgetting that clear fluid dripping from the nose after a head injury may be CSF, not mucus. A cribriform plate fracture tears the meninges and leaks cerebrospinal fluid — CSF rhinorrhoea — opening a route for meningitis, and it must be recognised, not wiped away as a runny nose.

🎓 Questions students ask

Why does a cold so often turn into a sinus infection?

Because a cold swells the very mucosa that lines the drainage openings. Each sinus communicates with the nose through a small ostium, and the frontal, maxillary and anterior ethmoidal sinuses all funnel through one narrow crossroads, the ostiomeatal complex, in the middle meatus. A viral cold inflames and thickens the mucosa there; the ostia — already narrow — block; and the sinus, cut off, fills with retained mucus that cannot escape and becomes a warm, closed pool ideal for bacterial overgrowth. The maxillary sinus suffers first and worst, because its ostium sits high on its medial wall and it was already draining uphill against gravity even when healthy. So sinusitis is usually not a new, separate infection: it is the mechanical consequence of a cold blocking a drain. That is also why treatment aims first at reopening the drainage — decongestants, steam, saline — before reaching for antibiotics.

Why does a runny nose accompany crying and cold weather?

Two different mechanisms, both anatomical. When you cry, the lacrimal glands produce more tears than can evaporate or be blinked away; the excess drains through the puncta into the nasolacrimal duct, which empties into the INFERIOR MEATUS of the nose. So the tears literally run into the nose and out of the nostril — the nose runs because the eyes are overflowing into it. Cold weather is different: breathing cold, dry air makes the vascular nasal mucosa work harder to warm and humidify it, so the mucous glands secrete more and the blood vessels engorge; some of that extra fluid, plus condensation as warm humid air from the lungs meets the cold air at the nostril, drips out. One is overflow plumbing from the eye; the other is the air-conditioning system running at full power. Both are the nose doing exactly what its anatomy is built to do.

Why can an infection on the nose spread to the brain?

Because of an unusual venous connection. The skin of the nose and the area between it and the medial corner of the eye — the so-called DANGER AREA of the face — is drained by the facial vein, which connects, through the angular and ophthalmic veins, directly to the CAVERNOUS SINUS inside the skull. Unlike most veins, these facial and ophthalmic veins have few or no valves, so under the wrong conditions blood, and with it infected clot, can flow BACKWARDS from a squeezed pimple or a boil on the nose, up into the cavernous sinus, and seed a CAVERNOUS SINUS THROMBOSIS — a life-threatening infection sitting on the internal carotid artery and the nerves to the eye. It is rare, and much rarer since antibiotics, but it is the anatomical reason for the old warning never to squeeze a spot in that triangle. Separately, a fracture of the cribriform plate in the roof of the nose opens a direct route for bacteria to ascend into the cranial cavity and cause meningitis. The nose sits closer to the brain, by both blood and bone, than its position on the face suggests.

Test yourself

A child with recurrent right-sided cheek pain and a purulent right nasal discharge is found to have maxillary sinusitis. Into which meatus does the maxillary sinus drain, and why is it the sinus most often infected?

🫁 In one breath

The nasal cavity is split by the SEPTUM (septal cartilage + perpendicular plate of ethmoid + vomer), roofed by the cribriform plate, floored by the hard palate, with a lateral wall of three (sometimes four) CONCHAE that warm, humidify and filter the air, and a MEATUS beneath each.
Drainage: inferior meatus = nasolacrimal duct only; MIDDLE MEATUS = frontal, maxillary and anterior/middle ethmoidal sinuses (the OSTIOMEATAL COMPLEX, the surgical target); superior meatus = posterior ethmoidal cells; sphenoethmoidal recess = sphenoid. The maxillary ostium is HIGH → drains against gravity → most often infected.
Dangerous sinus relations: maxillary floor ↔ upper teeth (dental spread, oroantral fistula); ethmoid ↔ orbit through the lamina papyracea (orbital cellulitis); sphenoid ↔ pituitary (trans-sphenoidal route), cavernous sinus and carotid.
Blood: both carotids meet in LITTLE'S AREA (Kiesselbach's plexus) on the anterior septum = ~90% of nosebleeds (anterior epistaxis, controlled by pinching the SOFT nose); Woodruff's plexus = posterior epistaxis. Sensation V1/V2; smell via CN I through the cribriform plate — a fracture causes anosmia and CSF rhinorrhoea.

📚 Sources

Drake RL, Vogl AW, Mitchell AWM. Gray's Anatomy for Students — Head and Neck: the nasal cavity, conchae and meatuses, and the paranasal sinuses and their drainage.
Moore KL, Dalley AF, Agur AMR. Clinically Oriented Anatomy — The nose and paranasal sinuses; Kiesselbach's area and epistaxis; the danger area of the face.
Netter FH. Atlas of Human Anatomy — Lateral wall of the nasal cavity, the openings into the meatuses, and the paranasal sinuses.
Snell RS. Clinical Anatomy by Regions — The nasal cavity and paranasal sinuses; sinusitis and the maxillary ostium; olfactory nerve and the cribriform plate.
Last RJ. Last's Anatomy: Regional and Applied — The nose, nasal septum and lateral wall; arterial anastomoses of the septum.
TeachMeAnatomy — The Nasal Cavity; The Paranasal Sinuses; Blood Supply of the Nose.