What is Gas Exchange in the Respiratory System?
The respiratory system moves air into the lungs and exchanges oxygen and carbon dioxide with the blood. Gas exchange happens in tiny air sacs called alveoli, where gases diffuse across a thin membrane.
Gas exchange is the diffusion of oxygen from alveolar air into blood and carbon dioxide from blood into alveolar air, driven by differences in partial pressure across the thin alveolar-capillary membrane.
- 1↓InhalationAir enters through the nose/mouth and travels down the pharynx, larynx and trachea.
- 2↓Bronchi & bronchiolesAir branches through progressively smaller airways into each lung.
- 3↓Alveoli fill with airAir reaches millions of tiny alveolar sacs surrounded by capillaries.
- 4↓DiffusionO2 moves from alveoli into blood; CO2 moves from blood into alveoli, along partial pressure gradients.
- 5↓Oxygen transportHemoglobin in red blood cells binds O2 and carries it to body tissues.
- 6ExhalationCO2-rich air is expelled back out through the airways.
Step-by-step worked examples
At rest, a healthy adult breathes about 12–20 times per minute. If someone breathes 16 times per minute, how many breaths do they take in one hour?
Breaths per minute = 16 Minutes per hour = 60 Breaths per hour = 16 × 60 = 960 breaths
Blood entering the lungs typically has an oxygen saturation of about 75%. After passing through the alveoli, healthy blood leaves with about 98% saturation. What is the increase?
Increase = final saturation − initial saturation Increase = 98% − 75% = 23 percentage points This rise happens as O2 diffuses from alveolar air into the capillary blood.
The human lungs contain about 300 million alveoli with a total surface area of roughly 70 m². If this area were a square, roughly how long would each side be?
Area = side² side = √70 ≈ 8.4 meters This huge surface area (close to the size of a tennis court) is what makes fast gas diffusion possible.
Flashcards
Quick quiz
Q1.Where does gas exchange take place in the lungs?
Q2.What causes oxygen to diffuse from alveoli into the blood?
Q3.What molecule carries most of the oxygen in blood?
Q4.Which gas is removed from the blood during gas exchange?
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Common mistakes
Thinking gas exchange happens in the trachea. — Correct: Gas exchange only occurs in the alveoli, where the membrane is thin enough for fast diffusion.
Believing oxygen is 'pumped' into the blood. — Correct: Oxygen moves by passive diffusion, following its partial pressure gradient — no energy is spent.
Assuming the lungs only remove CO2. — Correct: The lungs simultaneously bring in O2 and remove CO2 — it's a two-way exchange.
Confusing breathing (ventilation) with gas exchange (respiration). — Correct: Breathing moves air in and out; gas exchange is the diffusion of gases across the alveolar membrane.
FAQ
What is gas exchange in the respiratory system?
Gas exchange is the diffusion of oxygen into the blood and carbon dioxide out of the blood, occurring across the thin walls of alveoli in the lungs.
What is an example of gas exchange?
When you inhale, oxygen-rich air fills alveoli; oxygen then diffuses into nearby capillaries while CO2 diffuses out to be exhaled.
How is gas exchange efficiency measured?
It's often assessed via oxygen saturation (SpO2, normally 95–100%) and the surface area/thickness of the alveolar-capillary membrane.
Why are alveoli important for gas exchange?
Alveoli provide a huge, thin, moist surface area (about 70 m² total) that allows oxygen and carbon dioxide to diffuse quickly between air and blood.