Integumentary System

1. External covering of the body, made up of skin, hair, and nails
2. Multiple roles in homeostasis
3. Structure and function of skin
• vital organ that covers entire outside of body
• protective barrier against pathogens and injuries from surroundings
• largest organ, covering entire outside of body
• 2 mm thick
• shields body against heat, light, injury, and infection
• other functions
• regulate body temperature
• gather sensory information from environment
• stores water, fat, and vitamin D
• acts as physical barrier to protect us from disease
• skin gets cut, scratched, and exposed to radiation every day
• naturally shed skin cells every day
• body replaces damaged or missing skin by replacing it
• two layers make up skin
• epidermis and dermis
• fatty layer, called subcutaneous tissue, or hyperdermis, lies under dermis
• not considered part of skin
• two types of skin on body
• thin and hairy
• thick and hairless
4. Epidermis
• outermost layer of skin
• forms waterproof, protective wrap over body's surface
• divided into several layers
• epithelial cells are formed in lowest layer
• epithelial cells move up through layers of epidermis
• changes shape and composition and become filled with tough, fibrous protein called keratin
• cells now called keratinocytes
• at surface, they form layer of flattened, dead cells
• about as thick as a sheet of paper, but has 25-30 layers of keratinocytes
• gets scraped off during everyday activities, and usually shed after a month
• melanin: brownish pigment that gives skin and hair their color
• melanocytes (cells that produce melanin) are at the bottom layer of epidermis (stratum basale) 
• difference in skin color is due to melanocyte's level of activity
• amount of melanin depends on genetics and amount of UV light exposure
• melanin absorbs UV rays from the sun
• when UV rays penetrates skin and damage DNA, damaged DNA triggers synthesis of more melanin
• skin also makes vitamin D from energy of UV light
• melanin is like a UV filter, so more melanin means more time a person has to spend in sunlight to produce same amount of vitamin D
• epidermis contains cells to process certain antigens from microbes that enter through the skin
• helps immune system recognize microbe as intruder, and mount an attack on it
• epidermis has no blood vessels, so lower portion of epidermis is nourished by diffusion from blood vessels of dermis
5. Structure and function of dermis
• layer of skin directly under epidermis and made of tough elastic connective tissue
• tightly connected to epidermis by membrane made of collagen fibers
• dermis contains hair follicles, sweat glands, sebaceous glands, and blood vessels
• holds many nerve endings that provide sense of touch, pressure, heat, and pain
• arrector pili (tiny muscles) contract and pull on hair follicles, causing hair to stand up
• happens when you are cold or afraid, causing goose bumps
• dermis has two layers
• papillary region (upper layer) 
• made of loose connective tissue
• contains touch receptors that communicate with central nervous system
• named for finger-like projections called papillae
• extend toward epidermis and secures the dermis to epidermis
• provides dermis with "bumpy" surface that causes distinctive friction ridges
• called friction ridges because they help hands or feet grasp things by increasing friction
• friction ridges occur in patterns that are unique to the individual
• fingerprints and footprints used as a means of identification
• reticular region (lower layer) 
• made of dense elastic fibers (collagen) 
• contains hair follicles and roots, nerves, and glands
• gets name from dense concentration of protein fibers that weave throughout it
• protein fibers give dermis properties of strength, extensibility, and elasticity
• tattoo ink is injected into the dermis
• stretch marks also in dermis
6. Glands and follicles
• opens out to the epidermis, but originate from dermis
• sebaceous gland (oil gland) secrets oily substance, called sebum
• sebum is made of lipids and debris of lipid-producing cells
• "waterproofs" hair and skin surface to prevent them from drying out
• also inhibits growth of microorganisms on skin
• sebum is cause of oily appearance of skin and hair
• odorless, but breakdown of sebum can cause odors
• if sebaceous gland becomes plugged and infected it develops into a pimple
• sweat glands open to epidermal surface through skin pores
• controlled by sympathetic nervous system
• evaporation of sweat on skin helps lower skin temperature
• skin also functions as excretory organ because it releases excess water, salts, and other wastes in sweat
• two kinds of sweat glands
• eccrine glands are "regular" sweat glands that releases sweat to cool body
• apocrine glands are located in the armpits and groin areas
• act as scent glands because they produce a solution that bacteria breaks down, causing "body odor" 
• mammary glands are enlarged and modified sweat glands and are a major characteristic of mammals
7. Subcutaneous tissue (hypodermis) 
• contains fat and loose connective tissue that holds larger blood vessels and nerves
• attaches the skin to underlying bone and muscle to supply skin with blood vessels and nerves
• layer is important in regulation of body temperature
• mostly made of adipose tissue (fat cells) 
• subcutaneous tissue contains 50% of body fat
• functions include insulation and storage of nutrients
• size of layer varies throughout  body and person to person
8. Functions of skin: skin and homeostasis
• used for protection, control of body temperature, sensory reception, water balance, synthesis of vitamins and hormones, and absorption of material
• main function to act as barrier against microbes and viruses
• also to prevent water and extracellular fluid loss
• acidic secretions also stop the grown of fungi on skin
• heat and cold receptors located in skin
• as body temperature rises, hypothalamus sends signal to sweat-producing glands, to release sweat
• evaporation of sweat reduces temperature on skin surface
• hypothalamus also causes dilation of blood vessels near skin, so more heat is released from skin surface
• as temperature decreases, sweat glands decrease production
• if temperature continues to fall, shivering starts, to warm up the body
• homeostatic functions of skin
• protection of body's internal tissues and organs
• protection against invasion by infectious organisms
• protection of the body from dehydration
• protection of the body against large changes in temperature
• excretion of wastes through sweat
• acts as a receptor for the senses of touch, pressure, pain, heat, and cold
• makes vitamin D though exposure to UV radiation
• stores water, fat, and vitamin D
9. Homeostatic imbalances of the skin
• infrared light: thermal energy, or heat rays that you feel
• UV light helps body produce vitamin D, but damages DNA in skin cells
10. UV radiation beneficial effects
• causes production of vitamin D in the skin
• tens of thousands of people die of premature death because of vitamin D deficiency
• UV radiation used in treatment of skin conditions, like psoriasis and vitiligo
11. Harmful effects of UV radiation
• excessive exposure causes some cancers
• many people do not take dangers posed by sunlight seriously enough
• because of tanning bed popularity
• many skin cancers are easily cured
• melanomas, a cancer, is a potentially fatal disease
• UV radiation excites DNA molecules in skin, causing bonds to form between thymine bases
• produces thymine dimer that changes DNA helix
• dimers can lead to mutations, and mutations can lead to cancerous growths
• skin cancer is a very common condition
• increased popularity of sun bathing results in increased exposure to UV radiation
• lighter-skinned people are more at risk of developing skin cancer than darker-skinned people are
• because of amount of melanin in their bodies
• lighter-skinned people have less, so they also have less protection against UV radiation
• darker-skinned people have more, so they have more protection against UV radiation and need more time outside under the sun to get the needed amount of vitamin D
• body tans when exposed to moderate level of radiation
• tanning helps block UV penetration and prevents damage to vulnerable skin tissues
• suntan lotion (sun block) partly blocks UV rays and is widely available
• sun protection factor (SPF) only blocks UVB rays, responsible for sunburns
• does not block UVA rays, which are responsible for cancer and wrinkles
• clothing also blocks UV rays
• protective clothing works well in protecting against UV rays
12. Acne
• most common form of acne is acne vulgaris
• "common acne" 
• teenagers get common acne; affects 85% of teenagers worldwide
• mostly disappears in early twenties
• some people have severe acne even after puberty and their early twenties
• excessive secretion of sebum leads to plugging of hair follicle with dead skin cells (corneocytes) 
• blockage caused by failure of normal process of shedding skin cells that line the pores 
• skin inflames, causing red bump (pimple) 
13. Nails
• nails made up of specialized epidermal cells
• contains tough protein called keratin and is actually modified hair
• nail grows from nail bed, which is thickened to form lunula
• no nerve endings in nail
• fingernails have two purposes
• protective plate and enhances sensation of function
• sensation is very important because fingernails act as counterforce to fingertip, providing more sensory input when we touch an object
• parts of nails
• free edge: part of nail that extends past the finger
• nail plate: the hard and translucent portion, composed of keratin
• lunula: crescent shaped whitish area of nail bed (when visible) 
• cuticle: fold of skin at end of the nail
• grows at rate of about 1 cm per 100 days
• fingernails: 4-6 months to regrow completely
• toenails: 12-18 months to regrow completely
• actual growth rate dependent on age, season, exercise level, and hereditary factors
• growth record shows history for recent health and physiological imbalances
• major illness will cause deep horizontal groove in nails
• indicators of illness in other areas
• discoloration
• thinning
• thickening
• brittleness
• splitting
• grooves
• spots
• lines
• receded lunula
• changes in shape of nail
14. Hair
• filamentous fiber only found on mammals
• made mostly of keratin
• hair emerges from epidermis, but grows from hair follicles deep in the dermis
• three kinds of hair
• lanugo is fine hair that covers nearly entire body of fetuses
• vellus hair is short, fine hairs that grows in most places on human body except for palms of hands and soles of feet
• terminal hair is fully developed hair that is generally longer, coarser, thicker, and darker than vellus hair
• vellus covers entire human body except lips, palms of hands, soles of feet, navel, and scar tissue
• density of hairs varies from one person to another
• function of hair
• insulate
• protect
• sense the immediate surroundings
• curly hair has biologically different structure than straight hair
• much drier than straight hair because oil does not travel down curly hair as quickly as it does on straight hair
• people with curly hair find this hair type to be dry, hard to manage, and often frizzy
• 3-6 months, body hair growth stops
• follicle shrinks and root of hair grows rigid
• another growth cycle starts after period of dormancy
• head hair grows for long duration and to a great length before being shed
• terminal hair genetically programmed to be straight, curly, or wavy, and tends to change over time
• hair color is result of pigmentation due to different forms of melanin
• more melanin: darker
• less melanin: lighter