Question: which body structure provides actual diffusion of respiratory gases in lungs
[{'answer': 'circulatory systems', 'provenance': [{'wikipedia_id': '379303', 'title': 'Gas exchange', 'start_paragraph_id': 18, 'start_character': 28, 'end_paragraph_id': 18, 'end_character': 47, 'bleu_score': 1.0, 'section': 'Section::::Physical principles of gas-exchange.:Diffusion and surface area.\n'}]}, {'provenance': [{'wikipedia_id': '379303', 'title': 'Gas exchange', 'start_paragraph_id': 19, 'start_character': 0, 'end_paragraph_id': 19, 'end_character': 568, 'bleu_score': 0.9768747032089851, 'meta': {'yes_no_answer': 'NONE', 'annotation_id': 2898426849778891591}, 'section': 'Section::::Physical principles of gas-exchange.:Interaction with circulatory systems.\n'}]}]