Gas Exchange in Humans

Gas exchange is the process of absorbing oxygen (O₂) into the bloodstream and removing carbon dioxide (CO₂). In humans, this occurs in the alveoli —tiny air sacs in the lungs. Their structure is specialized to maximize efficiency:

1.1 Key Features of Gas Exchange Surfaces

Large Surface Area

Structure: The lungs contain millions of alveoli, which are densely packed like clusters of grapes.

Function: This creates a surface area of approximately 70 m² (equivalent to a tennis court!).

Why It Matters: A larger surface area allows more O₂ to diffuse into the blood and more CO₂ to diffuse out simultaneously.

Thin Membranes (One Cell Thick)

Structure: Both the alveolar walls and the surrounding capillaries are only one cell thick (made of squamous epithelium).

Function: Gases diffuse rapidly across these thin barriers.

Analogy: Imagine blowing air through a single sheet of tissue paper—there’s almost no resistance!

Dense Network of Capillaries

Structure: Alveoli are enveloped in a web of capillaries (tiny blood vessels).

Function: Blood flows continuously through these capillaries, maintaining a steep concentration gradient.

O₂-poor blood arrives from the body, creating a low O₂ concentration in capillaries.
O₂-rich air in alveoli creates a high O₂ concentration.

Result: O₂ diffuses into the blood, and CO₂ diffuses out into the alveoli.