Integumentary System PDF

Integumentary system • The skin is an organ comprised of all four tissue types: epithelial, connective, muscular, and nervous tissue. • The integumentary system consists of the skin and several accessory organs (hair, nails, sweat glands, and sebaceous glands). • In an adult, the skin has a surfac...

Integumentary system • The skin is an organ comprised of all four tissue types: epithelial, connective, muscular, and nervous tissue. • The integumentary system consists of the skin and several accessory organs (hair, nails, sweat glands, and sebaceous glands). • In an adult, the skin has a surface area of about 1.8 square meters (m2). • It accounts for nearly 15% of the weight of an average human. Regions of the Skin • Skin is made up of two layers that cover a third fatty layer. The outer layer is called the epidermis (ectoderm in origin); it is a tough protective layer that contains melanin (which protects against the rays of the sun and gives the skin its color). • The second layer (located under the epidermis) is called the dermis (mesoderm in origin); it contains nerve endings, sweat glands, sebaceous (oil glands), and hair follicles. • Under these two skin layers is a fatty loose connective tissue layer of subcutaneous tissue: the hypodermis ;( the word subcutaneous means "under the skin)  Functions of Skin Protection against injury, bacteria, dehydration (water proof layer of keratin), ultraviolet rays of sun (due to presence of melanin pigment).  Thermoregulation. Reception of sensation (touch, temperature and pain). Excretion of sweat. Absorption of ultraviolet radiation from the sun for synthesis of vitamin D. The Epidermis • The epidermis is made up of stratified squamous epithelium. • New epidermal cells for the renewal of skin are derived from stem (basal) cells. • The importance of these stem cells is observed when there is an injury to the skin. • If an injury, such as a burn, is deep enough to destroy stem cells, then the skin can no longer replace itself. • As soon as possible, the damaged tissue is removed and skin grafting begins. The skin needed for grafting is usually taken from other parts of the patient’s body. This is called autografting,as opposed to allografting. In allografting, the graft is received from another person and is sometimes obtained from dead bodies (called cadavers). Autografting is preferred, because rejection rates are low. Newly generated skin cells become flattened and hardened as they push to the surface . Hardening takes place because the cells produce keratin, a waterproof protein. These cells are also called keratinocytes. Outer skin cells are dead and keratinized, so the skin is waterproof. This prevents water loss and helps maintain water homeostasis. The skin’s waterproofing also prevents water from entering the body when the skin is immersed. Dandruff occurs when the rate of keratinization in the skin of the scalp is two or three times the normal rate. Genetically unique fingerprints and Two types of specialized cells are located deep in the epidermis. Dendritic cells (previously called Langerhans cells) are macrophages, white blood cells that phagocytize infectious agents and then travel to lymphatic organs. There they stimulate the immune system to react to the pathogen. Melanocytes, lying deep in the epidermis, produce melanin, the main pigment responsible for skin color. The number of melanocytes is about the same in all individuals, so variation in skin color is due to the amount of melanin produced and its distribution. When skin is exposed to the sun, melanocytes produce more melanin. This protects the skin from the damaging effects of the ultraviolet (UV) radiation in sunlight. The Dermis The dermis is a region of dense fibrous connective tissue beneath the epidermis. Dermis is made up of connective tissue containing collagen bundles, elastic fibers blood vessels, lymphatics, and nerve fibers. Dermis has two layers: 1. Papillary layer 2. Reticular layer. The collagen fibers are flexible but offer great resistance to overstretching. They prevent the skin from being torn. The elastic fibers maintain normal skin tension but also stretch to allow movement of underlying muscles and joints. The dermis also contains blood vessels that nourish the skin. The sensory receptors—primarily in the dermis—are specialized for touch, pressure, pain, hot, and cold. These receptors supply the central nervous system with information about the external environment. The Subcutaneous Layer The subcutaneous tissue layer consists of loose connective tissue that binds the skin loosely to the subjacent organs, making it possible for the skin to slide over them. This layer is composed of loose connective tissue and adipose tissue, which stores energy as fat. Adipose tissue helps thermally insulate the body from either gaining heat from the outside or losing heat from the inside. A well developed subcutaneous layer gives the body a rounded appearance and provides protective padding against external assaults. Accessory Organs of the Skin Nails are a protective covering of the distal part of fingers and toes, collectively called digits . Nails grow from epithelial cells at the base of the nail in the portion called the nail root. The cuticle is a fold of skin that hides the nail root. The whitish color of the half-moon-shaped base, or lunula, results from the thick layer of cells in this area. The cells of a nail become keratinized as they grow out over the underlying skin , called the nail bed. Hair follicles Hair follicles are the part of the epidermis that wraps around the hair itself. The hair root is the portion of the hair that exists beneath the skin where the hair shaft extends beyond the skin . Contraction of the arrector pili muscles attached to hair follicles causes the hairs to “stand on end” and goosebumps to develop. Hair color is largely due to the production of melanin by melanocytes present in the bulb. Each hair follicle has one or more oil glands called sebaceous glands, which secrete sebum. Sebum is an oily substance that lubricates the hair in the follicle and the skin. The oil secretions from sebaceous glands are acidic and retard the growth of bacteria. If the sebaceous glands fail to discharge (usually because they are blocked with keratinocytes), the secretions collect and form “whiteheads.” Over time, the sebum in a whitehead oxidizes to form a “blackhead.” Acne is an inflammation of the sebaceous glands, which most often occurs during adolescence due to hormonal changes. Sweat glands Sweat glands are numerous and present in all regions of the body. A sweat gland is a tubule that begins in the dermis and either opens into a hair follicle or, more often, opens onto the surface of the skin. Sweat glands play a role in modifying body temperature. When body temperature starts to rise, sweat glands become active. Sweat absorbs body heat as it evaporates. Once the body temperature lowers, sweat glands are no longer active