Temperature regulation what is a homeothermic animal? icon

Temperature regulation what is a homeothermic animal?

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What is a homeothermic animal?

An animal capable of maintaining its body temperature within very narrow limits is termed as homeothermic, e.g. human being.

What is the skin and core temperature?

Skin temperature is the surface temperature and it rises and falls with the temperature of the surroundings. Core temperature is the temperature of deep tissues of the body and it remains almost constant. Total heat content of the body is determined by the net difference between heat produced and heat lost. Maintaining constant body temperature implies that overall heat production must equal heat loss. Both these variations are subject to precise physiological control.

How much is normal core temperature? How is it measured?

Normal body temperature varies from97°Fto 99.5°F (36°C - 37.5°C). It is measured with the help of clinical thermometer kept either in mouth, or per rectum. Rectal temperature is 1"F greater than oral temperature. Average oral temperature is 98"F -98.6°F.

What are the physiological variations in the body temperature?

  1. Body temperature varies with exercise. During strenuous exercise, excessive heat is produced and rectal temperature can rise as high as 101°F - 104°F.

  1. Exposure to severe cold may reduce rectal temperature below 97°F.

  1. There is a characteristic circadian fluctuation in body temperature. It is lowest during sleep and highest when person is awake.

  2. In females, higher temperature is recorded during the last half of menstrual cycle.

Why is temperature regulation needed?

Maintaining the constant body temperature prevents biochemical reactions from fluctuating with external temperature. Large elevation in body temperature causes nerve malfunction and protein denaturation. Some people may get convulsions at body temperature of 41°C (106°F).

Name various mechanisms by which heat is produced in the body.

  1. Basal rate of metabolism in all the cells of the body.

  2. Muscular activity — Extra rate of metabolism due to muscle activity.

  1. Shivering. Characteristic muscle response to cold is called shivering. Shivering consists of oscillating rhythmic muscle tremors occurring at a rate of 10-20/s. During shivering, efferent impulses to skeletal muscles are controlled by descending pathways, primarily by hypothalamus, and heat production may increase several folds within seconds to minutes. As no work is performed during shivering, all the energy liberated by muscles appears as internal heat (shivering thermogenesis).

  2. ^ Non-shivering thermogenesis. On chronic exposure to cold there is an increase in metabolic rate due to increased secretion of epinephrine and to certain extent thyroid hormones. Such type of non-shivering thermogenesis is minimal in adult human beings.

Enumerate the mechanism by which heat is lost.

Most of the heat is produced by deep organs (liver, brain, muscles, heart). This heat is transferred from the deep organs to the skin through blood. The surface of the body exchanges heat with the environment by:

  1. Radiation

  2. Conduction

  3. Convection

  4. Evaporation of water.

Amount of heat loss is determined by two factors:

  1. Rapidity with which heat is conducted from the core to the skin.

  2. Rapidity with which heat is transferred from the skin to the surroundings.

Describe insulator system of the body.

Skin, subcutaneous tissue and the fat of subcutaneous tissue are heat insulators for the body. Fat conducts heat only one-third as readily as other tissues. This insulation is important in maintaining core temperature even though skin temperature varies with that of the surroundings.

The skin is not a perfect insulator (if it were so, no heat would ever be lost from the core) and so the temperature of its outer surface is somewhere between that of external environment and that of the core.

^ Describe the radiator system of the body.

Blood vessels penetrate fatty subcutaneous insulator tissue and are distributed profusely beneath the skin. There is a venous plexus underneath the skin which is supplied by inflow of blood from the skin capillaries. In exposed areas of the body such as hands, feet and ears, there is arteriovenous anastomoses (blood is supplied to the venous plexus directly from the small arteries). The rate of blood flow through venous plexus can be increased up to 30% of total cardiac output. When there is full vasoconstriction, rate of blood flow can be reduced to as much as 2-3% of cardiac output. Changing flow of blood through the skin is thus the effective mechanism of controlling heat loss. The skin is therefore an effective radiator system.

Temperature of 37.1 ° C is called set point of the temperature control mechanism (Fig.7.1). If temperature of the body increases beyond the set point, all mechanisms of heat loss come into play to bring body temperature back to normal. If body temperature falls below the set point, all the mechanisms of increased heat production and heat conservation are stimulated and body temperature is brought to the set point.

^ Describe the mechanism of heat loss by radiation, conduction and convection.

Radiation, conduction and convection are the mechanisms of heat loss only when the body temperature is higher than the surrounding temperature.

  1. Radiation. Radiation is the process by which surfaces of all the objects emit heat in the form of electromagnetic waves. The rate of emission depends upon the temperature of the surface. The rate of heat loss depends upon the temperature difference between the skin surface and the environment.

  2. Conduction. It is a gain or loss of heat due to transfer of thermal energy during collisions between adjacent molecules. Body surface loses or gains heat by conduction through direct contact with cooler or warmer object respectively.

  3. Convection. It is the process by which conductive heat loss is aided by movement of air or water next to the body. Air next to the body is heated by conduction of heat from the body. This air moves away and carries away the heat and its place is taken by fresh cool air. This air when gets heated follows the same pattern. Some amount of convection is always occurring because warm air becomes lighter and therefore rises but it is greatly facilitated by external forces like wind or fan. Thus convection aids conductive heat loss by continuously maintaining the supply of cool air.

^ What are the behavioural responses for altering heat loss by radiation and conduction?

There are three behavioural responses altering the heat loss:

  1. Changes in surface urea. Curling up into the bed and hunching of shoulders are the responses to cold which reduce the surface area exposed to the environment.

  2. Clothing. Outer surface of clothes forms the true exterior of body surface. Skin loses heat directly to the air space trapped between skin and clothes. Clothes in turn pick up the heat and transfer it to the environment acting as insulators. Insulator ability of clothing depends on thickness of the trapped air layer. Clothing is also important in summer. When environment is hot, radiation and conduction act as heat gain mechanisms. Person insulates himself against such temperature by clothing. White clothing is cooler since it reflects more radiant energy.

3. Behavioural mechanisms. Seeking wanner or colder surroundings is the behaviour response.

^ Describe mechanism of evaporation.

Water is evaporated from the surface of the skin and membranes of the respiratory tract. When liquid is converted to gaseous state about 580-600 cal of energy is required for the transformation. When water evaporates from the skin, heat required is conducted from the surface and causes cooling of the surface. Water that evaporates by diffusion through skin (in absence of sweating) and respiratory membrane is known as insensible perspiration. It amounts to 600 ml/day. But this heat loss is not under control and therefore cannot be changed as required. Evaporation of sweat from the surface causes evaporative heat loss which is under the control of nervous system. Sweat glands are supplied by sympathetic nerves. Stimulation of sympathetic nervous system causes sweating. Sweat is a dilute solution of sodium chloride. For cooling effect sweat must evaporate from the surface. Humidity or water vapour concentration of the air is the most important factor determining evaporation of sweat. On humid day, due to failure of evaporation, sweat simply remains on the skin. When the surrounding temperature is high, evaporation is the only means of heat loss.

^ Where is the temperature regulating centre?

Temperature regulating centre is present in the hypothalamus. It is also called hypothalamic thermostat.

Anterior hypothalamus and preoptic area contain large number of heat sensitive neurons and cold sensitive neurons. If preoptic area is heated, it initiates heat loss mechanisms and brings temperature back to normal. Thus this areamainly controls heat loss mechanism.

Fig. 7.1 Effect of hypothalamic temperature on evaporative heat loss and heat production

and shivering.

Posterior hypothalamic area near mamillary bodies receives signals from various peripheral thermoreceptors as well as preoptic anterior hypothalamic area. Posterior hypothalamic area in turn, sends signals to the periphery to provide heat producing and heat conserving mechanisms.

^ Name different thermoreceptors.

There are two groups of thermoreceptors.

  1. Peripheral thermoreceptors. Present in skin and mucous membrane, they are either warm or cold receptors.

  2. Central thermoreceptors. These are present in hypothalamus, spinal cord, various organs and in the great veins. Impulses from various receptors go to posterior hypothalamus at the level of mammary bodies. This area in turn sends signals to periphery for heat producing and heat conserving reactions of the body.

What are the mechanisms of heat conservation?

Mechanisms of heat conservation are the ones which decrease the heat loss from the body.

1. Skin vasoconsfriction. It is due to stimulation of posterior hypothalamic sympathetic centres.

2. Piloerection ('hair standing on'). This is also due to sympathetic stimulation causing contraction of erector pili muscles attached to hair follicles of the skin. Hair become upright thus increasing the thickness of insulated layer of air next to the skin preventing transfer of heat to the surroundings.

Enumerate the different mechanisms which bring the body temperature back to normal when body is too hot.

1. Increased heat loss due to:

  1. Vasodilation of skin vessels.

  2. Sweating.

  3. Behavioural response (put on cooler clothes, switch on the fan, etc.).

2. Decreased heat production due to:

  1. Decreased muscle tone and voluntary activity.

  2. Decreased secretion of epinephrine and thyroid hormones.

  3. Decreased food appetite.

Enumerate the different mechanisms which bring the body temperature back to normal when body is too cold.

1. Decreased heat loss due to:

  1. Vasoconstriction of skin vessels.

  2. Reduction of surface by curling up.

  3. Behavioural responses (to put warmer clothes).

2. Increased heat production due to:

  1. Increased muscle tone.

  2. Shivering.

  3. Increased secretion of epinephrine and thyroid hormones.

  4. Increased food appetite.

What is the role of peripheral thermoreceptors present in the skin in tempera­ture regulation?

Stimulation of skin receptors help as follows:

  1. Stimulation of skin receptors changes the set point of hypothalamus. Set point increases when skin temperature decreases (stimulation of cold receptors). Because set point is higher, all mechanisms for increasing body temperature set in, so that mechanisms of increased heat production and decreased heat loss are initiated in anticipation before the body temperature falls below normal.

  2. Stimulation of skin receptors also causes behavioural mechanisms of control of temperature.

  3. Local cord reflexes prevent excessive heat exchange. When any part of the body is kept under hot lamp, there is local vasodilatation and sweating caused through cord reflex, thus causing excessive heat loss from the local surface.

^ Describe the mechanism of sweat secretion.

Sweat glands have a deep subdermal coiled portion, a duct passing outward in the dermis and epidermis of the skin.

Epithelial cells lining the coiled portion secrete primary secretion. These cells are supplied by cholinergic sympathetic nerves. Stimulation of these nerves stimulates gland to secrete primary secretion which is similar to plasma except for plasma proteins and high NaCl concentration. When this secretion passes through the duct, there is absorption of sodium and chloride. Slight stimulation of sweat glands causes this primary secretion to pass through the duct. Slowly and essentially almost all sodium and chloride is reabsorbed. This causes osmotic absorption of water which concentrates the other constituents (urea, lactic acid, potassium ions) in the sweat.

With strong sympathetic stimulation of the glands, rate of secretion is high. Primary solution quickly passes through the duct and therefore sodium and chloride ions are absorbed only slightly. Water reabsorption is greatly reduced. Other dissolved constituents are only moderately increased.

^ What is acclimatization of sweating mechanism?

Usually rate of secretion of sweat is rarely more than 700 ml/h but when person is exposed to hot weather for a few (2-6) weeks, rate of secretion is greatly increased (2 litre/h). This is due to direct increased capability of sweat glands themselves. Later on there is adaptation known as acclimatization producing changes in sweat volume and composition. This is the effect of chronic exposure to hot weather. Acclimatization is due to increased secretion of aldosterone (stimulated due to slight decrease in NaCl

level of body fluids because of excessive sweating before acclimatization). Aldosterone increases absorption of sodium and chloride from the duct. This reduces excretion of salt to 3-5 g/day as compared to 15-30 g/day before acclimatization.

^ Describe different abnormalities in temperature regulating mechanism.

1. Fever. It is hyperthermia resulting from disturbance in temperature regulating centre, i.e. resetting of hypothalamic thermostat to level of higher temperature than the normal (370C) as a result of bacterial infections or physical trauma.

  1. Infections due to bacteria, viruses and protozoa (e.g. malaria) are the most common cause of fever. Toxins liberated from organisms act on neutrophils which in turn release a substance called endogenous pyrogen, a polypeptide. This pyrogen sets the hypothalamic thermostat at higher temperature.

  2. ^ Physical trauma. Tissue destruction as in cardiac infarction, uninfected neoplasma, serum sickness and rheumatic fever.

As the hypothalamic thermostat is set at a higher level, all mechanisms to increase the temperature of the body (to thermostatic set point) are initiated: There is increased heat production and decreased heat loss. In initial stages rigor or shivering occurs which increases heat production. Skin vessels are constricted to minimize the heat loss. A rapid rise in temperature thus occurs. When temperature is at its height the skin vessels dilate and skin is flushed. The sweat glands are inactive. The rise in temperature increases the rate of metabolism which in turn increases heat production to cause further rise in temperature.

If the factor causing high temperature is suddenly removed the set point of hypothalamic thermostat is reduced to a lower value — back to normal set point. Hence all mechanisms of heat loss set in because body temperature is higher than the set level of thermostat. This causes vasodilation in skin and profuse sweating. This sudden change in febrile condition is called 'crisis' or flush' (skin is red and hot because of vasodilation). After crisis, temperature of the person comes back to normal.

Antipyretic drugs (drugs reducing fever) such as aspirin are believed to reset hypothalamic thermostat to normal set point. This is how they bring the body temperature down.

^ 2. Heat stroke. It is a grave syndrome most commonly seen in tropical countries. It also occurs as a complication of diseases causing hyperpyrexia. The symptoms are due

to hyperpyrexia, salt loss and dehydration.

Tolerance of extreme heat depends on whether heat is dry or wet. If the air is dry t and sufficient convection currents are flowing to promote rapid evaporation from the body, the person can withstand several hours of air temperatures at 1300 F. On the other hand, if the air is wet and 100% humidified, the body temperature begins to rise even with atmospheric temperature about 94" F. In persons performing heavy work the | critical temperature level is still lower.

Heat stroke is a condition where hypothalamic thermostat loses its temperature regulating ability. When core temperature of a person rises above 105" to 1060 F the temperature regulating ability of the centre is greatly depressed and then the person is likely to develop heat stroke, and sweating stops or diminishes. As a result high body temperature tends to perpetuate itself unless some special measures are taken to reduce it.

The symptoms of heat stroke are high body temperature, dizzines, abdominal distress with vomiting, delirium and eventually unconsciousness if body temperature is not soon decreased. These symptoms are exacerbated by a degree of circulatory shock due to excessive loss of fluid and electrocytes in sweat. Hyperthermia is also damaging to body tissues especially to brain and is therefore responsible formany effects. Mortality is high. Even a few minutes of high body temperature can be fatal. Treatment therefore should be started promptly.

Treatment. It consists of energetic measures to cool the patient. This is done by immersing the patient in a tub of ice cold water. It sometimes evokes uncontrollable shivering with considerable increase in rate of heat production. Therefore sponge or spray cooling of skin is supposed to be more effective in reducing the temperature. Cooling measures are to be continued till body temperature falls to 38" C (101" F).

3. ^ Exposure to severe cold.

  1. Loss of temperature regulation at low temperature. When temperature of the body falls below 85"F the ability of hypothalamic thermostat to regulate temperature is lost. It is greatly impaired when body temperature falls below 94°F because decreased temperature decreases rate of chemical heat production in each cell. First sleepiness and then coma develop.

  2. Frostbite. When body is exposed to extremely low temperature, surface areas can freeze. This freezing is called frostbite, and it especially occurs in ear lobes, and in the digits of hands and feet. If the freeze is sufficient to cause development of ice crystals in cells, permanent damage results, e.g. permanent circulatory impairment which causes local tissue damage leading to gangrene. Frost-bitten areas must be removed surgically.

^ Write a note on hypothermia.

In animals hypothermia occurs during hibernation and the body temperature varies with that of environment over a range of 50 to 150 C.

In man hypothermia may be accidental or it may be used deliberately in surgery of brain and heart.

Accidental hypothermia. It may occur in Healthy persons exposed to excessive cooling, e.g. immersion in cold sea water after shipwreck. The capacity of thermoregulatory mechanisms is overcome and body temperature falls. At 330 C temperature regulation fails. Death occurs at body temperature below 25" C.

Accidental hypothermia can also occur in elderly persons living in inadequately heated rooms.

Hypothermia occurs in myxoedema because the patients cannot increase their metabolic rate on exposure to cold.

^ Deliberately induced hypothermia. Hypothermia is deliberately induced in some patients undergoing operations on heart and brain.

Several surgical procedures of heart and brain require temporary suspension of normal circulation. This is made possible by cardiopulmonary bypass machine. In this machine apump and oxygenator take over the functions of heart and lungs respectively. But it is not desirable to depend entirely on this arrangement. To reduce the probability and severity of hypoxic damage of the tissues under such conditions, metabolic requirements are reduced by producing hypothermia. The body temperature is lowered to 21"C to 25"C. This is done as follows — sedative is given to depress hypothalamic temperature regulating centre, followed by cooling with ice or cooling blankets. The low temperature is maintained by sprinkling of cold water or alcohol on the body. At 25"C the metabolic requirements are reduced to 40 to 45% of those of normal temperature. However care should be taken to prevent shivering which will raise O2 requirement. Cardiac arrest for 10 minutes under such conditions does not cause permanent damage to the heart or brain and allows the performance of operation in these organs which would be impossible without cooling. Certain drugs have been used to facilitate the lowering of body temperature, e.g. chlorpromazine.

Hypothermia may cause certain harmful effects on the body such as hypocapnia and hypoxia. Impaired oxygenation of tissues may lead to metabolic acidosis. Reduction in metabolic rate reduces the activity of metabolically fuelled pumps such as sodium pump. Hence K+ is lost and Na+ is gained by tissues. This leads to hyperkalaemia.

In spite of all the above problems hypothermia is used in cardiac surgery because it provides time to the surgeon for manipulating the heart without producing permanent ischaemic damage. The maximum period of circulatory arrest increases markedly at low body temperatures.

Describe the structure and functions of skin.

Structure of skin

Skin is made up of two layers:

  1. Epidermis — outer layer

  2. Dermis — inner layer.

1. Epidermis. It is made up of 5 layers.

  1. Stratum corneum (horny layer). It is the outermost layer consisting of dead cells, called corneocytes. These cells lose their nuclei due to pressure and become dead. Cytoplasm of these cells contains protein keratin, phospholipids and glycogen.

  2. Stratum lucidum. This layer consists of flat epithelial cells, some without nucleus and some with degenerated nucleus. Cells exhibit shiny character.

  3. ^ Stratum granulosum. It is formed of 2 to 5 layers of flattened rhomboid cells. Cytoplasm of these cells contains keratohyalin granules. Protein keratohyalin is the precursor of keratin.

Stratum germinativum. Thick layer consisting of polygonal cells superficially and cuboidal or columnar cells in deeper parts. New cells are constantly formed here by mitosis. Newly formed cells move continuously towards stratum corneum. From thislayer some projections extent down to dermis. These projections provide anchoring and nutrition. Cells in this layer contain pigment melanin and the colour of the skin depends on amount of melanin present.

(e) ^ Stratum spinosum (prickle cell layer). The cells in this layer possess spine-like projections, by which they are connected to each other.

2. Dermis. It is a connective tissue layer and is made up of two layers.

  1. Superficial papillary layer. It projects into the epidermis. It contains blood vessels, lymphatic vessels and nerve fibres. It also has chromatophores which are pigment containing cells.

  2. Deeper reticular layer. This layer is made up of reticular and elastic fibres which are found around hair bulbs, sweat glands, sebaceous glands.

^ Hair bulbs are present in dermis; from here, hair come out which partly remains in epidermis and rest shoots out from the skin.

Sweat glands lie in this layer, and are of two types:

(i)Eccrine glands with simple tubular structure, highly coiled are present in dermis. The rest of the duct is mostly straight and passes through superficial layer of dermis and then through epidermis ultimately opening as a pore into the exterior. Sweat is synthesized by coiled portion. Eccrine glands are common and occur all over the body but their population is most dense over the thick skin.

(ii) Apocrine glands are found in pubic region, axilla and around nipple. They are of less importance in human beings.

^ Sebaceous glands, usually open in hair follicle except in the face where they open into the exterior directly. The secretion from these glands is rich in oily substances, which prevents evaporation of water from the skin.

  1. Functions of skin

1. Protective function. Skin forms the covering of the body and protects inner organs from the following agents.

(a) Bacteria and toxic substances. Keratinized stratum corneum is responsible for protective functions against chemicals like acids and alkalies. Intact skin also offers resistance to many harmful agents like bacteria. If there is injury to the skin, bacteria invade from external environments.

  1. ^ Mechanical blow. Skin is somewhat lose and moves over the underlying subcutaneous tissue. Therefore mechanical impact of any blow to the skin is not transmitted to the underlying tissues.

  2. Ultraviolet rays. Ultraviolet rays of sun can cause damage to the skin. But melanin pigment and stratum corneum together protect from such injury. Exposure to ultraviolet rays stimulates melanin production and also increases the thickness of stratum corneum. Melanin and stratum corneum absorb light waves having wavelength of less than 360 millimillions. Such waves are often cancer producing.

2. ^ Temperature regulation. Role of skin in temperature regulation is already explained.

3. Asa sense organ. Skin is the largest sense organ of the body. It has many nerver endings which are specialized into receptors. Those cutaneous receptors are stimulated by touch, pain, pressure and temperature (hot and cold receptors). These sensations are conveyed to brain through afferent nerves. Thus different sensations are perceived.

  1. Storage function. Skin can store fat, water.

  1. Synthesis of vitamin D. Vitamin D is synthesized in skin by the action of ultraviolet rays on cholesterol.

  2. Excretory function. Skin can excrete small quantities of waste products like urea, salts and fatty substances in its secretion.

  1. Absorption. Skin can absorb certain soluble substances and some ointments.

  1. Secretory function. Sweat glands secrete sweat and sebaceous glands secrete sebum. Secretion of sweat helps in temperature regulation and water balance. Sebum keeps the skin smooth and moist.


Long Questions

•What is normal body temperature? How is it regulated?

Short Questions

  • Heat loss mechanisms.

  • Heat gain mechanisms.

  • Hypothalamic thermostat.

  • Sweating.

  • Hypothermia.

  • Heat stroke.


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