Diffusion of gases occurs in the alveolar region only and not in the other parts of respiratory system. Why?
The respiratory system is divided into two parts: the conducting part and the respiratory or exchange part. The conducting part, which extends from the external nostrils up to the terminal bronchioles, only transports atmospheric air to the alveoli, clears it from foreign particles, humidifies it, and brings it to body temperature. It is not designed for gas exchange.
The alveoli and their ducts form the respiratory or exchange part. The diffusion membrane in the alveolar region is extremely thin (much less than a millimetre) and consists of three layers: the thin squamous epithelium of alveoli, the endothelium of alveolar capillaries, and the basement substance in between them. This thin membrane, along with favourable partial pressure gradients of O₂ and CO₂, facilitates efficient diffusion of gases between blood and atmospheric air.
Explanation
The textbook clearly distinguishes between the conducting and exchange parts of the respiratory system. The conducting part (nostrils to terminal bronchioles) serves only to transport, filter, humidify, and warm the air. The alveolar region is specifically designed for diffusion with its thin membrane structure (less than a millimetre) comprising squamous epithelium, capillary endothelium, and basement substance. This structural adaptation, combined with partial pressure gradients, makes the alveoli the exclusive site of gas exchange.
Solution Steps
Step 1: Identify the two parts of respiratory system - conducting and exchange parts from the context.
Step 2: Explain that conducting part only transports, filters, humidifies air - no diffusion occurs there.
Step 3: Describe the thin diffusion membrane structure at alveoli (3 layers, less than 1mm thickness) that enables gas exchange.