In a prior life, I followed the enterprise service bus (ESB) and messaging markets pretty closely. Graham Glass (CTO of webMethods) has an interesting post that draws an analogy between the ESB and mechanisms of the human body (bold type added by me to highlight two key underlying capabilities):

The equivalent of an ESB in animals is the nervous system combined with the circulatory system. When an organ needs to communicate with another specific organ, it uses the nervous system to send a point-to-point message. When an organ needs to broadcast a message to other organs that might be interested, it releases a hormone into the bloodstream to send a multicast message.

Steve Shu