Ventilation & Gas Exchange

• Large Surface Area: The lungs contain millions of microscopic alveoli, providing a massive total area for gas molecules to cross simultaneously.

• Thin Diffusion Pathway: The alveolar wall and the capillary wall are each only one cell thick, consisting of flattened epithelial cells to minimize the distance gases must travel.

• Moist Surface: A thin layer of moisture lines the alveoli, allowing gases to dissolve before diffusing across the membrane, which increases the rate of exchange.

• Rich Blood Supply: A dense network of capillaries surrounds each alveolus, ensuring that oxygenated blood is quickly removed and deoxygenated blood is constantly supplied.

• Tidal Volume: The volume of air moved into or out of the lungs during a single, normal breath at rest.

• Breathing Rate: The frequency of ventilation, typically measured as the number of breaths taken per minute.

• Pulmonary Ventilation Rate (PVR): The total volume of air moved into the lungs per minute, calculated by the product of tidal volume and breathing rate.

Key Formula: $PVR = \text{Tidal Volume} \times \text{Breathing Rate}$

• Units: Usually expressed in $cm^3\,min^{-1}$ or $dm^3\,min^{-1}$. Ensure both inputs use consistent volume units before multiplying.

• Internal vs. External Intercostals: These are antagonistic muscles. External muscles are used for inspiration, while internal muscles are only used during forced (active) expiration.

• Terminology Precision: Never refer to 'cell walls' in animal respiratory systems; use 'alveolar wall' or 'epithelium'. Animal cells lack cell walls.

• Process Distinction: Clearly distinguish between ventilation (mechanical), gas exchange (diffusion), and respiration (chemical). Marks are often lost for using these interchangeably.

• Unit Conversion: Be prepared to convert between $cm^3$ and $dm^3$ ($1\,dm^3 = 1000\,cm^3$). Always check the units requested in the final answer.

• Graph Interpretation: When looking at lung volume graphs, the 'peaks' represent the end of inhalation and the 'troughs' represent the end of exhalation.