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Immunology and Thermoregulation summary - Thermoregulation part
The summary covers the theory explained during the thermoregulation lectures of the course Immunology and Thermoregulation and the lecture notes.
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THERMOREGULATION PARTINTRODUCTION ON THERMOREGULATION
Animals are housed for several reasons like:
- Control of: growth, production, reproduction, heat and diseases
- Manipulation of: behaviour, food, reproduction, longevity
- Modification/conditioning of: climate, health and diseases
Housing also developed for economic reasons, because by deciding how to manage animals we can
increase production and decrease management costs. Type of housing has influences on animals, people,
environment and third parties.
Animal captivation and housing can have positive effects, e.g. the supply of food, water and shelter, and
negative effects, which appear when the needs of the animal are not met since housing reduces the choices
that animals can make.
Housing and climate have a big influence on performance, health and welfare of animals. To measure
influence of housing, we can determine changes in behaviour (e.g. stereotypic behaviour like walking in
circles, ballo dell’orso), hormone levels (e.g. cortisol, testosterone, catecholamines), fertility (e.g. housing
can influence the ability of females to show when they are in oestrus), health, disease resistance,
production, growth, heat production and loss.
Homeostasis
Homeostasis is the ability of the body to maintain a constant internal environment in the body, e.g.
maintenance of body temperature, osmotic values, pH or blood sugar levels. When animals are exposed to
changes in their external environment or climate and they experience this as stressor, their homeostasis
can be disturbed. The animal will react by adjusting its neural and endocrine system after signals are sent to
the CNS so its behaviour or metabolism will change. Two possible outcomes: the animals have homeostasis
again, or they adapt to the change in the environment. There are 3 ways of adapting to a stressor:
- Acclimation: includes all reactions of an animal after a single environmental factor changes. E.g. a
chicken acclimates to a change in day length.
- Acclimatization: includes all reactions of an animal after more complex changes in climate and/or
housing conditions. E.g. a cow with outdoor access will acclimatize to weather changes (sun, wind,
temperature, day length).
- Habituation: it is the adaptation to a repeated stimulus that initially was experienced as a stressor.
Reallocation of energy
Animals require energy for maintenance, production and reproduction. Before energy is available, nutrients
must be absorbed, digested and metabolised.
Adaptation of animals to a stressor has consequences for the distribution of energy in the animal. The
energy that an animal takes from food is partly lost in faeces, and the rest is available nutrients. These
available nutrients are metabolised and used for maintenance (movement, circulation, respiration,
thermoregulation, immunological response etc.) and for performance (production of milk, meat, eggs, work
and reproduction).
With feed intake animals obtain Gross energy (GE). Part of it is lost as Faecal energy (FE) and the part that
is digestible is called Digestible energy (DE). A part of it is lost in urine (UE) and the rest is Metabolizable
energy (ME), which is used for maintenance (MEm) and production (MEp). The energy used for
maintenance is lost as heat, while the one used for production is called Net Energy (NE)/Energy Retention
(ER)/Energy Balance (EB). The efficiency of using MEp for production is called partial efficiency k; k is the
,part of MEp that is actually going to products, while 1-k is the part of MEp that goes to heat (e.g. at partial
efficiency of 25%, k=0.25 and 1-k=0.75). Therefore, the Net Energy (NE)/Energy Balance (EB) = MEp*k;
Energy balance can be positive or negative.
NB: heat production is indicated as M; heat loss is indicated as H.
M (heat production) = MEm + (1-k)*MEp = MEm + (1-k)*(ME-MEm). This equation is only valid in the
thermo-neutral zone. Thermo-neutral zone = range of ambient temperatures where the body is able to
maintain its temperature stable only by regulating heat loss.
Example: the energy balance (net energy) of an adult human who is not pregnant or lactating (under
normal conditions) is MEp=0, because this human is not producing anything, so all Metabolizable energy
(ME) is going for maintenance.
THERMOREGULATION AND HEAT BALANCE
Homeothermy and poikilothermy
- Homeotherm (tachy metabolic=high metabolism): body temperature is independent of the
environment and it is maintained into certain limits; body temperature is controlled by the
autonomic NS. Homeothermic animals are endothermic = their source of heat comes from their
body (it is produced with metabolism) and is regulated through regulation of activity. Mammals
and birds. NB: there are some exceptions like camels, that are heterotherms: their body temp.
varies between 34-40°C (large range) because in the desert there is a big thermic variation between
day and night.
- Poikilothermic (brady metabolic=low metabolism): body temperature is dependent on the
environment. Poikilothermic animals can be ectothermic, if they regulate their temperature
through behaviour (e.g. by laying in the sun or shadow), or endothermic so they regulate it through
activity. Insects, reptiles, amphibians and fish.
NB: deep body temperature (the one that we regulate) is different from skin temperature; skin
temperature can be different in different areas of your skin.
NB: body temperature is not entirely constant also in homeothermic animals, because metabolism is not
constant so also heat production is not constant.
Heat balance
M=heat production; H=heat loss. These two have to be balanced to have homeostasis, so the animals
regulate their heat production and loss to adapt to environmental changes.
M = H + ΔC; ΔC = BW* ΔT*c, where:
- ΔC=change in heat content in the body ; in homeothermic animals it is approx. 0, in poikilothermic it
is different from 0.
- BW=body weight
- ΔT=change in body temperature
- c=specific heat capacity of the body, e.g. water is 4.184 J/g/°C. It depends on the composition of
the body.
NB: if in homeothermic animals ΔC is approx. 0, it means that M=H approximately, so heat production is
approx. equal to heat loss balance.
,Heat production and heat loss
Factors influencing heat production: factors influencing heat loss:
- basal metabolic rate - vasoconstriction and vasodilation
- feed intake - insulation (fur, body tissue)
- shivering thermogenesis ST - sensible heat
- non-shivering thermogenesis NST - evaporation
- muscle activity - posture
HEAT LOSS
Heat loss or heat release
Ways to lose heat:
- radiation (Hr)
- convection (Hc) Sensible heat loss (Hs)
- conduction (Hd)
- evaporation (He) through skin and respiration. Also called latent heat loss
(Hr + Hc + Hd = Hs) heat loss H = Hs + He
Heat is always following a temperature gradient = it goes from an area of high temperature (body, skin, fur)
to an area of low temperature (ambient) temperature body>skin>fur>ambient
Heat passes the core of the body, skin and fur to be lost in the environment:
- heat loss from core to skin is through convection (blood, vasodilation) and conduction (fat tissue)
- from skin to fur through convection, conduction, radiation
- from fur to environment through conduction, convection, radiation
- from skin to environment (if there is no fur) is through evaporation.
Heat loss in general is H = A*c*ΔT, where A is the surface of the animal, c is a constant and ΔT is a
difference in temperature between animal and ambient. Hs = Tf-Ta; He = H – Hs DA SAPERE
Thermal insulation
Thermal insulation prevents or inhibits heat flow and therefore heat loss. A poorly insulated animal will lose
heat faster and will need to produce additional heat fast to maintain its body temperature. This has
consequences for its maintenance requirements, behaviour, sensitivity to climate changes and diseases.
Three types of insulation: IT IS AN EXAM QUESTION
- tissue insulation, happens inside the body. It = (Tb – Ts)/H
- fur insulation, happens in the fur. If = (Ts – Tf)/(H – He)
- air insulation. Ia = (Tf – Ta)/(H – He)
(Tb=body temp, Ts=skin temp, Tf=fur temp, Ta=air temp).
The sum of fur insulation and air insulation is external insulation Ie = If + Ia; tissue insulation is internal
insulation. The combination of internal and external insulation is body insulation Ib. Ib = (Tb – Ta)/(H – He)
Animals can influence heat loss H by changing internal insulation (tissue insulation), fur insulation and air
insulation.
Tissue insulation: It is the resistance of heat flowing from the internal body (core) to the skin surface . In the
short term, tissue insulation is mainly regulated by blood flow to the surface: vasoconstriction when it is
cold, to reduce heat loss, and vasodilation when it is hot to increase heat loss. In the long term, tissue
,insulation is regulated by adjusting the amount of subcutaneous fat: thicker fat results in a better
insulation, so it is present in cold environments and it is the best way of insulation in polar animals.
NB: in extreme cold, there is no vasoconstriction in the extremities of the body but vasodilation; this is to
prevent that the extremities freeze.
Another type of tissue insulation is morphological adaptation animals develop differently depending on
the temperature of the environment in which they live in the first stages of life, e.g. if you raise them in a
cold environment they will develop more fat to insulate better, or hares (lepri) have big ears if they live in a
hot environment because hares/rabbits lose heat from their ears, while arctic hares have smaller ears.
Tissue insulation is influenced by several factors such as age of the animal (young animals have lower tissue
insulation), condition of the animals (fat animals have higher tissue insulation) and prolonged exposure to
cold or heat (when exposure is prolonged, insulation is increased).
Fur insulation: Fur is responsible for a large part of insulation. The insulation value of fur
(how much it is able to insulate) depends on the amount of still air that can be retained
in the fur, since still air is a very good insulator: the more still air fur can retain, the
better the insulation.
Efficiency of insulation depends on hair density/hardness/length, the quality of
undercoat, age, season, species and breed. There are also factors that can disturb fur
and fur insulation:
- During low temperatures, hair or feathers of animals raise by pilo-erection.
When birds puff up their feathers, it is called “fluffing up”. However, when hair
or feathers are raised too much, too much moving air can get in the fur and
results in higher heat loss because of convection (=bad insulation).
- High wind speed disturbs fur because it moves fur, and it will contain less still
air.
- Lying down: when animals lay down, the layers of still air in fur are disturbed so insulation
decreases.
- Water: when the fur gets wet, less still air can be contained in fur so there is less insulation.
Air insulation: It depends on the layer of still air that surrounds the animal, which determines the amount
of heat lost from the fur to the environment. A thicker layer of still air around the animal ensures more
insulation; the thickness depends on shape, size and smoothness of the fur and by the air speed
surrounding the animal. When air speed increases, air insulation decreases and more heat is lost.
Radiation
With radiation, there is release of H from solid to solid without contact, because all objects emit heat in
form of electromagnetic waves. Heat loss by radiation between the skin surface of an animal and the
environment is measured by the Law of Stephan Boltzmann (non da imparare a memoria):
Hr = A*e1*e2*σ*(Tf4 – Ta4), where:
Hr=heat loss by radiation; A=surface of the animal; e1=emissivity of the surface;
e2=emissivity of the environment; σ=constant; Tf=temp. of fur; Ta=ambient temperature
Emissivity (e) depends on the wavelength of radiation, and the wavelength depends on the temperature of
the object emitting the radiation.
NB: radiation emission is very small at regular ambient temperature, so almost all radiation is absorbed and
not reflected.
,Heat loss by radiation can be influenced by changes in:
- climate: influence can be direct if there are changes in radiation temperature of the environment
e.g. when you turn on the heater, or indirect if there are changes in ambient temperature and wind
speed that influence skin temperature e.g. because sunlight enters a room and heats it.
- animals: influence is direct if there are changes in the surface area of animals as a result of
behaviour e.g. if the animal “curls up” and makes itself small, or indirect if there are changes in the
skin or fur temperature because of changes in tissue or fur insulation (vedi capitol sotto su
insulation)
NB: the color of fur or skin of animals has an effect on the emission and absorption of radiations : dark color
absorbs more and reflects less radiations so heat loss by radiation is more difficult; of course, this effect is
more important in animals that live outside because they receive direct solar radiation, while in animals
that live indoors this is less important.
Convection
Heat loss by convection happens when air or water flow along a body. Heat losses by convection can be
calculated with:
Hc = A*h*(Tf – Ta), where:
Hc=heat loss by convection; A=surface of animal; h=constant; Tf=fur temp; Ta=ambient temp.
Heat loss by convection is influenced by changes in:
- climate: when there are changes in ambient temperature, because the temperature gradient
between fur/skin and air also changes and it influences convection; and when there are changes in
air velocity because it affects “h”.
- animals: when there are changes in surface area of the animal due to changed behaviour, that
result in more or less heat loss; and when there are changes in insulation of fur and skin because
they influence skin and fur temperature.
NB: small animals lose more heat by convection than larger animals, because they have a larger
surface:weight ratio (=their weight is small but their surface is large in relation to it)
NB: when air velocity is 0, there is still some convection because the animals is still producing heat and this
heat moves towards the environment.
Conduction
Heat loss by conduction occurs when two objects with different temperatures are in contact, so there is an
exchange of temperature. Heat loss by conduction is calculated with:
Hd = A*c*((Tf-Ta)/d), where:
Hd=heat loss by conduction; A=surface area of the animal in contact with another object;
c=conductance constant of the other object; Tf=fur temp. Ta=ambient temp; d=distance
between temperature measurements of Tf and Ta.
Heat losses by conduction can be influenced by:
- climate: temperature differences between animal and environment, especially temperature of
walls and floors
- contact material: material used for the floor, wall or bedding where the animal is staying and lying
is important, because different materials are characterised by 1) different conductivity, so how
easily heat is transferred in this material and 2) different heat (or thermal) capacity, so how quick
the material reaches a balance between heat release and absorption. E.g. heat loss by conduction is
, higher when floors of farms are made of concrete compared to wood, because concrete has higher
conductivity and heat capacity; heat loss by conduction when straw is used as bedding material is
quite low because it has low conductivity and heat capacity. So, for example straw is a good
bedding material in cold countries because heat loss in animals is minimised.
NB: heat loss by conduction is higher immediately after the animal lays down, then it is constant
because a balance is reached between the temperature of the two objects and there is no more
exchange.
- Animal: animals change their surface area by changing behaviour so they increase or decrease their
heat loss, and can change their skin/fur temperature by changing skin/fur insulation.
Evaporation
It happens through diffusion (fluids pass through your skin without having glands), sweating (via sweat
glands) or respiration, and is increased when ambient temperature is high or during activity. Evaporative
heat loss is divided into:
- Continuous evaporation by diffusion, that occurs continuously and is not high.
- Evaporation by sweating: Different species have different capacity of sweating, e.g. humans sweat
easily so we can survive at very high ambient temperatures, while pigs do not sweat so they lose
heat by evaporation in other ways like covering with mud.
- Evaporation by respiration: it is very important in animals that do not sweat much, so they
increase breathing frequency and decrease respiratory volume to lose heat (they decrease
respiratory volume to prevent hyperventilation).
Evaporative heat loss is calculated with: He = Ae*d*(Cs – Ca), where:
He=evaporative heat loss; Ae=wet surface of the animal; d=evaporation coefficient;
Cs=water vapour pressure of the skin; Ca=water vapour pressure of the air.
Mollier diagram: (pag 35 reader) it represents the relationship between air temperature, relative humidity
of air and enthalpy (the heat content of a system). IMPARARE A LEGGERE IL GRAFICO non so come
Evaporation is influenced by:
- Climate: changes in air velocity directly influence evaporation (more heat loss); high relative air
humidity (RH) decreases evaporation because air is already saturated with water vapour.
- Animals: changes in the effective area for evaporation on the animal changes evaporation, e.g.
dogs take out their tongue to increase the surface area from which they can lose heat.
Example: water evaporates very fast from the skin, while mud takes about 3 hours to evaporate; therefore,
pigs use to cover in mud because it takes more time to evaporate so they are able to lose heat for a longer
time.
NB: a dog can cope more easily with a high relative humidity compared to a horse: horses can sweat well,
but if the relative humidity is high, they cannot lose heat by evaporation because air is already saturated.
On the other hand, dogs take out their tongue for evaporation through respiration, which still allows to lose
some heat even at high relative humidity.
NB: in chickens, dust bathing is important to lose heat because it helps them remove pathogens from their
feathers, so if feathers are healthy, they are also better in making fur insulation.
HEAT PRODUCTION
Animals produce heat as a by-product of physiological mechanisms such as metabolic activity. The energy
released during metabolism is stored in ATP. However, when these mechanisms of heat production fail,
, other mechanisms are used to produce heat as main product (not by-product like in metabolism); such
mechanisms are shivering thermogenesis and non-shivering thermogenesis.
The efficiency of ATP production during metabolism is higher for carbohydrates and lipids than proteins. A
higher efficiency of ATP production results in more production of heat, which can be measured with direct
or indirect calorimetry.
Three main sources of heat production:
- Maintenance MEm (protein turnover, respiration, heartbeat, normal activity..)
- Production inefficiency (1 – k), where k is production efficiency
- Extra thermoregulatory heat production ETH only in cold environment
Heat production is calculated with: M = MEm + (1-k)*(ME-MEm) (MEm is related to body weight, (1-k) is
related to physiological status, ME is related to feed intake).
Variation in heat production
Heat production is not constant, but can vary depending on several factors:
- Weight of the animal
- Feed provision: after feeding, heat production is higher because of higher metabolic activity due to
digestion and absorption of nutrients. In farm animals, it is useful to give feed during the day when
heat production is already higher (vedi punto sopra) to increase feed efficiency.
- Physiological state of the animal: is the animal growing? Lactating? pregnant?
- Day or night: heat production is normally higher during the day in diurnal animals, while nocturnal
animals have higher heat production during the night.
- Season: in periods with short day length, feed intake and consequently heat production are lower
than in periods with long day length. In addition, maintenance requirements are lower for animals
in periods with short days because activity is lower.
- Oestrus cycle: during oestrus, heat production and body temperature increase, but this increase
does not occur in all animals and is only a small increase. Therefore, this small increase cannot be
used to detect oestrus.
- Occurrence of diseases: during infections, heat production and body temperature increase as a
result of fever. An increased immune response as a result of an infection by definition results in a
higher heat production? depends on the type of immune response: in humoral immune
response it is not true, in cellular immune response it is true.
- Activity: animals that perform a task, e.g. draft animals, sport, reproduction, produce more heat.
Also standing can be considered an activity: heat production is indeed higher during standing than
during laying.
Direct calorimetry
It is a physical method to determine heat production. Lavoisier was the first to do it by putting a guinea pig
in an insulated box with ice cubes, and measure how much ice melted over time as a result of the animal’s
heat production. With direct calorimetry, we generally measure changes in temperature to determine heat
production.
Indirect calorimetry
With indirect calorimetry, the chemical composition of by-products of metabolism is measured and used to
determine heat production. Heat production is measured based on O2 consumption and production of CO2
and CH4 during metabolism. Heat production in mammals is calculated with Brower’s equation:
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