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Chapter: Essentials of Anatomy and Physiology: Organization and General Plan of the Body

Levels of Organization

The human body is organized into structural and functional levels of increasing complexity.


The human body is organized into structural and functional levels of increasing complexity. Each higher level incorporates the structures and functions of the previous level, as you will see. We will begin with the simplest level, which is the chemical level, and pro-ceed to cells, tissues, organs, and organ systems. All of the levels of organization are depicted in Fig. 1–1.


The chemicals that make up the body may be divided into two major categories: inorganic and organic. Inorganic chemicals are usually simple molecules made of one or two elements other than carbon (with a few exceptions). Examples of inorganic chemicals are water (H2O); oxygen (O2); one of the exceptions, car-bon dioxide (CO2); and minerals such as iron (Fe), cal-cium (Ca), and sodium (Na). Organic chemicals are often very complex and always contain the elements carbon and hydrogen. In this category of organic chemicals are carbohydrates, fats, proteins, and nucleic acids.


The smallest living units of structure and function are cells. There are many different types of human cells, though they all have certain similarities. Each type of cell is made of chemicals and carries out specific chemical reactions.


tissue is a group of cells with similar structure and function. There are four groups of tissues:

Epithelial tissues—cover or line body surfaces; some are capable of producing secretions with specific functions. The outer layer of the skin and sweat glands are examples of epithelial tissues. Internal epithelial tissues include the walls of capillaries (squamous epithelium) and the kidney tubules (cuboidal epithelium), as shown in Fig. 1–1.

Figure 1–1. Levels of structural organization of the human body, depicted from the simplest (chemical) to the most complex (organism). The organ system shown here is the urinary system.

QUESTION: What other organ system seems to work directly with the urinary system?

Connective tissues—connect and support parts of the body; some transport or store materials.Blood, bone, cartilage, and adipose tissue are examples of this group.

Muscle tissues—specialized for contraction, which brings about movement. Our skeletal muscles and the heart are examples of muscle tissue. In Fig. 1–1, you see smooth muscle tissue, which is found in organs such as the urinary bladder and stomach.


Nerve tissue—specialized to generate and transmit electrochemical impulses that regulate body functions. The brain and optic nerves are examples of nerve tissue.



An organ is a group of tissues precisely arranged so as to accomplish specific functions. Examples of organs are the kidneys, individual bones, the liver, lungs, and stomach. The kidneys contain several kinds of epithelial, or surface tissues, for their work of absorp-tion. The stomach is lined with epithelial tissue that secretes gastric juice for digestion. Smooth muscle tissue in the wall of the stomach contracts to mix food with gastric juice and propel it to the small intes-tine. Nerve tissue carries impulses that increase or decrease the contractions of the stomach.


An organ system is a group of organs that all con-tribute to a particular function. Examples are the uri-nary system, digestive system, and respiratory system. In Fig. 1–1 you see the urinary system, which consists of the kidneys, ureters, urinary bladder, and urethra. These organs all contribute to the formation and elimination of urine.

As a starting point, Table 1–1 lists the organ systems of the human body with their general functions, and some representative organs, and Fig. 1–2 depicts all of the organ systems. Some organs are part of two organ systems; the pancreas, for example, is both a digestive and an endocrine organ, and the diaphragm is part of both the muscular and respiratory systems. All of the organ systems make up an individual person. The balance of this text discusses each system in more detail.

Figure 1–2. Organ systems. Compare the depiction of each system to its description in Table 1–1

QUESTION: Name at least one organ shown in each system.

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