The Mechanism of Breathing Revision Notes for AQA A-Level Biology

The Mechanism of Breathing

What is breathing?

Breathing or ventilation is the process of moving air into and out of the lungs to maintain gas exchange across the alveolar epithelium. This movement depends on creating pressure differences between the atmosphere and the lungs.

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Pressure Gradient Principle

Air always moves from areas of high pressure to areas of low pressure. The respiratory system creates these pressure differences by changing the volume of the thoracic cavity.

Air moves according to pressure gradients:

Inspiration (breathing in) occurs when atmospheric pressure exceeds lung pressure
Expiration (breathing out) occurs when lung pressure exceeds atmospheric pressure

The muscles of breathing

Three main muscle groups control breathing by changing the volume of the thoracic cavity:

The diaphragm - a sheet of muscle separating the thorax from the abdomen

Intercostal muscles - located between the ribs, consisting of two distinct sets:

External intercostal muscles - their contraction causes inspiration by moving ribs upward and outward
Internal intercostal muscles - their contraction causes expiration by moving ribs downward and inward

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These muscles work together in a coordinated manner to create the precise volume and pressure changes necessary for efficient air movement. The diaphragm is the primary muscle of breathing, while the intercostal muscles provide fine control and assist in different phases of respiration.

Inspiration - breathing in

Inspiration is an active process requiring energy expenditure through muscle contraction. Understanding this sequence is crucial for comprehending how we draw air into our lungs.

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Step-by-Step Process of Inspiration:

Step 1: External intercostal muscles contract while internal intercostal muscles relax

Step 2: Ribs move upward and outward, increasing the thoracic volume

Step 3: Diaphragm muscles contract, causing the diaphragm to flatten and further increase thoracic volume

Step 4: Increased thoracic volume leads to decreased pressure within the lungs

Step 5: Atmospheric pressure now exceeds pulmonary pressure, forcing air into the lungs

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Key Principle: Increasing the volume of a closed container decreases the pressure of the gas inside it. This is the fundamental physical principle underlying inspiration.

Expiration - breathing out

During quiet breathing, expiration is largely a passive process requiring minimal energy. This is an important distinction from inspiration, which always requires active muscle work.

The mechanism involves:

Internal intercostal muscles contract while external intercostal muscles relax
Ribs move downward and inward, decreasing thoracic volume
Diaphragm muscles relax, allowing the diaphragm to return to its domed shape and further decrease thoracic volume
Decreased thoracic volume increases pressure within the lungs
Pulmonary pressure now exceeds atmospheric pressure, forcing air out of the lungs

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The Role of Elastic Recoil

During quiet breathing, the natural elastic recoil of lung tissue provides the main force for expiration. The lungs naturally want to return to their smaller, resting size after being stretched during inspiration.

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Only during strenuous activities do the intercostal muscles play a major active role in expiration. During exercise or forced breathing, expiration becomes an active process requiring energy.

Pulmonary ventilation

Pulmonary ventilation rate measures the total volume of air moved into and out of the lungs per minute. This calculation helps assess breathing efficiency and is an important clinical measurement.

The rate depends on two factors:

Tidal volume - the volume of air normally breathed in during each breath at rest (typically around 0.5 dm³)

Breathing rate - the number of breaths taken per minute (typically 12-20 breaths in healthy adults)

Calculating pulmonary ventilation rate

The formula is:

$\text{Pulmonary ventilation rate} = \text{tidal volume} \times \text{breathing rate}$

Units: $\text{dm}^3 \text{min}^{-1}$

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Worked Example: Calculating Ventilation Rate

Given:

Tidal volume = 0.6 dm³
Breathing rate = 17 breaths per minute

Solution: Pulmonary ventilation rate = 0.6 × 17 = 10.2 dm³ min⁻¹

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Key Points to Remember:

Breathing depends on pressure differences - gases always move from high to low pressure regions
Inspiration is active (requires energy) while quiet expiration is largely passive
External intercostals contract during inspiration, internal intercostals contract during expiration
Increasing thoracic volume decreases lung pressure, allowing air to flow in
Pulmonary ventilation rate = tidal volume × breathing rate (dm³ min⁻¹)

The Mechanism of Breathing (AQA A-Level Biology): Revision Notes