Thermal Physiology I: Heat Balance

Describe and understand the different definitions of animal thermoregulation

There are many different terms that can be used to describe how animals regulate body heat:

• Poikilothermic- Literally means "change heat." Body temperature changes according to ambient temperature.
• Homeothermic- Literally means "same heat." Body temperature remains constant despite changes in ambient temperature.
• Heterothermic- Somewhere between poikilothermic and heterothermic.
• Endothermic- Body heat is generated from metabolic processes.
• Ectothermic- The animal seeks an external source of body heat.

Describe, understand, and be able to calculate the Q10 effect of temperature on processes
Understand why changes in body temperature have profound effects on all life processes

The Q10 effect is basically a ratio of the reaction rates at a given temperature, and at another temperature that is 10 degrees higher than the given temperature. It is calculated as (reaction rate at higher temperature) / (reaction rate at given temperature). Generally, increasing temperature increases reaction rate (as the energy of the particles is brought closer to activation energy), but at high enough temperatures you might also have problems with denaturation of enzymes. As we're basically just big bags of chemical reactions, this is pretty bloody important.

Describe and understand the relationship between heat capacity and temperature

Heat capacity is the amount of energy that it takes to raise body temperature by 1 degree. For most animal tissue, heat capacity is around 3.5 joules/gram/°C. In other words, if the amount of heat energy generated (by metabolic processes etc.) in 1g of animal tissue is 3.5J higher than the amount of heat that is lost, then that 1g of animal tissue will become 1°C warmer.

Describe and understand the heat balance equation including its individual components, what they mean, and how they are affected by various environmental and biological factors
Be able to calculate the heat balance and its effect on body temperature given the relevant information

The heat balance equation is as follows:

Heat storage (S) = Metabolic heat production (M) ± Conductive heat exchange ± Convective heat exchange ± Radiative heat exchange ± Evaporative heat exchange (E) - External work (W)

which can be abbreviated to:

S = M ± Cond ± Conv ± Rad ± E - W

In order for body temperature to remain constant, S must be equal to 0. If S is not equal to 0, then body temperature will change. As mentioned above, since the heat capacity of animal tissue is 3.5 joules/gram/°C (or 3.5 kJ/kg/°C), for every 3.5J increase in S, 1g of animal tissue will increase in temperature by 1°C.

If you haven't done physics (like me :P), here are some quick definitions for the four main methods of heat exchange (conduction, convection, radiation, evaporation):

• Conduction- Transfer of thermal energy by direct contact (i.e. you transfer heat to things that you touch, and things that you touch transfer heat to you).
• Convection- Transfer of thermal energy to a moving fluid. This can be natural, like the buoyancy effect (heat rising), or forced (e.g. wind).
• Radiation- Transfer of thermal energy by electromagnetic radiation. The amount of radiation emitted depends on their surface temperature. The temperature and maximum wavelength are related according to Wien's law: λ_max = 2.898 * 10⁶ / T (°K).
• Evaporation- Transfer of thermal energy as latent heat of evaporation (2400J/g of water).

Describe and understand body temperature classifications of hypo- and hyperthermia

Hypothermia: Body temperature less than around 36°C.
Hyperthermia: Body temperature greater than around 38°C.

Describe and understand the basic components of the thermoregulatory control system in homeothermic animals

Just a warning from here on out: I wasn't able to attend this lecture in person, and the lecture recorded without sound. A lot of the other points on this post were not covered in the replacement lectures, so I'm basically just gonna wing the rest of this post by looking at the lecture slides.

Just like in other control systems, the thermoregulatory control system needs certain components. These components are: the variable to be regulated (skin temperature), receptors that measure the variable (thermosensors in the skin and hypothalamus), some kind of command centre where the information is integrated and then new information is sent out (presumably the CNS???), and the effectors that regulate the variable (skin blood flow, sweating, shivering and behavioural changes).

Describe and understand how peripheral and central temperatures are integrated by the control system to stimulate effector mechanisms that result in the regulation of body temperature

Both the skin and hypothalamus have cold and warm thermosensors. In the skin, the cold receptors are called A delta, and the warm receptors are C fibres. (Not sure if they have different names in the hypothalamus.) Warm sensors increase firing when they are warm, and cool sensors increase firing when they are cool.

Describe and understand the four main effector systems in thermoregulation and how they are controlled 

The four main effector systems are control of skin blood flow, sweating, shivering and behaviour changes.

Skin blood flow

At a comfortable temperature, our skin blood flow is around 25 mL/min.100g. This can decrease to 1 mL/min.100g in the cold, and increase to 150mL/min.100g in heat. The skin of the hands, feet, ears and nose have arteriovenous anastomoses (AVAs) which, when open, shunt blood directly to the veins (and not to the skin). AVAs are opened when warm, and closed when cold, thus directing blood flow to meet our needs for warmth. Indeed, changing skin temperature can adjust heat loss by a factor of 8.

Sweating

As you may (or may not) remember from second year, nerves that innervate sweat glands are part of the sympathetic nervous system, but they secrete acetylcholine rather than noradrenaline. These nerves are also called sudomotor nerves. There are two types of sweat gland: eccrine glands, which cover most of the body surface and are mainly responsible for thermoregulation, and apocrine glands, which are pretty much only found on the armpit and pubis. Sweating increases pretty much linearly once head temperature is higher than a set point. (Skin temperature also helps to determine the set point.)

Shivering

Shivering is essentially heat production by moving your muscles. It begins once head temperature drops below a set point. (Once again, skin temperature also plays a role in determining set point.)

Behavioural Changes

This wasn't covered at all, but I'm pretty sure that this just entails wearing more clothes when you're cold, and taking off clothes when you're hot.