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Human Body Study

Alveoli

The tiny air sacs at the very ends of the airways — the actual site of gas exchange. The lungs contain roughly 300 million.

The respiratory tract From the upper airway down through larynx, trachea and bronchial branching into the two lungs, with the diaphragm at the base. A separate inset shows alveoli — the microscopic gas-exchange sacs inside the lungs. Each label links to the article for that part. Alveoli Upper respiratory tract Larynx Trachea Bronchi Lungs Diaphragm The respiratory tract — schematic, not to scale.

Structure

Section titled “Structure”

Microscopic balloon-like sacs with walls one cell thick, wrapped in capillaries. Together they offer a gas-exchange surface roughly the size of a tennis court.

What they do

Section titled “What they do”

Oxygen passes from the air in the alveolus into the blood; carbon dioxide passes the other way — driven simply by differences in concentration. A thin liquid coating (surfactant) keeps the sacs from collapsing.

Key idea

Section titled “Key idea”

All of breathing’s purpose is realized here, across a barrier thinner than a sheet of paper.