The skin is the body’s outermost boundary and, in effect, its largest organ, accounting for
16 percent of total body weight. It is shed and renewed at a rate of 1 oz (25 g) a month. The
skin protects the body by waterproofing it, reacting to sunlight, regulating temperature,
and providing a barrier to invading organisms. It also supplies the brain with a range of sensory
information. Skin problems often result from regulatory imbalances (acne rosacea and psoriasis) or allergy (eczema).The skeleton is an extensive framework of bones, attached to which are skeletal muscles. Together they coordinate the body’s movements. Individual bones move against each other by means of joints. The skeleton also provides support and protection for some of the body’s most important organs, such as the brain, spinal cord, heart, and lungs. Diseases of the skeleton
often result from degeneration of joints (osteoarthritis) or autoimmune problems (rheumatoid arthritis).

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