7.1 Structure and function of the respiratory system

Respiratory System Anatomy

The respiratory system moves air from the outside environment to the alveoli deep in the lungs, where oxygen and carbon dioxide are exchanged with the blood. Every structure along this path either conditions the air (warming, filtering, humidifying) or helps move it to the right place. Understanding this anatomy is the foundation for everything else in respiratory physiology, from ventilation mechanics to gas transport to disease.

Nasal Cavity and Pharynx

The nose and nasal cavity are where inhaled air first gets conditioned. Hairs and mucus-producing goblet cells trap particles and pathogens, while a rich blood supply in the mucosal lining warms the air to near body temperature. The nasal cavity also houses olfactory receptors for the sense of smell.

The pharynx is a muscular tube connecting the nasal and oral cavities to the larynx and esophagus. It serves as a shared passageway for air and food, divided into three regions:

• Nasopharynx: upper portion, continuous with the nasal cavity
• Oropharynx: middle portion, continuous with the oral cavity
• Laryngopharynx: lower portion, leading to both the larynx (airway) and esophagus (digestive tract)

Larynx, Trachea, and Bronchi

The larynx (voice box) sits at the top of the trachea and has two major jobs: producing sound via the vocal cords and protecting the lower airway. The epiglottis, a flap of cartilage, folds over the laryngeal opening during swallowing to prevent food and liquid from entering the trachea.

The trachea (windpipe) is a flexible tube about 12 cm long and 2.5 cm in diameter that channels air from the larynx to the bronchi. C-shaped cartilage rings keep it open during pressure changes, while the open posterior side (facing the esophagus) allows the esophagus to expand during swallowing. The interior is lined with pseudostratified ciliated columnar epithelium and goblet cells, which continue to trap and sweep particles upward.

The trachea splits into two main (primary) bronchi, one for each lung. From there, the airways branch repeatedly into smaller and smaller tubes:

• The right main bronchus is wider, shorter, and more vertical than the left, which is why aspirated objects more commonly lodge on the right side.
• Primary bronchi divide into lobar bronchi, then segmental bronchi, then progressively smaller bronchioles and terminal bronchioles.

Lungs and Pleura

The lungs are paired, spongy organs that house the entire bronchial tree and the alveoli where gas exchange occurs. They are divided into lobes:

• Right lung: three lobes (upper, middle, lower)
• Left lung: two lobes (upper and lower), with a cardiac notch that accommodates the heart

Each lung is enclosed by the pleura, a double-layered serous membrane. The parietal pleura lines the inner wall of the thoracic cavity, while the visceral pleura adheres directly to the lung surface. Between them is a thin film of pleural fluid that serves two purposes: it lubricates the surfaces so the lungs slide smoothly during breathing, and its surface tension helps keep the lungs expanded against the chest wall.

Pulmonary Ventilation Mechanics

Breathing depends on pressure differences between the atmosphere and the lungs. The respiratory muscles change thoracic volume, which changes intrapulmonary pressure, which drives airflow in or out.

Inspiration and Expiration

Inspiration is an active process. The diaphragm and external intercostal muscles contract, expanding the thoracic cavity. This expansion lowers the pressure inside the lungs (intrapulmonary pressure) below atmospheric pressure, and air flows in down the pressure gradient.

The physics here follow Boyle's Law: at constant temperature, pressure and volume are inversely related.

$P_1V_1 = P_2V_2$

So when thoracic volume increases, intrapulmonary pressure decreases, and air rushes in.

Expiration during quiet breathing is largely passive. The diaphragm and external intercostals relax, the elastic recoil of the lungs and chest wall reduces thoracic volume, intrapulmonary pressure rises above atmospheric pressure, and air flows out. No muscular effort is needed for normal, resting exhalation.

During forced expiration (exercise, coughing), additional muscles actively compress the thoracic cavity to push air out faster and more completely.

Respiratory Muscles

The diaphragm is the primary muscle of breathing, responsible for roughly 75% of the volume change during quiet respiration.

• Innervated by the phrenic nerve (cervical roots C3, C4, C5). A useful mnemonic: "C3, 4, 5 keeps the diaphragm alive."
• At rest it's dome-shaped. During inspiration it contracts and flattens, increasing the vertical dimension of the thoracic cavity.

The external intercostal muscles assist inspiration by elevating the ribs and sternum, increasing the anterior-posterior and transverse dimensions of the chest.

• Innervated by intercostal nerves
• Their contraction swings the ribs upward and outward (often compared to a bucket-handle motion)

During forced expiration, two additional muscle groups engage:

• Internal intercostal muscles: pull the ribs downward and inward, compressing the thoracic cavity
• Abdominal muscles (rectus abdominis, external and internal obliques, transversus abdominis): compress the abdominal contents upward against the diaphragm, forcing it higher into the thorax and rapidly decreasing lung volume

Alveoli Structure and Function

Alveolar Anatomy

Alveoli are tiny, thin-walled sacs clustered at the ends of respiratory bronchioles. They are the sole site of gas exchange in the lungs. The adult lungs contain approximately 300 million alveoli, providing a total surface area of 70-80 square meters, roughly the size of a tennis court. This enormous surface area packed into a relatively small space is what makes efficient gas exchange possible.

Each alveolus is wrapped in a dense network of pulmonary capillaries. The barrier between alveolar air and capillary blood is extraordinarily thin (0.2-0.5 μm), consisting of:

1. A thin layer of alveolar epithelial cells
2. A fused basement membrane (shared by the epithelium and capillary endothelium)
3. The capillary endothelial cells

This minimal diffusion distance allows gases to cross in fractions of a second.

The alveolar wall contains two cell types:

• Type I pneumocytes: thin, squamous epithelial cells that make up about 95% of the alveolar surface area. Their extreme thinness is what keeps the diffusion barrier short.
• Type II pneumocytes: cuboidal cells that secrete pulmonary surfactant. They also act as progenitor (stem) cells that can divide and differentiate into type I cells to repair damaged alveolar walls.

Gas Exchange and Surfactant

Gas exchange across the alveolar-capillary membrane is driven by partial pressure gradients. Oxygen diffuses from the alveolar air (where its partial pressure is higher) into the blood, while carbon dioxide diffuses from the blood (where its partial pressure is higher) into the alveolar air.

The rate of diffusion is governed by Fick's Law:

$V_{gas} \propto \frac{A \times \Delta P}{T}$

where $A$ is surface area, $\Delta P$ is the partial pressure difference across the membrane, and $T$ is membrane thickness. Anything that increases surface area or the pressure gradient, or decreases membrane thickness, speeds up gas exchange. Diseases like emphysema (which destroys alveoli, reducing $A$) or pulmonary edema (which increases $T$) impair diffusion for exactly these reasons.

Pulmonary surfactant is critical for preventing alveolar collapse. Without it, the surface tension of the thin fluid lining inside each alveolus would cause smaller alveoli to collapse into larger ones (as predicted by the Law of Laplace). Surfactant reduces this surface tension, keeping alveoli open across a range of sizes.

• Dipalmitoylphosphatidylcholine (DPPC) is the primary phospholipid component. It forms a monolayer at the air-liquid interface that dramatically lowers surface tension.
• Surfactant proteins (SP-A, SP-B, SP-C, SP-D) help spread and stabilize the surfactant layer. SP-A and SP-D also contribute to innate immune defense in the lungs.

Premature infants often lack sufficient surfactant, leading to neonatal respiratory distress syndrome (NRDS), where alveoli collapse and gas exchange fails.

Upper vs. Lower Respiratory Tracts

The respiratory system is divided into upper and lower tracts at the level of the larynx. The upper tract conditions incoming air; the lower tract conducts it to the alveoli and performs gas exchange.

Upper Respiratory Tract

The upper respiratory tract includes the nose, nasal cavity, pharynx, and larynx. Its primary roles are filtering, warming, and humidifying inhaled air before it reaches the delicate lower airways.

Key structures and defenses:

• Nasal conchae (turbinates): scroll-like bony projections inside the nasal cavity that create turbulent airflow, increasing contact between air and the warm, moist mucosal surface. This maximizes heat and moisture transfer.
• Olfactory epithelium: specialized sensory tissue in the upper nasal cavity for detecting odors.
• Epiglottis and larynx: act as a valve system to route air toward the trachea and food toward the esophagus, preventing aspiration.

The upper tract also serves as a first line of immune defense. Goblet cells secrete mucus that traps inhaled particles and pathogens. The mucociliary escalator, the coordinated beating of cilia on respiratory epithelial cells, then propels this mucus-particle mixture upward toward the pharynx, where it's swallowed or coughed out.

Lower Respiratory Tract

The lower respiratory tract includes the trachea, bronchi, bronchioles, and alveoli. It can be divided into two functional zones:

• Conducting zone (trachea through terminal bronchioles): transports air but does not participate in gas exchange. These airways are sometimes called anatomical dead space because no diffusion occurs here.
• Respiratory zone (respiratory bronchioles, alveolar ducts, alveoli): where gas exchange actually takes place. Respiratory bronchioles are the transition point, containing scattered alveoli along their walls.

Structural features change as you move deeper into the bronchial tree:

• Cartilage support decreases: C-shaped rings in the trachea give way to irregular plates in the bronchi, and cartilage disappears entirely in the bronchioles.
• Smooth muscle becomes more prominent in bronchiolar walls, allowing regulation of airway diameter (bronchoconstriction and bronchodilation) to control airflow distribution.
• Epithelium transitions from pseudostratified ciliated columnar (trachea/bronchi) to simple cuboidal (bronchioles) to the extremely thin simple squamous type I pneumocytes in the alveoli.

Under normal conditions, the lower respiratory tract below the larynx is essentially sterile, maintained by the mucociliary escalator, alveolar macrophages, and the barrier function of the airway epithelium.