Which of the following causes important changes to begin to occur in the circulatory system? A. Introduction of iron into the blood B. Introduction of air into the lungs C. Introduction of carbon dioxide into the lungs D. Introduction of calcium into the blood

It depends,on your dog and the any trap.but i pray that he is fine.

Of that are from eating black  foods or iron supplements. the most common condition causing  it is a bleeding ulcer.  black poop caused by blood indicate a problem in the upper digestive tract.     along with pain, vomiting, or diarrhea is cause to see a doctor right go see a doctor