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Chapter 9 Human Body Orientation 457
Recall Activity
1. Simple columnar epithelial cells with __________________________ absorb digested macromolecules.
2. Which cells in the stomach release hydrochloric acid? ______________________________________________
3. Mucous neck cells produce __________________________ to protect the cells lining the stomach.
Concept 3: Tissue Level
A tissue is a group of cells that work together to do a job. On a tissue level, you
study groups of cells that work with one another. In the alimentary canal, for
example, muscle tissue contracts to move food through the canal. Through most
of the alimentary canal, muscle tissue is composed of two sheets of muscle cells.
The first sheet of smooth muscle cells is arranged in a circle around the tubelike
canal. When these muscle cells contract in unison, the tube becomes smaller in
diameter. The second sheet of muscle cells is perpendicular to the circular muscle
cells. In the second sheet, muscle cells are arranged lengthwise around the tube.
When these muscle cells contract, the tube becomes shorter in length.
Concept 4: Organ Level
An organ is a group of tissues that work together to do a job. An organ is com-
posed of different types of tissue. For example, the job of the small intestine is
to move food, break down food into macromolecules, and absorb macromol-
ecules into the blood. To achieve this, the small intestine contains several types
of tissue. Lining the inside of the small intestine is a layer of epithelial tissue
called mucosa. The cells of this tissue layer have microvilli and absorb mac-
romolecules from digested food. The mucosa is supported by a layer of con-
nective tissue called the lamina propria. Two layers of muscle tissue are next.
Cells in these tissue layers contract to move food through the small intestine.
Finally, another layer of epithelial tissue covers the outside of the small intes-
tine. This layer of epithelial tissue is called serosa (Figure 9.17).
Recall Activity
1. Through most of the alimentary canal, muscle tissue is composed of __________________________
sheet(s) of muscle cells.
2. In the alimentary canal, muscle tissue __________________________ to move food through the canal.
3. A(n) __________________________ is a group of cells working together to do a job.