Traditionally, invitations go out six to eight weeks before the wedding -- that gives guests plenty of time to clear their schedules and make travel arrangements if they don't live in town. If it's a destination wedding, give guests more time and send them out three months ahead of time.
The human body is made up of several organ systems that all work together as a unit to make sure the body keeps functioning. There are ten major organ systems in the body, each of which plays a different role in helping the body work. This system is made up of the heart, blood, blood vessels, and lymphatics.
The body has levels of organization that build on each other. Cells make up tissues, tissues make up organs, and organs make up organ systems. The function of an organ system depends on the integrated activity of its organs. For instance, digestive system organs cooperate to process food.