An office building is an analogy for cells where buildings protect occupants with entrances for people and materials to enter, just as the skin, lining of an organ or cell wall does. The individual cells resemble offices in a building, and they may be related or have specialized, unique functions.
A network of air and heating ducts, electrical wires and telephone lines connects the offices with each other and to the outside, similar to the way neurons and nervous systems carry messages and instructions in the form of electronic impulses to and from the brain, the master control mechanism. A thermostat, or brain, regulates temperature. Air circulates through ducts, just as the respiratory and circulatory systems of the body.
Deliverymen bring supplies and messages to individual offices, just as blood cells bring oxygen, nutrients and medicines to individual cells, and proteins carry various molecules to different parts of cells. Security guards protect against intruders, just as white blood cells and immune mechanisms protect bodies.
In offices, people retire and new people are hired. In a body, old cells die and new cells are produced to replace them. Some offices move out or close, just as organs sometimes shut down.