22.4 Gas Exchange

Breathing is more than just inhaling and exhaling. It's a complex process involving gas exchange between our lungs and blood. The air we breathe in differs from what's in our alveoli, the tiny air sacs where oxygen enters our bloodstream.

Our bodies adapt to ensure efficient gas exchange. Ventilation (airflow) and perfusion (blood flow) work together to maximize oxygen uptake and carbon dioxide removal. Understanding these processes helps us grasp how our respiratory system keeps us alive and functioning.

Composition and Mechanisms of Gas Exchange

Atmospheric vs alveolar air composition

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• Atmospheric air composition consists of ~78% nitrogen, ~21% oxygen, ~0.04% carbon dioxide, and variable amounts of water vapor (humidity)
• Alveolar air composition differs with ~74% nitrogen, ~14% oxygen, ~5% carbon dioxide, and saturated water vapor due to gas exchange occurring within the lungs (alveoli)

Mechanisms of lung gas exchange

• Diffusion involves the passive movement of gases from regions of high concentration to low concentration across the thin, moist alveolar-capillary membrane
• Partial pressure gradients drive gas exchange with oxygen moving from high $P_{O2}$ in alveoli to low $P_{O2}$ in blood and carbon dioxide moving from high $P_{CO2}$ in blood to low $P_{CO2}$ in alveoli
• Solubility of oxygen and carbon dioxide in water allows for efficient diffusion across the moist alveolar-capillary membrane facilitating rapid gas exchange
• Surfactant reduces surface tension in alveoli, preventing collapse and enhancing gas exchange efficiency

Ventilation, Perfusion, and Respiration

Ventilation and perfusion adaptation

• Ventilation-perfusion ratio (V/Q) represents the matching of ventilation (airflow) and perfusion (blood flow) in alveoli to ensure optimal gas exchange
• Hypoxic vasoconstriction occurs when pulmonary arterioles constrict in response to low alveolar oxygen levels diverting blood flow from poorly ventilated to well-ventilated alveoli to maintain an optimal V/Q ratio
• Alveolar recruitment and distension involve the opening of collapsed alveoli and stretching of open alveoli to increase the surface area available for gas exchange during increased ventilation (exercise)
• Pulmonary compliance refers to the lungs' ability to expand and contract, affecting the ease of breathing and gas exchange

External respiration process

1. Oxygen diffuses from alveoli (high $P_{O2}$) to blood (low $P_{O2}$) across the alveolar-capillary membrane
2. Oxygen binds to hemoglobin in red blood cells for transport throughout the body
3. Carbon dioxide diffuses from blood (high $P_{CO2}$) to alveoli (low $P_{CO2}$) across the alveolar-capillary membrane
4. Carbon dioxide is exhaled during ventilation removing it from the body

Internal respiration and tissue exchange

• Oxygen unloading occurs as oxygen dissociates from hemoglobin in capillaries and diffuses from blood (high $P_{O2}$) to tissues (low $P_{O2}$) where it is used in cellular respiration (ATP production)
• Carbon dioxide loading involves carbon dioxide diffusing from tissues (high $P_{CO2}$) to blood (low $P_{CO2}$) where it enters red blood cells and reacts with water to form carbonic acid
  • Carbonic acid dissociates into bicarbonate ($HCO_3^-$) and hydrogen ions ($H^+$)
  • Carbon dioxide is transported in blood as dissolved gas, bicarbonate, and carbamino compounds (bound to proteins) back to the lungs for exhalation
• The Bohr effect enhances oxygen unloading in tissues with high CO2 levels, improving oxygen delivery to active tissues

Ventilation Dynamics

Ventilation measurements

• Tidal volume is the amount of air inhaled or exhaled during normal breathing
• Minute ventilation represents the total volume of air moved in and out of the lungs per minute
• Dead space refers to the volume of air in the respiratory system that does not participate in gas exchange