A wildlife biologist is investigating a sudden increase in the death rate among several bird species. She has not been able to determine what disease or toxin is causing the deaths, but she has found that many of the sick birds are exhibiting a shortage of dissolved oxygen and a buildup of dissolved carbon dioxide in their cells. Which of these body systems in the sick birds is most likely functioning improperly?
Select one:

circulatory and respiratory

reproductive and nervous

muscular and circulatory

excretory and circulatory