The Integumentary System

   

   The integumentary system, formed by the skin, hair, nails, and associated glands, enwraps the
   body. It is the most visible organ system and one of the most complex. The integumentary
   system protects the body from the outside world and its many ________ substances. It utilizes
   the Sun's rays while at the same time shielding the body from their damaging effects. In
   addition, the system helps to regulate body ________, serves as a minor excretory organ, and
   makes the inner body aware of its outer ________ through sensory receptors.

   Integument comes from the Latin word integumentum, meaning "________" or "enclosure." The
   human integumentary system is an external body covering, but also much more. It ________,
   nourishes, insulates, and cushions. It is absolutely essential to life. ________ it, an
   individual would be attacked immediately by bacteria and die from heat and water loss.

  Skin

   Although the skin is not often thought of as an organ, such as the heart or liver, medically
   it is. An organ is any part of the body formed of two or more tissues that performs a
   specialized ________. As an organ, the skin is the largest and heaviest in the body. In an
   average adult, the skin covers about 2 square meters and accounts for approximately 7 percent
   of body weight, or about 5 kilograms in a 73-kilogram person. It ranges in thickness from 1 to
   2 millimetres, but can measure up to 6 millimetres thick on the ________ of the hands and the
   ________ of the feet. The skin in these areas is referred to as thick skin (skin elsewhere on
   the body is called ________skin).

   The skin has two principal ________: the epidermis and the dermis. The epidermis is the thin,
   ________ layer, and the dermis is the thicker, inner layer. Beneath the dermis lies the
   subcutaneous layer or ________, which is composed of fatty tissue. Although not technically
   part of the skin, it does anchor the skin to the underlying muscles. It also contains the
   major blood ________ that supply the dermis and houses many white blood cells, which destroy
   foreign invaders that have ________ the body through breaks in the skin.

   EPIDERMIS. The epidermis is made of stratified squamous epithelial tissue. Epithelial tissue
   covers the internal and external surfaces of the body and also forms glandular organs.
   Squamous cells are thin and flat like fish scales. Stratified simply means having two or more
   layers. In short, the epidermis is ________ of many layers of thin, flattened cells that fit
   closely together and are able to withstand a good deal of abuse or friction.

   The epidermis can be ________ into four or five layers. Most important of these are the inner
   and outer layers. The inner or deepest cell layer is the only layer of the epidermis that
   receives nutrients (from the underlying dermis). The cells of this layer, called basal cells,
   are constantly dividing and creating new cells daily, which ________ the older cells toward
   the surface. Basal cells produce keratin, an extremely durable and water-resistant fibrous
   protein.

   Another type of cell found in the lower epidermis is the melanocyte. Melanocytes produce
   melanin, a protein ________ that ranges in colour from yellow to brown to black. The amount of
   melanin produced determines skin colour, which is a hereditary characteristic. The melanocytes
   of dark-skinned individuals continuously produce large amounts of melanin. Those of
   light-skinned individuals produce less. Freckles are the result of melanin clumping in one
   spot.

   The outermost layer of the epidermis consists of about twenty to thirty rows of tightly joined
   flat dead cells. All that is left in these cells is their keratin, which makes this outer
   layer ________. It takes roughly fourteen days for cells to move from the inner layer of the
   epidermis to the outer layer. Once part of the outer layer, the dead cells remain for another
   fourteen days or so before flaking off slowly and steadily.

   DERMIS. The dermis, the second layer of skin, lies between the epidermis and the ________
   layer. Much thicker than the epidermis, the dermis contains the accessory skin structures.
   Hair, ________ glands, and ________ (oil) glands are all rooted in the dermis. This layer also
   contains blood vessels and nerve fibres. Nourished by the blood and oxygen provided by these
   blood vessels, the cells of the dermis are alive.

   Connective tissue forms the dermis. Bundles of elastic and collagen (tough fibrous protein)
   fibres blend into the connective tissue. These fibres provide the dermis strength and
   flexibility.

   The upper layer of the dermis has finger like projections that extend into the epidermis.
   Called dermal papillae, they contain blood capillaries that provide nutrients for the basal
   cells in the epidermis. On the skin surface of the hands and feet, especially on the tips of
   the fingers, thumbs, and toes, the dermal papillae form looped ridges. These print patterns,
   known as fingerprints or toeprints, increase the gripping ability of the hands and feet.
   Genetically determined, the patterns are ________ to every individual.

  Accessory structures

   The accessory structures of the integumentary system include hair, nails, and sweat and
   sebaceous glands.

   HAIR. Roughly 5 million hairs cover the body of an average individual. About 100,000 of those
   hairs appear on the ________. Almost every part of the body is covered by hair, except the
   palms of the hands, the soles of the feet, the sides of the fingers and toes, the lips, and
   certain parts of the outer genital organs.

   Each hair originates from a tiny tubelike structure called a hair ________ that extends deep
   into the dermis layer. Often, the follicle will project into the subcutaneous layer.
   Capillaries and nerves attach to the base of the follicle, providing nutrients and sensory
   information. Inside the base of the follicle, epithelial cells grow and divide, forming the
   hair bulb or enlarged hair base. ________, the primary component in these epithelial cells,
   coats and stiffens the hair as it grows upward through the follicle. The part of the hair
   enclosed in the follicle is called the hair ________. Once the hair projects from the scalp or
   skin, it is called a hair ________.

   The older epithelial cells forming the hair root and hair shaft die as they are ________
   upward from the nutrient-rich follicle base by newly formed cells. Like the upper layers of
   the epidermis, the hair shaft is made of dead material, almost entirely protein. The hair
   shaft is divided into two layers: the cuticle or outer layer consists of a single layer of
   flat, overlapping cells; the cortex or inner layer is made mostly of keratin.

   Hair shafts differ in size, shape, and colour. In the eyebrows, they are short and stiff, but
   on the scalp they are longer and more flexible. Elsewhere on the body they are nearly
   invisible. Oval-shaped hair shafts produce wavy hair. Flat hair shafts produce curly hair.
   Perfectly round hair shafts produce straight hair. The different types of melanin—yellow,
   rust, brown, and black—produced by ________ at the follicle base combine to create the many
   varieties of hair colour, from the palest blonde to the richest black. With age, the
   production of melanin ________, and hair colour turns gray.

   Attached to each hair follicle is a ribbon of smooth muscle called a hair ________ muscle.
   When stimulated, the muscle contracts and pulls on the follicle, causing the hair shaft to
   stand upright.

   NAILS. Nails in humans correspond to the hooves of horses and cattle and the claws of birds
   and reptiles. Found on the ends of fingers and toes, nails are produced by nail follicles just
   as hair is produced by hair follicles. The nail root is that portion of the nail embedded in
   the skin, lying very near the bone of the fingertip. Here, cells produce a stronger form of
   keratin than is found in hair. As new cells are formed, older cells are pushed forward,
   forming the nail body or the visible attached portion of the nail. The free edge is that
   portion of the nail that extends over the________of the finger or toe. Healthy fingernails
   grow about 1 millimetre per week, slightly faster than toenails.

   The nail body is made of dead cells, but the nail ________ (the tissue underneath the nail
   body) is alive. The blood vessels running through the nail bed give the otherwise transparent
   nail body a ________ colour. Near the nail root, however, these blood vessels are obscured.
   The resulting white crescent is called the ________ (from the Latin word luna, meaning
   "moon").

   SWEAT GLANDS. More than 2.5 million sweat glands are distributed over most surfaces of the
   human body. They are divided into two types: eccrine sweat glands and apocrine sweat glands.

   Eccrine glands, the more numerous of the two types, are found all over the body. They are
   especially numerous on the forehead, upper lip, palms, and soles. The glands are simply tubes
   that originate in the dermis. A duct extends from the gland to the skin's surface, where it
   opens into a ________ Eccrine glands produce sweat or ________, a clear secretion that is 99
   percent water. Some salts, traces of ________materials such as urea, and vitamin C form the
   remainder (the salts give sweat its characteristic salty taste).

   Depending on temperature and humidity, an average individual loses 0.3 to 0.8 litres of water
   every day through sweating. During rigorous physical activity or on a hot day, that amount
   could rise to 5 to 7 litres.

   Apocrine glands are found in the armpits, around the nipples, and in the groin. Like eccrine
   glands, apocrine glands are tubes found in the dermis. However, they are usually larger and
   their ducts empty into hair follicles. Also, apocrine glands do not function until puberty. At
   that time, they begin to ________ an odourless cloudy secretion that contains fatty acids and
   protein. If the secretion of apocrine glands is allowed to ________ on the skin for any length
   of time, bacteria that lives on the skin breaks down the fatty acids and protein for their
   growth, creating the unpleasant odor often associated with sweat. Apocrine glands are
   activated by nerve fibres during periods of pain and stress, but their function in humans is
   not well understood. Scientists theorize they may act as sexual attractants.

   SEBACEOUS GLANDS. Sebaceous glands, also known as ________glands, are found in the dermis all
   over the body, except for the ________ and ________. They ________ sebum, a mixture of lipids
   (fats), proteins, and fragments of dead fat producing cells. The function of sebum is to
   prevent the ________ of skin and hair. It also contains chemicals that kill bacteria present
   on the skin surface. While most sebaceous glands secrete sebum through ducts into hair
   ________, some secrete sebum directly onto the surface of the skin. Erector muscles, which
   contract to elevate hairs, also ________ sebaceous glands, forcing out sebum.

   

   

   Adapted from : http://www.faqs.org/health/Body-by-Design-V1/The-Integumentary-System.html