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General Facts about Skin
Skin protects internal organs .
It contains sensory nerve endings of pain, temperature and touch.
It is involved in regulation of body temperature.
Structure of skin
Skin is the largest organ in body and has a surface area of 1.5 to 2 m2 in adults and it includes glands,nails,hair.
There are 2 main layers:epidermis and dermis.
Between skin and underlying structures is subcutaneous layer composed of areolar tissue and adipose tissue.
The epidermis is composed of stratified keratinised squamous epithelium. It varies in thickness in different parts of body.It is thickest on palms and soles.Epidermis has no blood vessels and nerve endings.Deeper layers of epidermis are bathed in interstitial fluid from dermis.It provides oxygen and nutrients and drains away as lymph.
Several layers of epidermis cells extend from deepest germinative layer to stratum corneum.Stratum corneum is a thick horny layer.Surface cells are flat,thin,non nucleated,dead cells.Cytoplasm in these cells are replaced by fibrous protein keratin.
Maintenance of healthy epidermis depends on synchronization of 3 processes:
*keratinization of cells coming to surface.
*cell division in deeper layers with new cells being pushed to surface. Sweat gland ducts,sebaceous gland secretions, hairs pass through epidermis to come to surface.
Epidermis surface is ridged by cell projections in dermis called papillae.
Skin color is affected by various factors.
*Melanin,a dark pigment is absorbed by surrounding epithelial cells.
*Normal haemoglobin saturation and blood circulating in dermis give pink color to white skin.
*Excessive bile pigment in blood and carotenes in fat of subcutaneous give yellow color to skin.
Dermis is a tough and elastic layer.Connective tissue forms it.The matrix has collagen fibres which is interlaced with elastic fibres.Elastic fibre rupture occurs when skin is overstretched. Collagen fibres bind water and give tensile strength to skin.Fibroblasts, macrophages and mast cells are main cells in dermis.
The structure in the dermis are:
sensory nerve endings
sweat glands and ducts
hairs,sebaceous glands and arrector pili muscles
Functions of skin
The skin forms a waterproof layer provided mainly by its keratinised epithelium. It protects the deeper and more delicate structures.It acts as a barrier against
invasion by microorganisms
The epidermis has immune cells called Langerhans cells,which are microphage type.They phagocytose intruding antigens and travel to lymphoid tissue.Here they present antigen to T-lymphocytes thus stimulating immune response. The body reacts by reflex action by sensory nerve endings in skin.
Regulation of body temperature
Body temperature remains constant at 36.8 degrees Celsius across environmental temperatures. This is to ensure optimal range for enzyme activity. In health,variations are between 0.5 and 0.75 degrees Celsius although it rises slightly in evening during exercise. A negative feedback system maintains the balance between heat produced in body and heat lost to environment.
Body temperature rises when metabolic rate increases.Body temperature falls when it decreases.Energy released during metabolic activity is in form of heat.Most active organs produce most heat.Main organs involved are :
Skeletal muscle contraction produces a large amount of heat .More strenuous the muscular exercise greater is the heat produced.
Liver is very metabolic active and heat is produced as a by product.
These produce heat during peristalsis and during chemical action involved in digestion.
Heat loss is mainly by skin.Small loss of heat is in expired air,urine.Only loss of heat by skin can be regulated.
Loss of heat by skin is affected by difference between body and environmental temperatures. The exposed amount of body surface and type of clothes worn also affect heat loss.
Formation of Vitamin D
7-dehydrocholesterol is a lipid based substance in skin and ultraviolet rays of sun convert 7-dehydrocholesterol to vitamin D.
These include good nutritional status and general health.Infection, impaired immunity,poor blood supply reduce wound healing.
Local factors include good blood supply to provide oxygen and nutrients and remove waste and freedom from contamination.
Disorders of skin
Non infective inflammatory conditions
Complications of Burns
1.Dehydration and hypovolaemia