Chapter 14: The skin Flashcards
The skin
The skin completely covers the body and is continuous with the membranes lining the body orifices. It:
- protects the underlying structures from injury and from invasion by microbes
- contains sensory nerve endings that enable discrimination of pain, temperature and touch
- is involved in the regulation of body temperature.
Structure of the skin
The skin is the largest organ in the body and has a surface area of about 1.5–2 m2 in adults. In certain areas, it contains accessory structures: glands, hair, and nails. There are two main layers; the epidermis, which covers the dermis. Between the skin and underlying structures is a subcutaneous layer composed of areolar tissue and adipose (fat) tissue.
Epidermis
The epidermis is the thin, outer layer of the skin that is visible and works to provide protection for the body. This part of the skin does not contain any blood vessels and is, therefore, dependent on the dermis, which is the layer of the skin located directly underneath the epidermis, to provide access to nutrients and dispose of waste.
Epidermis (protection)
All layers of the skin, including the epidermis, are responsible for the protection of the body, including internal organs, muscles, nerves, and blood vessels. Additional roles of the epidermis include:
- Production of new skin cells
- Production of melanin to give color to the skin, as well as reduce the absorption and impact of UV radiation
- Physical protection of the body
- Immune protection of the body
Healthy epidermis
Healthy epidermis depends upon three processes being synchronized:
- desquamation (shedding) of the keratinized cells from the surface
- effective keratinization of cells approaching the surface
- continual cell division in the deeper layers with newly formed cells being pushed upwards to the surface.
Factors affecting the skin color
Skin color is affected by various factors.
- Melanin, a dark pigment derived from the amino acid tyrosine and secreted by melanocytes in the deep germinative layer, is absorbed by surrounding epithelial cells. The amount is genetically determined and varies between different parts of the body, between people of the same ethnic origin, and between ethnic groups.
- Normal saturation of hemoglobin and the amount of blood circulating in the dermis give white skin its pink color. When oxygen saturation is very low, the skin may appear bluish (cyanosis).
- Excessive levels of bile pigments in blood and carotenes in subcutaneous fat give the skin a yellowish color.
Dermis
• The dermis is tough and elastic. It is formed from connective tissue, and the matrix contains collagen fibers interlaced with elastic fibers. Rupture of elastic fibers occurs when the skin is overstretched, resulting in permanent striae, or stretch marks, that may be found in pregnancy and obesity. Collagen fibers bind water and give the skin its tensile strength, but as this ability declines with age, wrinkles develop. Fibroblasts, macrophages, and mast cells are the main cells found in the dermis. Underlying its deepest layer is the subcutaneous layer containing areolar tissue and varying amounts of adipose (fat) tissue. The structures in the dermis are:
- blood and lymph vessels
- sensory nerve endings
- sweat glands and their ducts
- hairs, arrector pili muscles, and sebaceous glands.
Blood and lymph
Arterioles form a fine network with capillary branches supplying sweat glands, sebaceous glands, hair follicles, and the dermis. Lymph vessels form a network throughout the dermis.
Sensory nerve endings
• Sensory receptors (specialized nerve endings) sensitive to touch, temperature, pressure, and pain are widely distributed in the dermis. Incoming stimuli activate different types of sensory receptors; for example, the Pacinian corpuscle is sensitive to deep pressure. The skin is an important sensory organ through which individuals receive information about their environment. Nerve impulses, generated in the sensory receptors in the dermis, are transmitted to the spinal cord by sensory nerves. From there, impulses are conducted to the sensory area of the cerebrum where the sensations are perceived
Sweat glands
- These are widely distributed throughout the skin and are most numerous in the palms of the hands, soles of the feet, axillae, and groins. They are formed from epithelial cells. The bodies of the glands lie coiled in the subcutaneous tissue. There are two types of sweat glands. Eccrine sweat glands are the more common type and open onto the skin surface through tiny pores, and the sweat produced here is a clear, watery fluid important in regulating body temperature. Apocrine glands open into hair follicles and become active at puberty. They may play a role in sexual arousal. These glands are found, for example, in the axilla. Bacterial decomposition of their secretions causes an unpleasant odor. A specialized example of this type of gland is the ceruminous gland of the outer ear, which secretes earwax.
- The most important function of sweat is the regulation of body temperature. Excessive sweating may lead to dehydration and serious depletion of sodium chloride unless intake of water and salt is appropriately increased. After 7–10 days’ exposure to high environmental temperatures, the amount of salt lost is substantially reduced but water loss remains high.
Hairs
- These grow from hair follicles, downgrowth’s of epidermal cells into the dermis or subcutaneous tissue. At the base of the follicle is a cluster of cells called the hair papilla or bulb. The hair is formed by the multiplication of cells of the bulb and as they are pushed upwards, away from their source of nutrition, the cells die and become keratinized. The part of the hair above the skin is the shaft and the remainder, the root.
- Hair color is genetically determined and depends on the amount and type of melanin present. White hair is the result of the replacement of melanin by tiny air bubbles.
Arrector pili
These are little bundles of smooth muscle fibers attached to the hair follicles. Contraction makes the hair stand erect and raises the skin around the hair, causing ‘goose flesh’. The muscles are stimulated by sympathetic nerve fibers in response to fear and cold. Erect hairs trap air, which acts as an insulating layer. This is an efficient warming mechanism, especially when accompanied by shivering, i.e., involuntary contraction of skeletal muscles.
Sebaceous glands
- These consist of secretory epithelial cells derived from the same tissue as the hair follicles. They secrete an oily antimicrobial substance, sebum, into the hair follicles and are present in the skin of all parts of the body except the palms of the hands and the soles of the feet. They are most numerous in the scalp, face, axillae, and groins. In regions of transition from one type of superficial epithelium to another, such as lips, eyelids, nipple, labia minora, and glans penis, there are sebaceous glands that are independent of hair follicles, secreting sebum directly onto the surface.
- Sebum keeps the hair soft and pliable and gives it a shiny appearance. On the skin it provides some waterproofing and acts as a bactericidal and fungicidal agent, preventing infection. It also prevents drying and cracking of the skin, especially on exposure to heat and sunlight. The activity of these glands increases at puberty and is less at the extremes of age, rendering the skin of infants and older adults prone to the effects of excessive moisture (maceration).
Nails
Human nails are equivalent to the claws, horns and hooves of animals. Derived from the same cells as epidermis and hair these are hard, horny keratin plates that protect the tips of the fingers and toes.
•The root of the nail is embedded in the skin and covered by the cuticle, which forms the hemispherical pale area called the lunula.
•The nail plate is the exposed part that has grown out from the nail bed, the germinative zone of the epidermis.
•Fingernails grow more quickly than toenails and growth is faster when the environmental temperature is high.
Functions of the skin (protection)
•The skin forms a relatively waterproof layer, provided mainly by its keratinized epithelium, which protects the deeper, more delicate structures. As an important non-specific defense mechanism, it acts as a barrier against:
-invasion by micro-organisms
-chemicals
-physical agents, e.g., mild trauma, ultraviolet light
-dehydration
•The epidermis contains specialized immune cells called dendritic (Langerhans) cells, which are a type of macrophage. They phagocytose intruding antigens and travel to lymphoid tissue, where they present antigen to T-lymphocytes, thus stimulating an immune response.
•Abundant sensory nerve endings in the dermis enable perception, discrimination, and location of internal and external stimuli. This allows responses to change in the environment, e.g., by reflex action (withdrawal) to unpleasant or painful stimuli, protecting it from further injury.
•The pigment melanin protects against harmful ultraviolet rays in sunlight.
Regulation of body temperature
Body temperature remains constant around 36.8°C across a wide range of environmental temperatures ensuring that the optimal range for enzyme activity required for metabolism is maintained. In health, variations are usually limited to between 0.5 and 0.75°C, although it rises slightly in the evening, during exercise, and in women just after ovulation. To maintain this constant temperature, a negative feedback system regulates the balance between heat produced in the body and heat lost to the environment.
Heat production
When metabolic rate increases, body temperature rises, and when it decreases body temperature falls. Some of the energy released during metabolic activity is in the form of heat; the most active organs produce the most heat. The principal organs involved are:
- skeletal muscles – contraction of skeletal muscles produces a large amount of heat and the more strenuous the muscular exercise, the greater the heat produced. Shivering also involves skeletal muscle contraction, which increases heat production when there is the risk of body temperature falling below normal.
- the liver is very metabolically active, which produces heat as a by-product. Metabolic rate and heat production are increased after eating.
- the digestive organs that generate heat during peristalsis and the chemical reactions involved indigestion.
Heat loss
- Most heat loss from the body occurs through the skin. Small amounts are lost in expired air, urine, and feces. Only heat loss through the skin can be regulated; heat lost by the other routes cannot be controlled.
- Heat loss through the skin is affected by the difference between body and environmental temperatures, the amount of the body surface exposed and the type of clothes worn. Air insulates against heat loss when trapped in layers of clothing and between the skin and clothing. For this reason, several layers of lightweight clothes provide more effective insulation against low environmental temperatures than one heavy garment.
Mechanisms of heat loss
In radiation, the main mechanism exposed parts of the body radiate heat away from the body. In evaporation, the body is cooled as body heat converts the water in sweat to water vapor. In conduction, clothes and other objects in direct contact with the skin take up heat. In convection, air passing over the exposed parts of the body is heated and rises, cool air replaces it, and convection currents are set up. Convection also cools the body when clothes are worn, except when they are windproof.
Control of the body temperature
•The temperature regulating center in the hypothalamus is sensitive to the temperature of circulating blood. This center responds to decreasing temperature by sending nerve impulses to:
-arterioles in the dermis, which constrict decreasing blood flow to the skin
-skeletal muscles stimulating shivering.
•As heat is conserved, body temperature rises and when it returns to the normal range again the negative feedback mechanism is switched off.
•Conversely when body temperature rises, heat loss is increased by dilation of arterioles in the dermis, increasing blood flow to the skin, and stimulation of the sweat glands causing sweating until it falls into the normal range again when the negative feedback mechanism is switched off.
The activity of the sweat glands
- When body temperature is increased by 0.25 to 0.5°C the sweat glands secrete sweat onto the skin surface. Evaporation of sweat cools the body but is slower in humid conditions.
- Loss of heat from the body by evaporation of water through the skin and expired air still occurs even when the environmental temperature is low. This is called insensible water loss (around 500 mL per day) and is accompanied by insensible heat loss.
Regulation of blood flow through the skin
- The amount of heat lost from the skin depends largely on blood flow through dermal capillaries. As body temperature rises, the arterioles dilate, and more blood enters the capillary network in the skin. The skin is warm and pink in color. In addition to increasing the amount of sweat produced, the temperature of the skin rises, and more heat is lost by radiation, conduction, and convection.
- If the environmental temperature is low or if heat production is decreased, the arterioles in the dermis are constricted. This reduces blood flow to the body surface, conserving heat. The skin appears paler and feels cool.
Fever
•This is often the result of infection and is caused by the release of chemicals (pyrogens) from inflammatory cells and invading bacteria. Pyrogens, e.g., interleukin 1, act on the hypothalamus, which releases prostaglandins that reset the hypothalamic thermostat to a higher temperature. The body responds by activating heat-promoting mechanisms, e.g., shivering and vasoconstriction until the new higher temperature is reached. When the thermostat is reset to the normal level, heat-loss mechanisms are activated. There is profuse sweating and vasodilation accompanied by warm, pink (flushed) skin until body temperature falls to the normal range again.
Hypothermia
- This means a core (e.g., rectal) temperature below 35°C. At a core temperature below 32°C, compensatory mechanisms that restore body temperature normally fail, e.g., shivering is replaced by muscle rigidity and cramps, vasoconstriction fails, and blood pressure, pulse, and respiration rates fall. Confusion and disorientation occur. Death usually occurs when the temperature falls below 25°C.
- Individuals at the extremes of age are prone to hypothermia as temperature regulation is less effective in young and older adults.
