The correct answer is "release of carbon dioxide by tissue cells".
Respiration is a medical term referring to the diffusion of oxygen from the external environment to the cells inside the tissues and the movement of carbon dioxide in the opposite direction. The process can be divided into external and internal respiration. The external respiration describes the gas exchange between the lungs and the outside environment, therefore it includes the processes of breathing, gas transport and gas diffusion across the respiratory membrane. The release of carbon dioxide by tissue cells is part of the internal respiration.
