Thermoregulation

1. Thermoregulation Thermoregulation is the balance between heat production mechanisms and heat loss mechanisms that occur to maintain a constant body temperature. heat production mechanisms and heat loss mechanisms that occur to maintain a constant body temperature.

2. Humans are homeothermic, meaning that internal body temperature is regulated through physiologicalmechanisms, usually keeping it in the resting range of 36.1 to 37.8 °C (97.0-100.0 °F) despite changesin environmental temperatures.

4. Heat flows from higher temperature to lower temperature. Physiologically, heat is generated in the muscles by metabolic chemical reactions, mainly in the liver. Some heat is lost through the lungs, although 90-95% is lost through the skin. Heat is transferred from the core to the skin by blood passing through peripheral blood vessels.

5. The rate of heat loss is determined by the extent to which the peripheral blood vessels dilate; fully dilated they will allow blood to travel 100 times faster than when constricted, thus losing body heat faster. Heat loss rates are also greatly increased by sweating, especially in dry environments.

6. The body controls heat loss by tightening the blood vessels under the skin, restricting the flow of blood - to the peripheral blood vessels ('Vasoconstriction'). The development of peripheral vasoconstriction allows a cooler, outer 'shell' to form an insulating barrier that slows heat loss from the body's core. Hands and feet have fewer large blood vessels, and when the flow of blood is restricted it is harder for the blood to keep flowing to these areas which quickly become cold.

7. When the body is too hot, it decreases heat production and increases heat loss. One way of increasing heat loss is through peripheral vasodilation, the dilation of blood vessels in the skin. When these vessels dilate, large quantities of warmed blood from the core of the body are carried to the skin, where heat loss may occur via radiation, convection, and conduction. Evaporation of fluids from the body also causes heat loss. Humans constantly lose fluids from the skin and in exhaled air. The unconscious loss of fluid is called insensible perspiration.

8. When the body is too cold, it increases heat production and decreases heat loss. Vasoconstriction, the constriction of the vessels of the skin, helps prevent heat loss. Shivering, which is a rhythmic contraction of skeletal muscles, produces heat. Heat can also be produced by nonshivering thermogenesis, an increase in metabolic heat production.

9. How the body controls heat loss The body controls heat loss by tightening the blood vessels under the skin, restricting the flow of blood - to the peripheral blood vessels ('Vasoconstriction'). The development of peripheral vasoconstriction allows a cooler, outer 'shell' to form an insulating barrier that slows heat loss from the body's core. Hands and feet have fewer large blood vessels, and when the flow of blood is restricted it is harder for the blood to keep flowing to these areas which quickly become cold.

10. Thermoregulatory Control thetemperature limits for living cells range from about0 °C (where ice crystals form) to about 45 °Cand humanscan tolerate internal temperatures below 35 °C or above41 °C for only very brief periods of time. To maintaininternal temperature within these limits, we havedeveloped very effective and, in some instances specialized,physiological responses to heat and cold. Theseresponses involve the finely controlled coordination ofseveral body systems.

11. Internal body temperature at rest is regulated atapproximately 37 °C (98.6 °F). During exercise, thebody is often unable to dissipate heat as rapidly as it isproduced. In rare circumstances, a person can reachinternal temperatures exceeding 40 °C (104 °F), with atemperature above 42 °C (107.6 °F) in active muscles.The muscles’ energy systems become more chemicallyefficient with a small increase in muscle temperature,but internal body temperatures above 40 °C canadversely affect the nervous system and reduce furtherefforts to unload excess heat.

12. The Preoptic-Anterior Hypothalamus:The Body’s Thermostat Sensoryreceptors called thermoreceptors detect changes intemperature and relay this information to the body’sthermostat, located in a region of the brain called thepreoptic-anterior hypothalamus (POAH). In response,the hypothalamus activates mechanisms that regulatethe heating or cooling of the body. Like a home thermostat,the hypothalamus has a predetermined temperature,or set point, that it tries to maintain. This isthe normal body temperature. The smallest deviationfrom this set point signals this thermoregulatory centerto readjust the body temperature.

13. Thermoreceptors are located throughout the bodybut especially in the skin and central nervous system.The peripheral receptors located in the skin monitorthe skin temperature, which varies with changes in thetemperature around a person. They provide informationnot only to the POAH but also to the cerebralcortex, which allows one to consciously perceive temperatureand voluntarily control one’s exposure to heator cold. Because the skin temperature changes longbefore core temperature, these receptors serve as anearly warning system for impending thermal challenges.

15. Body temperature is regulated by a system of sensors and controllers across the body. The brain receives signals regarding body temperature from the nerves in the skin and the blood. These signals go to the hypothalamus, which coordinates thermoregulation in the body. Signals from the hypothalamus control the sympathetic nervous system, which affects vasoconstriction, metabolism , shivering, sweating, and hormonal controls over temperature. In general, the posterior hypothalamus controls responses to cold, and the anterior hypothalamus controls responses to heat.

16. How heat transfers from the skin to the surrounding environment Heat loss is due to one or more of the following - convection, conduction, evaporation or radiation. In comfortable environments, about 65% is lost through radiation, with most of the rest through evaporation. In cold environment, most heat lost is via convection and conduction.

20. Convection(C) Convection happens when air or water with a lower temperature than the body comes into contact with the skin and then moves away. An example of convection is blowing on hot food to cool it down. The amount of heat loss depends on the temperature difference between the body and environment plus the speed with which air or water is moving.

22. Convection (C), on the other hand, involves transferringheat by the motion of a gas or a liquid acrossthe heated surface. When the body is still and there islittle air movement, a thin unstirred “boundary” layerof air surrounds the body. However, the air aroundus is usually in constant motion, especially so duringexercise as we move either the whole body or bodysegments (e.g., the arms pumping as we run) throughthe air. As air moves around us, passing over the skin,heat is exchanged with the air molecules. The greaterthe movement of the air (or liquid, such as water),the greater the rate of heat exchange by convection.

23. Convection is important on a daily basis, since itconstantly removes the metabolic heat we generate atrest and during activities of daily living, as long as theair temperature is lower than the skin temperature.

24. Conduction (K) Heat conduction (K) involves the transfer of heat fromone solid material to another through direct molecularcontact. As an example, heat can be lost from thebody when the skin is in contact with a cold object,Conversely, if a hot objectis pressed against the skin, heat from the object willbe conducted to the skin and heat will be gained bythe body. If the contact is prolonged, heat from theskin surface can be transferred to the blood as it flowsthrough the skin and transferred to the core, raisinginternal (core) temperature. During exercise, conductionis usually negligible as a source of heat exchangebecause the body surface area in contact with solidobjects (for example, soles of the feet on hot playingfields) is small.

26. Therefore, many environmental physiologiststreat conductive heat exchange as negligiblein their calculations of heat balance and exchange. If the temperature of the surroundingobjects is greater than that of the skin, thebody will experience a net heat gain via radiation. The more surface area in contact between two objects, the more quickly heat is transferred between them.

27. Radiation (R) At rest, radiation (R) and convection are the primarymethods for eliminating the body’s excess heat. Atnormal room temperature (typically 21-25 °C, or ~70-77°F), the nude body loses about 60% of its excess heat byradiation. The heat is given off in the form of infraredrays, which are a type of electromagnetic wave. The skin constantly radiates heat in all directions toobjects around it, such as clothing, furniture, and walls,but it also can receive radiant heat from surroundingobjects that are warmer.

28. A tremendous amount of radiant heat is received fromexposure to the sun. Taken together, conduction, convection, and radiationare considered avenues of dry heat exchange.Resistance to dry heat exchange is called insulation

29. Evaporation (E) evaporation (E) is the primary avenue for heatdissipation during exercise. As a fluid evaporates andturns into its gaseous form, heat is lost. Evaporationaccounts for about 80% of the total heat loss whenone is physically active and is therefore an extremelyimportant avenue for heat loss. Even at rest, evaporationaccounts for 10% to 20% of body heat loss, since someevaporation occurs without our awareness (termedinsensible water loss)

30. As body core temperature increases, once a thresholdcore temperature is reached sweat productionincreases dramatically. As sweat reaches the skin, it isconverted from a liquid to a vapor, and heat is lost fromthe skin in the process, the latent heat of vaporization.

31. Humidity and Heat Loss The water vapor pressure of the air (the pressureexerted by water vapor molecules suspended in theair) plays a major role in evaporative heat loss. Relativehumidity is a more commonly used term that relates thewater vapor pressure of the air to that of fully saturatedair (100% humidity). When humidity is high, the airalready contains many water molecules. This decreases its capacity to accept more water because the vapor pressure gradient between the skin and the air is decreased.Thus, high humidity limits sweat evaporation and heatloss, while low humidity offers an ideal opportunityfor sweat evaporation and heat loss. But this efficientcooling mechanism can also pose a problem. If sweatingis prolonged without adequate fluid replacement,dehydration can occur.

32. Heat Cramps Sweating excessively without replacing the lost fluid results in dehydration and an imbalance of body salt levels (electrolytes). As a consequence, painful cramps in the major muscles develop rapidly, but sometimes not until several hours after the event. Especially vulnerable are the hamstrings of your legs and the muscles of your arms and stomach. They become hard and painfully tense and often disable the affected person.

33. Electrolytes are chemicals that make fluids electrically conductive. You probably heard the term in relation to your car battery. The mechanic replenishes the electrolyte, in this case battery acid, when it is too low. Without electrolyte you wouldn’t have an electric current – your engine wouldn’t start. The body, too, requires electrolytes. Besides their conductive properties, body salts regulate the fluid levels in the body cells and control the function of the kidneys. The two major chemicals acting as electrolytes in the body fluids are sodium (table salt) and potassium.

34. After heavy sweating, replenish yourself with water and electrolytes. Half a teaspoon of salt dissolved in each litre of water is generally sufficient to top up the electrolyte levels. Sports drinks, or salty food together with water, are similarly effective. Rest in a cool place, out of the sun, to avoid a deterioration of the condition. See a doctor if you also have symptoms (see below) of heat exhaustion or heatstroke.

35. Heat Exhaustion The cause of heat exhaustion is similar to that of heat cramp – dehydration and/or an imbalance of body salts. In this case, however, the body’s temperature regulation system fails to adequately respond to an increase in body temperature as well. The disorder often follows overexertion in hot weather during sport or outdoor work. Elderly patients on diuretic medicines are also at great risk.  

36.  The signs and symptoms are similar to shock and include Weakness, exhaustion, fatigue Nausea and vomiting Diarrhea Heat cramps Lack of coordination, giddiness, faintness Rapid pulse and breathing Cold and clammy skin Profuse sweating.

37.  Someone showing these symptoms should be moved to a cool place, have their unnecessary garments removed and their body cooled. Lost fluids and electrolytes should be replaced. Consult a doctor if the person can’t keep the fluid down or doesn’t recover promptly. The condition is very similar to heatstroke, but the body temperature is usually less than 39°C.

38. Heat Rash The purpose of sweat is to evaporate and cool your body. Wearing non-porous covers, such as plastic baby diapers, oily make-up or tight-fitting garments, however, will hold the sweat within the glands. This may lead to an irritation of the glands and the formation of small red pimples or even blisters – symptoms of heat rash. It isn’t generally serious, but can develop into a secondary skin infection. Hot and humid weather is almost always the cause, but obesity, genetic factors and sensitive skin also add to your chances of heat rash.

39. Heat rash – also known as prickly heat and baby rash –  is more common amongst the very young, because their underdeveloped sweat glands clog easily. Even in winter an overdressed infant in a wet diaper can develop the pimples between the legs and on the buttocks.

40. Prevent heat rash by removing the cause of the sweat gland blockage. Don’t wear tight-fitting and non-porous garments in the summer heat. Avoid oily ointments and creams where possible and wash off any sweat or dirt. Change baby diapers regularly and apply moisture-absorbing powder. If prevention comes too late, your pharmacy has antiseptic cleansers or soothing remedies.

41. Heat Stroke Heatstroke is the most dangerous of all heat-related illnesses and requires immediate medical attention. I have previously explained the limitations of the body’s ability to regulate its temperature. When the self-cooling process is stressed beyond its capabilities, it may collapse completely. The condition becomes life threatening and, despite medical attention, approximately 10% of heatstroke patients die. The rate is much higher during heat wave conditions or in regions where medical help is limited.

42. A healthy person is not likely to succumb to heat and high humidity unless that person increases their body temperature during work or exercise in hot conditions. The elderly and the very young with a deficient or underdeveloped heat regulation mechanism, however, are always at risk to suffer from heatstroke, with or without physical activity. Chronic illnesses, genetic makeup and some types of medication can also increase the risk.

43. The signs and symptoms of heatstroke are: Body temperature climbs to 40.5°C or higher Headache Nausea, vomiting Visual disturbances Altered mental state whereby dizziness, irritability, confusion, progression to seizures and unconsciousness is possible Rapid pulse Flushed and usually dry skin. Sweat can be present in exertional heatstroke

44. Recognition of heatstroke symptoms is vital to allow prompt medical attention. If the patient isn’t cooled immediately, the high body temperature will damage the tissue of almost every organ. Muscle meltdown (rhabdomyolisis) and blood clotting (thrombosis) often accompany heatstroke.

45. Physiological Responses to Exercise in the Heat Heat production is beneficial during exercise in a cold environment because it helps maintain normal body temperature. However, even when exercise is per- formed in a cool environment, the metabolic heat load places a considerable burden on the mechanisms that control body temperature.

46. Cardiovascular Function Exercise increases the demands on the cardiovascular system. When the need to regulate body temperature is added during exercise in the heat, the burden placed on the cardiovascular system is enhanced. During exercise in hot conditions, the circulatory system has to continue to transport blood not only to working muscle but also to the skin, where the tremendous heat generated by the muscles can be transferred to the environment.

47. To meet this dual demand during exercise in the heat, two changes occur. cardiac output increases further (above that associated with a similar exercise intensity in cool conditions) by increasing both heart rate and contractility. blood flow is shunted away from nonessential areas like the gut, liver, and kidneys and to the skin.

48. The aerobic exercise increases both metabolic heat production and the demand for blood flow and oxygen delivery to the working muscles. This excess heat can be dissipated only if blood flow increases to the skin.

49. In response to the elevated core temperature (and to a lesser extent, the higher skin temperature), the SNS signals sent from the POAH to the skin arterioles cause these blood vessels to dilate, delivering more metabolic heat to the body surface. Sympathetic nervous system signals also go to the heart to increase heart rate and cause the left ventricle to pump more forcefully.

50. However, the ability to increase stroke volume is limited as blood pools in the periphery and less returns to the left atrium. To maintain cardiac output under such circumstances, the heart rate gradually creeps upward to help compensate for the decrease in stroke volume.