In areas of the body that are specialized for rapid diffusion, the surface area of the cell membranes may be increased by numerous folds called microvilli. The rapid passage of the products of digestion across the epithelial membranes in the intestine, for example, is aided by these structural adaptations. The surface

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■ Figure 3.6 Microvilli in the small intestine. Microvilli are seen in this colorized electron micrograph, which shows two adjacent cells joined together by junctional complexes.

area of the apical membranes (the part facing the lumen) in the intestine is increased by the numerous tiny fingerlike projections (fig. 3.6). Similar microvilli are found in the epithelium of the kidney tubule, which must reabsorb various molecules that are filtered out of the blood.

Test Yourself Before You Continue

1. Describe the structure of the plasma membrane.

2. Describe the different ways that cells can engulf materials in the extracellular fluid.

3. Explain the process of exocytosis.

4. Describe the structure and function of cilia, flagella, and microvilli.

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Essentials of Human Physiology

Essentials of Human Physiology

This ebook provides an introductory explanation of the workings of the human body, with an effort to draw connections between the body systems and explain their interdependencies. A framework for the book is homeostasis and how the body maintains balance within each system. This is intended as a first introduction to physiology for a college-level course.

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