What is the primary metabolic waste gas removed from the human bloodstream and exhaled by the lungs during the respiratory cycle?
Answer and explanation
Correct answer: A. Carbon dioxide (CO2)
- Step 1: Mitochondrial cellular respiration consumes oxygen and glucose to produce ATP, generating carbon dioxide as an end-product of the Krebs cycle.
- Step 2: CO2 is carried by venous blood as bicarbonate ions (HCO3-), carbaminohemoglobin, and dissolved gas to the pulmonary alveoli, where it diffuses across the respiratory membrane down its partial pressure gradient to be exhaled.
Answer
Carbon dioxide (CO2)
Why not the others: 'Oxygen (O2)' Oxygen is an essential metabolic reactant absorbed by the lungs and delivered to tissues; 'Methane (CH4)' Methane is produced by colonic gut flora and expelled in flatus, not a primary pulmonary waste gas; 'Nitrogen (N2)' Nitrogen gas is inhaled and exhaled in equilibrium without net metabolic consumption or generation.
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