Corrected sentences:
Our heart is a strong muscle about the size of our fist. The heart pumps blood through tubes called blood vessels. We live as long as our hearts keep beating.
Why I changed them (easy to understand):
- I added the word "about" to show the size is not exact.
- I changed "in" to "through" because blood moves through blood vessels.
- I used "our hearts" in the last sentence so it matches "we" (every person has a heart).
Another simple way to say it:
Your heart is a strong muscle about the size of your fist. It pumps blood through tubes called blood vessels. You live as long as your heart keeps beating.