Organization of the Body

How is the human body organized?

Every cell in the human body is both an independent unit and an interdependent part of a larger community—the entire organism. To complete a winning play, a player at first base has to use her eyes to watch the ball and use her brain to figure out how to position her body. With the support of her bones, muscles move her body to first base. Meanwhile, the player's lungs absorb oxygen, which her blood carries to cells for use during cellular respiration. Her brain monitors the location of the ball and sends signals that guide her glove hand to make the catch.

How can so many individual cells and parts work together so efficiently? One way to answer this question is to study the organization of the human body. The levels of organization in the body include cells, tissues, organs, and organ systems. At each level of organization, these parts of the body work together to carry out the major body functions.

Cells A cell is the basic unit of structure and function in living things. As you learned in Chapter 7, individual cells in multicellular organisms tend to be specialized. Specialized cells, such as bone cells, blood cells, and muscle cells, are uniquely suited to perform a particular function.

Tissues A group of cells that perform a single function is called a tissue. There are four basic types of tissue in the human body—epithelial, connective, nervous, and muscle. Figure 30–1 shows examples of each type of tissue.