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# The Integumentary System The integumentary system is the skin and its derivatives. It covers the entire body and is visible to the naked eye. At openings (such as the urethra and vagina), the skin transitions to mucous membranes. ## Functions of the Integumentary System * **Physical protecti...

# The Integumentary System The integumentary system is the skin and its derivatives. It covers the entire body and is visible to the naked eye. At openings (such as the urethra and vagina), the skin transitions to mucous membranes. ## Functions of the Integumentary System * **Physical protection:** Protects against mechanical, chemical, thermal, and radiation injuries. The skin also resists trauma and infection. * **Sensory organ:** Senses external stimuli through sensory receptors. * **Regulation of exchange:** Regulates temperature, water retention (working with sweat glands to maintain hydration), and excretes waste products. It also plays a role in absorbing medications. * **Vitamin D production:** A key component in bone formation. Insufficient vitamin D can lead to structural alterations. ## Skin Composition (90%) * 7-dehydrocholesterol * Cholecalciferol (Vitamin D₂) * 25-hydroxycholecalciferol (25-hydroxyvitamin D₃) * 1,25-dihydroxycholecalciferol (1,25-dihydroxyvitamin D₃) ## Vitamin D and Bone Formation Vitamin D is crucial for calcium and phosphorus absorption in the intestines, leading to calcium deposition in bones. The skin is responsible for approximately 90% of UV-induced vitamin D production. ## Skin Dimensions and Anatomy * Skin accounts for 15% of body weight. * Surface area ranges from 1.5 to 2 m². * Thickness varies (e.g., 0.5-1 mm on eyelids, 5-6mm on palms). * Skin appendages include hair, nails, and glands (sweat, sebaceous, mammary). * Skin can be divided into regions, each representing 9% of the total surface area (Wallace's Rule of 9s). ## Skin Topography The skin is not smooth; it has creases, lines, pores (exits of sweat glands), and folds. * Deep grooves on fingers are called **dermatoglyphs**.