Most blessings are said over food, over a mitzvah, or over something a person sees. Asher Yatzar is said over the fact that the body works. The Talmud gives its text (Berachot 60b), and it is said every morning and after using the restroom (Shulchan Aruch, Orach Chayim 7:1).
The text
“Blessed are You, Hashem, our God, King of the universe, who formed man with wisdom and created within him many openings and many hollow spaces. It is revealed and known before Your throne of glory that if one of them were to be opened, or one of them were sealed, it would be impossible to exist and stand before You. Blessed are You, Hashem, who heals all flesh and acts wondrously.”
The phrase “acts wondrously” is the Hebrew umafli la’asot. The wonder named is that the body stays in working order at all.

Where it sits in the morning
The order is Modeh Ani, then the washing of the hands, then Asher Yatzar, and then Elohai Neshamah (“My God, the soul You placed within me is pure”), which the Talmud places directly after it (Berachot 60b). The body is thanked first and the soul second, and the sequence mirrors the order of the levels in the five levels of the soul, from the Nefesh upward.
The blessing treats health as something given, and the Kabbalists regard the body as the vessel that carries the soul in this world. Our articles on Kabbalah and holistic healing and on the formula of a blessing go further into both ideas.