The document discusses the function of the heart, providing facts about how many times it beats in a day and gallons of blood it pumps in a lifetime. It explains that the heart functions like a pump to send oxygen-rich blood through the entire body, carrying oxygen and nutrients needed for all cells to survive. It also gives an overview of circulatory systems in fish, amphibians, and mammals, noting differences like amphibians having a double circulatory system but not always a completely separated two-pump heart, while fish have a two-chambered heart that pumps blood to the gills first before the rest of the body.