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Skin and Membranes
Chapter 7
Membrane :
o The term membrane refers to a thin, sheet like structure that covers and protects the body surface, line body cavities, and cover the inner surfaces of the hollow organs such as the digestive, reproductive, and respiratory passageways. o Some membranes anchor organs to each other or to bones, and others cover the internal organs. o In certain areas of the body, membranes secrete lubricating fluids that reduce friction during organ movements such as the beating of the heart or lung expansion and contraction. Membrane lubricants also decrease friction between bones in joints.
Classification of body membranes
There are two major categories or types of body membranes
- Epithelial membranes —composed of epithelial tissue and an underlying layer of connective tissue
- Connective tissue membranes —composed exclusively of various types of connective tissue, no epithelial tissue present.
Epithelial membranes
There are three types of epithelial tissue membranes in the body. a. Cutaneous membrane ( The skin) - The cutaneous membrane, or skin, is the primary organ of the integumentary system. It is the largest and most visible organs of the body. Skin composes 16% of the body weight. b. Serous membranes — Found only on surfaces within closed cavities. Example- o Pleura- covering of the lungs in the thoracic cavity o Peritoneum- covering of the abdominal organs in abdominal cavity o Pericardium- covering of the heart Examples of serous membrane : o Pleura— covering of the lungs in the thoracic cavity. Parietal layer covers the wall of the chest cavity and visceral layers line the lungs. o Peritoneum—covers our abdominal organs in abdominal cavity. Parietal and visceral layers line walls of abdominal cavity and cover the organs in that cavity. o Pericardium- covering of the heart. The heart is surrounded by a fibrous sac lined with a thin, slippery membrane that doubles back on itself to form a lubricating, fluid-filled pocket around the heart. Serous membranes secrete a thin, watery fluid between two layers and helps reduce friction and serves as a lubricant when organs rub against one another and against the walls of the cavities that contain them.
Diseases o Pleurisy —inflammation of the serous membranes that line the chest cavity and cover the lungs. o Pleural effusion- excessive collection of pleural fluid in the pleural space o Peritonitis —inflammation of the serous membranes in the abdominal cavity that line the walls and over the abdominal organs. o Ascites - Excessive collection of peritoneal fluid. c. Mucous membranes- o Line body surfaces that open directly to the exterior. For example-mouth cavity, vagina, anus, nasal cavity. o Produce mucus, a thick secretion that keeps the membranes soft and moist.
These two layers of the skin are supported by a thick layer of loose connective tissue and fat called subcutaneous tissue , or the hypodermis. Function of subcutaneous tissue : Fat in the subcutaneous layer insulates the body from extremes of heat and cold. It also serves as a stored source of energy for the body and can be used as a food source if required. In addition, the subcutaneous tissue acts as a shock-absorbing pad and helps protect underlying tissues from injury caused by bumps and blows to the body surface.
Epidermis
Outermost and thinnest primary layer of skin The tightly packed epithelial cells of the epidermis are arranged in up to five distinct layers. Stratum germinativum—innermost (deepest) layer of cells that continually reproduce; the new cells move toward the surface. These new cells "specialize" in ways that increase their ability to provide protection for the body tissues that lie below them. It enables the skin to repair itself if it is injured. It is sometimes called the pigment layer because it is responsible for the production of a pigment called melanin by the cell called melanocyte that gives color to the skin. Stratum corneum—outermost layer of keratin-filled (tough, waterproof protein) cells. These dry, dead cells filled with keratin "flake off" by the thousands onto our clothes and bedding, into our bathwater, and onto things we handle. Millions of epithelial cells reproduce daily to replace the millions shed. Skin color changes o Pink flush indicates increased blood volume or increased blood oxygen o Cyanosis—bluish gray color indicates decreased blood oxygen level o Vitiligo—patchy light skin areas resulting from acquired loss of epidermal melanocytes. A hereditary condition called albinism is characterized by a partial or total lack of melanin pigment in the skin and eyes. Causes includes genetic, autoimmunity or endocrine disorder. o Increased skin pigmentation caused by hormonal changes in pregnant women o Freckles—small, flat macules' common normal skin pigment variation
Dermis
o Deeper and thicker of the two primary skin layers; composed largely of connective tissue and scattered epithelial cells with many fibers in between. Some of the fibers are tough and strong (collagen or white fibers), and others are stretchable and elastic (elastic or yellow fibers). o The upper region of the dermis is characterized by parallel rows of peglike projections called dermal papillae.
o Thick skin has parallel friction ridges in dermis and no hairs. On the palms and soles, distinct rows of dermal papillae form the roughly parallel ridges that help us to walk upright without slipping and to make and hold tools. o Thin skin has irregular, shallow grooves and hair o Deeper area of dermis is filled with network of tough collagenous and stretchable elastic fibers. Number of elastic fibers decreases with age and contributes to wrinkle formation o Striae—"stretch marks"; elongated marks caused by overstretching of the elastic fibers of skin o Dermis also contains nerve endings, muscle fibers, hair follicles, sweat and sebaceous glands, and many blood vessels. Birthmarks — Developmental malformation of dermal blood vessels can result in pigmented birthmarks in significant numbers of newborns malformation of dermal blood vessels. For example- Strawberry hemangioma- a collection of dilated vessels that may initially appear as a bruise at birth and then grow rapidly during the first year into a bright red nodule called a strawberry hemangioma. A vast majority of these birthmarks shrink, fade, and disappear without treatment of any kind by age 7
Appendages of the skin
Hair o The human body is covered with millions of hairs. o Soft hair of fetus and newborn called lanugo. But soon after birth the lanugo is lost and replaced by new hair that is stronger and more pigmented. o Although only a few areas of the skin are hairless—notably the lips, the palms of the hands, and the soles of the feet—most body hair remains almost invisible. Hair is most visible on the scalp, eyelids, and eyebrows. o Hair follicle and hair papilla: Hair growth begins when cells of the epidermal layer of the skin grow down into the dermis, forming a small tube called the hair follicle. Hair growth begins from a small, cap-shaped
SKIN GLANDS
Two main types: sweat or sudoriferous , and sebaceous gland. A. Sweat, or sudoriferous glands : Sweat glands are the most numerous of the skin glands. They can be classified into two groups— eccrine and apocrine —based on type of secretion and location Eccrine sweat glands Most numerous, important, and widely spread sweat gland throughout the body. Produce perspiration or sweat, which flows out through pores on skin surface Function throughout life. Sweat assists in the elimination of waste products such as ammonia and uric acid. In addition to elimination of waste, sweat plays a critical role in helping the body maintain a constant temperature. Apocrine sweat glands Found primarily in axilla and around genitalia Secrete a thicker, milky secretion quite different from eccrine perspiration Apocrine glands enlarge and begin to function at puberty. Breakdown of secretion by skin bacteria produces odor B. Sebaceous glands o Secrets oil, or sebum and they grow where hairs grow. Their tiny ducts open into hair follicles so that their secretion, called sebum, lubricates the hair and skin. o It prevents drying and cracking of the skin. o Level of secretion increases during adolescence and amount of secretion regulated by sex hormones. o Frequently sebum accumulates in and enlarges some of the ducts of the sebaceous glands, forming white pimples. This sebum often darkens, forming a blackhead o Acne vulgaris — Acne vulgaris results from blocked sebaceous glands that become inflamed or infected. Formation of acne lesions can be minimized by careful cleansing of the skin to remove sebaceous plugs and to inhibit anaerobic skin bacteria FUNCTIONS OF THE SKIN o Protection — The skin acts as a “first line of defense” against a multitude of hazards. It protects us against the daily invasion of deadly microbes. The tough, keratin-filled cells of the stratum corneum also resist the entry of harmful chemicals and protect against physical tears and cuts. Because it is waterproof, keratin also protects the body from excessive fluid loss. Melanin in the pigment layer of the skin prevents the sun’s harmful UV rays from penetrating the interior of the body.
o Temperature regulation-Maintains body temperature by r egulation of sweat secretion and by regulation of flow of blood close to the body surface o Sense organ activity- Skin can detect sensations of light touch, pressure, pain, heat, and color o Excretion - Excretion of substances in sweat can influence the amounts of certain ions (such as sodium) and waste products (such as uric acid, ammonia, and urea) that are present in the blood. Excess vitamins, drugs, and even hormones in the blood can also be excreted onto the skin by sweat. o Synthesis of vitamin D - Synthesis of vitamin D occurs when the skin is exposed to UV light—usually from the sun. When UV light penetrates the skin, a precursor substance in skin cells forms, then is transported to the liver and kidneys where it is converted into an active form of vitamin D. DISORDERS OF THE SKIN Any disorder of the skin can be called a dermatosis, which simply means "skin condition. Many dermatoses involve inflammation of the skin, or dermatitis Skin lesions —any measurable variation from the normal structure. Lesions are not necessarily signs of disease; they may be benign variations that do not constitute a disorder. Lighting the skin from the side with a penlight is a method used to determine the category of a lesion: elevated, flat, or depressed. Elevated lesions —cast a shadow outside their edges o Papule—small, firm raised lesion (wart) o Plaque—large raised lesion o Vesicle—thin walled blister filled with fluid. o Pustule—elevated pus-filled lesion o Crust—scab; area with dried blood and exudate o Wheal (hive)—raised, firm lesion with a light center (allergic reaction) Flat lesions —do not cast a shadow o Macule—flat, discolored region (freckle) o Patch- Macules larger than 1 cm (vitiligo) Depressed lesions - cast a shadow within their edges o Excoriation—missing epidermis exposing dermis, as in a scratch wound o Ulcer—craterlike lesion caused by disintegration of skin (bed sore) o Fissure—deep crack or break from epidermis to dermis (athlete foot)
Estimating body surface area using the "rule of nines" in adults:
The severity of a burn is determined by the depth of the injury, as well as by the amount of body surface area affected. The "rule of nines" is one of the most frequently used methods of determining the extent of a burn injury (body surface area that has been burnt). With this technique the body is divided into 11 areas of 9% each, with the area around the genitals representing the additional 1% of body surface area
Skin Infections:
Impetigo —highly contagious staphylococcal or streptococcal infection most often in young children. It starts as a reddish discoloration, or erythema, but soon develops into vesicles and yellowish crusts. Tinea —fungal infection (mycosis) of the skin; several forms occur. Ringworm, jock itch, and athlete's foot are classified as tinea. Signs of tinea include erythema, scaling, and crusting. Antifungal agents usually stop the
acute infection. Recurrence can be avoided by keeping the skin dry because fungi require a moist environment to grow. Warts —benign neoplasm caused by papillomavirus. How ever, some warts do transform and become malignant. Boils— furuncles; staphylococcal infection in hair follicles. A group of untreated boils may fuse into even larger pus- filled lesions called carbuncles. Scabies — Scabies is a contagious skin condition caused by the itch mite (Sarcoptes scabiei). Transmitted by skin-to-skin contact.
Vascular and inflammatory skin disorders
Decubitus ulcers (bedsores/pressure sore) develop when pressure slows down blood flow to local areas of the skin covering bony prominences such as the heel. Ulcers form and infections develop because lack of blood flow causes tissue damage or death. Frequent changes in body position and soft support cushions help prevent decubitus ulcers. Urticaria or hives — Hives, or urticaria is a common condition characterized by raised red lesions called wheals caused by fluid loss from blood vessels. Urticaria is often associated with severe itching. Hypersensitivity or allergic reactions to drugs or food, physical irritants, and systemic diseases are common causes of urticarial. Scleroderma — Scleroderma is an autoimmune disease that affects the blood vessels and connective tissues of the skin characterized by hardening of the skin. Scleroderma begins as an area of mild inflammation that later develops into a patch of yellowish, hardened skin. Psoriasis —Psoriasis is a common, chronic, and often lifelong skin disease that affects 1% to 3% of the population. It is characterized by silvery white, scale like plaques that may remain fixed on the skin for months. Psoriasis is thought to have a genetic basis and tends to affect skin on the elbows, knees, and scalp most often. Eczema —common inflammatory condition characterized by papules, vesicles, and crusts ; not a disease itself but a symptom of an underlying condition. For example, an allergic reaction called contact dermatitis can progress to become eczematous.
