20.1–20.6 Animal body structures reflect their functions.

The teeth of a great white shark make it capable of consuming large prey.
20.1 Most animal bodies are organized in a hierarchy from cells to tissues, organs, and organ systems.

Form follows function. This simple statement captures one of the most universal relationships in the living world. For example, fast-swimming organisms, from penguins to tuna to sharks, share a common streamlined body shape (FIGURE 20-1). If a structure is adaptive—the product of natural selection—then its physical features closely reflect its function. And just as form follows function for large animal structures, the relationship also holds true for molecules, organelles, and cells. Form fits function in each.

Figure 20.1: Adapted for the life aquatic: the streamlined body of a penguin.

Animals are multicellular organisms. And multicellularity makes it possible for animals to attain much larger sizes and much greater physiological complexity than single-celled organisms. Increased size and complexity bring many benefits, including fewer potential predators, more potential prey, and, more generally, some protection from the influence of the external environment. For these reasons, among others, the transition from a single-celled to a multicellular body is one of the most important evolutionary transitions.

Perhaps the chief benefit of multicellularity is that it makes possible a division of labor and specialization at the cellular level. It is unnecessary for each cell to carry out every single process (such as generating movement, detoxifying harmful chemicals, digesting biological molecules, and sensing and responding to environmental changes). Instead, cells can be organized into groups, and groups organized into larger groups, to carry out specific life-sustaining functions such as exchanging gases between the organism and its environment, thinking and feeling, and fighting pathogens (FIGURE 20-2).

Figure 20.2: From cells to organ systems.

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One of the fundamental features of the animal body is its hierarchical organization. Cells, as you’ll recall from Chapter 3, are the smallest units of organization in all organisms. Even sponges, the structurally simplest of all animals, have cells specialized to perform a few distinct tasks. Some cells form the covering of the sponge’s body, and others obtain food to provide nutrition and energy to sustain the sponge’s activities. At the other end of the spectrum are humans—with more than 200 different cell types.

In most animals, groups of cells with similar structure, along with some products of those cells, form tissues, in which the cells act together to perform specific functions in the body. Adult animals generally have four main types of tissue (FIGURE 20-3). We’ll discuss them in more detail in Sections 20-2 through 20-5, but we introduce them here.

Connective tissue consists of cells embedded in a large amount of extracellular material, called matrix, which together contribute to body structure and support. Found throughout the human body, connective tissue can also serve to anchor cells, regulate communication between cells, and influence growth and wound healing. Some of the most important structures formed from connective tissue include tendons, cartilage, blood, adipose tissue (fat), and bone.

Epithelial tissue covers and lines most exterior and interior surfaces of the body. Skin is an epithelial tissue, as are the tissues that line the nose, throat, lungs, blood vessels, and digestive tract.

Muscle tissue is made up of cells that can contract. This characteristic gives muscle tissue the ability to generate movement or pump fluids through the body.

Nervous tissue, found throughout the human body, is specialized to send and receive electrical and chemical signals and, in doing so, can store and transmit information. The brain and spinal cord are made up of large amounts of nervous tissue.

Just as cells with similar functions are grouped into tissues, so tissues are often grouped into organs or organ systems. Organs are structures that serve specialized functions, and they can contain several (or even all four) types of tissue. The heart, liver, kidneys, and brain are examples of organs in the human body. Organ systems are groups of organs that work together to accomplish one or more, usually related, functions. The circulatory system, for example, includes the heart, blood vessels, and blood.

Figure 20.3: Tissues perform specific functions in the body.

In the remaining chapters of the book, we explore animal physiology—the complex functions carried out by each of the organ systems in animals. We describe the structures that are part of each system, how they function, and how they have evolved. As we do this, we see that an organism is greater than the sum of its parts. The working together of cells to form tissues, and of tissues to form organs and organ systems, enables multicellular organisms to reproduce, defend themselves, and communicate (among many other abilities), in ways that are not possible for a single cell or a single type of tissue.

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TAKE-HOME MESSAGE 20.1

Animal bodies are highly organized, and at all levels of organization, the physical features are closely related to function. In most animals, cells with similar structure and function are organized into tissues of four types: connective tissue, epithelial tissue, muscle tissue, and nervous tissue. Tissues are often organized into organs, which serve specialized functions and can contain several types of tissue. In turn, organs can be organized into organ systems that accomplish highly complex tasks.

Describe the relationship between cells, tissues, organs, and organ systems.

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