When I consider a living human, I would say that the skeleton/bones are most similar to the structure of a building, providing the base for everything else to exist within.
The finishes of a building are most akin to protein-based coverings such as hair, fingernails, or skin. These are easily replaceable, which is important as they grow older/weaker (on a short scale). Eventually the body runs out of budget and the replacements are phased out, and the building starts to fall apart.
The circulatory and breathing systems are like the electrical, mechanical, and plumbing/water systems in a building.