Gas Exchange and Respiratory Physiology Study Pack
Kibin's free study pack on Gas Exchange and Respiratory Physiology includes a 7-section study guide, 25 quiz questions, 30 flashcards, and 5 open-ended Explain review questions. Sign up free to track your progress toward mastery, plus upload your own notes and recordings to create personalized study packs organized by course.
Last updated May 27, 2026
Gas Exchange and Respiratory Physiology Study Guide
Trace the full path of gas exchange from alveolar diffusion to blood transport, covering partial pressure gradients, Dalton's Law, the six-layer respiratory membrane, hemoglobin binding, and the Bohr effect — everything you need to master respiratory physiology.
Key Takeaways
- •Gas exchange in the lungs and tissues relies on passive diffusion driven by partial pressure gradients — gases move from regions of higher partial pressure to lower partial pressure across the respiratory membrane.
- •The respiratory membrane in the alveoli is approximately 0.5 micrometers thick and consists of six distinct layers, including the alveolar epithelium, basement membranes, and the pulmonary capillary endothelium, all of which minimize diffusion distance.
- •Dalton's Law of Partial Pressures states that each gas in a mixture exerts pressure independently; in dry air at sea level, the partial pressure of oxygen (PO₂) is approximately 159 mmHg, but alveolar PO₂ drops to about 104 mmHg due to humidification and gas mixing.
- •Oxygen is transported in the blood primarily bound to hemoglobin (about 98.5%), while carbon dioxide travels in three forms: dissolved in plasma (~7%), bound to hemoglobin as carbaminohemoglobin (~23%), and as bicarbonate ions (~70%).
- •The oxygen-hemoglobin dissociation curve is sigmoidal and shifts right (reduced affinity) under conditions of increased temperature, decreased pH, and elevated PCO₂ — a relationship known as the Bohr effect.
- •Ventilation-perfusion (V/Q) matching determines how efficiently gas exchange occurs; mismatches between airflow and blood flow in the lungs reduce the overall efficiency of oxygen uptake and carbon dioxide removal.
Physical Laws Governing Gas Movement
Gas exchange is not driven by active transport or cellular machinery — it depends entirely on physical laws that describe how gases behave in mixtures and how they move across membranes.
Dalton's Law of Partial Pressures
- •Each gas in a mixture contributes independently to total pressure, proportional to its fractional concentration.
- •Dry atmospheric air at sea level has a total pressure of 760 mmHg; oxygen makes up 21% of that, yielding a PO₂ of approximately 159 mmHg.
- •Nitrogen (PN₂ ≈ 597 mmHg), carbon dioxide (PCO₂ ≈ 0.3 mmHg), and water vapor also contribute their own partial pressures.
Fick's Law of Diffusion
- •The rate of gas diffusion across a membrane is directly proportional to the surface area, the partial pressure gradient, and the diffusion coefficient of the gas, and inversely proportional to membrane thickness.
- •Carbon dioxide diffuses roughly 20 times faster than oxygen across biological membranes because of its greater solubility in aqueous solution, despite having a smaller partial pressure gradient.
- •Increasing surface area (as in healthy alveoli) or steepening the partial pressure gradient increases the diffusion rate; conditions like pulmonary fibrosis reduce efficiency by thickening the respiratory membrane.
Henry's Law and Gas Solubility
- •At the gas-liquid interface of the alveoli, the amount of a gas that dissolves into plasma is proportional to its partial pressure and its solubility coefficient.
- •Oxygen has a relatively low solubility coefficient, which is why hemoglobin — not dissolved plasma — serves as the primary oxygen carrier.
The Respiratory Membrane: Structure and Diffusion Pathway
Efficient gas exchange depends on a barrier thin enough for rapid diffusion yet durable enough to withstand constant mechanical stress — the respiratory membrane achieves both.
Six-Layer Architecture of the Respiratory Membrane
- •The respiratory membrane includes: (1) the alveolar fluid layer containing surfactant, (2) the type I alveolar epithelial cell, (3) the alveolar epithelial basement membrane, (4) a thin interstitial space, (5) the capillary basement membrane, and (6) the pulmonary capillary endothelial cell.
- •Total thickness averages 0.5 micrometers, which is far thinner than most cells and allows diffusion to occur in milliseconds.
Alveolar Cell Types and Their Roles
- •Type I pneumocytes are thin, squamous cells that form roughly 95% of the alveolar surface area and are the primary site of gas diffusion.
- •Type II pneumocytes secrete pulmonary surfactant, a phospholipid mixture that lowers alveolar surface tension, preventing alveolar collapse during exhalation.
- •Alveolar macrophages patrol the alveolar space and remove inhaled debris, protecting the integrity of the gas exchange surface.
Partial Pressures at the Alveolar Level
- •Inspired air is humidified as it travels through the airways; water vapor at body temperature exerts 47 mmHg, which dilutes the partial pressures of all other gases.
- •Alveolar PO₂ stabilizes around 104 mmHg and alveolar PCO₂ around 40 mmHg — values set by the balance between ventilation bringing in fresh air and perfusion removing gases via blood flow.
- •Pulmonary capillary blood arriving at the alveoli has a PO₂ of about 40 mmHg and a PCO₂ of about 45 mmHg, creating the gradients that drive oxygen into the blood and carbon dioxide into the alveoli.
Systemic Gas Exchange at the Tissues
Once oxygenated blood reaches the systemic capillaries, the partial pressure gradients reverse, driving oxygen into metabolically active cells and carbon dioxide back into the bloodstream.
Partial Pressure Gradients in Systemic Capillaries
- •Resting tissue cells maintain an intracellular PO₂ of roughly 40 mmHg because mitochondria continuously consume oxygen; arterial blood arrives at approximately 95–100 mmHg PO₂, creating a strong inward gradient.
- •Cellular respiration generates CO₂, raising intracellular PCO₂ to about 45–60 mmHg; venous blood leaving tissues has a PCO₂ near 45 mmHg, while resting arterial blood carries PCO₂ near 40 mmHg.
Effect of Metabolic Rate on Gas Exchange
- •During exercise, mitochondrial oxygen consumption increases sharply, dropping tissue PO₂ further and steepening the gradient — oxygen delivery automatically increases without any change in cardiac output alone.
- •Elevated CO₂ production during exercise lowers local pH (increases H⁺ concentration) and raises PCO₂ in working muscle, both of which trigger hemoglobin to release more oxygen via the Bohr effect.
Capillary Transit Time
- •Red blood cells spend approximately 0.75 seconds traversing a resting pulmonary capillary; equilibration of PO₂ between alveolar air and capillary blood is complete in about 0.25 seconds, leaving a large safety margin.
- •In high-demand states (e.g., intense aerobic exercise at altitude), transit time shortens and the safety margin narrows, which can limit oxygen uptake in extreme cases.
Oxygen Transport: Hemoglobin and the Dissociation Curve
Oxygen moves through the blood almost entirely bound to hemoglobin, and understanding how hemoglobin loads and unloads oxygen requires examining the oxygen-hemoglobin dissociation curve and its modulators.
Structure and Binding Capacity of Hemoglobin
- •Each hemoglobin tetramer contains four heme groups, each capable of binding one oxygen molecule, giving a maximum of four O₂ per hemoglobin molecule.
- •At full saturation, one gram of hemoglobin binds 1.34 mL of oxygen; with a normal hemoglobin concentration of about 15 g/dL, blood can carry roughly 20 mL O₂ per 100 mL blood.
- •The remaining ~1.5% of oxygen in blood travels dissolved in plasma, following Henry's Law, and contributes only minimally to total oxygen delivery.
Sigmoidal Shape of the Oxygen-Hemoglobin Dissociation Curve
- •The S-shaped (sigmoidal) curve results from cooperative binding: binding of the first O₂ molecule increases hemoglobin's affinity for subsequent molecules, accelerating saturation at higher PO₂.
- •The flat upper portion (PO₂ above ~60 mmHg) means that saturation remains high even with moderate drops in alveolar PO₂ — a built-in safety buffer at altitude or in mild respiratory disease.
- •The steep middle portion (PO₂ between 20 and 60 mmHg) corresponds to conditions in metabolically active tissues, where small drops in PO₂ cause large releases of oxygen.
Factors That Shift the Dissociation Curve
- •A rightward shift (decreased hemoglobin-O₂ affinity, increased O₂ unloading) is caused by increased temperature, decreased pH, elevated PCO₂, and increased 2,3-bisphosphoglycerate (2,3-BPG) — all conditions present in active tissues.
- •A leftward shift (increased affinity, decreased O₂ unloading) occurs in cooler, more alkaline, or lower-CO₂ environments — conditions in the pulmonary capillaries that favor oxygen loading.
- •2,3-BPG, produced by red blood cells during glycolysis, binds to deoxyhemoglobin and stabilizes it, shifting the curve right; elevated 2,3-BPG levels in chronic anemia or high-altitude acclimatization enhance oxygen delivery.
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What is the partial pressure of oxygen (PO₂) in dry atmospheric air at sea level?
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Partial Pressure and Fick's Law of Diffusion
Explain how partial pressure gradients drive gas exchange in the lungs and tissues. How do Dalton's Law and Fick's Law work together to determine how fast and in which direction oxygen and carbon dioxide move across the respiratory membrane?
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