LEVELS OF ORGANIZATION
CELLS
Cells are the basic functional units of animal life—the smallest subdivisions of the body that are capable of life. A simple, single-celled animal like an ameba has to carry out all the life functions necessary to support itself within its one cell.
It must do things such as: grow; respond to positive and negative stimuli; seek out, engulf, and absorb food; eliminate wastes; and reproduce. It has no ability to influence its environment and has to take things as they come. If environmental conditions are favorable, the amoeba survives. If not, it dies.In the complex animals we discuss in this book, the body's cells must divide the work. The sheer size of a dog or horse results in most of the animal's cells being far removed from the outside environment. The animal's body must create and support an internal environment that allows all of its cells to live and function. To accomplish this, cells must specialize in some functions and eliminate others. For example, some cells specialize in absorbing nutrients (intestinal lining cells), others in carrying oxygen (red blood cells), and still others in organizing and controlling body functions (nerve cells). A particular cell in the body depends on the rest of the body's cells all doing their jobs to ensure its survival. At the same time, all the other cells in the body rely on that cell doing its job to contribute to their survival.
TISSUES
When specialized cells group together, they form tissues. The entire animal body is made up of only four basic tissues: epithelial tissue, connective tissue, muscle tissue, and nervous tissue. The basic characteristics and functions of the four body tissues are summarized in Table 1-4.
Epithelial tissue is composed entirely of cells, and its main jobs are to cover body surfaces, secrete materials, and absorb materials. The surface of the skin is covered by epithelium, as are the linings of the mouth, intestine, and urinary bladder.
Epithelial tissue also forms glands, which are structures that secrete useful substances and excrete wastes. The secreting units of sweat glands, salivary glands, and mammary glands are all composed of specialized epithelial tissues. The epithelium that lines the GI tract is specialized to absorb nutrients from the lumen of the tube.Connective tissue holds the body together (connects its cells) and gives it support. Cells are very soft and cannot support themselves without outside help. Connective tissues
| TABLE 1-4 | Body Tissues | |
| TISSUE | CHARACTERISTICS | FUNCTIONS |
| Epithelial | Composed only of cells | Covers and protects (surfaces) |
| Secretes (glands) | ||
| Absorbs (intestinal lining) | ||
| Connective | Composed of living cells and non-living intercellular substances | Binds cells and structures together and supports the body |
| Muscle | Skeletal (voluntary) | Movements |
| Cardiac (heart) | ||
| Smooth (involuntary) | ||
| Nervous | Composed of nerve cells (neurons) and supporting cells | Transmits information around body; Coordinates and controls activities |
range from very soft, such as adipose tissue (commonly called fat) to very firm, such as cartilage and bone. Connective tissues are composed of cells and a variety of nonliving intercellular substances, such as fibers, that add strength.
Muscle tissue moves the body inside and out. It exists as three types: skeletal muscle, cardiac muscle, and smooth muscle. Skeletal muscle moves the bones of the skeleton and is under conscious nervous system control. Cardiac muscle makes up the heart and works “automatically” (no conscious effort is required).
Smooth muscle is found in internal organs such as the digestive tract and urinary bladder. It also works pretty much automatically.Nervous tissue transmits information around the body and controls body functions. It transmits sensory information from the body to the brain, processes the information, and sends instructions out to tell the body how to react to changing conditions.
Epithelial and connective tissues are discussed in more detail in Chapter 5. Muscle and nervous tissues are more complicated, so each has its own chapter.
ORGANS
The next level up from tissues is organs. Organs are made up of groups of tissues that work together for common purposes. For example, the kidney is an organ composed of various tissues that function together to eliminate wastes from the body. Some organs, such as the eyes, lungs, and kidneys, occur in pairs. Others, such as the brain, heart, and uterus, are single structures.
SYSTEMS
Systems are the most complex level of body organization. Systems are groups of organs that are involved in a common set of activities. For example, the digestive system is concerned with obtaining, digesting, and absorbing nutrients to fuel the rest of the body. It is composed of the organs that make up the digestive tube, such as the esophagus, stomach, and intestine, as well as accessory digestive organs, such as the salivary glands, pancreas, and liver. Table 1-1 lists all the major systems of the body.
TEST YOURSELF 1-4
1. What is the difference between a cell, a tissue, an organ, and a system in an animal's body?
2. What are the four basic tissues that make up an animal's body?