Alveoli: Structure and Function as the Exchange Surface
Biology · Breathing and Exchange of Gases · NEET
Alveoli are tiny, balloon-like air sacs at the ends of the alveolar ducts. Their wall is a single layer of very thin squamous epithelium wrapped in a dense network of blood capillaries, so O2 and CO2 can diffuse across in one quick step. Memory hook: think of alveoli as millions of soap-bubble-thin balloons — thin wall + huge surface area + rich blood supply = the perfect place for gas exchange.
The alveolus is lined by thin squamous epithelium and wrapped in blood capillaries. Gases cross a 3-layer diffusion membrane: alveolar squamous epithelium, basement substance, and capillary endothelium.
Your doubts, answered
Why are alveoli the exchange surface and not the trachea or bronchi?
Gas exchange needs a very thin, wet, richly vascularised surface. Only the alveoli (and their ducts) have a wall of thin squamous epithelium wrapped in blood capillaries, so O2 and CO2 can diffuse across easily. The trachea and bronchi have thick walls with cartilage and only carry air — they are the conducting part, not the exchange part.
What type of epithelium lines the alveoli?
Alveoli are lined by a single layer of thin (simple) squamous epithelium. Squamous means flat, sheet-like cells. Because this layer is so thin, the diffusion distance for gases is very short. NCERT states the diffusion membrane includes 'the thin squamous epithelium of alveoli'.
What are the three layers of the diffusion membrane?
NCERT lists three layers: (1) the thin squamous epithelium of the alveoli, (2) the endothelium of the alveolar capillaries, and (3) the basement substance in between (a thin basement membrane supporting the squamous epithelium plus the basement membrane around the capillary endothelium). Even with three layers, the total thickness is very small, so diffusion is fast.
Why do alveoli have such thin walls and a large surface area?
Thin walls give a short diffusion path, and the large combined surface area (millions of alveoli) gives lots of room for gases to cross at once. Both facts follow Fick's idea: diffusion is faster when the surface is thin and wide. This is exactly what makes the lungs efficient at loading O2 and unloading CO2.
Are alveoli part of the conducting part or the respiratory part?
Alveoli and their ducts form the respiratory (exchange) part. NCERT says the part from the external nostrils up to the terminal bronchioles is the conducting part, while 'the alveoli and their ducts form the respiratory or exchange part'. Actual diffusion of gases happens only here.
⚠️ The NEET trap ✗ The diffusion membrane is a single thin layer of squamous cells. ✓ The diffusion membrane has three layers: alveolar squamous epithelium, capillary endothelium, and the basement substance between them. 🧠 NCERT clearly says 'three major layers'. Students remember only the squamous epithelium and forget the capillary endothelium and basement substance — count to three.
Real NEET questions
2021
The partial pressures (in mm Hg) of oxygen (O2) and carbon dioxide (CO2) at alveoli (the site of diffusion) are:
A · pO2 = 95 and pCO2 = 40
B · pO2 = 159 and pCO2 = 0.3
C · pO2 = 104 and pCO2 = 40 ✓
D · pO2 = 40 and pCO2 = 45
Solution: At the alveoli (the actual site of diffusion) the partial pressures are pO2 = 104 mm Hg and pCO2 = 40 mm Hg (NCERT Table 14.1). The values 159/0.3 belong to atmospheric air, 95/40 to oxygenated blood, and 40/45 to tissues. This question tests that the alveolus is the exchange surface where these key values apply.
2016
The partial pressure of oxygen in the alveoli of the lungs is
A · Equal to that in the blood
B · More than that in the blood ✓
C · Less than that in the blood
D · Less than that of carbon dioxide
Solution: Alveolar pO2 is 104 mm Hg while the deoxygenated blood arriving at the alveoli has pO2 of only 40 mm Hg. Because alveolar pO2 is greater, O2 diffuses from the alveoli into the blood across the diffusion membrane. So the alveolar pO2 is MORE than that in the blood.
2016
Lungs do not collapse between breaths and some air always remains in the lungs which can never be expelled because
A · There is a negative pressure in the lungs
B · There is a negative intrapleural pressure pulling at the lung walls ✓
C · There is a positive intrapleural pressure
D · Pressure in the lungs is higher than the atmospheric pressure
Solution: The alveoli-filled lungs sit in an air-tight thoracic chamber and are held expanded by a negative (sub-atmospheric) intrapleural pressure that pulls outward on the lung walls. So the alveoli never fully collapse and some air (residual volume) always stays, keeping the exchange surface ready.
Solved Breathing and Exchange of Gases NEET PYQs
Try the real previous-year questions from this chapter — each with the answer and a full solution.
How many alveoli are there and why does the number matter?
Human lungs have millions of alveoli. Their huge combined surface area lets a very large amount of O2 and CO2 diffuse at the same time, which is why the lungs are so efficient. For NEET, remember 'many alveoli = large surface area = fast gas exchange'.
Do alveoli have cartilage or muscle in their walls?
No. Alveoli walls are only a thin single layer of squamous epithelium wrapped in capillaries — no cartilage and no muscle. Cartilage is found in the conducting airways (trachea, bronchi) to keep them open, not in the exchange surface.
What is the difference between an alveolus and an alveolar duct?
Alveolar ducts are the small passages that lead into the alveoli, and both together make up the respiratory (exchange) part. The alveolus itself is the balloon-like air sac where diffusion happens. NCERT groups them as 'the alveoli and their ducts'.
Why is CO2 exchanged so easily at the alveoli?
CO2 is 20-25 times more soluble than O2, so much more CO2 can diffuse through the same diffusion membrane per unit pressure difference. The gradient runs from tissues to blood and from blood to alveoli, so CO2 leaves the blood into the alveoli to be breathed out.
Is the alveolus part of the conducting or respiratory part for NEET?
Respiratory (exchange) part. This is a very common one-mark point: conducting part = external nostrils up to terminal bronchioles; respiratory part = alveoli and their ducts, where gas diffusion actually occurs.
