Describe the structural adaptations of alveoli that make them efficient for gas exchange.

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ALVEOLI (singular: alveolus) are tiny, balloon-like air sacs at the end of bronchioles in the lungs. They are the sites of gas exchange — where O₂ moves from air into blood and CO₂ moves from blood into air. STRUCTURAL ADAPTATIONS: (1) LARGE NUMBER: There are about 300 million alveoli in each lung, giving an enormous total surface area of about 70 m² (the size of a badminton court) — maximises the area for gas exchange. (2) THIN WALLS: Alveolar walls are only ONE CELL THICK — this minimises the diffusion distance for gases. O₂ and CO₂ cross the wall rapidly. (3) RICH BLOOD SUPPLY: Each alveolus is surrounded by a dense network of capillaries — blood flows past continuously, maintaining a steep concentration gradient. O₂ moves from high concentration in alveoli to low in blood; CO₂ moves the opposite way. (4) MOIST SURFACE: The alveolar surface is coated with a thin film of water — gases must dissolve in this layer to diffuse. The moisture ensures efficient dissolution. (5) ELASTIC WALLS: Alveoli expand during inhalation and recoil during exhalation, aiding ventilation.

Practice — Respiration — Aerobic and Anaerobic

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