22. Describe the four main characteristics of an ani- mal's respiratory surface.

23. Trace the pathway of air through the human res-
piratory system, from the nostrils to the lungs. At
what point within the lungs do the oxygen mol-
ecules really enter the body?

Ascientist is designing an investigation to study the impact of invasive bird populations on local, native bird populations at backyard bird feeders. what population is most likely to be a control group for the investigation? amount of food number of bird feeders invasive bird populations local backyard birdfeeders without food