Complex sentence: The frog acts rather like the heart, blood flows down the horses leg, into the frog, and the horses weight compresses the frog on the ground, and essentially squeezes the blood out of the frog, pushing it back up the horse's legs.
Split into simpler sentences: The frog acts rather like the heart, in that blood flows down the horse's leg into the frog. The horse's weight then compresses the frog on the ground, essentially squeezing the blood out of the frog, pushing it back up the horse's legs.