ThermoregulationThermoregulation

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

heat production mechanisms and heat loss heat production mechanisms and heat loss mechanisms that occur to maintain a constant mechanisms that occur to maintain a constant body temperature. body temperature.

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

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.

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.

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.

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.

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.

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.

Thermoregulatory Control

the temperature limits for living cells range from about 0 °C (where ice crystals form) to about 45 °Cand humans can tolerate internal temperatures below 35 °C or above 41 °C for only very brief periods of time. To maintain internal temperature within these limits, we have developed very effective and, in some instances specialized, physiological responses to heat and cold. These responses involve the finely controlled coordination of several body systems.

Internal body temperature at rest is regulated at approximately 37 °C (98.6 °F). During exercise, the body is often unable to dissipate heat as rapidly as it is produced. In rare circumstances, a person can reach internal temperatures exceeding 40 °C (104 °F), with a temperature above 42 °C (107.6 °F) in active muscles. The muscles’ energy systems become more chemically efficient with a small increase in muscle temperature, but internal body temperatures above 40 °C can adversely affect the nervous system and reduce further efforts to unload excess heat.

The Preoptic-Anterior Hypothalamus:The Body’s Thermostat

Sensory receptors called thermoreceptors detect changes in temperature and relay this information to the body’s thermostat, located in a region of the brain called the preoptic-anterior hypothalamus (POAH). In response, the hypothalamus activates mechanisms that regulate the 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 is the normal body temperature. The smallest deviation from this set point signals this thermoregulatory center to readjust the body temperature.

Thermoreceptors are located throughout the body but especially in the skin and central nervous system. The peripheral receptors located in the skin monitor the skin temperature, which varies with changes in the temperature around a person. They provide information not only to the POAH but also to the cerebral cortex, which allows one to consciously perceive temperature and voluntarily control one’s exposure to heat or cold. Because the skin temperature changes long before core temperature, these receptors serve as an early warning system for impending thermal challenges.

Temperature (C) Symptoms

28 muscle failure

30 loss of body temp control

33 loss of consciousness

37 normal

42 central nervous system breakdown

44 death

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.

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.

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.

Convection (C), on the other hand, involves transferring heat by the motion of a gas or a liquid across the heated surface. When the body is still and there is little air movement, a thin unstirred “boundary” layer of air surrounds the body. However, the air around us is usually in constant motion, especially so during exercise as we move either the whole body or body segments (e.g., the arms pumping as we run) through the air. As air moves around us, passing over the skin,heat is exchanged with the air molecules. The greater the movement of the air (or liquid, such as water), the greater the rate of heat exchange by convection.

Convection is important on a daily basis, since it constantly removes the metabolic heat we generate at rest and during activities of daily living, as long as the air temperature is lower than the skin temperature.

Conduction (K)

Heat conduction (K) involves the transfer of heat from one solid material to another through direct molecular contact. As an example, heat can be lost from the body when the skin is in contact with a cold object, Conversely, if a hot object is pressed against the skin, heat from the object will be conducted to the skin and heat will be gained by the body. If the contact is prolonged, heat from the skin surface can be transferred to the blood as it flows through the skin and transferred to the core, raising internal (core) temperature. During exercise, conduction is usually negligible as a source of heat exchange because the body surface area in contact with solid objects (for example, soles of the feet on hot playing fields) is small.

Therefore, many environmental physiologists treat conductive heat exchange as negligible in their calculations of heat balance and exchange.

If the temperature of the surrounding objects is greater than that of the skin, the body 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.

Radiation (R)

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

A tremendous amount of radiant heat is received from exposure to the sun.

Taken together, conduction, convection, and radiation are considered avenues of dry heat exchange. Resistance to dry heat exchange is called insulation

Evaporation (E)

evaporation (E) is the primary avenue for heat dissipation during exercise. As a fluid evaporates and turns into its gaseous form, heat is lost. Evaporation accounts for about 80% of the total heat loss when one is physically active and is therefore an extremely important avenue for heat loss. Even at rest, evaporation accounts for 10% to 20% of body heat loss, since some evaporation occurs without our awareness (termed insensible water loss)

As body core temperature increases, once a threshold core temperature is reached sweat production increases dramatically. As sweat reaches the skin, it is converted from a liquid to a vapor, and heat is lost from the skin in the process, the latent heat of vaporization.

Humidity and Heat Loss

The water vapor pressure of the air (the pressure exerted by water vapor molecules suspended in the air) plays a major role in evaporative heat loss. Relative humidity is a more commonly used term that relates the water vapor pressure of the air to that of fully saturated air (100% humidity). When humidity is high, the air already 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 heat loss, while low humidity offers an ideal opportunity for sweat evaporation and heat loss. But this efficient cooling mechanism can also pose a problem. If sweating is prolonged without adequate fluid replacement, dehydration can occur.

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.

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.

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.

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.  

 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.

 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.

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.

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.

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.

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.

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.

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

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.

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.

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.

To meet this dual demand during exercise in the heat, two changes occur.

1. cardiac output increases further (above that associated with a similar exercise intensity in cool conditions) by increasing both heart rate and contractility.

2. blood flow is shunted away from nonessential areas like the gut, liver, and kidneys and to the skin.

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.

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.

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.

Because blood volume stays constant or even decreases (as fluid is lost in sweat), another phase of cardiovascular adjustment occurs simultaneously. Sympathetic nerve signals to the kidneys, liver, and intestines cause vasoconstriction of those regional circulations, which allows more of the available cardiac output to reach the skin without compromising muscle blood flow.

Exercising in hot environments sets up a competition between the active muscles and the skin for a limited blood supply. The muscles need blood and the oxygen it delivers to sustain activity; the skin needs blood to facilitate heat loss to keep the body cool.

The sweat glands are controlled by stimula- tion of the POAH. Elevated blood temperature causes this region of the hypothalamus to transmit impulses through the sympathetic nerve fibers to the millions of sweat glands distributed over the body’s surface. The sweat glands are fairly simple tubular structures extending through the dermis and epidermis, opening onto the skin.

Loss of both electrolytes and water in the sweat trig- gers the release of both aldosterone and antidiuretic hormone (ADH), also known as vasopressin or arginine vasopressin. Recall that aldosterone is responsible for maintaining appropriate sodium concentrations in the blood and that ADH plays a key role in maintain- ing fluid balance

Acclimation to Exercise in the Heat

Does repeated exercise in the heat make us better able to tolerate thermal stress?

Effects of Heat Acclimation:- Repeated bouts of prolonged, low-intensity

exercise in the heat cause a relatively rapid improvement in the ability to maintain cardiovascular function and eliminate excess body heat, which reduces physiological strain.

This process, termed heat acclimation, involves changes in plasma volume, cardiovascular function, sweating, and skin blood flow that allow for subsequent exercise bouts in the heat to be performed with a lower core temperature and heart rate response.

Because the body’s heat loss capacity at a given rate of work is enhanced by acclimation, core temperature during exercise increases less than before acclimation and heart rate increases less in response to standardized submaximal exercise after heat acclimation

In addition, after heat acclimation, more work can be done before adverse symptoms occur or a maximal tolerable core temperature or heart rate is reached. This series of positive adaptations typically takes a period of 9 to 14 days of exercise in the heat to fully occur.

Well-trained individuals need fewer exposures than untrained individuals to fully acclimate. A critical physiological adjustment that occurs over the first one to three days of acclimation is the expansion of plasma volume. The exact mechanism by which plasma volume expands after these initial exercise-heat exposures is not universally agreed upon

. The process likely involves (1) proteins being forced out of the circulation as muscles contract, (2) these same proteins then being returned to the blood through the lymph, and (3) fluid moving into the blood because of the oncotic pressure exerted by the increased protein content.

However, this change is temporary, and blood volume usually returns to original levels within 10 days. This early expansion of blood volume is important because it supports stroke volume, allowing the body to maintain cardiac output while additional physiological adjustments are made.

An athlete must exercise in a hot environment to attain full acclimation that sustains exercise in the heat. Simply sitting in a hot environment, such as a sauna or steam room, for long periods each day will not fully or adequately prepare the individual for physical exertion in the heat, at least not to the same extent as will exercising in the heat.

Exercise in the Cold

The hypothalamus has a temperature “set point” of about 37 °C (98.6 °F), but daily fluctuations in the body temperature can be as much as 1 °C. A decrease in either skin or blood temperature provides feedback to the thermoregulatory center (POAH) to activate mechanisms that conserve body heat and increase heat production.

The primary means by which our bodies avoid excessive heat loss (in the order in which they are invoked) are peripheral vasoconstriction, nonshivering thermogenesis, and shivering.

When exercising in the cold, people should not overdress. Overdressing can cause the body to become hot and initiate sweating. As the sweat soaks through the clothing, evaporation removes the heat, and heat is lost at an even faster rate.

Peripheral vasoconstriction

It occurs as a result of sympathetic stimulation of the smooth muscle layers of the arterioles in the skin. This stimulation causes the vascular smooth muscle to contract, which constricts the arterioles, reduces the blood flow to the shell of the body, and minimizes heat loss.

When changing skin blood flow alone is not adequate to prevent heat loss, non- shivering thermogenesis—stimulation of metabolism by the SNS—is increased. Increasing the metabolic rate increases heat production.

The next line of defense of body temperature during cold stress is shivering, a rapid, involuntary cycle of contraction and relaxation of skeletal muscles, which can cause a four- to fivefold increase in the body’s rate of heat production.

Body Size and Composition

Insulating the body against the cold is the most obvious form of protection against hypothermia. Insulation is defined as resistance to dry heat exchange through radiation, convection, and conduction. Both inactive peripheral muscles and subcutaneous fat are excellent insulators. Subcutaneous fat thickness are a good indicator of an individual’s tolerance for cold exposure.

The thermal conductivity of fat (its capacity for transferring heat) is relatively low, so fat impedes heat transfer from the deep tissues to the body surface. People who have more fat mass conserve heat more efficiently than smaller, leaner individuals in the cold. The rate of heat loss also is affected by the ratio of body surface area to body mass. Larger individuals have a small surface area-to-mass ratio, which makes them less susceptible to hypothermia.

Physiological Responses to Exercise in the Cold

Muscle Function

Cooling a muscle causes it to contract with less force. The nervous system responds to muscle cooling by altering the normal muscle fiber recruitment patterns for force development, which may decrease the effi- ciency of the muscle’s actions. Both muscle shortening velocity and power decrease significantly when muscle temperature is lowered.

Metabolic Responses

Prolonged exercise increases the mobilization and oxidation of free fatty acids (FFAs) as a fuel source. The primary stimulus for this increased lipid metabolism is the release of catecholamines (epinephrine and norepinephrine).

Exposure to cold markedly increases epinephrine and norepinephrine secretion, but FFA levels increase substantially less than during prolonged exercise in warmer conditions. Cold exposure triggers vasoconstriction in the vessels supplying not only the skin but fatty subcutaneous tissues as well.

The subcutaneous fat is a major storage site for lipids (as adipose tissue), so this vasoconstriction reduces the blood flow to an important area from which the FFAs would be mobilized. Thus, FFA levels do not increase as much as the elevated levels of epinephrine and norepinephrine would predict.

Blood glucose plays an important role in both cold tolerance and exercise endurance. For example, hypo-glycemia (low blood sugar) suppresses shivering. The reasons for these changes are unknown.

Health Risks During Exercise in the Cold

Hypothermia Individuals immersed in near-freezing water

will die within a few minutes when their rectal temperature decreases from a normal level of 37 °C (98.6 °F) to 24 or 25 °C (75.2 or 77 °F).

Once core temperature falls below about 34.5 °C (94.1 °F), the hypothalamus begins to lose its ability to regulate body temperature. This ability is completely lost when the internal temperature decreases to about 29.5 °C (85.1 °F).

Cardiorespiratory Effects

Hypothermia critically affects the heart’s sinoatrial node, decreasing the heart rate, which in turn reduces cardiac output. Breathing cold air does not freeze the respiratory passages or the lungs because the inspired air is progressively warmed as it moves through the respiratory tract. Exposure to extreme cold decreases respiratory rate and volume.

Frostbite

Exposed skin can freeze when its temperature is lowered just a few degrees below the freezing point (0 °C or 32 °F). peripheral vasoconstriction helps the body retain heat. Unfortunately, during exposure to extreme cold, the circulation in the skin can decrease to the point that the tissue dies from lack of oxygen and nutrients. This is commonly called frostbite. . If not treated early, frostbite injuries can be serious, leading to gangrene and loss of tissue.

Exercise-Induced Asthma

Exercise-induced asthma is a common problem that affects as many as 50% of winter sport athletes. The main cause of this syndrome is the drying of the airways due to the combination of the high respiration rate associated with exercise and the extremely dry air as temperature drops.

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