Skin Flashcards
Skin
Covers entire surface of the body. Forms s protective, elastic, waterproof sensitive covering. Continuous with mucous membranes lining the body. Surface area of 1.5-2m2 in adults. Combination of epithelial and connective tissues.
Structure:
Epidermis - surface epithelial layer, keratinised stratified squamous epithelium.
Dermis- underlying connective tissue layer, fibrous connective tissue ( some elastic fibres).
Most parts of the body have 3rd layer between skin and underlying structures.
Sub cutaneous layer - made up of loose are play connective tissue and adipose tissue.
Other structures associated with skin are hairs, nail , sweat glands and sebaceous glands.
Epidermis
Keratinised stratified squamous epithelium.
Most superficial layer.
0.07mm trunk, 0.8mm palm of hands, 1.4mm soles of feet.
No blood vessels or nerve endings . Receives nutrition from underlying connective tissue.
Pierced by ducts of sweat glands and hairs
Made up of several layers.
Epidermis
Germinative layer
- Stratum germinativum - one cell thick. Cells are cuboidal or columnar. Mitosis constantly happening. Junction between this layer and underlying connective tissue follows a wavy form. Ridges of epithelium = rete pegs and ridges of dermis= dermal papillae. This arrangement is to aid nutrition of epithelial cells and stabilise layers.
- Stratum spinosum - 20 cells thick. Cells in deepest part of this layer can still divide. Cells gradually move up through layer as they mature. Cells many different shapes but gradually become flattened as move nearer surface. Cells in this layer held together by inter cellular bridges known as desmosomes. Provide strong adhesion between epithelial cells.
Desmosomes
Intercellular bridges holding 2 epithelial cells tightly together. Found in stratum spinosum.
Structure: intercellular region of epithelial cell membranes is bridged by the filaments with a band of darkly staining material sandwiched between. This is intercellular contact layer. He epithelial cell membranes are coated on inner aspect with layer of dense protein known as attachment plaque. Many fine filaments inserted into these attachment plaques( tono filaments).
Hemidesmosomes- structures providing strong adhesion between epithelium and connective tissue. An exception is junctional epithelium. Epithelium and connective tissue must be held together as epithelium is avascular.
Stratum granulosum
Stratum lucidum
Stratum corneum
STRATUM GRANULOSUM - layer is 3-5 cells wide. The cells have now become flattened and contain granules in their cytoplasm of protein, kerato-hyalin.
STRATUM LUCIDUM - cells lose their nuclei and their cell membrane and undergo various chemical changes. Microscopically this layer looks clear.
STRATUM CORNEUM- cells have become completely flat with no nucleus or membrane and called squames in which cytoplasm replaced protein keratin. Squames constantly shed from surface. Keratin layer thickest in areas subject to most wear and tear. Keratin contributes to waterproof quality of skin. Absorbs some water but will never go past keratin .
Maintaining healthy epidermis
Intact epidermis is first line of body defence against infection and chemical and physical trauma. Maintenance of a healthy epidermis depends on 3 main processes.
- desquamation of keratinised cells from surface
- adequate keratinisation of cells approaching surface
- continual cell divison in deeper layers
These processes must be carefully balanced
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Dermis
Consists of layer of thick fibrous connective tissue. Both collagen and elastic fibres present plus normal ct cells. Other main structures:
BLOOD/ LYPMPH VESSELS- arterioles form fine network with capillary network supplying glands , hair follicles, dermis.
SENSORY NERVE ENDINGS- sensitive to touch, temp, pressure, pain widely distributed. Pascinian corpuscle sensitive to deep pressure. Nerve impulses transmitted to spinal chord where sensations perceived.
SWEAT GLANDS - widely distributed. Most numerous in palms of hands, soles of feet. Formed from epithelial cells. Two type, eccrine- more common, open up in skin surface as tiny pores. Sweat produced here is clearer watery fluid important in regulating body temp. Apocrine open into hair follicles , activate at puberty. Play role in sexual arousal.
HAIRS - grow outwards from hair follicles. Down growth of epithelial cells into dermis. At base of follicle is cluster of cells called hair papilla. Hair formed by multiplication of cells and as hairs pushed up away from nutrition they die and become keratinised. Shaft and root.
ARRECTOR PILI - little bundles of smooth muscle fibres attached to hair follicles. Contraction makes hairs stand up and skin around it stimulated by cold and fear by sympathetic nerve fibres. Erect hairs trap air.
SEBACEOUS GLANDS - consist of secretory epithelial cells derived from same tissue as hair follicles. Secrete oily antimicrobial substance , sebum into hair follicles, numerous in scalp, face, groin. Not on palms of soles . Sebum keeps hair soft and plainly and gives shiny appearance. Waterproofs skin and acts as bactericidsl and fungicidal agent.
Colour of skin
Affected by various factors.
Melanin - pigment secreted by melanocytes. These are cells found in Germinative layer of epiddermis. Pigment absorbed by nearby epithelial cells. Amount of melanin is genetically determined and varies between ethnic groups. Exposure to sunlight increases production of melanin. Helps protect against effects of melanin.
Amount of blood circulating in dermis- plus the %saturation of haemoglobin. These combine to give skin a pink colour . Depending on melanin levels.
Presence of bile pigments - excessive levels of bile pigments can build up in blood and subcutaneous layer giving yellow colour to skin.
Functions of skin
- Protection- skin protects against invasion of microorganisms, chemicals, trauma , uv light, dehydration.
- Regulation of body temp - 36.8
- Sensory - touch, pressure, pain, temp
- Absorption- drugs / chemicals. Pass through epidermis and into blood vessels in dermis. Eg painkillers , nicotine .
- Excretion- minor role eg sodium chloride, urea, garlic
- Manufacture of vitamin D- 7-dehydrocholesterol is a fatty substance in skin which is converted to vitamin D by action of UV light. Wouldn’t penetrate to dermis.
Regulation of body temperature
Core body temperature remains fairly constant at 36.8 ensuring optimal range for enzyme activity required for metabolism. In health variations usually limited to 0.5-0.75 although temp raises slightly in the evening, during exercise and just after ovulation. To maintain this constant temp a negative feedback system regulates balance between heat production and heat loss.
Heat production
When metabolic rate increases body temperature rises and vice versa. Some things of the energy release during metabolic activity is in the form of heat, the most active organs produce the most heat. Principal organs involved:
- skeletal muscle contraction, shivering where risk of body temp falling.
- liver very active metabolically and so generates considerable heat as by product . Metabolic rate and heat increased after eating.
- digestive organs generate heat during peristalsis and chemical reactions involved in digestion.
Heat loss
Most heat loss occurs through skin but some in urine, faeces, expired air but only can be controlled through skin. Heat loss through skin can be affected by difference between body and environmental temps, amount of body exposed and clothes. Air insulates when trapped in layers.
Mechanisms:
Radiation- when exposed parts of body radiate heat away.
Evaporation- evaporation of sweat
Conduction- when clothes in contact take up heat.
Convection- when air passing over exposed parts of body is heated and rises cool air replaces it, setting up convection currents.
Control of body temp
Temperature regulating centre in hypothalamus sensitive to temp of circulating blood. This centre responds to decreasing temp by sending nerve impulses to: arterioles in dermis which constrict and skeletal muscles stimulating shivering. As heat conserved body temp rises , when normal negative feedback mechanism switched off.
Activity of sweat glands - when body temp increases by 0.25-0.5 sweat glands secrete sweat onto skin surface. Evaporation then cools body. Loss of body heat via evaporation of water through skin and expired air occurs when environmental temp is low and called insensible water loss(500ml per day).
Regulation of blood flow through skin- amount of heat loss depends on blood flow through dermal capillaries. As body temp rises arterioles dilate and more blood enters capillaries in skin. Skin warm and pink in colour. Opposite when temp is low.
Fever
Often result of infection and caused by release of chemicals ( pyrogens) from inflammatory cells and invading bacteria. Pyrogens ie interleukin 1 acts on hypothalamus which releases prostaglandins that reset hypothalamic thermostat to higher temp. Body responds by activating heat promoting mechanisms eg shivering and vasoconstriction until new temp reached. When the thermostat is reset to normal level heat loss mechanisms activated . There’s profuse sweating and vasoconstriction and pink skin. This shivering ( chill) followed by profuse sweating known as rigor.
Hypothermia
Defined as core temperature below 35 degrees. At a core temp below 32 compensatory mechanisms that normally restore body temp fail eg shivering replaced by muscle rigidity and cramps, vasoconstriction fails and blood pressure , pulse and respiration rates fall. Death occurs below 25.
Extremes of age prone
