Thermoregulation Why Regulate Heat With the exception of some prokaryotes, life can only exist between temperatures of about -2C and about 50C. There are several reasons for this: Enzymes are temperature sensitive Many membrane proteins can only
function properly if they float freely in the lipid bilayer, which is only possible if it is liquid; at low temperature the bilayer freezes. Why Regulate Heat At around 85C, the hydrogen bonds holding the two strands of DNA together break, causing it to become single
stranded. Thermoregulation This is the regulation of body temperature. Homeothermy is the ability to regulate body temperature. Includes birds and mammals Remember poikilothermic and homeothermic.
Homeothermic is hard to define as there are animals that can partially regulate their temperature by their behaviour. Thermoregulation The most useful distinction is based on how temperature is regulated. Ectotherms regulate their body temperature by their behaviour; they can
only maintain their body temperatures above the ambient temperature by absorbing radiant heat. Ecto means outside. Thermoregulation Endotherms keep warm using heat generated inside the body; they regulate their heat loss by physiological
mechanisms in the skin. Endo means inside. Body Temperature in Endotherms Most mammals have body temperatures between 37-39C Birds have slightly higher temperatures between 40-42C Our body temperatures varies slightly for a
number of reasons. In healthy humans the average is about 35.8C in the early morning and about 37.3C in the evening. (for precise comparisons temp should be taken at the same time each day) Body Temperature in Endotherms Temperature varies with level of activity, and may rise to 40C in vigorous exercise.
Women in the second phase of their menstrual cycle have temperatures about 0.3C higher than the first phase. Body temperature varies from person to person suggesting that there may be genetic factors involved. Body Temperature in Endotherms Many endotherms hibernate in winter,
body temperature falling to a degree or two above the ambient temperature. Temperature also varies from one part of the body to another Though the core temperature in deeper parts of the body does not fluctuate much , in the outer shell (especially the limbs) it varies considerably. Different organs have slight differences,
reflecting variation in heat production. How We Lose and Gain Heat Heat can be gained or lost from any place where the body is in contact with the environment (skin and lungs) Heat can be gained or lost by: Conduction Convection
Radiation Evaporation Direct and reflected solar radiation Evaporation
from lungs Evaporation from Skin Conduction and Convection
Radiation Conduction from ground Conduction to ground Conduction
When your body is in direct contact with a cooler object you lose heat by conduction. Heat is also lost by conduction through the gut wall and the lungs. The rate at which heat is lost by conduction is affected by 2 factors: The thermal conductivity of the material next to the skin. A naked person is quite comfortable in air at 20C, but quickly feels cold in water at the
same temperature. Water conducts heat faster than air. Conduction The temperature gradient, the steeper the thermal gradient, the faster the conduction. Convection This is the transfer of heat by currents in
gases or liquids. This is why cold air feels much colder when there is a wind blowing. i.e. wind chill and how a fan makes you feel cooler. 40F = 4.4C 10F = -12.2C
Wind Chill Radiation Absorption of heat from the sun or a heat source such as a fire. Evaporation The change from liquid to gas absorbs latent heat, which produces a cooling
effect. To keep cooler than the surroundings animals have to lose water sweating. In very hot weather a person may produce as much as 1.5L of sweat per hour. Water is also lost from the respiratory system.
The Skin This is the largest organ in the human body. The skin plays a vital role in thermoregulation in two ways: It contains receptors that detect changes in the environmental temperature It contains effectors that can vary the rate of heat loss from the body.
The Skin The Skin Consists of two distinct layers: An outer epidermis Has to withstand wear and tear. The innermost cells are in constant mitotic division and form the Malpighian layer
As they differentiate the epidermal cells produce large amounts of the tough, fibrous protein, Keratin becoming strongly bonded together. Cells near the surface make up the dead cornified layer. The Skin An inner dermis
This is much thicker than the epidermis and contains a network of fibrous proteins, collagen and elastin. It contains several structures that are important in thermoregulation. The Inner Dermis Hair Follicles These are pits from which hairs grow.
The angle of the hair can be adjusted by contraction of a small erector muscle attached to each follicle. In most mammals this allows the thickness of the fur to be increased. In humans this thermoregulatory role has been lost, though the muscles still contract to produce goose pimples when the body is cold.
The Inner Dermis Sweat Glands Deep in the dermis these are supplied by sympathetic nerve fibres. In humans most sweat glands are eccrine glands and secrete a dilute salt solution which cools the skin when it evaporates. In most mammals most sweat glands open
into hair follicles and secrete pheromones and play no part in thermoregulation. The inner Dermis Blood Vessels Besides nourishing the skin, these bring heat to the body surface. Stimulation of the smooth muscle in the arterioles causes them to constrict and thus
reduce the delivery of heat to the skin. Inner Dermis Afferent nerve endings Some of these are sensitive to changes in temperature Beneath the dermis is a layer of adipose tissue which contains fat that acts as an
energy store and a thermal insulator. Heat Production Most energy taken into the body is eventually lost as heat produced in metabolism, this is the main source of heat in endotherms. Respiration or ATP production occurs in the mitochondria.
There are three main steps. Step 1 A series of enzymatic reactions known as the TCA Cycle or Krebs Cycle where carbon from glucose and lipids is converted into CO2, while electrons are transferred to the inner membrane of the mitochondria.
This CO2 is exhaled. Step 1 Step 2 In the inner mitochondrial membrane a series of oxidation/reduction reactions take place (electron transport or oxidative phosphorylation).
Electrons are transferred through a series of specialized molecules in the membrane. This results in the movement of H+ ions to the space between the inner and outer membranes these will build up unless a channel opens up allowing them back across the membrane. Step 2
Meanwhile the electrons are transferred to O2 to make H2O. Importantly this chemical reaction releases heat and is a major site of heat production in the body. Step 2 Step 3
A by-product of this process is that it creates an ion gradient across the membrane. This difference in H+ ions drives a protein ATP Synthase that is embedded in the membrane, which adds phosphate to ADP to make ATP. In the process H+ ions are transported back across the membrane so the electron
transport in step 2 can only keep going if ATP synthase is active. Step 3 Exercise and Heat Production When a person exercises, muscles have increased demand for ATP. Feedback signals are sent to increase the
rate of oxidative phosphorylation. This means there is greater demand for O2 and increased production of CO2. As a by-product of this heat is produced. This is why you get hot when you exercise. Shivering This is a feedback mechanism that occurs
when a mammal is faced with cold temperatures. Shivering is a result of involuntary muscle contractions, causing a rise in oxidative phosphorylation and thus heat production. This is known as shivering-induced thermogenesis. Non-Shivering Thermogenesis
Babies are not able to shiver. They use non-shivering thermogenesis to keep warm. This happens in a special tissue called brown fat. In brown fat, there are a large number of mitochondria that express a special protein called uncoupling protein (UCP).
Non-Shivering Thermogenesis UCP is a channel that allows H+ ions to travel back across the membrane without having to produce ATP. This means that the transfer of electrons to oxygen and the pumping of H+ ions outwards can happen without the need for ATP. This means that heat production can
happen without making extra ATP. Non-Shivering Thermogenesis Babies have a significant amount of brown fat, and they use it until they develop the ability to shiver. Other animals, e.g. bears, rely on brown fat when they hibernate, as it allows them to produce heat without having to
exercise. Non-Shivering Thermogenesis Feedback Mechanisms to Restrict Heat Loss Skin is an area where animals can lose heat to the environment. Blood is a major way to transfer heat around
the body. Animals can rapidly reduce blood flow to the skin, reducing heat transfer and thus reducing heat loss. This is why your fingers and toes go blue when they are very cold. Feedback Mechanisms to Restrict Heat Loss
In a cold environment, small muscles in the skin are activated that make the hair stand upright. This is called piloerection (goosebumps). This blocks the flow of air around the skin slowing heat loss. How does the Body Cool Down when it is too Hot?
Sensors in the skin detect temperature changes, send signals to the brain and induce responses that lower temperature. Blood Flow to the skin can be increased when body temperature is high, so increasing the rate of heat loss to the atmosphere by radiation. e.g. large ears in elephants.
How does the Body Cool Down when it is too Hot? Sweating occurs mostly in humans and other primates. Sweating occurs as a feedback mechanism in response to a rise in external temperature or due to heat produced during exercise. Sweating occurs when signals from the brain activate sweat glands throughout the skin.
The sweat is a saline solution. It cools the body as it evaporates from the skin. How does the Body Cool Down when it is too Hot? Many animals use panting as a means of losing heat. Panting transfers heat to the water in the
lining of the airways leading to the lungs. These are normally kept moist so by panting the animal is able to induce more water evaporation and thus more cooling. How is it Controlled? The major control centre for regulating body temperature is the hypothalamus. This has its own sensors, which can sense
very small changes in temperature and send out signals to regulate cooling mechanisms. This is called the central regulatory mechanism as it is sensing temperature at the core of the body. The Hypothalamus
How is it Controlled? It takes time for core temperatures to be affected by the environment so the body also has temperature sensors in the skin that tell the brain what is happening to the environmental temperature, enabling the body to react more quickly. These sensors are a special type of neuron found only in the skin. They
contain a series of protein channels called TRPV channels. How is it Controlled? These TRPV channels are found in the plasma membrane of these nerve cells, and their structure is highly tuned to a particular temperature. It the temperature changes, the structure
of the TRPV proteins changes., then their function changes. When TRPV proteins are at their optimum temperature, they make a channel through the membrane. How is it Controlled? This allows a number of ions to cross the membrane changing the electrical charge
across the membrane. This triggers an electrical pulse that moves along the nerve cell to the next nerve cell until it reaches the hypothalamus, where it adds to information provided by the central regulatory sensors. Fever
This is associated with infection. The immune system produces fever-inducing chemicals called pyrogens, which travel through the blood to the hypothalamus, where they trick it into raising the body temperature. Aspirin and paracetamol lower temperature by blocking the production of pyrogens.
Frostbite The blood vessels in the skin constrict in very cold conditions to preserve heat. If this is prolonged it starves the cells peripheral tissue of nutrient and heat. When nerve cells stop working no signals get sent to the brain, the fingers and toes go numb. If this happens for a short time it can be
reversed. If it happens for longer the cells die i.e. frostbite Frostbite Why do Chillis Burn? Capsaicin, a chemical found in chillis binds directly to the TRPV channels in the lining of the mouth.
This activates these receptors in the same way as if the food were genuinely too hot, so the brain generates the same type of signal.
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