🎯Diagnostic: Test Your Starting Level24 questions (~2 per section). No prior reading required, see what you already know.
Aim to answer every question before checking. Missed questions point you to the sections you need most.
1. (6.1) Air passes through the respiratory tract in which order?
C. The upper airway filters/warms/humidifies air. The conducting zone ends at terminal bronchioles; gas exchange begins at respiratory bronchioles and alveoli.
2. (6.1) The epiglottis functions to:
A. The epiglottis is a cartilaginous flap at the top of the larynx. Vocal cords produce sound; ciliated epithelium filters air; type II pneumocytes secrete surfactant.
3. (6.2) Alveoli are specialized for gas exchange because they have:
D. ~300 million alveoli give ~70 m² of surface area. Type I pneumocytes (squamous) provide a minimal diffusion distance to the pulmonary capillaries.
4. (6.2) Pulmonary surfactant, produced by type II pneumocytes:
B. Surfactant (mainly DPPC phospholipid) breaks up water hydrogen bonding at the alveolar surface. Without it, small alveoli would collapse (Laplace's law), as in neonatal RDS.
5. (6.3) During inspiration:
C. Inspiration is active: diaphragm and external intercostals contract, thoracic volume rises, intrapulmonary pressure drops below atmospheric, and air flows in (Boyle's law).
6. (6.3) Normal quiet exhalation is driven by:
A. Quiet exhalation requires no muscle effort; the stretched lungs spring back. Forced exhalation does recruit abdominals and internal intercostals.
7. (6.4) Tidal volume is:
D. Normal tidal volume is about 500 mL at rest. Increases with exercise by recruiting IRV and ERV.
8. (6.4) Vital capacity equals:
B. Vital capacity is the maximum volume a person can exhale after maximal inhalation. Total lung capacity = VC + residual volume.
9. (6.5) Gas exchange at the alveolus occurs by:
A. O₂ diffuses from alveolus (PO₂ ~100 mmHg) to capillary (PO₂ ~40 mmHg); CO₂ diffuses the opposite way. Diffusion is governed by Fick's law (area, thickness, gradient).
10. (6.5) Thickening of the alveolar-capillary membrane (e.g., in pulmonary fibrosis):
C. By Fick's law, diffusion rate is inversely proportional to membrane thickness. Fibrosis lays down collagen between alveolus and capillary, slowing O₂ transfer.
11. (6.6) Most oxygen in blood is transported:
B. About 98.5% of arterial O₂ is bound to hemoglobin; only about 1.5% is dissolved. Each Hb tetramer binds up to 4 O₂ cooperatively.
12. (6.6) A rightward shift of the oxygen-hemoglobin dissociation curve means:
D. CADET, face Right: rises in CO₂, Acid (low pH), 2,3-BPG/DPG, Exercise, Temperature shift the curve right, helping tissues extract O₂.
13. (6.7) The majority of CO₂ in blood is transported as:
A. About 70% travels as HCO₃⁻, 20-25% bound to Hb as carbaminohemoglobin, and ~7% dissolved. This bicarbonate pool is also the body's main extracellular buffer.
14. (6.7) The interconversion of CO₂ and bicarbonate inside red blood cells is catalyzed by:
C. CO₂ + H₂O ⇌ H₂CO₃ ⇌ H⁺ + HCO₃⁻, catalyzed by carbonic anhydrase. Without it, CO₂ transport would be far too slow.
15. (6.8) Central chemoreceptors in the medulla respond primarily to:
B. CO₂ crosses the blood-brain barrier readily; it reacts with CSF water to make H⁺. Rising CSF H⁺ drives increased ventilation to blow off CO₂.
16. (6.8) Peripheral chemoreceptors (carotid/aortic bodies) are most sensitive to:
D. Peripheral bodies are the only sensors of arterial PO₂. They become the dominant ventilatory drive when hypoxia is severe or central drive is blunted (chronic CO₂ retention).
17. (6.9) V/Q matching is optimal when:
A. In the healthy lung, overall ventilation and perfusion are reasonably matched. V/Q varies regionally: higher at the apex (more V than Q) and lower at the base (more Q than V).
18. (6.9) A pulmonary embolus that blocks a pulmonary artery produces:
C. Blocked perfusion with intact ventilation creates alveolar dead space (V/Q → ∞). A shunt (V/Q = 0) is the opposite: perfusion with no ventilation, as in atelectasis.
19. (6.10) The acute ventilatory response to high altitude is:
B. Low PO₂ activates carotid bodies, which drive an increase in ventilation. The resulting hypocapnia leads to respiratory alkalosis until the kidneys compensate.
20. (6.10) Prolonged acclimatization to high altitude involves:
D. Renal EPO rises with chronic hypoxia, boosting red-cell production. 2,3-BPG also rises, shifting the Hb curve right and improving tissue O₂ unloading.
21. (6.3) The pleural cavity between the visceral and parietal pleurae maintains:
C. Intrapleural pressure is about -4 to -5 cmH₂O at rest. Puncturing the pleura (pneumothorax) breaks this seal, and the lung collapses inward.
22. (6.6) The Bohr effect describes:
A. Active tissues are warm, acidic, and CO₂-rich, all of which lower hemoglobin's O₂ affinity at that local site, unloading more oxygen exactly where it is needed.
23. (6.5) Alveolar PO₂ at sea level is approximately:
D. Inspired air is ~160 mmHg O₂; the alveolus drops to ~100 mmHg after humidification and mixing with CO₂. Venous blood returns at ~40 mmHg, setting the diffusion gradient.
24. (6.7) The chloride shift in red blood cells refers to:
B. As HCO₃⁻ is produced inside the RBC and leaves via the AE1 antiporter, Cl⁻ enters to preserve electroneutrality. The reverse happens in the pulmonary capillaries.
Previous Attempts
Take a deep breath. Right now, without thinking about it, your diaphragm just pulled downward, your rib cage expanded, and air rushed into your lungs - not because you pushed it in, but because you created a vacuum. In a fraction of a second, oxygen molecules crossed a membrane thinner than a soap bubble, hitched a ride on hemoglobin molecules in your blood, and began their journey to every cell in your body. Simultaneously, carbon dioxide - the waste product of cellular metabolism - traveled the opposite direction, from your blood into your lungs, and left your body on your next exhale.
You do this roughly 20,000 times per day without conscious effort. The respiratory system is so well-designed that it automatically adjusts your breathing rate based on blood chemistry, delivers more oxygen to muscles that are working harder, and even helps regulate your blood pH. Understanding how it accomplishes all of this is one of the highest-yield topics on the MCAT.
Your Lungs Are a Bellows
Before we get into anatomy and physiology, anchor this one mental image: your respiratory system works like a blacksmith’s bellows. When you pull the handles apart (your diaphragm contracts downward), the chamber expands, pressure inside drops below atmospheric pressure, and air rushes in. When you let go (diaphragm relaxes), the chamber shrinks, pressure rises, and air is pushed out. You never “suck” air in - you create negative pressure and the atmosphere does the pushing.
This single concept - that breathing is driven by pressure gradients, not suction - unlocks everything from Boyle’s Law applications to understanding why a punctured chest wall causes a lung to collapse.
