When blood reaches the respiring tissues, haemoglobin

  • A. releases carbon dioxide and binds more oxygen
  • B. is destroyed by the cells
  • C. holds its oxygen more tightly
  • D. releases oxygen and picks up carbon dioxide

Explanation

Low oxygen pressure and high CO2 in the tissues make haemoglobin surrender oxygen and take on CO2 and H+, reversing what happened in the lungs. One molecule thus shuttles both gases on the same round trip.

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