Answer: internal (tissue).
- A internal (tissue)
- B purely external
- C fully anaerobic
- D only cellular
Correct answer: A. internal (tissue)
Explanation: Internal respiration is gas exchange between blood and tissues; external respiration occurs in the lungs.
The S-shaped (sigmoid) curve reflects cooperative binding between Hb's four O₂ sites; the Bohr effect shifts the whole curve to the right under conditions found in actively respiring tissue (more CO₂, more acid, more heat), making haemoglobin release oxygen more readily exactly where it's needed.
Concept context
The mechanics of breathing, lung volumes, gas transport in blood, the Bohr effect, and respiratory regulation.