Mechanism of Ventilation in Mammals

Spec Mapping — OCR H420 Module 3.1.1 — Exchange surfaces, content statements covering the mechanism of ventilation in mammals, the roles of the diaphragm and intercostal muscles, the application of Boyle's law to the thoracic cavity, and the distinction between quiet (passive expiration) and forced (active expiration) breathing (refer to the official OCR H420 specification document for exact wording). This lesson supplies the mechanical engine that maintains the alveolar concentration gradient introduced in Lesson 2.

Ventilation — the mechanical process of moving air in and out of the lungs — is essential for maintaining the steep concentration gradients on which gas exchange depends. Without ventilation, oxygen in the alveolar air would be rapidly depleted by the blood and carbon dioxide would accumulate, halting diffusion. Mammals use negative pressure breathing: the thoracic cavity is actively expanded, pressure inside the lungs falls below atmospheric pressure, and air flows down the pressure gradient into the lungs. This lesson examines the anatomy of the ventilation apparatus, the roles of the diaphragm and intercostal muscles, and the subtle differences between quiet and forced breathing.

Negative-pressure breathing is itself an evolutionary innovation. Amphibians use positive-pressure breathing (buccal-pumping air into the lungs), and the transition to the mammalian negative-pressure mechanism is associated with the evolution of a muscular diaphragm — an anatomical novelty that distinguishes the synapsid lineage (which led to mammals) from reptiles. The diaphragm allows the lungs to be filled passively under their own elastic tension rather than being inflated by an active mouth pump, freeing the buccal cavity for other roles (chewing, vocalisation, suckling).

Key Definitions:

• Inspiration (inhalation) — the active phase of breathing in which air enters the lungs.
• Expiration (exhalation) — typically passive at rest; air leaves the lungs.
• Boyle's law — at constant temperature, pressure is inversely proportional to volume ($p \propto 1/V$p∝1/V).

---

The Thoracic Cavity

The thoracic cavity is sealed from the abdomen below by the diaphragm, a sheet of skeletal muscle shaped like a dome. Its walls are formed by the ribcage, to which the external and internal intercostal muscles attach. The lungs themselves are enclosed in a double-walled membrane called the pleura, containing a thin film of pleural fluid that lubricates movement and couples the lung surface to the chest wall.

• Parietal pleura — outer layer, attached to the ribcage.
• Visceral pleura — inner layer, attached to the lung surface.
• Pleural cavity — the narrow, fluid-filled space between them.

Because the pleural fluid is incompressible, expansion of the thoracic wall automatically pulls the lung surface outwards with it — this is the basis of negative-pressure breathing. If air enters the pleural cavity (a pneumothorax, typically following a chest wound or spontaneous bleb rupture), the coupling is broken and the affected lung collapses inwards under its own elastic recoil — clinical proof that the pleural seal is what holds the lungs open.

Boyle's law: PV = constant at fixed T

---

Inspiration (Active)

Inspiration is an active, energy-requiring process driven by muscle contraction.

1. External intercostal muscles contract — they run downwards and forwards between the ribs. When they shorten, they pull the ribs upwards and outwards, rather like a bucket handle being lifted.
2. Diaphragm contracts — it flattens from its dome shape downwards.
3. These combined movements increase the volume of the thoracic cavity.
4. By Boyle's law, an increase in volume causes a decrease in pressure inside the thorax (and therefore inside the lungs), to just below atmospheric pressure.
5. Air flows down the pressure gradient from the atmosphere into the lungs, inflating them.

flowchart LR
    A[External intercostals contract] --> C[Thoracic volume increases]
    B[Diaphragm contracts and flattens] --> C
    C --> D[Intra-thoracic pressure falls]
    D --> E[Air flows into lungs down pressure gradient]

---

Expiration (Passive at Rest)

At rest, expiration is largely a passive process, driven by elastic recoil.

1. External intercostal muscles relax; the ribs return downwards and inwards under gravity.
2. Diaphragm relaxes and returns to its dome shape as the elastic tissue in the lungs recoils and abdominal pressure pushes it upwards.
3. The elastic fibres in the lung tissue (abundant in alveolar walls and bronchiole walls) recoil, reducing lung volume.
4. Thoracic volume decreases, so by Boyle's law pressure inside the lungs rises above atmospheric pressure.
5. Air is pushed out of the lungs to the atmosphere.

---

Forced Expiration (Active)

During vigorous exercise, forced expiration (e.g., blowing out a candle, sneezing), the internal intercostal muscles contract. These run at right angles to the externals — downwards and backwards — and pull the ribs downwards and inwards, actively reducing thoracic volume. At the same time, the abdominal muscles contract to push the diaphragm further upwards. Accessory muscles such as the sternocleidomastoid, scalenes and pectoralis minor are also recruited for forced inspiration in heavy exercise — patients with severe airway obstruction visibly use their neck muscles to breathe, a clinical sign called accessory muscle use.

Exam Tip: The internal and external intercostals are antagonistic: their fibres run in opposite directions and they move the ribcage in opposite directions. This is a classic example of antagonistic muscle pairs in OCR exam questions.

The respiratory control centre

Although the mechanics of breathing are muscular, the rhythm is set by neurones in the medulla oblongata of the brainstem. The respiratory centre receives inputs from:

• Central chemoreceptors in the medulla itself, sensitive to CSF pH (which falls when blood pCO₂ rises).
• Peripheral chemoreceptors in the carotid bodies and aortic bodies, sensitive to blood pO₂, pCO₂ and pH.
• Stretch receptors in the bronchioles (the Hering-Breuer reflex limits over-inflation).
• Higher centres in the cerebral cortex (voluntary control — singing, breath-holding, speech).

When blood pCO₂ rises (during exercise or with sleep apnoea), the respiratory centre increases firing frequency to the phrenic and intercostal motor neurones, driving deeper and faster breathing. The dominant driver of ventilation is carbon dioxide — not, as is often misstated, oxygen. Hypoxia drives breathing only at low pO₂ values below about 8 kPa.

---

Summary Table

Stage	External intercostals	Internal intercostals	Diaphragm	Thoracic volume	Intra-thoracic pressure	Air flow
Quiet inspiration	Contract	Relaxed	Contracts (flattens)	Increases	Decreases	In
Quiet expiration	Relax	Relaxed	Relaxes (domes)	Decreases	Increases	Out
Forced expiration	Relax	Contract	Forced up by abdominals	Decreases more	Increases more	Out (forcefully)

---

Why Ventilation is Essential

Ventilation maintains the steep concentration gradient required for efficient gas exchange:

• Fresh air entering the alveoli has a high partial pressure of oxygen (pO₂) and low partial pressure of carbon dioxide (pCO₂).
• Stale air leaving the alveoli contains less O₂ and more CO₂.
• Without ventilation, alveolar pO₂ would fall and pCO₂ would rise, reducing the gradient across the alveolar membrane and slowing diffusion.

Combined with a continuous blood supply that removes oxygen and delivers carbon dioxide on the other side of the membrane, ventilation ensures that Fick's law conditions are maintained and gas exchange can proceed at a sufficient rate to support metabolism.

---

The Role of Elastic Fibres

The walls of the bronchioles and alveoli contain extensive elastic fibres composed of the protein elastin. These fibres stretch during inspiration as the lungs inflate and recoil during expiration, providing much of the passive driving force for exhalation. Destruction of elastic fibres by chronic inflammation (as in emphysema, almost always caused by long-term cigarette smoking) greatly reduces lung recoil and makes expiration very difficult — patients must actively contract their intercostal muscles simply to exhale.

Elastic recoil also has a structural consequence: the lung tissue surrounding small bronchioles exerts radial traction on the airway walls, holding them open during expiration when intra-thoracic pressure rises. When emphysema destroys this surrounding parenchyma, the small bronchioles collapse on expiration and trap air — the classic gas trapping of obstructive disease, visible on a spirometer as a prolonged FEV₁ and reduced FEV₁/FVC ratio (covered in the next lesson).

Compliance and elastance

Compliance is the change in lung volume per unit change in pressure:

$\text{Compliance} = \frac{\Delta V}{\Delta P}$Compliance=ΔPΔV​

A normal lung has high compliance — small pressure changes produce large volume changes. In pulmonary fibrosis, scarring stiffens the lung and lowers compliance; patients must generate larger pressure differences to inflate their lungs, increasing the work of breathing. In emphysema, paradoxically, compliance is high (the lungs are floppy from elastin destruction), but the FEV₁/FVC ratio is dramatically reduced because the airways collapse during expiration. The same spirometer can therefore distinguish "stiff" (restrictive) from "floppy" (obstructive) disease.

---

Common Exam Mistakes

• Saying "the lungs contract to push air out". The lungs have no muscle tissue of their own (other than the smooth muscle in bronchiole walls). They are passive bags, moved by the actions of the diaphragm and ribcage.
• Confusing inspiration with "breathing in oxygen". Inspiration draws in whole air, which contains about 21% oxygen.
• Mixing up the intercostals. External intercostals pull ribs up and out (inspiration); internal intercostals pull them down and in (forced expiration).
• Forgetting Boyle's law. Exam questions often expect the phrase "volume increases, pressure decreases" explicitly, linked back to the cause of airflow.
• Saying "the diaphragm relaxes downwards". It relaxes upwards, back into its dome shape. It contracts downwards.
• Ignoring the role of elastic recoil in passive expiration.

Exam Tip: When describing ventilation, always use the sequence: muscle action → volume change → pressure change → direction of airflow. Marks are awarded for each linked step.

Why CO₂ is the dominant ventilation driver

A surprising twist of human physiology is that breathing is regulated principally by pCO₂, not pO₂. The reasoning is mechanistic:

1. Resting alveolar pO₂ (~13 kPa) sits on the plateau of the haemoglobin dissociation curve. Small fluctuations have very little effect on saturation, so the body has no need to defend pO₂ precisely.
2. Resting alveolar pCO₂ (~5.3 kPa) is on the steep middle portion of the relationship between pCO₂ and blood pH. Small rises in pCO₂ cause large falls in pH, which the brain cannot tolerate.
3. Therefore the medullary chemoreceptors prioritise pCO₂ feedback, and ventilation is finely tuned to keep arterial pCO₂ at ~5.3 kPa.

At altitude, the pO₂ drop eventually becomes severe enough (below ~8 kPa) for peripheral chemoreceptors to override the CO₂ set-point and stimulate hyperventilation. This compensatory mechanism is what altitude-acclimatised mountaineers depend on.

---

Pleural pressures: a closer look