Mammalian skin often contains hairs, which in sufficient density is called fur. The hair mainly serves to augment the insulation the skin provides, but can also serve as a secondary sexual characteristic or as camouflage. On some animals the skin is very hard and thick, and can be processed to create leather. Reptiles and fish have hard protective scales on their skin for protection, and birds have hard feathers, all made of tough β-keratins. Amphibian skin is not a strong barrier to passage of chemicals. A frog sitting in an anesthetic solution will quickly go to sleep. The skin is often known as "the largest organ in the human body". This applies to exterior surface, as it covers the body, appearing to have the largest surface area of all the organs. Moreover, it applies to weight, as it weighs more than any single internal organ, accounting for about 15 percent of body weight. For the average adult human, the skin has a surface area of between 1.5-2.0 square metres, most of it is between 2-3 mm thick. The average square inch of skin holds 650 sweat glands, 20 blood vessels, 1000 melanocytes, and more than a thousand nerve endings. Your skin is your armor, your shield from the elements. Yet when was the last time you really thought about what you need to do to take care of it? Throughout your life, genetic and environmental factors will cause a lot of wear and tear to your skin. But you are never too young or too old to start taking care of your skin. As we age, our body undergoes various changes, and so does our skin. In our skin, we have tissues known as collagen and elastin. These tissues are mainly responsible for making our skin firm and stretchable. As we grow older, the amount of collagen and elastin in our skin naturally diminishes. As a result, our skin becomes less elastic and weaker, thus creating "wrinkles".
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