Mammalian Gas Exchange System

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, matching OCR A-Level Biology A specification 3.1.1 (d)–(f).

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.

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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.