Respiratory System — revision sheet

The respiratory system conducts air from the nose to about 480 million alveoli, where oxygen diffuses into the pulmonary capillaries and carbon dioxide diffuses out across a membrane only ≈0.5 µm thick. Ventilation is powered by the diaphragm and thoracic wall acting on the pleura-coupled lungs, and is regulated breath by breath by brainstem centres that respond mainly to arterial PCO2 and pH.

Respiratory rate (adult, rest): 12–20 breaths/minTidal volume: ≈500 mL (≈6–8 mL/kg)Total lung capacity: ≈6 L (male); ≈20–25% less in femalesAnatomical dead space: ≈150 mL (≈2 mL/kg)Alveolar ventilation: ≈4.2 L/minNumber of alveoli: ≈480 million (range ≈270–790 million)Gas-exchange surface: ≈70 m²Blood–gas barrier: ≈0.2–0.6 µm thickArterial PO2 / PCO2: ≈75–100 mmHg / 35–45 mmHgMean pulmonary arterial pressure: ≈15 mmHg (PH if > 20 mmHg)Overall V/Q ratio: ≈0.8O2 consumption / CO2 production: ≈250 / ≈200 mL/min (RQ ≈0.8)

Upper respiratory tract — nose & sinuses

External nose Organ part: The pyramidal projection of the face whose bony and cartilaginous framework supports the nostrils (nares), the entry point for inspired air.
Nasal cavity Region: Paired air passages from the nostrils to the choanae, divided by the nasal septum and enlarged by the conchae, which warm, humidify and filter inspired air and house the organ of smell.
Nasal septum Organ part: The midline partition dividing the nasal cavity into right and left halves, made of bone posteriorly and cartilage anteriorly.
Nasal conchae and meatuses Organ part: Three curved shelves of bone covered with vascular mucosa on the lateral nasal wall; the air channels beneath them (meatuses) receive the openings of the paranasal sinuses and nasolacrimal duct.
Paranasal sinuses Space: Four paired air-filled extensions of the nasal cavity within the frontal, ethmoid, sphenoid and maxillary bones, lined with respiratory mucosa and draining into the nasal cavity.

Upper respiratory tract — pharynx

Pharynx Organ: A 12–15 cm fibromuscular tube from the skull base to the lower border of the cricoid cartilage (C6), shared by the respiratory and digestive tracts and divided into nasopharynx, oropharynx and laryngopharynx.
Nasopharynx Region: The uppermost, purely respiratory part of the pharynx behind the choanae, housing the adenoids and the openings of the auditory tubes.
Oropharynx Region: The middle part of the pharynx, behind the mouth, from the soft palate to the upper border of the epiglottis, carrying both air and food and housing the palatine and lingual tonsils.
Laryngopharynx Region: The lowest part of the pharynx, behind the larynx, from the upper border of the epiglottis to the lower border of the cricoid cartilage (C6), where it continues as the oesophagus.

Larynx

Larynx Organ: The cartilaginous valve and voice box between the laryngopharynx and trachea (C3–C6) that protects the lower airway, generates voice and allows effort closure of the thorax.
Thyroid cartilage Cartilage: The largest laryngeal cartilage: two quadrilateral laminae fused anteriorly to form the laryngeal prominence ("Adam's apple"), shielding the vocal folds.
Cricoid cartilage Cartilage: A signet-ring-shaped cartilage at C6 — the only complete cartilaginous ring in the airway — forming the base of the larynx and marking the start of the trachea and oesophagus.
Arytenoid cartilages Cartilage: Paired pyramidal cartilages seated on the cricoid lamina whose rotation and gliding open and close the vocal folds.
Epiglottis Cartilage: A leaf-shaped flap of elastic cartilage behind the tongue base that folds back over the laryngeal inlet during swallowing.
Vocal folds Organ part: Paired mucosal folds stretched from the thyroid angle to the arytenoid vocal processes, containing the vocal ligament and vocalis muscle, that vibrate to produce voice and close to protect the airway.
Vestibular folds Organ part: Paired thick, pink mucosal folds above the vocal folds that protect the airway and lubricate the vocal folds but take little part in normal voice production.
Intrinsic laryngeal muscles Muscle group: Small skeletal muscles connecting the laryngeal cartilages that open (abduct), close (adduct), tense and relax the vocal folds and constrict the laryngeal inlet.

Tracheobronchial tree (conducting zone)

Trachea Organ: A flexible 10–12 cm tube held open by 16–20 C-shaped hyaline cartilage rings, running from the cricoid cartilage (C6) to its bifurcation at the carina (T4/T5).
Carina of trachea Organ part: The keel-shaped ridge formed by the last tracheal cartilage at the bifurcation of the trachea into the main bronchi, at the level of the sternal angle (T4/T5).
Bronchi Organ part: The cartilage-supported branching airways from the carina to the bronchioles: main (primary) → lobar (secondary) → segmental (tertiary) bronchi and their subdivisions.
Right main bronchus Organ part: The short (≈2.5 cm), wide and steep main bronchus to the right lung — the usual destination of aspirated material and misplaced endotracheal tubes.
Left main bronchus Organ part: The longer (≈5 cm), narrower and more horizontal main bronchus, passing beneath the aortic arch to the left lung.
Lobar bronchi Organ part: The second-generation airways, one supplying each lobe: right superior, middle and inferior; left superior (including the lingular bronchus) and inferior.
Segmental bronchi Organ part: Third-generation airways, each ventilating one bronchopulmonary segment — ten in the right lung and typically eight to ten in the left.
Bronchioles Organ part: Small airways (< ≈1 mm) without cartilage or glands, held open by the elastic pull of surrounding lung and controlled by a relatively thick layer of smooth muscle.
Terminal bronchioles Organ part: The smallest purely conducting airways (generation ≈16, ≈0.5 mm), each supplying one pulmonary acinus — the end of the anatomical dead space.

Respiratory zone & alveolar wall

Respiratory bronchioles Organ part: Transitional airways (generations ≈17–19) whose walls are interrupted by scattered alveoli — the first site where gas exchange occurs.
Alveolar ducts and sacs Organ part: Elongated passages (generations ≈20–22) whose walls consist almost entirely of alveolar openings, ending in grape-like alveolar sacs (generation 23).
Pulmonary alveoli Organ part: ≈480 million thin-walled air sacs (≈200 µm across) providing ≈70 m² of surface, wrapped in capillaries, where O2 and CO2 are exchanged.
Type I pneumocyte Cell: Extremely thin, broad squamous epithelial cells that cover ≈95% of the alveolar surface and form the epithelial part of the blood–gas barrier.
Type II pneumocyte Cell: Cuboidal alveolar epithelial cells, typically at septal corners, that synthesise and secrete surfactant from lamellar bodies and act as the stem cells of the alveolar epithelium.
Alveolar macrophage Cell: Resident phagocytes on the alveolar surface ("dust cells") that ingest inhaled particles, microbes and spent surfactant — the main defence of the gas-exchange region.
Pulmonary surfactant Molecule: A lipoprotein film secreted by type II pneumocytes that lowers alveolar surface tension, stabilising alveoli, increasing compliance and keeping the alveoli dry.
Respiratory membrane Membrane: The ultrathin barrier (≈0.2–0.6 µm) separating alveolar air from capillary blood, across which O2 and CO2 diffuse.
Respiratory epithelium Tissue: Ciliated pseudostratified columnar epithelium with goblet cells that lines most of the conducting airways and drives mucociliary clearance.

Lungs & pleura

Right lung Organ: The larger lung (≈55% of total lung volume), divided by the oblique and horizontal fissures into superior, middle and inferior lobes and ten bronchopulmonary segments.
Left lung Organ: The smaller lung (≈45% of lung volume), divided by a single oblique fissure into superior and inferior lobes, with a cardiac notch and lingula accommodating the heart.
Bronchopulmonary segments Organ part: Pyramidal, independently ventilated and perfused subdivisions of the lobes, each supplied by a segmental bronchus and artery — the anatomical basis of segmentectomy and of localising disease.
Hilum and root of the lung Region: The hilum is the wedge-shaped area on the mediastinal surface where the structures of the lung root — bronchus, pulmonary artery, pulmonary veins, bronchial vessels, nerves and lymphatics — enter and leave the lung.
Pleura Membrane: The serous membrane enclosing each lung: visceral pleura on the lung surface and parietal pleura lining the thoracic cavity, continuous at the root and separated by a thin film of pleural fluid.
Visceral pleura Membrane: The thin, shiny serous membrane intimately covering the lung surface and lining its fissures; insensitive to pain.
Parietal pleura Membrane: The outer pleural layer lining the thoracic wall, diaphragm and mediastinum; supplied by somatic nerves and therefore exquisitely sensitive to pain.
Pleural cavity Space: The potential space between the visceral and parietal pleura, containing a thin film of fluid and held at subatmospheric pressure, which couples the lungs to the chest wall.

Muscles of respiration

Diaphragm Muscle: The dome-shaped musculotendinous partition between thorax and abdomen and the principal muscle of inspiration, supplied by the phrenic nerves (C3–C5).O Xiphoid process (sternal part); inner surfaces of the lower six costal cartilages and ribs 7–12 (costal part);…I Central tendon.A Contraction flattens and lowers the domes, enlarging the vertical (and, via the lower ribs, transverse) dimensions of the thorax for…N Motor: phrenic nerves (C3–C5), each supplying its own half. Sensory: central part by the phrenic nerves;…

Physiology — mechanics of ventilation

Pulmonary ventilation (mechanics of breathing) Physiological process: The bulk movement of air into and out of the lungs, driven by pressure gradients that the respiratory muscles create by changing thoracic volume (Boyle's law).
Lung volumes and capacities Physiological process: The four standard lung volumes (TV, IRV, ERV, RV) and the capacities formed by adding them (IC, FRC, VC, TLC), measured by spirometry and gas-dilution or plethysmographic methods.
Lung compliance and elastic recoil Physiological process: Compliance (ΔV/ΔP) describes how easily the lungs and chest wall stretch; it is set by elastic fibres and, above all, alveolar surface tension, which surfactant reduces.
Dead space and alveolar ventilation Physiological process: The part of each breath that does not take part in gas exchange: anatomical dead space (≈150 mL of conducting airways) plus alveolar dead space (ventilated but unperfused alveoli).

Physiology — gas exchange & transport

Pulmonary gas exchange Physiological process: Passive diffusion of O2 from alveolar gas into pulmonary capillary blood and of CO2 in the opposite direction, each down its partial-pressure gradient across the respiratory membrane.
Oxygen transport and the oxyhaemoglobin dissociation curve Physiological process: O2 is carried ≈98.5% bound to haemoglobin and ≈1.5% dissolved; the sigmoid dissociation curve and its shifts determine how readily O2 is loaded in the lungs and unloaded in tissues.
Carbon dioxide transport Physiological process: CO2 travels from tissues to lungs as bicarbonate (≈70%), carbamino compounds (≈20–23%) and dissolved gas (≈7–10%), using carbonic anhydrase and the chloride shift in red cells.
Ventilation–perfusion (V/Q) matching Physiological process: Efficient gas exchange requires each alveolus to receive ventilation in proportion to its blood flow; the overall V/Q ratio is ≈0.8, varying from ≈3 at the apex to ≈0.6 at the base.

Physiology — control of breathing & defence

Brainstem respiratory centres Brain region: Networks of neurons in the medulla and pons that generate the automatic rhythm of breathing and adjust its rate and depth in response to chemical, mechanical and higher-centre inputs.
Central chemoreceptors Receptor: Chemosensitive neurons near the ventral surface of the medulla that respond to the H+ concentration of brain extracellular fluid/CSF — and therefore to arterial PCO2 — providing most of the drive to breathe.
Peripheral chemoreceptors (carotid and aortic bodies) Receptor: Small, highly vascular sensory organs at the carotid bifurcation and aortic arch that detect falls in arterial PO2, and rises in PCO2 and H+, signalling via CN IX and X.
Cough and sneeze reflexes Physiological process: Protective reflexes that expel irritants: the cough clears the larynx, trachea and bronchi (vagal afferents); the sneeze clears the nasal cavity (trigeminal afferents).
Mucociliary clearance Physiological process: The "mucociliary escalator": a mucus blanket that traps inhaled particles and is propelled by coordinated ciliary beating towards the pharynx, where it is swallowed or expectorated.

Mnemonics

Diaphragm apertures: "I 8 10 Eggs At 12": IVC passes at T8, oEsophagus at T10, Aorta at T12.
Phrenic nerve roots: "C3, 4, 5 keeps the diaphragm alive" — the phrenic nerve is the sole motor supply of each hemidiaphragm.
Right shift of the O2–Hb curve: "CADET, face Right": CO2, Acid (H+), DPG (2,3-BPG), Exercise, Temperature — all lower affinity and favour O2 unloading.
Pulmonary artery at the hilum: "RALS": on the Right the pulmonary artery is Anterior to the main bronchus; on the Left it is Superior. The right upper lobe bronchus arises above the artery (eparterial bronchus).
Surface markings of lungs and pleura: "6–8–10" for the inferior lung border and "8–10–12" for the inferior pleural reflection at the midclavicular line, mid-axillary line and paravertebral line respectively. The difference is the costodiaphragmatic recess.
Nasal meatus drainage: "Sphenoid sits high, tears drain low, everything else goes to the middle": sphenoid sinus → sphenoethmoidal recess; posterior ethmoidal cells → superior meatus; frontal, maxillary, anterior and middle ethmoidal cells → middle meatus; nasolacrimal duct → inferior meatus.
Vocal fold abductor: "PCA Pulls Cords Apart" — posterior cricoarytenoid is the only abductor. All intrinsic laryngeal muscles are supplied by the recurrent laryngeal nerve except cricothyroid (external laryngeal nerve), the "singer's muscle" that tenses the cords.

From the Corpus anatomy atlas — for education only.