2.2.3 Respiratory

Overview of Respiratory Physiology 🌬️

Respiratory physiology for USMLE Step 1 focuses on how the lungs, chest wall, airways, and blood work together to move gases and maintain acid base balance. You do not need engineering level detail, but you must understand the core concepts well enough to apply them to clinical scenarios like asthma, COPD, pulmonary embolism, and respiratory failure.

At its core, the respiratory system has three main jobs. First, it brings oxygen from the atmosphere into the blood. Second, it removes carbon dioxide produced by metabolism. Third, it helps maintain normal pH by adjusting CO₂ excretion. On exams, you will be tested on how changes in breathing pattern, disease, altitude, or drugs disturb these processes and how the body attempts to compensate.

In this chapter, the focus is on the unique physiology of the respiratory system. Related topics such as detailed cardiovascular transport or renal acid base compensation are handled in their own chapters. Here you will learn how air moves, how gases cross membranes, how ventilation is controlled, and how these processes fail in disease.

Mechanics of Breathing 🫁

Breathing mechanics describe how the lungs and chest wall move to allow air to enter and leave the lungs. Inspiration is an active process at rest, driven mainly by contraction of the diaphragm. When the diaphragm contracts, it moves downward, increasing thoracic volume and lowering intrathoracic pressure, so air flows into the lungs. In exercise or respiratory distress, accessory muscles of inspiration in the neck and chest also contract to expand the rib cage further.

Expiration at rest is usually passive. The diaphragm relaxes, the lungs recoil because of their elastic properties, and air flows out as thoracic volume decreases. During forced expiration, such as during exercise or asthma attack, abdominal muscles and internal intercostals actively compress the thoracic cavity and increase intrapulmonary pressure to push air out.

The respiratory system behaves like a springy structure. The lungs naturally recoil inward, while the chest wall tends to spring outward. At functional residual capacity, or FRC, these two opposing forces are in balance. FRC is the lung volume at the end of a normal, quiet expiration. It is very important clinically because gas exchange continues from this resting volume between breaths.

Compliance is a key quantitative property of the lungs and chest wall. Compliance describes how easily a structure expands when a pressure is applied.

High compliance means the lung inflates easily with little pressure change, as in emphysema. Low compliance means the lung is stiff and hard to inflate, as in pulmonary fibrosis or acute respiratory distress syndrome. On exams you must recognize how diseases shift the pressure volume curve of the lung and how that affects FRC.

Surface tension in the alveoli also affects mechanics. Wet alveolar surfaces tend to collapse because of surface tension forces. Pulmonary surfactant, produced by type II pneumocytes, reduces surface tension, increases compliance, and helps prevent alveolar collapse, especially at low lung volumes. Deficiency of surfactant, as in neonatal respiratory distress syndrome, leads to stiff lungs, low compliance, and respiratory failure. The effect of surfactant is often tested together with Laplace’s law for spheres, which relates surface tension, pressure, and radius, but the detailed equation is handled in biophysics contexts.

Airway resistance is another mechanical factor. Resistance is highest in medium sized bronchi and is influenced by airway radius. Bronchoconstriction increases resistance and makes breathing more difficult, as seen in asthma. Bronchodilation decreases resistance and is the goal of many respiratory drugs. Resistance also depends on lung volume because higher lung volumes pull airways open, reducing resistance.

Lung Volumes and Capacities 📏

Lung volumes and capacities are core topics that appear frequently in questions and spirometry interpretation. Lung volumes are basic measurable components of lung size. Capacities are combinations of volumes.

Key static lung volumes include tidal volume, inspiratory reserve volume, expiratory reserve volume, and residual volume. Tidal volume is the volume of air inhaled or exhaled in a normal quiet breath. Inspiratory reserve volume is the additional volume that can be inhaled with maximal effort after a normal inspiration. Expiratory reserve volume is the extra volume that can be exhaled with maximal effort after a normal expiration. Residual volume is the air that remains in the lungs after a maximal forced expiration and cannot be exhaled. Because of residual volume, there is always air in the lungs, which helps prevent collapse and allows continuous gas exchange.

Capacities combine these volumes. Vital capacity is the maximum volume of air that can be exhaled after a maximal inspiration. Total lung capacity is the volume of air in the lungs after a maximal inspiration and is the sum of all lung volumes. Functional residual capacity, mentioned earlier, is the volume remaining in the lungs after a normal expiration. FRC cannot be measured directly with simple spirometry because it includes residual volume.

It is very helpful to track which measurements include residual volume. The table below summarizes the main static measures and whether they can be obtained by simple spirometry.

Spirometry is used to diagnose and classify lung disease. In obstructive diseases such as asthma and COPD, there is difficulty getting air out. Forced expiratory volume in 1 second, or FEV₁, is reduced, and the FEV₁/FVC ratio falls below normal. Total lung capacity may be normal or increased due to air trapping, and residual volume often increases. In restrictive diseases, such as pulmonary fibrosis, total lung capacity and vital capacity are reduced, but FEV₁ and FVC fall proportionally, so the FEV₁/FVC ratio is normal or increased. Recognizing these patterns is a common exam task.

Ventilation and Perfusion 💨💉

Ventilation refers to the movement of air in and out of the alveoli, while perfusion refers to the blood flow through pulmonary capillaries. Effective gas exchange requires both adequate ventilation and adequate perfusion. Their relationship is expressed as the ventilation perfusion ratio, written as $V/Q$.

Alveolar ventilation, $\dot V_A$, is the portion of total ventilation that actually reaches the gas exchanging alveoli. Total, or minute, ventilation is the total volume of air entering or leaving the lungs per minute. Dead space is the volume of air that does not participate in gas exchange. There are two broad categories of dead space. Anatomic dead space is air in the conducting airways that never reaches alveoli. Alveolar dead space is air that reaches alveoli that are ventilated but not perfused. The sum of these is called physiologic dead space.

If dead space increases, as in pulmonary embolism, alveolar ventilation falls unless total ventilation increases. This leads to impaired CO₂ removal and hypoxemia. Many questions test your ability to reason through how changing respiratory rate and tidal volume alter alveolar ventilation and blood gases.

Ventilation perfusion matching varies within the lung due to gravity. In an upright person, both ventilation and perfusion are higher at the base than at the apex, but perfusion increases more than ventilation, so $V/Q$ is lower at the base and higher at the apex. Typical exam statements say that the apex of the lung has high $V/Q$ with higher alveolar PO₂ and lower alveolar PCO₂, while the base has low $V/Q$ with lower PO₂ and higher PCO₂. Extreme cases occur in disease. In pure shunt, $V/Q$ equals zero because alveoli are perfused but not ventilated, as in airway obstruction or fluid filled alveoli. In pure dead space, $V/Q$ is infinite because alveoli are ventilated but not perfused, as in large pulmonary embolism.

The lung has local mechanisms to improve $V/Q$ matching. Hypoxic pulmonary vasoconstriction is the most important. When alveolar PO₂ falls, the small pulmonary arteries constrict, diverting blood away from poorly ventilated regions to better ventilated ones. This is opposite to systemic circulation. This mechanism is adaptive locally, but if the entire lung is hypoxic, as at high altitude or severe lung disease, global vasoconstriction increases pulmonary arterial pressure and can lead to right heart strain.

Gas Exchange and Transport 🧪

Gas exchange occurs across the very thin blood gas barrier in the alveoli. Oxygen moves from alveolar air into blood, and carbon dioxide moves from blood into alveolar air. This movement follows partial pressure gradients and is described by diffusion principles. The rate of diffusion depends on the surface area available, the thickness of the barrier, and the difference in partial pressures.

Oxygen transfer from alveoli to blood approaches equilibrium under normal conditions, so arterial PO₂ is close to alveolar PO₂. If the barrier is thickened, as in pulmonary fibrosis, or if blood flows too quickly, as in severe exercise in disease, oxygen may not fully equilibrate and diffusion limitation occurs. Carbon dioxide usually equilibrates more easily, so isolated diffusion limitation mainly affects oxygen and causes hypoxemia without major hypercapnia at first.

In blood, oxygen is transported mostly bound to hemoglobin. Only a small fraction is dissolved in plasma, but dissolved oxygen determines the partial pressure of oxygen. The hemoglobin oxygen dissociation curve describes the relationship between PO₂ and hemoglobin saturation. It has a sigmoidal shape due to cooperative binding. The key concept for USMLE is that various factors shift this curve, which changes how readily hemoglobin releases oxygen to tissues.

Conditions that increase metabolic activity in tissues, such as higher temperature, increased PCO₂, and lower pH, shift the curve to the right. A right shift means at a given PO₂, hemoglobin is less saturated and more oxygen is released. 2,3 bisphosphoglycerate, or 2,3 BPG, also shifts the curve to the right. These changes help active tissues receive more oxygen. A left shift occurs with lower temperature, decreased PCO₂, increased pH, and fetal hemoglobin. In this case, hemoglobin holds onto oxygen more tightly.

Carbon dioxide transport has three main forms. A small amount is dissolved in plasma. Some CO₂ binds to hemoglobin as carbaminohemoglobin. Most CO₂ is transported as bicarbonate ion in plasma. The conversion of CO₂ and water to carbonic acid and then to bicarbonate and hydrogen ions is catalyzed by carbonic anhydrase, particularly in red blood cells. This reaction is central to acid base balance. In tissues, CO₂ produced by metabolism diffuses into blood and is converted to bicarbonate, which is transported to the lungs. In the lungs, the reaction reverses so that CO₂ can be exhaled.

The relationship between CO₂ transport and oxygen binding is described by the Haldane effect, while the effect of CO₂ and pH on oxygen binding is called the Bohr effect. These named effects often appear in conceptual questions about gas exchange and blood transport.

Regulation of Breathing 🧠

Breathing is regulated automatically by the nervous system to maintain appropriate blood levels of O₂, CO₂, and H⁺. Voluntary control from the cerebral cortex can temporarily modify breathing, for example to hold your breath, but automatic control from brainstem centers is dominant in normal life.

The medulla contains central pattern generators that produce the basic rhythm of breathing. These neurons integrate sensory information from chemoreceptors and mechanoreceptors and send signals via spinal motor neurons to the diaphragm and intercostal muscles. The pons has additional centers that fine tune the pattern of inspiration and expiration.

Chemoreceptors are the primary sensors for chemical control. Central chemoreceptors, located near the ventral surface of the medulla, respond mainly to changes in pH of the cerebrospinal fluid. They are most sensitive to changes in arterial PCO₂, because CO₂ crosses the blood brain barrier and is converted to H⁺ in the CSF. When arterial PCO₂ rises, central chemoreceptors increase ventilation to blow off CO₂ and restore pH. Peripheral chemoreceptors are located in the carotid bodies at the bifurcation of the common carotid arteries and in the aortic bodies. They respond to low arterial PO₂, high PCO₂, and low pH. Peripheral chemoreceptors are the main sensors for hypoxemia. When arterial PO₂ falls below a critical level, they strongly stimulate ventilation.

Mechanoreceptors in the lungs and chest wall also modulate breathing. Stretch receptors in airway smooth muscle respond to lung inflation and help terminate inspiration to prevent overinflation. Irritant receptors respond to smoke, dust, and chemicals and can trigger reflex bronchoconstriction and coughing.

Under certain pathological conditions, the control of breathing shifts. In chronic hypercapnia, such as long standing COPD, central chemoreceptors become less sensitive to CO₂, and hypoxic drive from peripheral chemoreceptors becomes more important. This has clinical implications when giving supplemental oxygen to such patients, which is discussed elsewhere in clinical chapters.

Breathing patterns can vary significantly. Hyperventilation refers to ventilation that is too high for metabolic needs and leads to reduced PCO₂, or hypocapnia. Hypoventilation is ventilation that is too low and leads to elevated PCO₂, or hypercapnia. Cheyne Stokes respiration is a periodic pattern of gradual increase and decrease in tidal volume with intermittent apnea. It occurs in certain forms of heart failure and neurological disease and is a favorite pattern for test questions.

Acid–Base Balance and the Lungs ⚖️

The respiratory system has a central role in acid base balance because CO₂ behaves as an acid through its conversion to carbonic acid and hydrogen ions. Arterial PCO₂ is directly related to the acid load from respiratory sources. The kidneys then handle the metabolic component, but that is covered in renal physiology.

The link between alveolar ventilation and arterial PCO₂ is very strong. As a first approximation, if CO₂ production is constant, arterial PCO₂ is inversely related to alveolar ventilation.

Respiratory acidosis occurs when alveolar ventilation is inadequate and PCO₂ rises. Common causes include central nervous system depression, severe obstructive lung disease, or neuromuscular weakness. The immediate effect is increased hydrogen ion concentration and decreased pH. Over time, the kidneys retain bicarbonate to partially compensate. Respiratory alkalosis occurs when alveolar ventilation is excessive and PCO₂ falls. It can be seen in anxiety induced hyperventilation, high altitude adaptation, or salicylate toxicity in its early phase. Here, the kidneys gradually excrete more bicarbonate to compensate.

On the USMLE, you are often given arterial blood gas values and asked to determine if there is respiratory acidosis or alkalosis and whether it is acute or chronic. Recognizing that the primary respiratory disorder is defined by the direction of PCO₂ change is crucial. The magnitude of pH change and accompanying bicarbonate shift help determine acuteness and whether the kidneys have compensated.

Respiratory Responses to Exercise and Altitude 🏃‍♂️⛰️

Exercise and high altitude provide physiologic stress tests of the respiratory system. They are favorite exam settings that integrate mechanics, gas exchange, and control of breathing.

During moderate exercise, ventilation and cardiac output increase to match higher metabolic demand. Oxygen consumption and CO₂ production increase, but arterial PO₂ and PCO₂ usually remain close to normal because increased ventilation keeps up with demand. Ventilation initially rises in proportion to CO₂ production. At high exercise intensities, anaerobic metabolism produces lactic acid, which lowers pH and further stimulates ventilation. Venous blood shows marked changes with lower PO₂ and higher PCO₂, but arterial blood gas values remain near normal in healthy individuals.

At high altitude, barometric pressure is lower, so inspired PO₂ and consequently alveolar and arterial PO₂ fall. Low arterial PO₂ powerfully stimulates peripheral chemoreceptors, which increase ventilation. Increased ventilation lowers PCO₂ and causes respiratory alkalosis. Initially, this alkalosis blunts further hyperventilation, but over several days the kidneys excrete more bicarbonate, partially correcting the alkalosis. This renal compensation allows ventilation to rise further, improving arterial PO₂. Over time, other adaptations include increased 2,3 BPG, which shifts the hemoglobin oxygen dissociation curve to the right, enhanced erythropoietin production and increased red blood cell mass, and pulmonary vascular changes that can increase pulmonary arterial pressure.

High altitude questions often ask about specific directions of change in ventilation, PCO₂, pH, 2,3 BPG, and hematocrit over different time scales. It is helpful to separate immediate respiratory responses from slower renal or hematologic adaptations.

Common Pathophysiologic Patterns 🧩

Many clinical respiratory problems on USMLE can be understood through a few key physiologic patterns. Although disease details are discussed in other chapters, you should recognize the physiologic signatures.

Hypoxemia, meaning low arterial PO₂, can arise from several mechanisms. Hypoventilation reduces alveolar ventilation and raises PCO₂, which can lower alveolar and arterial PO₂. Diffusion impairment thickens the alveolar barrier and limits oxygen transfer, especially during exercise. Shunt allows venous blood to bypass ventilated alveoli, so its low PO₂ dilutes arterial blood. Ventilation perfusion mismatch is the most common cause in lung disease and reflects regions with low or high $V/Q$. Importantly, some causes of hypoxemia improve with supplemental oxygen while others improve poorly.

Hypercapnia, or elevated arterial PCO₂, usually results from hypoventilation or severe $V/Q$ mismatch. Pure diffusion impairment rarely causes significant hypercapnia. When both hypoxemia and hypercapnia are present, a ventilatory problem is often involved.

Obstructive and restrictive lung patterns are recurrent themes. Obstructive physiology is characterized by increased airway resistance, difficulty exhaling, low FEV₁/FVC, increased residual volume, and often increased total lung capacity due to air trapping. Restrictive physiology shows reduced compliance, small lung volumes, reduced TLC and VC, and normal or high FEV₁/FVC ratio. These patterns can be recognized on flow volume loops and spirometry results.

Finally, the concept of work of breathing is tested indirectly. Diseases that decrease compliance, such as fibrosis, increase the elastic work needed to expand the lung, which encourages patients to take small, rapid breaths. Diseases that increase airflow resistance, such as COPD, increase the resistive work of breathing. These patients often adopt slow, deep breathing patterns to minimize resistance related work. Recognizing these patterns can help you reason through clinical vignettes.