Questions: Respiratory System Anatomy and Ventilation

Question 1

A region of alveoli becomes partially obstructed, reducing local oxygen levels significantly. How do the local pulmonary arterioles respond, and why is this response different from the systemic circulation?

• A. They dilate, to deliver more oxygenated blood to the hypoxic region — the same response as systemic arterioles
• B. They constrict, redirecting blood toward better-ventilated alveoli to optimize gas exchange
• C. They dilate, because low oxygen always causes smooth muscle relaxation in vascular walls
• D. They constrict, to reduce blood pressure and protect the lung from further damage

Answer: (B)

This is hypoxic pulmonary vasoconstriction (HPV) — a response unique to the pulmonary circulation that is opposite to the systemic response. In systemic tissues, low O₂ signals that the tissue needs more delivery, so arterioles dilate to increase flow. But in the lung, the alveolus is the *source* of oxygen, not a consumer. If alveoli are hypoxic (poorly ventilated), sending more blood there would waste cardiac output on non-functioning gas exchange. Instead, local pulmonary arterioles constrict, shunting blood to better-ventilated regions. This V/Q matching mechanism optimizes overall oxygenation at the whole-lung level. Option A is the common misconception that confuses systemic and pulmonary vascular responses.

Question 2

A healthy person at rest takes a normal breath. Which muscles are active during inhalation, and what happens during exhalation?

• A. Both inhalation and exhalation are passive — the diaphragm is not a skeletal muscle and works automatically
• B. Inhalation is active (diaphragm contracts); exhalation at rest is passive, driven by elastic recoil of the lungs
• C. Inhalation is active (diaphragm contracts); exhalation is also active (internal intercostals contract)
• D. Inhalation is passive — the negative pleural pressure automatically pulls air in without muscle activity

Answer: (B)

At rest, inhalation is driven by active diaphragm contraction (and external intercostals), which increases thoracic volume and decreases intrapulmonary pressure below atmospheric, drawing air in. Resting exhalation requires no active muscle contraction — when the diaphragm relaxes, the elastic recoil of the lung tissue restores volume, raising intrapulmonary pressure above atmospheric and expelling air passively. Active forced exhalation (during exercise, coughing, or blowing) additionally recruits internal intercostals and abdominal muscles to increase the expiratory pressure gradient. Option C correctly identifies active inhalation but wrongly makes resting exhalation active.

Question 3

Exhalation at rest is passive — it requires no muscle contraction and is driven entirely by the elastic recoil of the lungs.

• T. True
• F. False

Answer: True

This is correct. At rest, exhalation is a passive mechanical process: when the diaphragm relaxes, the lungs and chest wall recoil toward their equilibrium position, reducing thoracic volume and increasing intrapulmonary pressure above atmospheric. Air flows out along the pressure gradient without any active muscle work. This is why resting breathing is energetically inexpensive — only inhalation requires active muscular effort. Forced exhalation during exercise or against resistance adds internal intercostals and abdominal muscles, making it active. Understanding which phase is active vs. passive is important for interpreting breathing disorders like emphysema, which destroys elastic recoil and impairs passive exhalation.

Question 4

The conducting airways (trachea, bronchi, bronchioles) participate meaningfully in gas exchange and contribute to overall oxygen uptake.

• T. True
• F. False

Answer: False

The conducting zone (nose to terminal bronchioles) is anatomical dead space — it conducts air to the respiratory zone but performs no gas exchange. The walls of conducting airways are too thick and their surfaces are lined with ciliated epithelium and mucus, not the ultra-thin type I pneumocytes of alveoli. Gas exchange occurs only in the respiratory zone: respiratory bronchioles, alveolar ducts, and alveolar sacs. The ~150 mL of air in the conducting zone at rest is 'wasted' ventilation that never reaches alveoli. This is why shallow rapid breathing is less efficient than deep slow breathing — a larger fraction of tidal volume remains in dead space.

Question 5

Why must ventilation and perfusion be matched at the alveolar level, and what happens physiologically when they are mismatched?

Model answer: Gas exchange requires both a supply of fresh air (ventilation) and blood flow to carry away oxygen and deliver CO₂ (perfusion). Alveoli that are ventilated but not perfused waste respiratory effort — they receive fresh air but no blood comes to take up the oxygen (dead space ventilation). Alveoli that are perfused but not ventilated waste cardiac output — blood flows past collapsed or blocked alveoli without being oxygenated, and returns to the left heart still hypoxic (shunt). In both cases, the overall efficiency of gas exchange falls. The body compensates through hypoxic pulmonary vasoconstriction: when alveoli have low O₂ (indicating poor ventilation), local arterioles constrict, redirecting blood to better-ventilated regions and minimizing the shunt effect.

V/Q mismatch is the most common cause of hypoxemia in clinical medicine. Conditions like pneumonia (alveoli filled with fluid — ventilation drops, perfusion continues), pulmonary embolism (perfusion drops, ventilation continues), and ARDS (widespread V/Q mismatch) all impair gas exchange through this mechanism. Understanding V/Q matching explains why supplemental oxygen helps some causes of hypoxemia (low V/Q regions) but not others (complete shunt, where no alveolar ventilation reaches the blood).