Cilia (SIL-ee-uh; singular, cilium10) (figs. 3.11c-e and 3.12) are hairlike processes about 7 to 10 ^m long. Nearly every human cell has a single, nonmotile primary cilium a few micrometers long. Its function in many cases is still a mystery, but some of them are sensory. In the inner ear, they play a role in the sense of balance; in the retina of the eye, they are highly elaborate and form the light-absorbing part of the receptor cells; and they are thought to monitor fluid flow through the kidney tubules. In some cases they open calcium gates in the plasma membrane. Sensory cells in the nose have multiple nonmotile cilia which bind odor molecules.
Motile cilia are less widespread, occurring mainly in the respiratory tract and the uterine (fallopian) tubes. There may be 50 to 200 of these cilia on the surface of one cell. Cilia beat in waves that sweep across the surface of an epithelium, always in the same direction (fig. 3.13). Each cilium bends stiffly forward and produces a power stroke that pushes along the mucus or other matter. Shortly after a cilium begins its power stroke, the one just ahead of it begins, and the next and the next—collec-tively producing a wavelike motion. After a cilium completes its power stroke, it is pulled limply back by a recovery stroke that restores it to the upright position, ready to flex again.
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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.