Q.Diffusion of gases occurs in the alveolar region only and not in the other parts of respiratory system. Why?

Diffusion of gases is confined to the alveolar region because only the alveoli have the structural features that make exchange possible.

• The alveoli are thin, richly-vascularised, bag-like sacs, so blood and air are brought extremely close together there.
• The barrier between air and blood is a very thin diffusion membrane, far less than a millimetre thick, which gases can cross easily.
• The rest of the system — from the nostrils down to the terminal bronchioles — is the conducting part. Its walls are not built for exchange; its job is only to carry, clean, humidify and warm the incoming air.
• With no thin, blood-rich surface elsewhere, gases have nowhere else to diffuse.

The respiratory system can be divided by function into two very different portions, and understanding this split explains why diffusion is restricted to one region.

The first portion is the conducting part, running from the external nostrils all the way down to the terminal bronchioles. Its role is purely to move atmospheric air towards the lungs while preparing it: it transports the air, cleans it of foreign particles, humidifies it and brings it to body temperature. Its walls are not designed for gas exchange — the larger airways are even held open by rings of cartilage — so no oxygen or carbon dioxide crosses here.

The second portion is the respiratory or exchange part — the alveoli and their ducts. This is where diffusion actually takes place, and the alveoli are structurally built for it:

• they are very thin-walled, irregular, bag-like sacs,
• they are richly supplied with blood capillaries, bringing blood right up against the air, and
• the barrier between the alveolar air and the blood, the diffusion membrane, is made of just a few very thin layers and is much less than a millimetre thick.

Because diffusion depends on gases crossing a thin surface between air and blood, it can only happen where such a surface exists. The alveolar region provides exactly that — a huge, thin, blood-rich interface — while the conducting airways offer thick walls and no close contact with blood.

Method: Explaining Why Gas Exchange Happens Only at a Particular Site

Use this reasoning pattern whenever asked why diffusion or exchange is restricted to one region of a transport system.

Steps

Step 1: Split the system into its functional zones

Identify the zones involved — here, the conducting part (nostrils to terminal bronchioles) versus the respiratory/exchange part (the alveoli) — and work out which zone the question is really asking about.

Step 2: Recall the structural conditions diffusion needs

Effective diffusion needs a large surface area, a very thin barrier, and a rich blood supply pressed right up against that barrier.

Step 3: Test each zone against those conditions

Check each zone in turn. The conducting part has thick walls (some even cartilage-supported) and no close blood contact, so it fails the test. The alveoli are thin-walled and richly vascularised, so they satisfy every condition. The site where all three conditions are met — the alveoli — is where exchange happens, and nowhere else in the tract meets them.