The Human Gas Exchange System

Ventilation is driven by Boyle's Law, which states that pressure is inversely proportional to volume ($P \propto 1/V$). By changing the volume of the thoracic cavity, the body creates pressure gradients that force air to move.

During Inspiration, the diaphragm contracts (flattens) and the external intercostal muscles contract, pulling the ribcage upward and outward. This increases thoracic volume, lowering the internal pressure below atmospheric pressure, causing air to rush in.

During Expiration at rest, the diaphragm and external intercostals relax. The elastic recoil of the lung tissue decreases thoracic volume, increasing internal pressure above atmospheric pressure, which forces air out.

The Surface Area to Volume Ratio is maximized by the presence of millions of alveoli. This provides a massive total area (roughly the size of a tennis court) for gas molecules to cross simultaneously.

The Diffusion Distance is minimized because the alveolar wall and the capillary wall are each only one cell thick (squamous epithelium). This creates a total barrier of less than $1 \mu m$.

A Steep Concentration Gradient is maintained by two factors: continuous ventilation (bringing in $O_2$ and removing $CO_2$) and constant blood flow in the surrounding capillaries (carrying $O_2$ away and bringing $CO_2$ to the lungs).

Identify the Muscle Pairs: Always specify whether the intercostal muscles are 'internal' or 'external'. Using the generic term 'intercostal' often loses marks in descriptions of the breathing mechanism.

Pressure-Volume Relationship: When explaining ventilation, always link the volume change to the pressure change, and then to the movement of air. Air moves from high pressure to low pressure.

Diffusion vs. Active Transport: Remember that gas exchange is a purely passive process. No ATP is used to move oxygen or carbon dioxide across the alveolar membrane; it relies entirely on concentration gradients.