Gas exchange in humans

Section: Biology | Syllabus: Cambridge IGCSE Combined Science (0653)

Features of Gas Exchange Surfaces

The lungs are specialised for efficient gas exchange between air and the blood. Effective gas exchange surfaces share four key features:

Large surface area - more area for the diffusion of O₂ and CO₂; around 700 million alveoli give the lungs a total surface area of about 70 m²
Thin surface - alveolar walls are only one cell thick, giving a short diffusion distance
Good blood supply - a dense network of capillaries around each alveolus maintains steep concentration gradients by constantly carrying O₂ away and bringing CO₂ to the surface
Good ventilation with air - breathing movements continuously replenish fresh air in the alveoli, keeping the diffusion gradients steep

Larynx - the voice box at the top of the trachea, through which air passes from the throat
Trachea - the windpipe, the main airway from the larynx down to the chest, kept open by rings of cartilage
Bronchi - two branches of the trachea, one entering each lung
Bronchioles - smaller branches of the bronchi throughout the lungs
Alveoli - tiny air sacs at the end of the bronchioles, the site of gas exchange
Lungs - a pair of spongy organs in the thorax, containing the bronchi, bronchioles and alveoli
Diaphragm - a dome-shaped sheet of muscle beneath the lungs, separating the thorax from the abdomen
Ribs - the bony cage protecting the lungs, which move during breathing
Intercostal muscles - muscles between the ribs that move them during breathing
Associated capillaries - a dense network of blood capillaries surrounding each alveolus

Air passes through a branching system of airways before reaching the alveoli, where gas exchange occurs

During exercise, the rate and depth of breathing both increase:

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