How do astronauts walking on the moon keep their waste body fluids from freezing in the extremely cold environment? I understand that if they need to release waste body fluids, they do so into specifically prepared "diapers" or some other such system. But still, all body fluids, such as blood, cranial, urinary, etc., must be kept at warm enough temperature to flow. How is this done? Are radioisotope heat generators used in any way?
The challenge of keeping an astronaut in a suit on the moon comfortable turns out to be in keeping him cool, not keeping him warm.
Heat is transferred in three ways: (infrared) radiation, conduction (direct contact), and convection (transfer via a fluid medium like air). For a suited astronaut, conduction is minimal and convection is nonexistent; the only way the suit loses heat is by radiating it. The exterior of the suit is white to minimize heating in direct sunlight, but that also minimizes radiative ability. So all the heat produced by the astronaut's body in the suit stays in the suit, and that's a substantial amount.
So you actually have to circulate coolant through tubes in an undergarment of the space suit to draw heat out. Freezing urine is not at all a concern!