Chapter 26

Cells called melanocytes that lie deep in the epidermis produce a pigment, called melanin, that gives skin its color. Exposure to ultraviolet light-from the sun or a tanning bed -causes melanocytes to produce more melanin, which they pass along to the skin cells above them, yielding a suntan. But why would sun exposure prompt this increased melanin production? What is the function of a \(\tan ,\) in other words?

The text notes that bone density can increase in response to exercise. Imagine two fairly inactive people who decide to start exercising. One becomes a runner while the other becomes a swimmer. Who experiences the greater increase in bone density: the swimmer (in the arm bones) or the runner (in the leg bones)?

Acids secreted by the stomach give the materials within it a pH level that can lie between that of lemon juice and battery acid. Yet our small intestines, which the stomach feeds into, have chemical buffers flowing into them that raise the pH level of the material coming from the stomach. Why should the stomach need such an extreme \(p H\) level while the small intestines do not?