ACROSS
3. airway connecting the trachea to a lung.
5. in a mammalian lung, one of many tiny air sacs where gas exchange with the blood occurs.
7. maximum amount of air moved in and out of lungs with forced inhalation and exhalation.
9. thin, moist layer of cells across which gases are exchanged between animal cells and the external environment.
10. of an aquatic animal, a folded or filamentous respiratory organ in which blood or hemolymph exchanges gases with water.
11. a protein that reversibly binds oxygen when the oxygen concentration is high and releases it when oxygen concentration is low. Hemoglobin is an example.
13. one inhalation and one exhalation.
15. membrane consisting of alveolar epithelium, capillary endothelium, and their fused basement membranes.
16. short airway that contains the vocal cords; the “voice box.”
18. a small airway that leads from a bronchus to alveoli.
19. airway to the lungs; windpipe.

DOWN
1. internal respiratory organ in which blood exchanges gases with the air.
2. of insects, a system of air-filled tubes that convey gases between openings at the body surface and internal tissues.
4. physiological processes that collectively supply animal cells with oxygen and disposes of their waste carbon dioxide.
6. tissue flap that folds down to prevent food from entering the airways when during swallowing.
8. exchange of substances between two fluids moving in opposite directions.
12. opening formed when the vocal cords relax.
14. throat; opens to airways and digestive tract.
17. sheet of smooth muscle between the thoracic and abdominal cavities; contracts during inhalation.