Explain gas exchange

Explain how oxygen and carbon dioxide cross the alveolus by diffusion, from high concentration to low.

Builds on (2 levels back)inferred · high confidence

Diffusion (high to low): Gas exchange is diffusion; you must know molecules move from where they are crowded to where they are sparse.
Alveolus and capillary structure: The thin, moist alveolar wall sits next to a capillary: knowing this layout explains why gases cross there.

Prerequisites are inferred: pending teacher review.

Re-learn the skill with worked practice and clear examples.

At the alveolus, oxygen diffuses from the air (high O2) into the blood (low O2), and carbon dioxide diffuses from the blood (high CO2) into the air (low CO2).

Step 1: Set up the concentrations

Fresh air in the alveolus is high in oxygen and low in carbon dioxide. Blood arriving from the body is the opposite: low in oxygen and high in carbon dioxide.

Step 2: Let each gas diffuse

Each gas moves from high to low: oxygen diffuses from the alveolar air into the blood; carbon dioxide diffuses from the blood into the alveolar air to be breathed out.

Step 3: Note what speeds it up

The alveolar wall is extremely thin and moist, and the lungs have a huge total surface area of millions of alveoli. Both features make diffusion fast and efficient.

Practice

The diagram shows an alveolus next to a capillary. Based on the labeled concentrations, which way does each gas diffuse?

Reviewed

An alveolus air sac labeled high oxygen and low carbon dioxide sits beside a capillary labeled low oxygen and high carbon dioxide, separated by a thin moist wall.

A.Oxygen moves blood to air; carbon dioxide moves air to blood
B.Both oxygen and carbon dioxide move from the air into the blood
C.Oxygen moves air to blood; carbon dioxide moves blood to air
D.Both gases stay put because the wall blocks them

Show the worked solution ▾

Answer: C. Oxygen moves air to blood; carbon dioxide moves blood to air

Step 1: Read each gas's concentrations: Oxygen: high in the air, low in the blood. Carbon dioxide: high in the blood, low in the air.
Step 2: Apply high-to-low: Oxygen diffuses from the air into the blood; carbon dioxide diffuses from the blood into the air.

Why it's right: Each gas diffuses from high to low: oxygen goes air to blood, carbon dioxide goes blood to air.

Why the others miss:

A: This reverses both directions, sending each gas the wrong way.
B: Carbon dioxide is high in the blood, so it leaves the blood, not enters it.
D: The thin, moist wall lets gases cross easily; it does not block them.

Aligned to Human Body Systems: gas exchange at the alveolus · reading level ~grade 9

Where you'd see this

A diagram in a clinic explains shortness of breath by showing oxygen failing to diffuse across thickened alveolar walls.

Careers that use this

Respiratory Therapist: Explains to patients how oxygen crosses the alveolus so they understand their breathing treatments.

Video library

Watch: Explain gas exchange

Respiratory System

Amoeba Sisters · ~8 min

Guided notes

Fill these in as you work through the lesson.

Big idea: In the alveolus, oxygen diffuses from the air into the blood and carbon dioxide diffuses from the blood into the air, each moving from high concentration to low.

Key terms: write the meaning

Alveolus (tiny air sac where gases cross):
Diffusion (movement from crowded to sparse):
Oxygen (O2) (the gas going into the blood):
Carbon dioxide (CO2) (the waste gas leaving the blood):

The rule

In the alveolus, diffuses from the air into the blood and diffuses from the blood into the air, each moving from concentration to concentration.

Check yourself

Where in the lung does gas exchange happen?
Which way does oxygen move, and why?
Name two features of the alveolus that make diffusion fast.

Work one example

Air in the alveolus is high in oxygen and low in carbon dioxide; blood arriving from the body is the opposite. Use diffusion to explain which way each gas moves and why the thin, moist alveolar wall helps.