01 / Location & anatomical relationships
At the ends of the smallest airways within the lungs.
02 / Structure & organization
Thin type I pneumocytes form much of the gas-exchange surface. Type II pneumocytes produce surfactant and contribute to epithelial renewal. Closely apposed capillary endothelium and supporting layers create a short diffusion path.
03 / Function
Their thin walls allow oxygen and carbon dioxide to move between air and blood.
04 / Mechanism
A dense network of capillaries surrounds these tiny sacs. Their moist, thin walls allow gases to cross a very short distance. A substance called surfactant helps keep the sacs from collapsing as air moves out.
Working together: respiratory system
Your diaphragm contracts to help draw air in. Air reaches your lungs, where oxygen crosses into the blood through the walls of tiny air sacs.
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Connect structure with function
Surfactant reduces surface tension and helps stabilize alveoli. Alveolar macrophages provide defense without replacing the thin epithelial barrier required for gas exchange.
Check your understanding
Locate the alveoli in relation to its neighbors. Explain how the structural features described above support its function, then distinguish it from a nearby structure with a different role.
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Continue with OpenStax Anatomy and Physiology for system-level study. This atlas is educational and has not undergone independent medical peer review.