Gas Exchange and Respiratory Physiology Study Pack

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Last updated May 21, 2026

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Gas Exchange and Respiratory Physiology Study Guide

Trace the full journey of oxygen and carbon dioxide across the respiratory membrane, from partial pressure gradients in the alveoli to hemoglobin binding, the oxyhemoglobin dissociation curve, and the Bohr effect. This pack covers CO₂ transport as bicarbonate and carbaminohemoglobin, V/Q ratios, and the structural factors — membrane thickness, surface area — that maximize diffusion efficiency.

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 thin, moist membranes.
  • •In the alveoli, oxygen diffuses from inhaled air (PO₂ ≈ 104 mmHg) into pulmonary capillary blood (PO₂ ≈ 40 mmHg), while carbon dioxide moves in the opposite direction from blood (PCO₂ ≈ 45 mmHg) into alveolar air (PCO₂ ≈ 40 mmHg).
  • •Approximately 98.5% of oxygen in the blood is carried bound to hemoglobin as oxyhemoglobin; the remaining 1.5% is dissolved directly in plasma.
  • •Carbon dioxide is transported in three forms: about 70% as bicarbonate ions (HCO₃⁻) in plasma, roughly 23% bound to hemoglobin as carbaminohemoglobin, and about 7% dissolved in plasma.
  • •The oxygen-hemoglobin dissociation curve is sigmoidal and shifts right (releasing more O₂) in response to increased temperature, elevated PCO₂, decreased pH, and increased 2,3-bisphosphoglycerate (2,3-BPG) — collectively described by the Bohr effect.
  • •Effective gas exchange depends on matched ventilation and perfusion (V/Q ratio); significant mismatches reduce blood oxygenation and CO₂ clearance.
  • •Alveolar surface area (~70 m² in healthy adults), a thin respiratory membrane (~0.5 µm), and continuous pulmonary blood flow collectively maximize diffusion rates.

Physical Principles Governing Gas Movement

Gas exchange is fundamentally a physical process governed by the behavior of gases in mixtures and across membranes, so understanding the underlying principles is essential before examining where and how exchange occurs in the body.

Dalton's Law and Partial Pressure

  • •Each gas in a mixture exerts pressure independently of other gases present; this individual contribution is called its partial pressure.
  • •In dry atmospheric air at sea level (760 mmHg total), oxygen (21% of air) has a partial pressure (PO₂) of about 159 mmHg, while nitrogen contributes most of the remainder.
  • •Water vapor in the airways reduces the effective partial pressure of incoming gases; after humidification in the nasal passages and trachea, alveolar PO₂ is approximately 104 mmHg rather than 159 mmHg.

Fick's Law of Diffusion

  • •The rate of gas diffusion across a membrane is directly proportional to the surface area of the membrane, the partial pressure gradient across it, and the solubility of the gas, and inversely proportional to membrane thickness.
  • •Carbon dioxide diffuses about 20 times more readily than oxygen through biological membranes because of its much higher solubility in aqueous environments, which compensates for CO₂'s smaller partial pressure gradient.
  • •Any condition that thickens the respiratory membrane (e.g., pulmonary edema, fibrosis) or reduces alveolar surface area (e.g., emphysema) directly impairs diffusion rates according to these relationships.

Structure of the Respiratory Membrane and Alveolar Architecture

The efficiency of gas exchange in the lungs depends on specialized anatomical features of the alveoli and their surrounding capillaries that collectively minimize diffusion distance and maximize available surface area.

Layers of the Respiratory Membrane

  • •The respiratory membrane is the collective barrier gases must cross between alveolar air and pulmonary capillary blood; it consists of the alveolar epithelial cell (primarily type I pneumocytes), the shared basement membrane, and the pulmonary capillary endothelial cell.
  • •Total thickness of the respiratory membrane is approximately 0.5 µm, which is thin enough to allow rapid passive diffusion of both O₂ and CO₂.
  • •Type II pneumocytes embedded in the alveolar wall secrete surfactant, a phospholipid-rich fluid that lowers surface tension and prevents alveolar collapse between breaths.

Alveolar and Capillary Geometry

  • •The adult human lungs contain roughly 300–500 million alveoli, creating a total respiratory surface area of approximately 70 m² — comparable to the floor area of a studio apartment.
  • •Pulmonary capillaries are so narrow (about 5–8 µm in diameter) that red blood cells must deform slightly as they pass through, maximizing the contact time each cell has with alveolar air.
  • •The dense capillary meshwork surrounding each alveolus ensures that virtually the entire alveolar surface is in close proximity to blood flow at any given moment.

About this Study Pack

Created by Kibin to help students review key concepts, prepare for exams, and study more effectively. This Study Pack was checked for accuracy and curriculum alignment using authoritative educational sources. See sources below.

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