Lesson 1, Topic 1
In Progress

Levels of Organization

March 28, 2021

Before you begin the study of the structure and function of the human body and its many parts, it is important to think about how those parts are organized and how they might logically fit together into a functioning whole.

Examine Figure 1-2. It illustrates the differing levels of organization that influence body structure and function. Note that the levels of organization progress from the least complex (chemical level) to the most complex (organism level).

Because you already know that “structure fits function,” it should not surprise you that the highly complex and coordinated functions of the whole body can be understood by discovering the many basic processes that occur in the smaller parts, such as organs, tissues, and cells.

Organization is one of the most important characteristics of body structure. Even the word organism, used to denote a living thing, implies organization.

Although the body itself is considered a single structure, it is made up of trillions of smaller structures. Atoms and molecules are often referred to as the chemical level of organization. The existence of life depends on the proper levels and proportions of many chemical substances in the cells of the body.

Many of the physical and chemical phenomena that play important roles in the life process are reviewed in Chapter 2. 6Such information provides an understanding of the physical basis for life and for the study of the remaining levels of organization that are so important in the study of anatomy and physiology—cells, tissues, organs, and systems.

Cells are considered to be the smallest “living” units of structure and function in our bodies. Although long recognized as the simplest units of living matter, cells are far from simple. They are extremely complex, a fact you will discover in Chapter 3.

Tissues are somewhat more complex than cells. By definition a tissue is an organization of many cells that act together to perform a common function. The cells of a tissue may be of several types, but all work together in some way to produce the structural and functional qualities of the tissue. Cells of a tissue are often held together and surrounded by varying amounts and varieties of gluelike, nonliving intercellular substances. The varied properties of different tissues are explored in Chapter 4.

Organs are larger and even more complex than tissues. An organ is a group of several different kinds of tissues arranged in a way that allows them to act together as a unit to perform a special function. For instance, the brain shown in Figure 1-2 is an example of organization at the organ level. Unlike microscopic molecules and cells, some tissues and most organs are gross (large) structures that can be seen easily without a microscope.

Systems are the most complex units that make up the body. A system is an organization of varying numbers and kinds of organs arranged in ways that allow them to work together to perform complex functions for the body. The organs of the nervous system shown in Figure 1-2 function to monitor and regulate the overall functioning of the body.

The body as a whole—the human organism—is all the atoms, molecules, cells, tissues, organs, and systems that you will study in subsequent chapters of this text. Although capable of being dissected or broken down into many parts, the body is a unified and complex assembly of structurally and functionally interactive components, each working together to ensure healthy survival.