Nutrition Science and Everyday Application - beta v 0.1

130 ALICE CALLAHAN, PHD, HEATHER LEONARD, MED, RDN, AND TAMBERLY POWELL, MS, RDN
Figure 3.2. The cell structure
Tissues, Organs, Organ Systems, and Organisms
A tissue is a group of many similar cells that share a common structure and work together to
perform a specific function. There are four basic types of human tissues: connective, which
connects tissues; epithelial, which lines and protects organs; muscle, which contracts for
movement and support; and nerve, which responds and reacts to signals in the environment.
An organ is a group of similar tissues arranged in a specific manner to perform a specific
physiological function. Examples include the brain, liver, and heart. An organ system is a
group of two or more organs that work together to perform a specific physiological function.
Examples include the digestive system and central nervous system.
There are eleven distinct organ systems in the human body (Figure 3.3). Assigning organs
to organ systems can be imprecise since organs that “belong” to one system can also have
functions integral to another system. In fact, many organs contribute to more than one
system. And most of these organ systems are involved in nutrition-related functions within
the body (Table 3.1). For example, the cardiovascular system plays a role in nutrition by
transporting nutrients in the blood to the cells of the body. The endocrine system produces
hormones, many of which are involved in regulating appetite, digestive processes, and
nutrient levels in the blood. Even the reproductive system plays a role in providing nutrition
to a developing fetus or growing baby.