The skin bears the brunt of most physical injuries to the body, but it resists and recovers from trauma better than other organs do. The toughness of keratin and strength of the epidermal desmosomes make the skin a barrier that is not easily breached. Few infectious organisms can penetrate the skin on their own. Those that do either rely on accidental breaks in the skin or have life cycles that involve vectors—animals such as mosquitoes, fleas, lice, and ticks, with mouthparts strong enough to puncture the skin.
The epidermal surface is populated by bacteria, fungi, and other pathogens poised for any opportunity to get inside. Even vigorous scrubbing in a hot shower does not rid the skin of bacteria. They are, however, discouraged from multiplying on the skin by its relatively dry, unfavorable habitat. The sebum (oil of the skin) contains bactericidal substances, and sweat forms a film called the acid mantle (pH 4-6) that is unfavorable to microbial growth. Even when a pathogen breaches the epidermis, it faces an army of dermal macrophages and leukocytes that can quickly migrate to the site of infection and mount a defense.
The skin is also important as a barrier to water. It prevents the body from absorbing excess water when you are swimming or bathing, but even more importantly, it prevents the body from losing excess water. This function becomes especially evident when skin is lost; in patients who have suffered extensive burns, fluid replacement is one of the most critical needs for survival.
The skin is also a barrier to solar radiation, including ultraviolet (UV) rays. Most UV rays are filtered out by atmospheric ozone, but even the small fraction that reaches us is enough to cause sunburns and skin cancer. Scientists fear that we could see a catastrophic increase in skin cancer because of the loss of atmospheric ozone. Ozone is destroyed by chemicals called chlorofluorocar-bons (CFCs), used in air conditioners, refrigerators, spray cans, and other sources.
Although the skin is impermeable to most chemicals, there are exceptions. The blood receives 1% to 2% of its oxygen by diffusion through the skin, and it gives
off some carbon dioxide and volatile organic chemicals. Amino acids and steroids diffusing through the skin are one factor that attracts mosquitoes to people. The fat-soluble vitamins A, D, E, and K can be readily absorbed through the skin, as can many drugs and poisons (see insight 6.2).
Insight 6.2 Clinical Application
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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.