In this task, you are given a paragraph, and your job is to generate comma-separated entities present in the given paragraph. Generate entities from a given passage in such a way that (i) they are present in the paragraph, (ii) they are non-duplicate, (iii) they underwent a state change during the process. Avoid creating entities that (i) aren't present in the process paragraph and (ii) aren't significant. 

Let me give you an example:  Magma rises from deep in the earth. The magma goes into volcanos. The volcanos pressure the magma upwards. The pressure causes the magma to push through the surface of the volcano. The lava cools. The lava forms new rock. New magma is pressured to the surface of the volcano. The volcano bursts through the rock the formed after the last eruption. 
The answer to this example can be:  magma ,lava ,new rock 
Here is why: The entities are present in the process paragraph and they undergo state change. So, this is a positive example.

OK. solve this:
Air enters the nose or mouth and travels through the windpipe into the lungs. The lungs have a series of smaller and finer tubes and sacs. Air reaches the smallest and finest air sacs called alveoli. Oxygen travels from the alveoli into the blood. Oxygen-rich blood is carried to the heart. Oxygen-rich blood is pumped throughout the body by the heart. Cells take in the oxygen rich blood and release carbon dioxide rich blood. The carbon dioxide rich blood travels to the lungs. Carbon dioxide travels through the throat and is exhaled. 
Answer:
air ,oxygen ,oxygen-rich blood, oxygen rich blood ,carbon dioxide rich blood ,carbon dioxide 