Mammalian Gas Exchange System

Spec Mapping — OCR H420 Module 3.1.1 — Exchange surfaces, content statements covering the structure and function of the mammalian gas exchange system from nasal cavity to alveolus, the histology of each component (cartilage, ciliated epithelium, goblet cells, smooth muscle, elastic fibres), and the role of surfactant in alveolar function (refer to the official OCR H420 specification document for exact wording). This lesson builds directly on Fick's law (Lesson 1) and prepares the ground for ventilation mechanics (Lesson 3).

Mammals combine a very high metabolic rate with a low surface area to volume ratio, which creates a large demand for oxygen and a matching need to remove carbon dioxide. To meet this demand, mammals possess an elaborate gas exchange system with a huge internal surface area — the lungs — connected to the outside world by a branching series of airways. Each section of this system is specialised for a particular role, from filtering and conditioning incoming air to the final site of gas exchange in the alveoli. This lesson works systematically from the nostrils to the alveoli.

The intellectual roots run deep. John Mayow (1668) recognised that breathing "consumed" something in the air — what we now call oxygen — long before Lavoisier identified it. Marcello Malpighi (1661) used the early light microscope to show that the lungs were not solid organs but a sponge of tiny sacs (alveoli) traversed by capillaries. August and Marie Krogh in the early twentieth century formalised the partial-pressure framework of pulmonary gas exchange and won the 1920 Nobel Prize. The architecture of the mammalian lung — a branching tree culminating in a vast alveolar surface — is the engineering solution to the Fick's-law constraint set out in the previous lesson.

Key Definitions:

• Gas exchange surface — the alveolar epithelium, where oxygen enters and carbon dioxide leaves the blood.
• Ventilation — the mechanical movement of air into and out of the lungs.
• Airway — a tube (trachea, bronchus, bronchiole) that conducts air to or from the alveoli.

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Overall Plan of the System

flowchart TB
    N[Nasal cavity]
    P[Pharynx]
    L[Larynx]
    T[Trachea]
    B1[Left and right bronchi]
    B2[Bronchioles]
    TB[Terminal bronchioles]
    AL[Alveolar ducts and alveoli]
    N --> P --> L --> T --> B1 --> B2 --> TB --> AL

Air is drawn in through the nostrils, passes through the nasal cavity, pharynx and larynx, and enters the trachea. The trachea branches into two primary bronchi, one to each lung, which then subdivide into smaller and smaller bronchioles, eventually ending at the alveoli where gas exchange takes place. There are approximately 23 generations of branching between the trachea and the alveoli, generating an internal surface area of around 70 m² in an adult human — roughly the size of a tennis court — within a chest cavity volume of only about 5 dm³.

23 generations of branching → ~300–500 million alveoli → ~70 m² total surface area

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Nasal Cavity

The nasal cavity is the first stage of conditioning the inhaled air. It is lined with a moist, highly vascularised mucous membrane and performs three functions:

1. Warming — the dense capillary network in the nasal lining warms air to close to body temperature before it reaches the lungs.
2. Moistening — evaporation from the mucous lining humidifies the air, protecting the delicate alveolar walls from drying out.
3. Filtering — coarse hairs and mucus trap dust, pollen and microorganisms, which are then moved away by ciliated epithelium.

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Trachea

The trachea is a tube about 10–12 cm long and 2–2.5 cm in diameter in adults. It has several important structural features:

• C-shaped rings of cartilage — support the trachea and prevent it from collapsing during inhalation, when air pressure inside falls below atmospheric pressure. The open (incomplete) side of each 'C' faces the oesophagus, allowing it to bulge during swallowing.
• Smooth muscle — lies between the ends of each cartilage ring; can contract to narrow the airway.
• Elastic fibres — allow stretching and recoil during breathing.
• Ciliated pseudostratified columnar epithelium — lines the lumen; cilia beat mucus upward towards the pharynx (the mucociliary escalator).
• Goblet cells — scattered among the epithelial cells; secrete mucus that traps pathogens and debris.
• Submucosal glands — also secrete mucus.

Tissue	Function
Cartilage	Keeps airway open; prevents collapse
Smooth muscle	Adjusts airway diameter (dilation/constriction)
Elastic fibres	Recoil after stretch during breathing
Ciliated epithelium	Sweeps mucus upwards
Goblet cells	Secrete trapping mucus

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Bronchi and Bronchioles

The trachea bifurcates at the carina into a left bronchus and a right bronchus. Each bronchus has a structure similar to the trachea — cartilage plates (rather than complete rings), smooth muscle, ciliated epithelium and goblet cells — but smaller in diameter.

As the bronchi branch further, they become bronchioles:

• Large bronchioles: still contain some cartilage, ciliated epithelium and goblet cells; diameter typically > 1 mm.
• Small bronchioles (< 1 mm): cartilage disappears entirely; walls contain mainly smooth muscle and elastic fibres. Without cartilage, their patency depends on the outward pull of the surrounding lung tissue.
• Terminal bronchioles: no goblet cells, cuboidal epithelium, lead into respiratory bronchioles and alveolar ducts.

The smooth muscle in bronchioles is clinically significant: in asthma, it contracts and narrows the airways, increasing resistance to airflow. Drugs such as salbutamol cause it to relax (bronchodilation).

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Alveoli — the Gas Exchange Surface

Each lung contains around 300–500 million alveoli, giving a combined surface area of approximately 70 m² — about the size of a tennis court. Each alveolus is a tiny, thin-walled sac, 200–300 µm across, surrounded by a dense network of capillaries. The walls contain three main cell types:

• Type I pneumocytes (squamous alveolar cells) — extremely thin (~0.1 µm), flattened cells that form the majority of the alveolar surface. They are the main site of gas exchange.
• Type II pneumocytes — cuboidal cells that secrete surfactant, a phospholipid-rich fluid that reduces the surface tension of the alveolar lining. Without surfactant, surface tension would tend to pull the alveoli closed, making breathing extremely difficult (as occurs in premature babies with respiratory distress syndrome).
• Alveolar macrophages — phagocytes that engulf inhaled particles and pathogens.

The combined barrier that a molecule of oxygen must cross from alveolar air to red blood cell is only about 0.5 µm thick and consists of:

1. Thin fluid lining with dissolved surfactant
2. Alveolar epithelium (single layer of type I pneumocytes)
3. Thin basement membrane
4. Capillary endothelium (single squamous endothelial cell)

flowchart LR
    A[Alveolar air: high pO2, low pCO2]
    B[Surfactant layer]
    C[Alveolar epithelium]
    D[Basement membrane]
    E[Capillary endothelium]
    F[Plasma]
    G[RBC: haemoglobin]
    A -->|O2 in| B --> C --> D --> E --> F --> G
    G -->|CO2 out| F --> E --> D --> C --> B --> A

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Features that Make the Alveoli an Ideal Exchange Surface

The combined barrier — surfactant film + Type I pneumocyte + basement membrane + capillary endothelium — is only about 0.5 µm thick, the same as a typical cell membrane-plus-cytoplasm distance. This is why $\Delta x$Δx in the Fick's-law equation is so small at the alveolus.

Applying Fick's law to the alveolus

$J = D \cdot A \cdot \frac{\Delta c}{\Delta x}$J=D⋅A⋅ΔxΔc​

For oxygen across the alveolar–capillary barrier:

• $A \approx 70\,\text{m}^2$A≈70m2 (total alveolar surface area)
• $\Delta x \approx 0.5\,\mu\text{m} = 5 \times 10^{-7}\,\text{m}$Δx≈0.5μm=5×10−7m
• $\Delta c$Δc maintained large by continual ventilation (replacing alveolar air) and pulmonary blood flow (removing oxygenated blood)
• $D$D for O₂ in lipid/aqueous membrane is fixed by physical chemistry

The lung is engineered to maximise every term it can manipulate. The only term it cannot freely change is $D$D. Surfactant from Type II pneumocytes lowers the surface tension of the alveolar lining, preventing the smallest alveoli from collapsing into the larger ones (by the Laplace relation $P = 2\gamma/r$P=2γ/r), thereby keeping all 300–500 million alveoli inflated and contributing to $A$A.

Feature	Contribution to Fick's law
Huge total surface area (~70 m²)	Increases $A$A
Wall one cell thick (~0.1 µm)	Reduces $\Delta x$Δx
Surrounded by dense capillary network	Maintains $\Delta c$Δc on the blood side; reduces $\Delta x$Δx to ~0.5 µm total
Red blood cells flattened against capillary walls	Brings Hb close to alveolar air; effective increase in $A$A
Continual ventilation	Replaces alveolar air, keeping alveolar O₂ high and CO₂ low (maintains $\Delta c$Δc)
Moist inner lining	Dissolves gases for diffusion
Surfactant	Reduces surface tension; prevents alveolar collapse; keeps small alveoli open and contributing to $A$A