Answer 2:
When we breathe air into our lungs, it reaches
small branched air sac structures within our lungs
called alveoli. They are very small and
have a lot of surface area. As blood is pumped
by the heart past the alveoli, oxygen from the air
diffuses into the blood. I hope this helps!
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Answer 3:
The heart just pumps blood through the body,
which is a huge job. Oxygen gets into the blood
through the lungs. Blood gets oxygen from
the lungs and moves it to the cells of the body,
and then the blood goes back to the lungs and gets
more oxygen. The heart and lungs are close
together, so the oxygen-poor blood from the
body goes into the heart on its way to the lungs,
and the oxygen-rich blood from the lungs goes into
the heart on its way to the lungs. You can read
more about this:
here
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