Body temperature regulation during exercise. . Humans regulate heat generation and preservation to maintain internal body temperature or core temperature. Normal core temperature at rest varies between 36.5 and 37.5 °Celsius (°C), which is 97.7 to 99.5 °Fahrenheit (°F). Core temperature is regulated by the hypothalamus (in the brain), which is often called the body’s thermostat.
During all types of exercise the body’s ability to thermo regulate is challenged. Heat is produced as a bi-product of metabolism (metabolism is defined as all of the reactions that occur in the human body). However, the human body is only 25% efficient, therefore you lose approximately 75% of energy as heat. During exercise, heat is produced mainly from working muscle contractions and core temperature can go above 40 °C (104 °F).
How does the body lose heat? • . Heat can be lost through the processes of conduction, convection, radiation, and evaporation. Conduction is the process of losing heat through physical contact with another object or body.
Convection is the process of losing heat through the movement of air or water molecules across the skin. The use of a fan to cool off the body is one example of convection.The amount of heat loss from convection is dependent upon the airflow or in aquatic exercise, the water flow over the skin.
Radiation is a form of heat loss through infrared rays. This involves the transfer of heat from one object to another, with no physical contact involved. • . Evaporation is the process of losing heat through the conversion of water to gas (evaporation of sweat).
How much water is in the body? • Water makes up approximately 60% of your total body composition. In addition, 73% of lean body mass or muscle is composed of water. It is the essential nutrient for survival and is required for all cell functions. Water is also an important constituent in thermoregulation, because it is a major component of blood volume.
Sweat Basics • The average person has 2.6 million sweat glands. Sweat is made up of water and electrolytes such as sodium, chloride, and potassium. When the hypothalamus senses an increase in core temperature it will act by increasing blood flow to the skin, stimulating the sweat glands. The result is an increase in the rate of water lost through sweating.
During low- to moderate-intensity exercise of less than one hour, there are minimal electrolyte losses because the body reabsorbs most of the electrolytes from the sweat.
However, during moderate- to high-intensity exercise of greater than one hour, the electrolyte loss in sweat becomes significant and the sweat rate is too fast for re-absorption of electrolytes.
Heat Production During Exercise • Increased muscular activity during exercise causes an increase in heat production in the body due to the inefficiency of the metabolic reactions involved in providing energy for muscle force development.
For every litter of oxygen consumed during exercise such as cycling or running approximately 16 kJ of heat is produced and only 4 kJ is actually used to perform mechanical work.
Thus, for an athlete consuming oxygen at a rate of 4 litters/minute during exercise, the rate of heat production in the body will be about 16000 x 4 / 60 = 800 J/s or watts (W) or 16 x 4 x 60 / 1000 = 3.84 MJ/hour.
Only a small proportion of the heat produced in active skeletal muscle is lost from the overlying skin. Rather, most of the heat is passed to the body core via the flow of venous blood returning to the heart.
. This rate of heat storage cannot persist, as the muscle contractile proteins and enzymes would be inactivated by heat-induced denaturation within 10 minutes.
Thus, most of the heat generated in the muscle is transferred to the body core and increases in body core temperature are sensed by thermo receptors located in the hypothalamus.
This area of the brain also receives sensory input from skin thermo receptors and integrates this information is used to produce appropriate reflex effectors responses - increasing blood flow to the skin and initiating sweating - to increase heat loss and limit further rises in body temperature.
The set point of the hypothalamic thermostat does not change during exercise, but some heat storage does occur. When the rate of heat loss from the body equals the rate of heat production, the rise in body temperature plateaus.
Heat Loss Through Sweating • Environmental heat stress is determined by the ambient temperature, relative humidity, wind velocity and solar radiation: both directly from the sun and reflected from the ground. The relative humidity is the most important of these factors, since a high humidity will severely compromise the evaporative loss of sweat..
Sweat must evaporate from the body surface in order to exert a cooling effect. Evaporation of 1 liter of water from the skin will remove 2.4 MJ of heat from the body. It follows that sweat rate during exercise would have to be at least 1.6 litters/minute if all the heat produced was to be dissipated by evaporative loss alone
. A reduction in skin blood flow and sweat rate as the body becomes progressively dehydrated or a high humidity limiting evaporative loss of sweat will lead to further rises in core temperature, resulting in fatigue and possible heat injury to body tissues. The latter is potentially fatal.
environmental heat stress • A useful index of environmental heat stress is the wet bulb globe temperature (WBGT). • This is calculated as follows: • WBGT = 0.7Twb + 0.2 Tbg + 0.1 Tdb • where Twb is the temperature (in oC) of a wet bulb thermometer, Tdb is the temperature of a dry bulb thermometer and Tbg is the temperature of a black globe thermometer.
Heat loss via the evaporation of sweat will be largely determined by the water vapour pressure (humidity) of the air close to the body's surface. The local humidity may be high if inappropriate, poorly ventilated clothing is worn, reducing the flow of air over the skin surface.
heat stroke • . Body temperatures of 36-38oC can be considered as the normal range at rest and may rise to 38-40oC during exercise. Further increases are commonly associated with heat exhaustion and occasionally with heat stroke, a life-threatening disorder
It is characterised by a lack of consciousness following exertion and by clinical symptoms of damage to the brain, liver and kidneys. It is important to realise that the elevated core temperature associated with exercise is not regulated at its elevated level
Heat Loss Via Increased Skin Blood Flow • The other crucial effector mechanism in thermoregulation during exercise in the heat is the increased flow of blood through the skin capillaries. This allows increased loss of heat from the body core to the environment by radiation and convection.
To a large degree the effectiveness of this route of heat loss is also dependent on the body surface available for heat exchange with the environment and the temperature gradient between the body surface and the surrounding atmosphere.
Heat Sensors (Thermo receptors) In The Body • peripheral thermo receptors, located in the skin, provide advance warning of environmental heat input. • Central thermo receptors located in the hypothalamus (and spinal cord in some species) are sensitive to changes in internal core temperature and effectively monitor the temperature of blood flowing to the brain.
nervous signals • include nervous signals from osmoreceptors and pressure receptors, so that changes in • plasma osmolarity and blood volume are capable of affecting sweating and skin blood vessel responses to rises in core temperature.
1. Some hormones (e.g. estrogen) and cytokines (e.g. interleukin-1) are also capable of influencing thermoregulatory responses. Interleukin-1, also known as endogenous pyrogen, is secreted from macrophages and is responsible for raising the set point temperature of the hypothalamic "thermostat"" causing the rise in core temperature during fever.
training • it seems that there is a decreased set point temperature as a result of training in endurance athletes. It has also been reported that these individuals have a lower resting metabolic rate in thermo neutral conditions and a lower skin temperature.
Exercise training improves thermoregulation in the heat by an earlier onset of sweat secretion, and increasing the total amount of sweat that can be produced. Thus, training induces an increase in the sensitivity of the sweat rate/core temperature relationship
Other adaptations to training include an increase in total blood volume and maximal cardiac output. As a result, blood flow in muscle and skin, with its heat flux, is better preserved during strenuous exercise in the heat.
There Is No Adaptation To Dehydration • It is important to know that the body does not adapt to dehydration, so that exercising in the heat without fluid intake does not confer an additional adaptation in thermoregulation. In fact, progressive dehydration during exercise in the heat reduces the sensitivity of the sweat rate/core temperature relationship
Cardiovascular Function • . To accomplish this task during exercise in the heat, a large portion of cardiac output must be taken up by the skin and the exercising muscles. Because the volume of blood in circulation is limited, exercise poses a complex problem: An increase in blood flow to one of these areas automatically decreases blood flow to the others. This can lead to potentially problematic situations.
the cardiovascular system has to make some incredible adjustments. The subsequent redistribution of blood reduces the overall volume of blood that returns to the heart, which reduces the end-diastolic volume. This, in turn, reduces stroke volume.
Above 80% of maximal cardiac output, blood flow to the skin will decrease due to the blood being diverted to the muscles. As a result, adequate heat will not be able to dissipate into the external environment. Performance will be adversely affected.
Energy Production • Exercise in the heat hastens glycogen depletion and increases muscle lactate levels, both of which are antecedents of sensations of fatigue and exhaustion. Increased muscle temperature may impair skeletal muscle function and metabolism, which can also lead to fatigue
Moreover, increased carbohydrate utilization appears to be directly linked to the increased secretion of epinephrine with elevated body temperature (hyperthermia(
impulses are transmitted from the hypothalamus through the sympathetic nerve fibers to the millions of sweat glands distributed over the body’s surface. Sweat glands are tubular structures that extend through the dermis and the epidermis, and open onto the surface of the skin.
As the filtrate sweat passes through the duct of the gland, sodium and chloride are gradually reabsorbed back into the surrounding tissues, and then into the blood. During light sweating, the filtrate sweat travels slowly through the tubules
When sweating is heavy, however, the filtrate moves more quickly through the tubules; therefore the sweat that reaches the surface is high in sodium and chloride content.
Aldosterone and Anti-Diuretic Hormone • Aldosterone is a naturally occurring steroid hormone, which is produced and secreted by the adrenal cortex. The stimulation and release of this hormone is initiated by the Na+/K+ ratio of the body and also by the hormone angiotensin (an octapeptide produced in the blood from an inactive precursor—angiotensinogen—by the enzyme rennin secreted by the kidney).
aldosterone can strongly stimulate the sweat glands, causing them to reabsorb more sodium and chloride. Potassium, calcium and magnesium are not reabsorbed by the sweat glands; therefore their concentrations in the sweat remain constant.
Aldosterone’s primary effect is on the kidney tubules, where it stimulates sodium retention and potassium excretion. The kidney ducts are stimulated to reabsorb sodium back into the bloodstream
The loss of minerals and water via sweating triggers the release of aldosterone and ant diuretic hormoneADH . ADH maintains fluid balance, while aldosterone (as discussed above) maintains appropriate sodium levels.
Heat-Related Injuries • Heat Rash: Heat rash, also called prickly heat, is a benign condition associated with a red, raised rash accompanied by sensations of prickling and tingling during sweating. • Continually drying the body with a towel can help prevent the formation of a rash.
Heat Syncope: Also known as heat collapse, heat syncope is associated with rapid physical fatigue during overexposure to heat. • . Heat syncope can be quickly relieved by laying in a cool environment and ingestion of fluids.
Heat Cramps: Heat cramps are characterized by severe cramping of the skeletal muscles, particularly the muscles involved in movement during the activity in which the athlete is participating. • intake of fluids to treat heat injuries has been well known for thousands of years.
. Heat Exhaustion: This injury is commonly accompanied by such symptoms as extreme fatigue, breathlessness, dizziness, vomiting, fainting, cold and clammy skin, or hot and dry skin, hypotension (low blood pressure), and a weak, rapid pulse. Heat exhaustion is caused by the cardiovascular system’s inability to adequately meet the body’s needs.