Questions: Respiratory System Anatomy and Ventilation Mechanics

Question 1

A patient with asthma has airway narrowing (bronchoconstriction) that slows airflow out of the lungs. Which spirometry pattern would you expect?

• A. Reduced total lung capacity and vital capacity with a normal FEV₁/FVC ratio
• B. Low FEV₁/FVC ratio with near-normal total lung volumes, because narrowed airways slow forced expiration disproportionately
• C. Normal spirometry, since asthma affects gas exchange but not mechanical airflow
• D. Increased FEV₁/FVC ratio, because bronchospasm forces air out more rapidly

Answer: (B)

Asthma is an obstructive disease: airway narrowing increases resistance, particularly during forced expiration (which tends to collapse already-narrowed airways further). FEV₁ falls because air cannot move out rapidly in 1 second, while FVC may be more preserved. The FEV₁/FVC ratio below ~0.70 indicates obstruction. In contrast, restrictive diseases like pulmonary fibrosis shrink total lung volumes but leave the FEV₁/FVC ratio normal — both numerator and denominator shrink proportionally when the lung is simply stiff and small, not obstructed.

Question 2

During normal quiet inhalation, which statement most accurately describes how air enters the lungs?

• A. The lungs actively expand by contracting smooth muscle in the alveolar walls, creating suction pressure
• B. The diaphragm and external intercostals contract, increasing thoracic volume; intrapulmonary pressure falls below atmospheric pressure, and air flows in along the pressure gradient
• C. The trachea dilates, reducing airway resistance enough to allow passive airflow driven by body heat
• D. Surfactant secretion at the alveoli creates a chemical gradient that pulls air molecules inward

Answer: (B)

Breathing is an application of Boyle's law: at constant temperature, pressure and volume are inversely related. When the diaphragm flattens and the rib cage lifts (via the external intercostals), thoracic volume increases. Air already in the lungs now occupies a larger space, so its pressure drops below atmospheric (~760 mmHg). This gradient — high pressure outside, lower inside — drives air in. The lungs do not 'suck' air; they create a low-pressure zone that the atmosphere fills. Exhalation at rest is the reverse: muscle relaxation allows elastic recoil to decrease volume and raise pressure above atmospheric.

Question 3

Normal quiet exhalation requires no muscular effort because relaxation of the inspiratory muscles allows the thorax and lungs to recoil passively, raising intrapulmonary pressure above atmospheric.

• T. True
• F. False

Answer: True

The lungs and chest wall have elastic properties: stretched during inhalation, they naturally recoil toward their resting position when the diaphragm and external intercostals relax. This recoil decreases lung volume, raises intrapulmonary pressure above atmospheric, and drives air out — all without active muscular contraction. Forced exhalation (as in blowing hard or during exercise) does recruit internal intercostals and abdominal muscles, but quiet tidal breathing relies entirely on passive elastic recoil.

Question 4

Surfactant prevents alveolar collapse primarily by lubricating adjacent alveolar surfaces so they can slide freely past each other during breathing.

• T. True
• F. False

Answer: False

Surfactant works by reducing surface tension at the air-liquid interface of alveoli, not by lubrication. By Laplace's law (P = 2T/r), surface tension at a curved surface creates a collapsing pressure — and for tiny alveoli, this would be enormous without intervention. Surfactant — a phospholipid mixture secreted by type II pneumocytes — inserts into the air-liquid interface and dramatically lowers surface tension, reducing the collapsing pressure. Without surfactant, alveoli collapse (atelectasis) at the end of each breath. This is why premature infants lacking mature type II cells develop respiratory distress syndrome, treated by administering synthetic surfactant.

Question 5

Explain, using Boyle's law, why contracting the diaphragm causes air to flow into the lungs.

Model answer: Boyle's law states that at constant temperature, pressure and volume are inversely related (PV = constant). When the diaphragm contracts and flattens, it increases the volume of the thoracic cavity. The air inside the lungs now occupies a larger volume, so its pressure drops below atmospheric. This creates a pressure gradient — atmospheric pressure (~760 mmHg) outside is greater than intrapulmonary pressure (now ~758 mmHg) — and air flows from high to low pressure, from the atmosphere into the lungs, until the pressures equalize. The diaphragm does not suck air in; it creates the low-pressure zone that the atmosphere fills.

The key insight is that breathing is passive air movement driven by pressure gradients, not active suction. The respiratory muscles change volume; Boyle's law translates that volume change into a pressure change; and the pressure difference drives bulk airflow. This is also why a penetrating chest wound (pneumothorax) can be immediately life-threatening: air enters the pleural space instead of the lungs, collapsing the pressure gradient that drives inhalation.