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🫀 Science · Human Anatomy and Physiology · Lesson 8 of 12 · 5 min

Respiratory system: airway, gas exchange and breathing

🫁 Every breath follows the same road and every exchange happens in a space thinner than a cell. Know the road and the pressure rule, and most respiratory items fall into place.

💡 Key idea

Volume up means pressure down means air in; gas exchange is diffusion in the alveoli.

🧠 The idea

Air enters the nose or mouth, then passes the pharynx, larynx (voice box, guarded by the epiglottis), trachea, right and left primary bronchi, smaller bronchioles, and finally the ALVEOLI. Gas exchange happens by DIFFUSION across the thin alveolar and capillary walls: oxygen moves from the alveoli into the blood, and carbon dioxide moves from the blood into the alveoli, each from high to low partial pressure. Most oxygen rides on hemoglobin; most carbon dioxide travels as bicarbonate. Breathing is driven by the DIAPHRAGM. On inhalation it contracts and flattens, the external intercostals lift the ribs, chest volume rises, pressure in the lungs falls below room air, and air flows in. Quiet exhalation is passive: the muscles relax, volume falls, pressure rises, and air flows out. Surfactant keeps alveoli from collapsing, and the medulla oblongata sets the breathing rhythm, responding mostly to blood carbon dioxide.

✏️ Worked example

Order these: bronchioles, larynx, trachea, alveoli, pharynx. Step 1: air hits the throat first, so pharynx. Step 2: the voice box sits at the top of the windpipe, so larynx, then trachea. Step 3: the airways keep branching smaller, so bronchioles. Step 4: the dead end where exchange happens is the alveoli. Answer: pharynx, larynx, trachea, bronchioles, alveoli.

📌 Takeaways

  • Path: nose, pharynx, larynx, trachea, bronchi, bronchioles, alveoli.
  • Diaphragm contracts on inhalation; quiet exhalation is passive recoil.
  • The main chemical drive to breathe is rising carbon dioxide, sensed by the brainstem.

✅ Check yourself

Question 1 of 3

Where in the respiratory system does gas exchange with the blood take place?

  1. Primary bronchi
  2. Trachea
  3. Alveoli
  4. Larynx
Show the answer and rationale

Answer: C

Alveoli are tiny air sacs wrapped in capillaries, with walls thin enough for oxygen and carbon dioxide to diffuse. The trachea, bronchi and larynx only conduct air and are too thick for exchange.

Question 2 of 3

What happens to the diaphragm during a normal inhalation?

  1. It relaxes and flattens
  2. It contracts and rises
  3. It relaxes and rises
  4. It contracts and flattens
Show the answer and rationale

Answer: D

The diaphragm contracts and moves downward (flattens), enlarging the chest cavity so lung pressure drops and air flows in. Relaxing and rising into a dome is what happens during exhalation.

Question 3 of 3

Which structure closes over the larynx during swallowing so food does not enter the airway?

  1. Soft palate
  2. Glottis
  3. Uvula
  4. Epiglottis
Show the answer and rationale

Answer: D

The epiglottis is a flap of cartilage that folds over the opening of the larynx when you swallow. The soft palate and uvula close off the nasal cavity instead, and the glottis is the opening between the vocal cords that the epiglottis covers.

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