What the Lungs Exhale: Carbon Dioxide as Metabolic Waste

The lungs' primary excretory function is eliminating carbon dioxide (CO₂), the waste product of cellular respiration.

Process

1. Cells: Glucose + O₂ → ATP + CO₂ + H₂O (mitochondria).
2. CO₂ diffuses into capillaries → transported as bicarbonate (HCO₃⁻), dissolved, carbaminohemoglobin.
3. Lungs: CO₂ diffuses from pulmonary capillaries → alveoli → exhaled air.
4. Simultaneously: O₂ inhaled → diffuses into blood.

Gas Exchange Efficiency

• Alveolar surface area: ~70 m².
• Diffusion distance: <1 μm.
• Ventilation/perfusion matching optimizes exchange.

Other Exhaled Substances

• Water vapor (humidified air)
• Volatile organic compounds (acetone in ketosis, ethanol, disease biomarkers)
• Trace gases (nitric oxide, used in asthma monitoring)

CO₂ is the major metabolic waste gas; its removal is essential for pH homeostasis.