Anatomy and Physiology 1 - Skin and Membranes

Introduction to Body Membranes The term membrane refers to a thin, sheet-like structure that may have many important functions in the body. There are two main types of body membranes: epithelial membranes and connective tissue membranes. Epithelial Membranes Epithelial membranes are composed of epithelial tissue and an underlying layer of connective tissue. There are three types of epithelial membranes: Cutaneous membrane or skin: covers the surface of the body Serous membranes: line the walls of body cavities and cover organs Mucous membranes: line body surfaces that open directly to the exterior “A mucous membrane is a type of epithelial membrane that produces mucus, a thick secretion that keeps the membrane soft and moist.” Serous Membranes Serous membranes are composed of simple squamous epithelium on a connective tissue basement membrane. They have two layers: Parietal layer: lines the walls of body cavities Visceral layer: covers organs found in body cavities Examples of serous membranes include: Pleura: lines the walls of the thoracic cavity and covers the lungs Peritoneum: lines the walls of the abdominal cavity and covers the organs in the abdominal cavity Pericardium: lines the walls of the pericardial cavity and covers the heart Mucous Membranes Mucous membranes are wet membranes with a connection to the exterior. They produce mucus, which keeps the membrane soft and moist. The lamina propria is a layer of areolar tissue that connects the epithelium to underlying structures. Connective Tissue Membranes Connective tissue membranes do not contain epithelial components. They produce a lubricant called synovial fluid. Examples of connective tissue membranes include: Synovial membranes: found in the spaces between joints and in the lining of bursal sacs Integumentary Structure and Function The integumentary system consists of the skin, hair, nails, and glands. The skin is the most vulnerable organ and is exposed to radiation, trauma, infection, and chemicals. Skin Layers The skin has several layers: Layer Description Epidermis outermost and thinnest primary layer of skin Dermis deeper and thicker of the two primary skin layers Subcutaneous tissue deep to skin, also called hypodermis Epidermis The epidermis is composed of several layers of stratified squamous epithelium. It is avascular, meaning it does not have blood vessels, and is nourished by diffusion from capillaries of the papillary layer of the dermis. The layers of the epidermis are: 1. Stratum germinativum: innermost layer of cells that continually reproduce 2. Stratum spinosum: layer of cells that are held together by desmosomes 3. Stratum granulosum: layer of cells that are filled with keratin 4. Stratum lucidum: thin, transparent layer of cells 5. Stratum corneum: outermost layer of keratin-filled cells Dermis The dermis is composed largely of connective tissue. It has two layers: Papillary layer: upper layer of dermis characterized by parallel rows of tiny bumps called dermal papillae Reticular layer: deeper layer of dermis filled with a network of tough, interlacing, collagenous, and stretchable elastic fibers Accessory Organs of the Skin The accessory organs of the skin include: Hair: a nonliving keratinized structure that extends beyond the surface of the skin Nails: hard, keratinized structures that protect the tips of the fingers and toes Glands: produce sebum and sweat to help regulate body temperature and protect the skin Hair Hair is a nonliving keratinized structure that extends beyond the surface of the skin. It is composed of: Hair follicle: a tube-like structure that surrounds the hair root Hair root: the part of the hair that is hidden in the follicle Hair shaft: the visible part of the hair There are three types of hair: Lanugo: fine, soft hair that covers the body of a fetus Vellus hair: fine, soft hair that covers the body of an adult Terminal hair: coarse, thick hair that grows on the scalp, eyebrows, and eyelashes Hair Color Hair color is determined by the amount and type of melanin produced by melanocytes in the hair follicle. Melanin is a pigment that gives hair its color and helps protect it from the sun.## Hair Color and Texture The color and texture of hair are determined by the presence of melanin and the shape of the hair follicle. Eumelanin is responsible for brown and black hair, while pheomelanin produces red hair. Blond hair has an intermediate amount of pheomelanin and very little eumelanin. Gray and white hair have little or no melanin. The texture of hair is related to its cross-sectional shape: Straight hair is round Wavy hair is oval Curly hair is relatively flat Nail Structure Nails form on the dorsal surface of the tips of the fingers and toes. The nail structure consists of: 1. Nail body: covers the nail bed 2. Nail root: where nail production occurs, an epithelial fold not visible from the surface 3. Blood vessels: give the nail its characteristic pink color, but near the root, these vessels may be obscured, leaving a pale crescent known as the lunula Skin Receptors The skin contains specialized nerve endings that make it possible for the skin to act as a sense organ. These receptors include: Lamellar corpuscle (Pacini corpuscle): capable of detecting pressure Tactile corpuscle (Meissner corpuscle): capable of detecting light touch Glands in the Skin The skin contains several types of glands, including: Sweat glands: produce perspiration or sweat, which flows out through pores on the skin surface Sebaceous glands: secrete oil for the hair and skin Ceruminous glands: produce earwax (cerumen) Mammary glands: modified apocrine sweat glands that produce milk “The sweat glands are responsible for thermoregulation, excretion, and protection. They produce perspiration or sweat, which cools the surface of the skin and reduces body temperature.” Functions of the Skin The skin has several important functions, including: Protection: against infection, ultraviolet rays, and harmful chemicals Temperature regulation: the skin can release almost 3000 calories of body heat per day Sense organ activity: the skin functions as an enormous sense organ, with receptors that serve as receivers for the body Excretion: the skin rids itself of wastes, including excess vitamins, drugs, and hormones Synthesis of vitamin D: occurs when the skin is exposed to ultraviolet light Vitamin D Production Vitamin D production begins in the skin and aids in calcium absorption. Vitamin D (calcitriol) is a hormone that stimulates the uptake of calcium and phosphate from the intestines. Functions of the Skin Table Function Description Protection against infection, ultraviolet rays, and harmful chemicals Temperature regulation the skin can release almost 3000 calories of body heat per day Sense organ activity the skin functions as an enormous sense organ, with receptors that serve as receivers for the body Excretion the skin rids itself of wastes, including excess vitamins, drugs, and hormones Synthesis of vitamin D occurs when the skin is exposed to ultraviolet light Aging and the Integumentary System The skin is affected by aging, with changes including: Thinning of the epidermis Decrease in collagen Wrinkling due to decrease in elastic fibers Decrease in blood supply, causing poor ability to regulate body temperature Decrease in functioning melanocytes, leading to age spots Effects of Sunlight on Skin Sunlight can cause: Aging of the skin Replacement of normal elastic fibers with an interwoven mat of elastic-like material Decrease in collagen fibers Impaired ability for keratinocytes to divide Skin Cancer There are several types of skin cancer, including: Kaposi sarcoma (KS): caused by Kaposi sarcoma-associated herpesvirus (KSHV) Squamous cell carcinoma: a common type of skin cancer that is slow-growing Basal cell carcinoma: the most common type of skin cancer, which originates in cells at the base of the epidermis Malignant melanoma: the most serious form of skin cancer, which may develop from benign, pigmented moles or excess UV radiation Burns Burns can be classified into four degrees: 1. First-degree (partial-thickness) burns: only the surface layers of the epidermis are involved 2. Second-degree (partial-thickness) burns: involve the deep epidermal layers and always cause injury to the upper layers of the dermis 3. Third-degree (full-thickness) burns: characterized by complete destruction of the epidermis and dermis 4. Fourth-degree (full-thickness) burns: extend below subcutaneous tissue and reach muscle or bone Classification of Burns Table Degree Description First-degree only the surface layers of the epidermis are involved Second-degree involve the deep epidermal layers and always cause injury to the upper layers of the dermis Third-degree characterized by complete destruction of the epidermis and dermis Fourth-degree extend below subcutaneous tissue and reach muscle or bone Body Membranes Overview Body membranes are comprised of epithelial tissue and connective tissue. There are four types of body membranes found in the body: Mucous membranes Serous membranes Synovial membranes Cutaneous membrane Cutaneous Membrane The cutaneous membrane, also known as the skin, is composed of a top layer of epithelium and a bottom layer of connective tissue. “The cutaneous membrane is defined as the outermost layer of the body that protects the underlying tissues and organs.” The top epithelial layer is comprised of keratinized stratified squamous epithelium, in which the top superficial layers of cells are dead. The bottom connective tissue layer is made up of connective tissue proper, which contains many collagen fibers. Mucous Membranes Mucous membranes line many passageways that open up to the external environment. They are also composed of epithelial and connective tissue. “Mucous membranes are defined as membranes that line the body's internal surfaces that are exposed to the external environment and are covered with a layer of mucus.” In many areas of the body, these membranes are covered with a mucus secreted by goblet cells that are part of the epithelial layer. Mucous membranes line the: Digestive tract Respiratory tract Urinary tract Reproductive tract Serous Membranes Serous membranes line many of the body cavities that are not open to the external environment. They are comprised of simple squamous epithelium and connective tissue. “Serous membranes are defined as membranes that line the body's internal cavities and produce a serous fluid that helps to reduce friction between the layers.” Serous membranes have two layers: a parietal layer and a visceral layer. In between these two layers is the serous cavity, which contains serous fluid. Serous membranes line the: Location Description Heart Surrounds the heart Lungs Surrounds the lungs Abdominal cavity Surrounds the abdominal organs Abdominal organs Surrounds organs such as the stomach Synovial Membranes Synovial membranes line some of the joints in the body. They are composed of connective tissue with a thin layer of synovial cells. “Synovial membranes are defined as membranes that line the joints and produce a synovial fluid that helps to reduce friction between the bones.” The synovial cells secrete synovial fluid that fills the joint cavity, which helps to reduce friction. Introduction to Skin Structure The integument or cutaneous membrane, also known as human skin, is made up of two layers: the epidermis and the dermis. Epidermis The epidermis consists of four to five layers, depending on the type of skin. “The main function of the epidermis is to protect the body from external factors such as water loss, temperature, and damage from the environment.” The layers of the epidermis are: Stratum Basal: the bottom or deep layer, made up of a single layer of cells attached to a basement membrane. This layer contains: Keratinocytes: the most common cell in this layer, responsible for producing keratin, a tough structural protein that strengthens the skin and makes it almost waterproof. Melanocytes: scattered among the keratinocytes, producing the pigment melanin in reaction to exposure to ultraviolet light. Tactile cells: scattered among the keratinocytes, serving as light touch receptors. Stratum Spinosum: the next layer moving upwards, made up of daughter keratinocytes and epidermal dendritic cells. The daughter keratinocytes connect to neighboring cells via desmosomes. Stratum Granulosum: this layer is comprised of three to five layers of keratinocytes, where the process of keratinization begins. Stratum Lucidum: only found in thick skin, this layer is comprised of clear, flat, closely packed keratinocytes with no nucleus or organelles. Stratum Corneum: the top layer, comprised of dead keratinized cells. Dermis The dermis is comprised of connective tissue proper, with collagen being the most plentiful type of fiber found throughout the dermis. The dermis houses other structures such as: Blood vessels Hair follicles Sweat glands Sebaceous glands Sensory nerve endings Nail roots Erector pili muscles The dermis has two layers: Layer Composition Function Papillary loose connective tissue layer forms bumps or projections called dermal papillae that fit with the epidermal ridges of the epidermis Reticular dense connective tissue with some elastic fibers and layer many bundles of collagen fibers extends to the deeper subcutaneous layer Subcutaneous Layer The subcutaneous layer, also known as the hypodermis, is located below or deep to the dermal layer of the skin. This layer is composed of: Loose connective tissue Adipose connective tissue or fat tissue It helps bind the skin to underlying structures, acts as a cushion, protects the body, provides insulation, and provides for energy storage. Introduction to Skin Functions The skin, also known as the integument, has many important functions in the body. It provides protection, acting as a barrier against injury or trauma. Protective Functions The skin barrier helps to minimize the damage to underlying structures. For example, if you cut your finger, the skin barrier helps to prevent further damage. The skin also contains dendritic immune cells in the epidermis, which signal an immune response against foreign invaders that have entered the skin. Metabolic Regulation The skin is important in metabolic regulation. Keratinocyte cells synthesize or produce vitamin D3 due to exposure to ultraviolet radiation from the sun. This increases the absorption of calcium and phosphate in the small intestine, which are both important to bone health. “The process by which the skin produces vitamin D3 can be defined as: the skin's ability to synthesize vitamin D3 from ultraviolet radiation, which is essential for maintaining strong bones.” As little as 10 to 15 minutes of exposure to direct sunlight per day provides the body with sufficient vitamin D3. Secretion and Absorption The skin also functions in secretion and absorption. When body temperature rises, the body discharges substances from the skin in sweat. The composition of sweat can be broken down into: Water Salts A nitrogen substance called urea, which is a cellular waste product The skin can also absorb certain chemicals, for instance nicotine from nicotine patches. Temperature Regulation The skin plays a role in temperature regulation. If the body is cold, blood vessels in the dermis can narrow, which is called vasoconstriction. This diminishes the amount of blood flowing through them, causing a person to look paler. The effects of vasoconstriction and vasodilation can be seen in the following table: Condition Effect on Blood Vessels Appearance Cold Vasoconstriction (narrowing) Paler Warm Vasodilation (widening) More pinkish or reddish Sensory Reception The skin functions in sensory reception as tactile corpuscles located in the papillary layer of the dermis are touch receptors that detect cold, vibrations, and pressure. Strength and Flexibility The skin provides strength and flexibility, as the collagen fibers in skin give its strength and the elastic fibers allow the skin to stretch and recoil, so the body can move in multiple directions without tearing the skin. The main functions of the skin can be summarized as: Protection Metabolic regulation Secretion and absorption Temperature regulation Sensory reception Providing strength and flexibility Introduction to Skin Functions The skin, also known as the integument, has many important functions in the body. It provides protection, acting as a barrier against injury or trauma. Protective Functions The skin barrier helps to minimize the damage to underlying structures. “The skin's protective function is defined as its ability to prevent external factors, such as pathogens and physical damage, from entering the body and causing harm.” Some key components of the skin's protective function include: Dendritic immune cells in the epidermis, which signal an immune response against foreign invaders Melanocyte cells, located in the epidermis, which produce the pigment melanin in reaction to exposure to ultraviolet light Metabolic Regulation The skin is important in metabolic regulation as keratinocyte cells synthesize or produce vitamin D3 due to exposure to ultraviolet radiation from the sun. The following table summarizes the role of vitamin D3 in the body: Substance Function Vitamin D3 Increases the absorption of calcium and phosphate in the small intestine, which are both important to bone health Secretion and Absorption The skin also functions in secretion and absorption. Some key examples include: Sweat, which consists of water, salts, and a nitrogen substance called urea, a cellular waste product Nicotine from nicotine patches, which can be absorbed through the skin Temperature Regulation The skin plays a role in temperature regulation. The following table summarizes the two main mechanisms of temperature regulation in the skin: Mechanism Description Vasoconstriction Narrowing of blood vessels in the dermis, which diminishes the amount of blood flowing through them, causing a person to look paler Vasodilation Increase in diameter of blood vessels in the dermis, allowing more blood to flow through them, causing a person to look more pinkish or reddish Sensory Reception The skin functions in sensory reception as tactile corpuscles located in the papillary layer of the dermis are touch receptors that detect cold, vibrations, and pressure. Strength and Flexibility The skin provides strength and flexibility, as the collagen fibers in skin give its strength and the elastic fibers allow the skin to stretch and recoil, so the body can move in multiple directions without tearing the skin. Some key benefits of the skin's strength and flexibility include: Allowing the body to move freely without restriction Providing protection against external forces and injuries Introduction to Skin Functions The skin, also known as the integument, has many important functions in the body. It provides protection, acting as a barrier against injury or trauma. Protective Functions The skin barrier helps to minimize the damage to underlying structures. The skin's protective function can be thought of as a shield that guards the body against external harm, much like a suit of armor protects a knight in battle. This shield is made up of several layers, including the epidermis, which contains dendritic immune cells that signal an immune response against foreign invaders. Regulation of Metabolic Processes The skin is important in metabolic regulation. Keratinocyte cells synthesize or produce vitamin D3 due to exposure to ultraviolet radiation from the sun. This increases the absorption of calcium and phosphate in the small intestine, which are both important to bone health. The following table summarizes the role of the skin in metabolic regulation: Substance Function Vitamin D3 Increases absorption of calcium and phosphate Calcium Essential for bone health Phosphate Essential for bone health Secretion and Absorption The skin also functions in secretion and absorption. When body temperature rises, the body discharges substances from the skin in sweat. Sweat consists of: Water Salts A nitrogen substance called urea, which is a cellular waste product The skin can also absorb certain chemicals, for instance nicotine from nicotine patches. Temperature Regulation The skin plays a role in temperature regulation. If the body is cold, blood vessels in the dermis can narrow, which is called vasoconstriction. This diminishes the amount of blood flowing through them, causing a person to look paler. The blood vessels in the dermis can also increase in diameter, called vasodilation, allowing more blood to flow through them. The following table summarizes the role of the skin in temperature regulation: Condition Response Cold body temperature Vasoconstriction Warm body temperature Vasodilation Sensory Reception The skin functions in sensory reception as tactile corpuscles located in the papillary layer of the dermis are touch receptors that detect: Cold Vibrations Pressure Strength and Flexibility The skin provides strength and flexibility, as the collagen fibers in skin give its strength and the elastic fibers allow the skin to stretch and recoil, so the body can move in multiple directions without tearing the skin. The skin's ability to stretch and recoil can be thought of as a rubber band that can be stretched to its limits and then return to its original shape, allowing the body to move freely without restriction.