HLTH 2015: Physiology of Training and Environmental Extremes - Thermoregulation

Thermoregulation

Thermoregulation is the body's way of maintaining a stable core temperature, which is especially important during exercise in different environments. This process involves mechanisms like sweating, shivering, and changes in blood flow to manage heat. Understanding thermoregulation includes learning about heat, how the body gains and loses heat, the theories behind thermoregulation, how to assess environmental temperatures, and what happens in extreme temperatures.

What is Heat?

Heat is a form of energy that measures the vibration of molecules. It naturally moves from areas of higher temperature to lower temperature, and it's impossible to have negative heat.

Mechanisms of Heat Gain and Loss

The body loses heat through radiation, conduction, convection, evaporation, and circulatory adjustments. It gains heat through metabolic processes, muscular activity, hormones, the thermic effect of food, posture, the environment, and circulatory adjustments.

Interacting Mechanisms of Heat Balance

Heat is exchanged with the environment through radiation, conduction, convection, evaporation, and adjustments in blood circulation.

Radiation

Objects emit infrared rays, causing heat to move from warmer to cooler objects. At rest, radiation accounts for about 60% of excess heat loss when the ambient temperature is between 21 and 25^\circ C.

Conduction

Conduction is the transfer of heat through direct contact. Blood circulation helps carry heat to the skin's surface for this process.

Convection

Convection involves the transfer of heat through the movement of liquids or gases. The rate at which air or water is replaced near the body affects how quickly cooling occurs.

Evaporative Cooling

Evaporative cooling is a major defense against overheating. As sweat evaporates from the skin and respiratory passages, it cools the body. Each liter of sweat that evaporates removes 580 kcal of heat.

Thermoregulation

Humans are homeotherms, meaning they maintain a constant body temperature to support vital processes like oxygen transport and metabolism.

Measuring Body Temperature

Core temperature (Tc) is measured rectally, which is typically 0.6^\circ C higher than oral measurements. It can also be estimated using auditory canal and esophageal measurements. Skin thermistors (Tsk) are also used.

Core Temperature

Normal core temperature ranges from 36.5 to 37.5^\circ C. The body regulates temperature efficiently between 35 and 40^\circ C. If Tc exceeds 41^\circ C, it can cause cell damage, while a Tc below 34^\circ C can slow metabolism and lead to arrhythmias.

Mean Body Temperature (MBT)

Mean body temperature (MBT) is calculated using the formula \MBT = (0.33 \times Tsk) + (0.67 \times TR), which helps determine the direction of heat transfer. Skin temperature (Tsk) is determined by \Tsk = (0.5 \times Tst) + (0.14 \times Tfa) + (0.36 \times Tca).

Hypothalamic Temperature Regulation

The hypothalamus, receiving input from skin receptors, centrally controls thermal balance. It manages heat storage by either increasing heat production or heat loss as needed.

Hypothalamic Reaction to Heat

When the hypothalamus overheats, it stimulates heat loss by inhibiting the vasomotor center, which increases skin blood flow and sweat gland activity.

Stimulus for Thermoregulation?

Thermoregulation can be triggered by changes in core temperature (feedback) or environmental changes (feedforward). Regulatory models include the set-point model and the heat regulation model.

Set-Point Temperature Regulation Model

In the set-point model, the body defends a specific core temperature, and the range between sweating and shivering is small (0.6°C). The set point can vary due to factors like menstrual timing, dehydration, starvation, and fever.

Heat Regulation Model

The heat regulation model regulates overall heat storage rather than just temperature. However, it's difficult to prove due to the lack of flow sensors.

Set-Point vs. Heat Regulation

These models explain why sweat rate declines after stopping exercise and why sweating begins immediately in hot conditions.

Evaluating the Environment

Factors to consider include air temperature, humidity, air currents, radiant heat, exercise intensity, training level, acclimatization, and clothing.

Evaluating the Environment

Assess the environment using dry bulb, wet bulb, and globe temperatures.

Relative Humidity (RH%)

Relative humidity measures the amount of water in the air relative to the total possible moisture.

Relative Humidity

Relative humidity is critical for evaporative heat loss. High humidity impairs cooling, leading to dangerous rises in core temperature above 35^\circ C when humidity exceeds 60%.

Measuring Hot

The heat stress index combines temperature and humidity. The Wet Bulb Globe Temperature (WBGT) uses ambient, wet bulb, and globe temperatures. The formulas for WBGT vary depending on sun exposure:

Outdoors: \WBGT = 0.7 \times Temp{wet bulb} + 0.2 \times Temp{globe} + 0.1 \times Temp_{air}

Indoors: \WBGT = 0.7 \times Temp{wet bulb} + 0.3 \times Temp{globe}

Australian Open Heat Stress Scale

This scale considers radiant heat, humidity, wind speed, air temperature, hydration, playing conditions, breaks, and play suspension.