2.3The Hierarchical Structure of the Human Body

Although the diverse cells of multicellular organisms are influenced by the cells around them, they independently carry out metabolism and multiply. Furthermore, these diverse cells perform their functions as a unified, individual organism. How is cell activity integrated into individuals? Various cells combine to form tissues, and these tissues organize into larger units called organs. This integration of hierarchy of functions supports the basic mechanisms of life. In humans, organ systems integrating these organs are at the top of this hierarchy (Table 2-1). For example, the circulatory system controls the circulation of blood and lymph fluid throughout the body, and the nervous system controls the transmission of information between parts of the body and the brain and spinal cord. In addition, the endocrine system uses hormones to regulate the activities of various parts of the body. Moreover, in many cases, these organ systems work together to maintain homeostasis, such as in the regulation of blood glucose levels (see Chapter 8).
Conversely, as we delve further into cells, we see that they contain organelles, which are made up of molecules such as proteins, lipids, and sugars, which in turn are made of various atomic elements.

Table 2-1. Principal organs and organ systems of humans

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