(This topic has been written by a student from
Hajvery University, Lahore)
The skin is the largest organ of the body and has a surface area of about 1.5 to 2m2 in adults and it contains glands, hairs and nails. The skin completely covers the body and is continuous with the membranes lining the body orifices.
Structure of the skin
Structurally skin is divided into following layers:
1. Epidermis
2. Dermis
Between the skin and underlying structures is a subcutaneous layer of fat.
Epidermis:
Most superficial layer of the skin is epidermis and is composed of stratified keratinized squamous epithelium which varies in thickness in different parts of the body. It is thickest in the palms of the hands and soles of the feet. There are no blood vessels or nerve endings in the epidermis, but its deeper layers are bathed in interstitial fluid from the dermis, which provides oxygen and nutrients, and is drained away as lymph. There are several layers of cells in the epidemic which extend from the deepest germinative layer to the surface stratum corneum which is thickest Horney layer. The cells on the surface are flat, thin, non-nucleated, dead cells, or squamis, inwhich the cytoplasm has been replaces by the fibrous protein keratin. These cells that originated in the germinative layer and have undergone gradual change as they progressed towards the surface. Complete replacement of the epidermis takes about a month. The maintenance of the healthy epidermis depends upon three processes being synchronized:
• desquamation (shedding ) of the keratinized cells form the surface
• effective keratinisation of the cells approaching the surface
• Continual cell division in the deeper layers with newly formed cells being pushed to the surface.
Hairs, secretions from sebaceous glands and ducts of sweat glands pass through the epidermis to reach the surface .The surface of the epidermis is ridged by projections of cells in the dermis called papillae. The pattern of ridges is different in every individual and the impression made by them is the 'fingerprint’. The downward projections of the terminative layer b.w the papillae are believed to aid nutrition of epidermal cells and stabilize the two layers, prevention damage due to shearing forces. Blisters and epidermis and serous fluid collects b/w the two layers.
Color of the skin
Various factors affect the color of the skin; some of these factors are as following;
• Melanin :It is a dark pigment derived from the amino acid tyrosine and secreted by melanocytes in the deep germinatve layer , is absorbed by surrounding epithelial cells .The amount is genetically determined and varies b/different parts of the body, b/w people of the seem ethnic origin and b/w ethnic groups. The number of melanocytes is fairly constant so the differences in color depends on the amount of melanin secreted It protects the skin from the harmful effects of sunlignt.Exposure to sunlight promotes synthesis of melanin.
• The percentage saturation of heaemoglobin and the amount of blood circulating in the dermis give white skin white skin its pink color.
Excessive levels of bile pigments in blood and carotenes in subcutaneous fat give the skin a yellowish colour.
Dermis
The dermis is tough and elastic .It is formed from connective tissue and the matrix contains collagen fibers interlaced with elastic fibres.Rupture of elastic fibers occurs when the skin is overstretched, resulting in permanent striate ,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 ate the main cells found in the dermis .Underlying its deepest layer there is areola tissue and varying amounts of
adipose tissues. The structures in the dermis are:
• blood Bessel
• lymph vessels
• sensory nerve endings
• sweat glands and their ducts
• Hairs, arrector pili muscles and sebaceous glands.
Blood vessels
Arterioles from a fine network with capillary branches supplying sweat glands, sebaceous glands, hair follicles and the dermis .The epidermis has no blood supply. It obtains nutrients and oxygen from interstitial fluid derived from blood vessels I the papilles of the dermis.
Lymph vessels
These 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 deferent types of sensory receptors through which individuals receive unformation about their environment. Nerve impulses, generated in the sensory receptors in the dermis, are conveyed to the apical cord by sensory nerves, then to the sensory area of the cerebrum where the sensations are perceived.
Sweat glands
These are widely dies tribute throughout the skin and are most numerous in the palms of the hands, soles of the feet, axillae and groins. They are composed of epithelial cells. The bodies of the glands lie coiled in the subcutaneous tissue. Some ducts open onto the skin surface at tiny depressions, or pores, and others open into hair follicles. Glands opening into hair follicles do not become active until puberty. IN the maxilla they secrete odourless milky fluids which, if decomposed by surface microbes, caudles unpleasant odour .The functions of this secretion are not known. Sweat glands are stimulated by sympathetic nerves in response to raised body temperature and fear. The most wanted and important function of sweat secreted by glands opening onto the skin surface is in the regulation of body temperature. Evaporation of sweat from body surface takes heat from the body core and the amount of sweat produced is governed by the temperature -regulating centre in the hyppothalamus.Excessive sweating may lead to dehydration and serious depletion o f sodium chloride unless intake of water and salt is appropriately increased .After 7 to 10 days exposure to high environmental temperatures the amount of salt lost is substantially reduced but water loss remains high.
Hairs
These are formed by a down -growth of epidermal cells into the dermis or subcutaneous tissue, called hair follicles. At the base of the follicle is a cluster of cells called the bulb? The hair is formed by 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 are the shaft and the remainder, the root. The color of the hair is genetically determined and depends on the amount of melanin present. White hair is the result of the replacement of melanin by tiny air bubbles.
The arrestor pile
These are little bundles of smooth muscles fibers attached to the hair follicles .Contraction makes the hair stand erect and raises the skin around the hair, causing 'goose flesh'.Teh muscles are stimulated by sympathetic neurone fibers in response to fear and cold. Erect hairs trap air, which acts as an insulating layer. This is an efficient warming machine especially when accompanied by shivering, i.e. involuntary contraction of skeletal muscles.
The sebaceous glands
These consist of secretary epithelial cells derived from the same tissue as the hair follicles .they secrete an oily 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 skin of the scalp, face, axillae,and groins. In regions of transition from one type of superficial epithelium to another, such as lips, eyelids, nipple, labiaminora, and glens 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 water-proofing and acts as a bactericidal and fungicidal agent, preventing infection. It also prevents drying and cracking of skin, especially on exposure to heat and sunshine .The activity of these glands increases at puberty and is less at the extremes of age, rendering infants and older adults prone to the effects of excessive moisture.
Nails
Human nails are equivalent to the claws, houns and hoofs of animals. They are derived from the same cells as epidermis and hair and consist of hard, honey keratin plates. They protect the tips of the fingers and toes. The root of the nail is embedded in the skin is covered by the cuticle and forms the hemispherical pale area called the lunula.The nail plate is the exposed partthat has grown out from the terminative zone of the epidermis called the nail bed.Fingernails grow mire quickly than toe nails and growths quicker when the environmental temperature is high.
Functions
Protection: The s of skin kin forms a relatively waterproof layer, provided mainly by its keratinized epithelium, which protects the deeper and more delicate structures .As an important non-specific defense mechanism it acts as a barrier against;
• Invasion by microbes
• Chemicals
• Physical agents, e.g. mild trauma, U.V light
• Dehydration
The epidermis contains specialized immune cells called Algerians cells .They phagocytes intruding
antigens and travel to lymphoid tissue, where they present
antigen to T-lymphocytes, thus stimulating an immune response.
Due to the presence of the sensory nerve endings in the skin the body reacts by reflex action to unpleasant or painful stimuli, protecting it from further injury .The pigment melanin affords some protection against harmful U.B rays in sunlight.
Regulation of body temperature
The temperature of the body remains fairly constant at about 36.8 across a wide range of environmental temperature .In health, variations ate usually limited to b/w 0.5 and 0.75 although it is raised slightly in the evening, during exercise and in women just after ovulation .When metabolic rate in creases ,body temperature rises, and when it decreases body temperature falls .To ensure this constant temperature ,a balance is maintained b/3 heat produced in the body and heat lost to the environment .
Heat production
Some of the energy released in the cells during metabolic activity is in the form of heat and the must active organs produce the most heat .The principal organ s involved are as follows:
• The muscles: contraction of skeletal muscles produces a large amount of heat and the more strenuous the muscular exercise, the grater the heat produced .Shivering also involves skeletal muscles contraction and produces heat when there is the risk of the body temperature falling below normal.
• The liver is very metabolically active, and heat is produced as a by-product .Metabolic rate and heat production is increased after eating.
• The digestive organs produce heat during peristalsis and during the chemical reaction involved in digestion.
Heat loss
Most heat loss from the body occurs through the skin .Small amounts are lost in expired air, urine and faces .Only heat loss through skin can be regulated; there is no control over heat lost by the other routes .Heat loss through the skin is affected by the difference b.w body and environmental temperatures, the amount of the body surface exposed to the air and the type of clothes worn. Air is a poor conductor of heat and when layers of air are trapped in clothing and b/w the skin and clothing they act as effective insulators against excessive heat loss. For this reason several layers of lightweight clothes provide more effective insulation against a low environmental temperature than one heavy garment. A balance is maintained b.w heat production and heat loss.
Mechanisms of heat loss
In evaporation, the body is cooled when heat is used to convert the water in sweat to water vapour. In radiation, exposed parts of the body radiate heat away from the body .In conduction, clothes and other objects in 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. Heat is also lo9st from the clothes by convection.
Control of body temperature
Nervous control: the temperature regulating centre in the hypothalamus is responsive to the temperature of circulating blood. This centre controls body temperature through autonomic nerve stimulation of the sweat gland when body temperature rises. The vasomotor centre in the medulla oblongata controls the diameter of the small arteries and arterioles, and therefore the amount of blood which circulates in the capillaries in the dermis. The vasomotor centre is influence4d by the temperature of its blood supply and by temperature rises, the skin capillaries dilate and the extra blood near the surface increases heat loss by radiation, conduction and convection .The skin is warm and pond in colour .When body temperature falls arteriolar constriction conserves heat and the skin is whiter and feels cool.
Formation of Vitamin – D: 7-dehydrocholesterol is a lipid based substance in the skin, and ultraviolet light from the sun converts it to vitamin D. This circulates into e blood and is used, with
calcium and phosphate, in the formation and maintenance of bone.
Coetaneous sensation: sensory receptors consist t of nerve endings in the dermis that are sensitive to touch, pressure, temperature of pain. Stimulation generates nerve impulses in sensory nerves that transmitted to the cerebral cortex. Some areas have more sensory receptors than others causing them to be especially sensitive, e.g. the lips and fingertips.
Absorption
This property is limited but substances that can be absorbed include:
• Some drugs, in transdermal patches, e.g. hormone replacement therapy during the menopause, nicotine as an aid to stopping smoking.
• Some toxic chemicals e.g. mercury.
Excretion
The skin is a minor excretory organ for some substances including:
• Sodium chloride in sweat; excess sweating may lead to low blood sodium levels
• Urea, especially when kidney function is impaired
• Aromatic substances, e.g. garlic and other spices.
Wound healing
The important function of the skin is quick healing of wounds to save the human body from different diseases.
Reference:
Ross and Wilson Anatomy and Physiology in Health and Illness
Physiology: with STUDENT CONSULT Online Access (Costanzo Physiology)
Textbook of Medical Physiology: With STUDENT CONSULT Online Access
Physiology Coloring Book, The (2nd Edition)
Human Physiology
(These are helpful in the preparation of "The Skin" in Physiology)
