- ✓ STRUCTURE OF THE SKIN
- ✓ THE SWEAT GLANDS
- ✓ FUNCTIONS OF THE SKIN
- ✓ REGULATION OF BODY TEMPERATURE
- ✓ CLINICAL ASPECTS
STRUCTURE OF THE SKIN
Skin is the dividing line between the individual and his external environment. The whole surface of the body is covered by skin. It clothes and supports the underlying soft tissue. It is a protective, elastic, waterproof and sensitive covering.
The skin, which varies in thickness from less than 0.5 mm to 3 or even 4 mm, consists of two main parts as Epidermis of ectodermal origin and an underlying dermis derived from the mesoderm. Beneath the dermis is the subcutaneous layer of connective tissue (Fig. V-16).
Epidermis: The epidermis is a stratified squamous epithelium. No blood vessels are seen in this layer. It gets its nutrition from lymph. Nerves are found in this layer. It consists of the following layers from outside inwards.
- The Stratum Corneum
- The Stratum Lucidum
- The Stratum spinosum or Rete mucosum of Malpighian layer.
The Stratum Corneum
This is the outermost horny layer, formed by a keratinised stratified epithelial cells. This layer is thickest in the sole and palm, and is thinnest in the lips. Hairs are special growths of these keratinised cells (Nail is a modified epidermis).
The Stratum Lucidum
This is immediately below the first layer. This is a thin, more or less transparent layer in which the cells are indistinct.
The Stratum Granulosum
This is the next layer, It is formed by a few layer of flattened cells, filled up with granules which stain deeply with haematoxylin. These granules are composed of a substance known as “Eleidin”. Due to the presence of these granules this layer is called stratum granulosum.
The Retemucosum
This layer is broad and thick and is made up of large cells. Some of these cells are branched. These cells are called prickle cells. The cells in the deepest layer of the retemucosum contain granules of a pigment called melanin which gives the dark colour to skin. These cells are called melanoblasts. They contain an enzyme Tyrosinase (Oxidase) which converts Tyrosin to Melanin. From this layer finger like processes pass into dermis.
The Dermis
Cuti-vera or corium is composed of dense connective tissue and contain number of structures.
Large cells belonging to the reticulo-endothelial system
Fine elastic fibres
Capillary blood vessels
Lymphatics
The most superficial layer of the corium is prolonged into minute papillae over which the epidermis is moulded. Sensory nerve endings of various types.219
Hair roots of hair follicle
Sweat glands
Sebaceous glands
Involuntary muscle fibres
In scrotum, penis, nipple and areola involuntary muscle fibres are found.
Arrectorespilorum
Small bundles of involuntary muscle fibres which are attached to the hair follicle and pass out into more superficial layer of corium. When these fibres contract the hairs become more erect.
Nail
The nail consists of modified epidermis. It is made up of:
- Free edge, the unattached extension.
- A body, the attached and visible uncovered portion.
- A root the part lying beneath the skin. Nail bed is richly supplied with nerves and blood vessels.
THE SWEAT GLANDS
The sweat glands are distributed all over the body except at the margin of the lips, in the nail-bed, the prepuce and glans penis. They are especially abundant in the skin of palm 3000 per sq. inch surface area and sole of the foot. They are composed of single unbranched coiled tubes, which lie in the subcutaneous tissue and send their ducts up to open on the surface of skin by cork-screw like channels. The secreting part of the tube consists of a basement membrane lined by double layer of cells; the innermost of these are cubical and represent the secreting cells proper. Between the secreting cells and basement membrane is a layer of unstriated muscle fibres. The duct of the gland has an epithelium consisting of two or three layers of cell but there is no muscle fibre.
The sweat glands are of two types:
- Eccrine Glands
Eccrine Glands
These glands constitute the major portion and generally found throughout the surface. They are most numerous on the palms and soles. They secrete dilute sweat containing NaCl, urea, lactates and other salts.
Apocrine Glands
These are large sweat glands found only in certain situations such as axilla, areola, mons pubis and labia majora etc. Their secretions vary in composition. They secrete a fluid containing a fatty material of peculiar odour.
The Ceruminous glands of external ear are similar and secrete a waxy material.
SWEAT
Sweat mainly consists of secretion of eccrine glands and it is the most dilute of all animal fluids. Specific gravity is 1.002 – 1.003.
Usually it is acidic in reaction but may become neutral or alkaline in profuse sweating. P.H. 4.75.
Composition
Water 99.2–99.6%
Inorganic Constituents
NaCl is the most important substance 0.2–0.5%
Muscular exercise increases the salt concentration. Small quantities of K and sulphates are present and Ammonium Carbonates are present.
Organic Constituents
Urea 0.03% same quantity as in plasma
Lactic acid 0.7%
Sugar 0.004%
The total quantity of sweat secreted in 24 hours is about 1 litre. In hot climates it is increased to 3 litres to 10 litres.
MECHANISM OF SWEAT SECRETION
Innervation
The sweat glands are innervated by the sympathetic nervous system, but these sympathetic fibres act by liberation of acetylcholine at their post ganglionic terminals (i.e.) cholinergic sympathetic fibres. Stimulation of sympathetic nerves increases sweat secretion, Acetylcholine and pilocarpine cause abundant sweating.
The usual stimulus to sweat secretion is rise in blood temperature which exerts is effect in two ways, directly upon the nervous centres, which is of more important, and reflexly by stimulation of heat receptors in the skin. Sweating is abolished by sectioning the nerves to a part and is therefore not due to direct stimulation of sweat glands.
Destruction of the sympathetic nerve fibres to a part completely abolishes the sweating response to a rise in temperature. As sweat comes from the blood, rapid sweating demands a large cutaneous blood flow. Therefore, sweating is usually associated with vasodilatation of skin blood vessels. This vasodilatation is brought about by:
- External heat acting directly on the vessels.
- Reflexly by stimulating cutaneous thermoreceptors.
- Rise in blood temperature acting directly on the vasomotor centre.
- More recently a substance called “bradykinin” has been suggested as an important factor causing active vasodilation in human skin containing eccrine sweat glands. During activity of sweat glands, a proteolytic enzyme is released into the tissues, where it acts on an unknown protein to produce the polypeptide bradykinin. This later substance is a powerful vasodilator.
Adrenaline has no action on sweat secretion.
Atropine inhibits sweat secretion.
- Cerebral cortex (Motor and premotor areas).influence sweet secretion.
- Hypothalamus—Electrical stimulation and local warming evokes sweat secretion.
- Spinal centres transection of the spinal cord does not suppress sweat secretion below the lesion.
High Environment Temperature evokes excessive sweating (Cl− loss increased). The action is directly on the nerve centres, by the rise of blood temperature as well as reflexly by stimulation of heat receptors in the skin. This is known as Thermal Sweating.
Psychic Influences: Extreme emotional conditions nervousness, fear, fatigue, mental worry etc, provoke sweating which occurs mostly on the forehead, palms and soles. This form is called Mental Sweating. In these conditions sweating maybe associated with vaso-constriction (cold sweat) showing that sweating is not entirely dependent upon circulation, although usually in other forms of sweating it is associated with vasodilation. Mental sweating is due to impulses from higher cortical region.
Muscular exercise evokes sweating, it is partly mental and partly thermal sweating. In initial stages, sweating is due to impulses from the cerebral cortex and later on due to rise in the body temperature.
Gustatory Sweating: Reflex sweating confined to face induced by eating spicy food occurs in many persons.
Sleep: Secretion of sweat is considerably increased.
Water is lost from the skin in 2 ways:
- Sensible Perspiration: sweating.
- Insensible perspiration: Imperceptible loss of water from skin by a process of diffusion (not visible and not felt). The amount is about 600–800cc. It is not due to any active secretion, but due to passage of water by diffusion through epidermis.
THE HAIRS
The hairs are developed from the epidermis and are present all over the body except the palms, the soles (the lateral surface, backs of second phalanges of the fingers and the toes, the lips and parts of ext. genitalia). The hair consists of a Shaft projecting above the surface of the skin and a root which is part within the skin. At its deep end the root expands into a bulb, the hair-bulb, which is moulded over a specialised vascular portion of the dermis called the hair papilla. The hair follicle encloses the root of the hair. The hairs are set at acute angle in the skin and do not project vertically. Sebaceous glands are related with hairs and these open into the upper part of hair follicle.
Sebaceous Glands
Sebaceous glands are small sac like glands found in the skin all over the body except in the palms and soles. They are usually associated with hairs, one or several glands open into one hair follicle. The sebaceous gland is a pear-shaped body, consisting of a secretory part and a short neck opening into the hair follicle. Their secretion is called sebum.
Secretion of sebum is a continuous process, though it is probably quickened in conditions of increased vascularity of the skin. The secretion is not under the control of central nervous system. When arrectores pilorum muscle contracts the sebum is squeezed out into the root of the hair and the surrounding skin. Sebum is rich in fatty acids and cholesterol and has characteristic odour. It keeps the skin surface moist and has got destructive action over streptococci and thereby protects the skin.
FUNCTIONS OF THE SKIN
- Protection: The skin protects underlying parts against mechanical injuries, bacterial infections and the actinic rays of the sun. The nails are also defensive appendages of the skin.
- Important Sense Organ: The skin is an important sense organ. It contains receptors which mediate the sensations of touch, pain, warmth, cold and pressure.
- Regulation of Body Temperature: The skin is the most important organ which takes part in the regulation of body temperature. (This will be dealt in more detail separately).
- Touch by Meissnere' corpuscle
- Pain by bare nerve endings
- Warmth by Ruffini's corpuscle
- Cold by end organs of Krause
- Pressure by Paccinian corpuscle
- Secretory Function:
- Sebum
- Sweat (Milk)
- Excretory Function: Water, some salts and metabolites are excreted through skin and some drugs (Hg+, Arsenic, Quinine and Codine).
- Absorption: Oily substances are absorbed through intact skin. Watery solutions are not absorbed.
- Formation of Vit D: Vitamin D is synthesised by ultra violet rays of the sun acting upon the ergosterol present in the skin and subcutaneous tissue.
- Respiratory Function: A considerable amount of cutaneous respiration is carried out in frogs. It is said that in man a small amount of CO2 is eliminated through skin probably dissolved in sweat. N2 and O2 are reabsorbed.
- Acid Base Equilibrium: Sweat being an acid in reaction, a good amount of acid is excreted through it. In acidosis, sweat becomes more acidic and in this way skin plays some role in the maintenance of acid-base equilibrium.
- Water Balance: Formation and evaporation of sweat is an important factor in the regulation of water balance of the body.
- Psychic Functions: Skin is an organ of emotional expression, e.g. Fear causing blanching of skin and cold sweats.
REGULATION OF BODY TEMPERATURE
THE NORMAL HUMAN BODY TEMPERATURE: The normal body temperature at rest is about 97°F – 99°F (Average is 98–4°F or 37°C).
Body temperature is measured by placing a clinical thermometer under the tongue, in the axilla or in the rectum. The axillary temp is 1°F less and rectal temperature is 1°F more. The rectal temperature is the most reliable but the mouth temperature is usually taken.
Variations in Body Temperature
- Diurnal Variations: Temperature is highest in the evening between 5–8 PM (6 PM) after the day's labour and lowest in the morning between 3–5 AM after a night's rest. In night workers this may be reversed.
- Age: In infants heat regulating mechanism is not very stable and is poorly developed and its variations are wider. In old age T° may be subnormal due to low BMR. Excitement or other strong emotions even in old children may raise the temperature by as much as 20°F.
- Sex: In female there is a variation related to menstrual cycle. At the moment of ovulation (on the 14th day) basal body temperature falls.
- Food: Ingestion of food (especially protein diet due to the high SDA) stimulates metabolic activity and may cause rise in body T°.
- Exercise: Muscular activity increases heat production as 75% energy liberated is converted to heat (only 25% to work).
- Atmospheric Conditions: Temperature, Humidity, air movement etc., may affect body temperature.
- Other Conditions: Affecting BMR will alter body temperature, e.g. Hyperthyroidism, Malnutrition. The lowest limit of T° compatible with life is 80.6°F–84.2° F (27–29°C). Increase in temperature to 107.6° f (42° C) is seldom observed. Rise above 111°F lead to fatal disturbances in CNS since the tissues consist largely of water which has a higher thermal capacity and is also a relatively good conductor of heat.
The internal parts of the body circulation helps to distribute heat evenly throughout the tissue. Temperature of skin is usually lower than the T° in the depth of the body. It varies in different parts of body with environmental temperature, humidity, clothing, state of circulation in skin, evaporation of sweat etc. This provides a T° gradient which leads to heat loss from the depth to the surface of the body, hence to the environment. In hot climate the temperature gradient may be reversed.
Temperature of viscera is 1–1.5°F higher than T° in axilla. Temperature is highest in the liver since it is most active tissue, the central organ of metabolism. Temperature in blood vessels also varies.
The maintenance of a suitable body temperature is an adaptive process (like metabolism for supplying energy for activity of cells, regulation of fluid and electrolyte balance and PH for maintaining homeostasis for cellular activity). This adaptive process of temperature regulation is relatively a later development and is essential only to bird and mammal even now.
According to their capacity to regulate their body temperature animals are classified into:
- Poikilothermic Animals: (Cold blooded animals) in which temperature varies with environmental temperature (e.g.) invertebrates, fishes, amphibians, frogs, turtle and reptiles. Temperature is usually a few degree above that of environment. Vital processes are easily modified by changes in environmental temperature.
Significance of Constant Body Temperature
Of all the factors responsible for the normal pattern of biochemical reactions in man or any other mammal, a relatively constant body temperature is one of the most important homeostatic conditions. All the biochemical reactions in the body require enzyme activity. Fall in body T° will inhibit the activity of enzymes and metabolism will be retarded. On the other hand if body T° rises to high level, normal functions of the tissues, especially cells of CNS are interfered with. Tissue proteins, and the enzymes become irreversibly inactivated when subjected to more heat. For the continuation of the metabolic processes and for the normal function of the body, optimum temperature is essential. For human body this is 98.4°F.
Temperature Regulation: Normal body temperature is maintained at a fairly constant level though many factors tend to upset the constancy. The temperature is maintained by accurately balancing heat production and heat loss. This is brought out by one or two of both ways.
- Increasing or decreasing heat production
- Increasing or decreasing heat loss (Fig. V-17)
It will be convenient to deal under two heading separately.
HEAT PRODUCTION (THERMOGENESIS)
Energy derived from metabolic processes may be liberated as work, may be used to synthesise other compounds which are stored in the body for later use, or changed into heat energy.
Heat is produced in the body as a result of metabolic processes. Therefore heat production depends upon the metabolic rate and ultimately all heat produced in the body come from the oxidative processes. In the oxidative processes responsible for heat production all the tissues participate. In the regulation of heat production, muscles probably play the leading part and liver is one of the most constant and active source of heat. If the O2 utilisation of the body is increased, the heat production is raised and if the heat loss is not also proportionally increased the body temperature will be increased until heat loss again equals heat production.
Factors of thermogenesis (Heat production)
- Food: Oxidation of Food : (Protein. Fat, Carbohydrate) Specific dynamic action of food stuff increases heat production. The effect of protein 30% is greater than that of carbohydrate 6% and fat 4%.
- Proteins—4 cal/gm.
- Carbohydrate—4 cal/gm.
- Fats—9 cal/gm.
- Skeletal Muscular Activity: Increased tone of the muscles and muscular exercise increase the heat production. During severe muscular exercise more than 75% of the metabolic energy appears as heat. Only 25% appears as external work.
- Glandular and Visceral Activity: increases the heat production (Liver, kidney, other viscera and glands).
The processes involved in heat production are all chemical processes, and the regulation of heat production referred to as Chemical Regulation of heat balance. The heat production of an average man doing light work is about 3000 calories in 24 hours. Under resting basal conditions 1500 – 1700 calories, i.e. I cal/kg. body wt./hr.
Low environment temperature stimulates heat production by increasing the metabolic rate and muscular tone, by causing shivering. At air temperature below 28°C the body (nude) loses heat rapidly to such an extent that metabolism must be increased by means of shivering to maintain the balance (at 25°C). Within the temperature range between heat 28°C and 31°C the naked male body is able quite easily to maintain the balance between heat production and heat loss.
There is neither sweating nor shivering and a male subject feels comfortable. This range of temperature is known as the “Comfort zone”.224
The comfort zone is broader in women (27° to 33°) because women have a more efficient heat regulating mechanism than men. Women are able to resist cold better and feel more comfortable at high temperature. The external temperature below which heat production must be increased in order to maintain normal temperature is called the Critical Temperature. The critical temperature and temperature of comfort will vary the amount and nature of clothing.
Shivering: Shivering occurs as an effort to increase heat production to raise the body temperature to normal when the external temperature falls to 23°C or when the body temperature had dropped by about 0.6°-1°C. The fall in skin temperature acts as a stimulus which calls into play the mechanism of heat production. Shivering which consists of fine contraction of the muscles, muscle tone increases and smooth muscles of skin contract, thereby heat production is increased. Shivering consists of phasic skeletalon muscular contraction. In man it becomes apparent first in the head (masseter muscle), later in the arms, body, and legs. The extreme shivering is obviously influenced by surface thermoreceptors and by thermo-receptors in the tracheal mucosa.
Shivering can be of reflex origin or central origin. Cold stress shows increased motor activity and alertness whereas local hypothalamic warming will produce drowsiness. The hypothalamic thermodetectors project upon and to some extent modulate the activating system in the brainstem thereby influence wakefulness and skeletal muscle tone through motor system. The central nervous thermoregulatory mechanism has been compared to a “thermostat”. In fever and during hard muscular work, when thermoregulatory balance is activated at higher level of body T° than is normally the case at rest, the “Thermostat” is said to be ‘reset’. Vessels are constricted, the warm blood which is responsible for normal feeling of warmth by stimulation of cutaneous sense organs is shifted to deep regions and the chill is felt. The centre for shivering (heat production) is situated in the posterior part of Hypothalamus.
FACTORS OF THERMOLYSIS (HEAT - LOSS)
Heat is lost from the body through the following processes.
- Radiation, convection and conduction from skin
- Evaporation of water from the skin and lungs.
- In rising the T° of inspired air to body T°.
- In urine and faeces.
Heat loss is dependent on the physical factors. Therefore it is called physical regulation. Under ordinary circumstances of every day life over 95% of heat loss occurs through 1 and 2 mechanisms.
3,000 calories of heat produced are lost in the following proportions through various channels.
HEAT LOSS MECHANISMS
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- Radiation, convection and conduction
All bodies when placed at a temperature below their own lose heat by conduction, radiation and convection. The T° gradient of the skin to the surrounding cooler objects determine the heat loss from the body surface.
Radiation is the transfer of heat from the surface of one object of that of another with which it is not in contact, by way of electromagnetic waves. The human body radiates heat not to the surrounding air, but through air too the cold objects in the environment (e.g.) walls, floor, furniture, other objects). The amount of heat lost by radiation depends upon the area of body surface, the temperature difference between skin surface and surrounding objects and the humidity of air. By radiation, 55% of total heat is lost. At high environmental T° it is interfered with the radiating surface of the human body is only 85% of total surface area, for opposed surfaces, e.g. axilla. Inner surface of thigh and upper arms do not loose heat by radiation (Profile area) It is for this reason one huddles during cold.
Convection: In free convection a thin layer of air in contact with the body is moistened and warmed, thereby it becomes lighter and rises up and its place is taken by cooler drier 225air. As a result of these convection current heat is continuously conveyed from body surface to the air. Heat lost through this method depends upon air temperature, air movement, nature and amount of clothing, 15% is lost by this process.
Conduction is the transfer of heat from one molecule to another through gas, liquids and solids by direct contact and the rate of transfer depends upon thermal conductivity of the object and difference in temperature, between the two objects.
- Evaporation of Water from skin, lungs, Heat is lost from the body through
- Secretion and evaporation of sweat.
- Exhalation of water in the expired air.
Heat is lost through lungs by evaporation of water in the expired air.
On the Skin there are two ways of evaporation
- Sweating (Sensible perspiration or visible sweat).
- Insensible perspiration.
The heat lost in the evaporation of 1 litre of sweat amounts to 580 calories. At ordinary room temperature when there is no visible perspiration the heat from evaporation through skin and lungs amounts to 27% of total heat loss. Upon 28°-30°C of air temperature there is no significant loss of heat by evaporation but above 30°C to 35°C more heat is lost by evaporation. The rate of evaporation of water from skin and lungs depends upon the T° difference between body and environment and is assisted by air movement. It is inversely proportional to the relative humidity of the air (The degree too which the atmosphere is saturated with water vapour).
- Some heat is lost in raising the T° of the inspired air to body temperature. 2–3% is lost by this way.
- Heat lost in urine and faeces accounts for only 1% or less of the total heat loss.
Clothing plays some part in controlling heat loss. This depends upon:
- Area of body covered.
- Nature, thickness and colour of material.
- Number of layer of clothes.
FACTORS OF REGULATION OF BODY TEMPERATURE
When there is tendency for wide variation in the body T° either due to more heat production, or due to more heat loss, or due to external environmental conditions, the heat regulating mechanism will come into play and set right the heat balance. This heat balance is controlled by:
- Nervous system
- Endocrines.
THE NERVOUS REGULATION OF BODY T°
a. Reflex Regulation
Afferent impulses set up by stimulation of temperature receptors (Heat and cold spots) in skin, and other receptors set up appropriate reflex bodily responses—sweating, vasomotor changes etc.
b. Central Regulation
The heat regulating centre or Thermo Regulating Centre or Thermotaxic Centre is situated in the Hypothalamus. There are separate centres for response to heat (Centre for heat loss) and to cold. (Centre for heat production). The centre which respond to heat is in the Anterior hypothalamus (Heat loss) and that for response to cold is in the posterior hypothalamus (Heat production centre).
Afferent Fibres
The hypothalamus because of its close connection with the thalamus receives all afferent impulses concerned with heat regulations.
Efferent Fibres
Efferent impulses from hypothalamus are carried along with both somatic and autonomic nerves. The impulses cause muscular or glandular activity (Heat production) and acts on cutaneous circulation, sweat secretion and pulmonary ventilation.
The peripheral nerves concerned are:
- Motor nerves, which stimulate movements and tone in muscles.
- Vasomotor nerves which constrict and dilate the cutaneous vessels.
- Sympathetic nerves to sweat glands.
ENDOCRINE REGULATION OF BODY T°
Thyroid
Exposure for a long time to low temperature (in winter months) thyroid shows signs of greater activity. BMR is increased (These effects are not obtained in hypophysectomised animals). It appears that cold acts on the hypothalamus and provokes a discharge of impulses which increase the secretion of thyrotropic hormone of the anterior hypophysis which stimulates thyroid.
In hypothyroidism the body T° is subnormal, BMR is low. In hyperthyroidism, the body T° is high, High BMR.
Adrenal Medulla
Exposure to cold stimulates secretion of adrenaline, which causes increased metabolic rate and increased heat production. It also constricts vessels and diminishes heat loss. Adrenal secretion in response to cold in a short time, is a primary delicate reaction to cold and of short duration while shivering is the second grosser compensatory mechanism. In other words adrenaline release is a fine adjustment where as shivering is coarse adjustment.
Adrenal Cortex - influences thermal regulation by its metabolic effects.
Adrenalectomised animals do not resist cold well mainly due to adrenal cortical insufficiency.
Hypophysis
The Anterior pituitary exerts a continuous indirect action on the heat production by maintaining the normal level of thyroid and adrenal function.
Anterior hypophysis insufficiency causes a decrease in the resistance to cold and a fall in BMR.
Relative roles of chemical and physical mechanisms for regulation of body temperature, under varying environment temperatures.
1. At Low Environmental Temperature:
(7°-15°C or 44.5°-59°F)
Predominant mechanism is increase in heat production (Chemical regulation) by voluntary movements, shivering and increase metabolic rate. Heat loss especially by radiation is minimised by cutaneous vasoconstriction.
2. At moderate Environmental Temperature:
(20°-30°C or 68°-86°F)
Predominant mechanism is physical process (heat-loss) main factor being control of heat loss by alteration of blood flow through skin etc.
3. At high Environmental Temperature
Heat loss by radiation is reduced and above 37°C body tends to gain heat and heat loss may be increased by rise into body temperature. The predominant mechanism here is secretion of sweat, evaporation of which has cooling effect thereby heat loss and maintenance of normal T°.
CLINICAL ASPECTS
DISTURBANCES OF TEMPERATURE REGULATION
Heat Stroke
This syndrome develops in men who are working hard, unsuitably clothed in hot environment. The symptoms are due to hyperpyrexia, salt loss and dehydration, As the external temperature is high heat cannot be lost by radiation and convection. (If in addition air is moist and still evaporation cannot take place either). Heat regulating mechanism breaks down and the body temperature rises (Rectal temperature 108°-112°F). The subject becomes suddenly unconscious. Cessation of sweat secretion immediately preceded by the loss of consciousness. Hyperthermia causes severe damage to the CNS and death occurs. Sunstroke is a form of heat stroke.
HEAT CRAMP OR STOKER'S CRAMP
Men working very hard in hot moist atmospheres (stokers, miners) sweat profusely. NaCl is very much lost in the sweat; due to deficiency of NaCl, intense and exceedingly painful contraction of muscles occurs in abdomen and leg.
Fever or Pyrexia or Hyperthermia
The commonest type of disturbance in temperature regulation is fever. The body temperature is raised. The body is believed to be able to deal more effectively with invading organisms, when the temperature is raised and so pyrexia may be regarded as a protective mechanism. Heat production is increased but heat loss is reduced simultaneously due the cutaneous vasoconstriction. Temperature regulation is disturbed. Body temperature is adjusted to a higher level than normal. The higher centres (Hypothalamus) are reset at higher temperature. As the fever recedes, sweating occurs, the heat balance is restored to its normal and the heat is lost and temperature falls.
Fever is only a symptom manifested in many diseases. The major causes are bacterial, parasitic and viral infections and consequent inflammation thereof.227
Other causes may be chemicals, toxins, metabolic and tumor interfering with hypothalamic functions.
Due to various causes endotoxins are liberated. To these toxins the body defence mechanism by monocytes and macrophages, and other reticuloendothelial cells respond and produce cytokines [IL-IB, IL-6, β - interferon, γ - interferon and tumor necrosis factor - α (TNFα)]. These cytokines act as pyrogens and produce fever directly or these cytokines act on the hypothalamus which in turn produce prostaglandins (PGE2) which raises the set point and cause fever. The antipyretic effect of aspirin is due to its inhibitor effect on prostaglandin synthesis. When the T° is 40°C brain cells are damaged and if T° exceeds 43°C heat stroke and death occurs.
Hypothermia is a condition in which body T° is below normal. In hypothermia at the body temperature of 28°C the metabolism slows down. Respiration and heart rate are slow and BP is low. The individual becomes unconscious. but survive without ill effects. Now-a-days hypothermia is induced during heart and brain surgery. This helps to prolong surgery time and to minimise bleeding.228