What do we call separate body structures composed of multiple tissues that each perform specific functions?

The term used to describe separate body structures made up of multiple tissues that work together to perform specific functions is "organs." Each organ in the body is a complex structure composed of different tissue types, such as muscle tissue, connective tissue, epithelial tissue, and nervous tissue, all collaborating to accomplish particular tasks essential for maintaining overall health and function.

For instance, the heart is an organ consisting of cardiac muscle tissue, connective tissue, and nerve tissue. Its primary function—pumping blood throughout the body—is made possible by the coordinated work of these various tissues.

The other options refer to different biological structures: cells are the basic units of life; tissues are groups of cells that perform specific functions but do not encompass multiple types; and systems refer to groups of organs that work together to perform complex functions. Understanding the roles and relationships of these structures is essential in studying human anatomy and physiology.