There's the peritoneum which lines the abdominal cavity.

It's got a bunch of blood vessels to exchange chemicals so it's common to do kidney dialysis right into the abdominal cavity instead of the blood directly. Unlike hemodialysis, the machines can be taken home and operated by the patient.

And in vaginoplasties when they don't have enough tissue to do just a penile inversion, they can now pull through the peritoneum to help construct the vaginal canal instead of doing a colon graft.