Respiratory Physiology for USMLE Step 1: Key Concepts and Review
Respiratory physiology is a high-yield area of basic science for USMLE Step 1. Questions can test lung volumes, ventilation, perfusion, gas exchange, oxygen and carbon dioxide transport, pulmonary mechanics, airway resistance, and respiratory control.
The original ValueMD discussion collected respiratory physiology multiple-choice questions covering many of these concepts. This updated resource reorganizes those concepts into a structured review for medical students.
Lung Functions and Respiratory Anatomy
The lungs have several important functions beyond gas exchange. They participate in oxygen and carbon dioxide exchange, contribute to acid-base regulation, filter small emboli from the circulation, and have important metabolic and endocrine functions.
The conducting airways transport air but do not normally participate directly in gas exchange. Gas exchange begins in the respiratory bronchioles, which connect the conducting portion of the airway to the respiratory portion.
An acinus consists of the respiratory structures supplied by a terminal bronchiole, including respiratory bronchioles, alveolar ducts, and alveoli. Terminal bronchioles themselves belong to the conducting zone.
Lung Volumes and Capacities
Understanding the relationships between lung volumes is fundamental to respiratory physiology.
Important terms include:
- Tidal volume (TV): air moved during a normal quiet breath
- Inspiratory reserve volume (IRV): additional air that can be inhaled after a normal inspiration
- Expiratory reserve volume (ERV): additional air that can be exhaled after a normal expiration
- Residual volume (RV): air remaining after maximal expiration
- Vital capacity (VC): maximum volume exhaled after maximum inspiration
- Functional residual capacity (FRC): volume remaining after a normal expiration
- Total lung capacity (TLC): volume in the lungs after maximal inspiration
A useful relationship is:
TLC = VC + RV
and:
FRC = ERV + RV
Spirometry can measure several important lung volumes, but residual volume and capacities containing residual volume cannot be measured directly by simple spirometry.
Pulmonary Mechanics
During inspiration, contraction of the diaphragm and other inspiratory muscles increases thoracic volume. This causes intrapulmonary pressure to fall below atmospheric pressure, allowing air to enter the lungs.
During quiet expiration, the respiratory muscles generally relax and elastic recoil of the lungs contributes to expiration.
Transpulmonary pressure is the pressure difference between the alveolar space and the pleural space. It helps maintain lung expansion.
Lung Compliance
Lung compliance describes the change in lung volume produced by a given change in pressure.
Compliance = ΔV / ΔP
High compliance means the lungs can undergo a relatively large change in volume for a given pressure change. Low compliance means greater pressure is required to achieve the same volume change.
Pulmonary fibrosis is associated with decreased compliance, whereas emphysema is associated with increased lung compliance.
In obstructive disease such as asthma, airway resistance can change substantially with airflow and lung volume. Dynamic compliance may therefore differ from static compliance.
Surfactant and Alveolar Stability
Pulmonary surfactant is produced primarily by type II pneumocytes. It reduces surface tension at the air-liquid interface of the alveoli.
Surfactant is particularly important for preventing alveolar collapse and reducing the work required for breathing.
The relationship between pressure, radius, and surface tension can be understood using Laplace’s law. Without surfactant, smaller alveoli would be more susceptible to collapse. Surfactant reduces surface tension and helps stabilize alveoli of different sizes.
Premature infants may have insufficient surfactant production, contributing to neonatal respiratory distress syndrome.
Airway Resistance
Airway resistance is influenced strongly by airway radius. Small changes in airway diameter can produce substantial changes in resistance.
Airway resistance is particularly important in obstructive diseases such as asthma and chronic obstructive pulmonary disease.
During forced expiration, dynamic airway compression can produce portions of the flow-volume curve that are relatively independent of additional expiratory effort. This concept is important when interpreting forced expiratory maneuvers.
Dead Space and Alveolar Ventilation
Not all inspired air reaches regions where gas exchange occurs.
Anatomic dead space consists primarily of the conducting airways.
Alveolar ventilation can be calculated as:
Alveolar ventilation = (Tidal volume − Dead space) × Respiratory rate
For example, if tidal volume is 500 mL, dead space is 150 mL, and respiratory rate is 12 breaths per minute:
(500 − 150) × 12 = 4,200 mL/min
Therefore, approximately 4.2 L/min of air reaches gas-exchanging regions under these assumptions.
This relationship explains why rapid, shallow breathing can be less effective for alveolar ventilation than slower breathing with larger tidal volumes.
Ventilation-Perfusion Relationships
The ventilation-perfusion ratio, or V/Q ratio, describes the relationship between ventilation and pulmonary blood flow.
An alveolus with ventilation but little or no perfusion has a high V/Q ratio and behaves more like dead space.
An alveolus that is perfused but poorly ventilated has a low V/Q ratio and behaves more like a shunt.
At the apex of the lung, both ventilation and perfusion are lower than at the base, but perfusion decreases more substantially. Consequently, the V/Q ratio is higher at the apex.
At the base, ventilation and perfusion are both greater, but the V/Q ratio is lower.
Gas Exchange and Diffusion
Gas movement across the alveolar-capillary membrane is influenced by factors described by Fick’s law of diffusion.
Diffusion increases with:
- Greater surface area
- Greater partial-pressure gradient
- Greater diffusion coefficient
Diffusion decreases with:
- Greater membrane thickness
Conditions that reduce alveolar surface area or increase the thickness of the respiratory membrane can impair gas exchange.
Carbon monoxide is commonly used to assess pulmonary diffusing capacity because its transfer into pulmonary capillary blood is strongly diffusion dependent.
Oxygen Transport
Most oxygen in blood is transported bound to hemoglobin, with a smaller amount dissolved directly in plasma.
Oxygen content depends on hemoglobin concentration, hemoglobin saturation, and dissolved oxygen.
The oxygen-hemoglobin dissociation curve shifts to the right when hemoglobin affinity for oxygen decreases. Important causes include:
- Increased temperature
- Increased carbon dioxide
- Increased 2,3-BPG
- Decreased pH
A rightward shift facilitates oxygen unloading in metabolically active tissues.
A leftward shift indicates increased hemoglobin affinity for oxygen.
Carbon Dioxide Transport
Carbon dioxide is transported in blood in several forms:
- Primarily as bicarbonate
- Dissolved in plasma
- Bound to proteins, including hemoglobin as carbamino compounds
Within red blood cells, carbon dioxide combines with water to form carbonic acid. Carbonic acid then dissociates into hydrogen ions and bicarbonate.
The enzyme carbonic anhydrase greatly accelerates this reaction.
In the lungs, the process reverses, allowing carbon dioxide to be regenerated and exhaled.
Alveolar Gas Equation
The alveolar gas equation is useful for estimating alveolar oxygen pressure:
PAO₂ ≈ PIO₂ − (PaCO₂ / R)
At sea level, inspired oxygen partial pressure is approximately 150 mmHg after accounting for humidification. With a respiratory quotient around 0.8 and a PaCO₂ of 40 mmHg, alveolar oxygen pressure is approximately 100 mmHg under typical assumptions.
The equation is useful for understanding the relationship between inspired oxygen, carbon dioxide production, and alveolar oxygen concentration.
Control of Breathing
Respiration is regulated by central and peripheral mechanisms.
Central chemoreceptors respond primarily to changes related to carbon dioxide and hydrogen ion concentration in the central nervous system.
Peripheral chemoreceptors, located mainly in the carotid and aortic bodies, respond to changes in arterial oxygen, carbon dioxide, and pH.
A significant reduction in arterial oxygen can increase ventilation through peripheral chemoreceptor stimulation.
Higher carbon dioxide levels are also a major stimulus for increased ventilation.
High-Yield USMLE Concepts
For Step 1 preparation, focus on understanding the relationships rather than memorizing isolated facts.
Key concepts include:
- Respiratory bronchioles are the first airways with gas-exchange capability.
- Type II pneumocytes produce pulmonary surfactant.
- Surfactant reduces alveolar surface tension.
- Transpulmonary pressure helps keep the lungs expanded.
- Alveolar ventilation depends on tidal volume minus dead space.
- Pulmonary perfusion is greater at the lung bases than at the apices.
- V/Q ratio is generally higher at the apex than at the base.
- Oxygen is transported primarily bound to hemoglobin.
- Carbon dioxide is transported primarily as bicarbonate.
- Increased temperature, increased CO₂, decreased pH, and increased 2,3-BPG shift the oxygen-hemoglobin curve to the right.
- Central chemoreceptors are strongly influenced by CO₂-related changes in CNS pH.
- Peripheral chemoreceptors are particularly important in detecting significant arterial hypoxemia.
How to Approach Respiratory Physiology Questions
When solving a respiratory physiology question, identify the underlying relationship before calculating or selecting an answer.
For example, if a question changes tidal volume and respiratory rate, determine whether it is asking about minute ventilation or alveolar ventilation. If the question involves an oxygen-hemoglobin curve, identify whether hemoglobin affinity has increased or decreased. If it involves different regions of the lung, consider how gravity affects ventilation and perfusion.
Understanding these physiological relationships makes it easier to solve unfamiliar clinical scenarios rather than relying only on memorized questions.
Continue the Discussion
Looking for more practice with respiratory physiology, ventilation-perfusion relationships, pulmonary mechanics, or USMLE-style questions?
Join the ValueMD forum discussion to compare study approaches and discuss respiratory physiology with other medical students.
Disclaimer: This resource is intended for medical education and general informational purposes. It does not replace current USMLE preparation materials, medical textbooks, clinical guidelines, or professional medical advice.