66. The heat balance, and the effect of the environmental temperature. Flashcards
Forms of heat balance:
Animals can be divided into three major groups:
- poikilothermic animals: body temperature changes depending on the environmental temperature.
- heterothermic animals: body temperature is independent of the environmental temperature. However, in certain periods these animal behave as a poikilothermic organisms.
- homeothermic animals: the body temperature is independent of the environmental temperature during all periods of life.
Domestic animals belong to the 3rd group. They maintain a constant body temperature in the deep layers of their body (core temperature). This is accomplished by different methods of changing the temperature (peripheral temperature) of tissues, which are in close contact with the environment. The constant core temperature is a diagnostic parameter. Its value is is altered by the time of day, feed intake, muscular activity, sex and age.
Transiently heterotherm: Hibernation, aestivation
The hibernating animals become transiently poikilothermic. At the beginning of hibernation in about half a day the original metabolic rate (100%) drops to 5-7%. During the longer - 3-month - hibernation period the central nervous system is the only one which stays near around the previous core temperature. Achieved by special “heating” elements surrounding mainly the brain (adipose depots and venous plexi).
- The hibernation is interrupted by a short (a few hours long) awakening phase. This happens every 2 - 3 - weeks periods. This time the metabolic rate rises over 100% (120-140%), which results in extremely rapid warming and awakening. During that time the animals take up feed, micturate and defecate. These processes are followed by a new period of rest.
- Some species do not have a real hibernation period. For example the body temperature of bears does not diminishes drastically. Bears accumulate very high reserves of nutrients. Heat loss is small during winter sleep (high bodyweight/body surface area ratio).
- Similarly to hibernation animals can be inactivated by higher environmental temperatures (several kinds of snakes, the American ground squirrel etc). In the middle of summer they get into a special inactive metabolic condition avoiding dangers caused by dryness and heat. As the hot weather is over the animals restore normal metabolic rate. This summer inactivation period is called the aestivation (aethivum=dryness, drought)
Transiently heterotherm: Torpor
Many warm-blooded small animals have such small size that they are not able to ingest or reserve the amount of food that provides enough energy for maintaining high body temperature during the cold nights. The small bodyweight and the relatively large body surface area of these animals play a major role in this phenomenon. The heat-loss per bodyweight is exponentially higher than in case of a large animal (mammals).
-These animals are highly active at daytime, their body temperature is constant and independent of the environmental temperature. During the evening as the environment becomes colder their metabolic speed suddenly diminishes and body temperature comes near to the environmental temperature. The animal’s body is in a still, rigid state (“Torpor”). As a result of very low energy consumption and the radical diminishing of the thermal gradient between the body and the environment the animal can withstand the cold night with minor thermal losses. In the morning the heat of the sun and the increase of the metabolic rate activates these species and they become homeothermic again.
Temperature of body parts
The temperature of different body parts is not uniform. Temperature of the limbs can be much lower than that of other parts of the body. Studies showed that by polar animals – living in extremely low environmental temperature – the composition of cellular fat of limbs is considerably different from the ones of the other parts of the body: the ratio of unsaturated fatty acids is much higher in here. This assures sufficient membrane fluidity needed for normal metabolism even at 3 - 5 °C.
Importance of heat exchange
One-way head exchanger –
The one-way heat exchanger is well known from physics. In this kind of heat exchange the efficiency of the process is comparatively low.
The effluent temperature gives approximately the average of the incoming temperatures.
One of the most effective defense against heat loss across the surfaces of the extremities touching the ground is the so-called looping heat exchange.
Warm blood flowing towards the extremities can loose so much heat that close by the ground the thermal gradient - between the body and the ground - becomes minimal. So the heat-loss is close to zero here.
Counter-current head exchanger –
The so-called counter-current heat exchange is more effective than the unidirectional heat exchange system.
The heat flowing out from the warm branch can be almost fully transferred to the substance flowing in on the cold branch. So the temperature of the substance leaving the warm branch can become near to the temperature of the one coming in along the cold branch.
Our domestic animals have this kind of heat exchange between the deep arterial and venous systems
fluence of age on body temperature:
higher body temperature in young animals is caused by higher metabolic rate. Higher temperature means higher temperature gradient against the environment. This increases the speed of heat loss as well. In the picture you can see the oxygen consumption of chickens of different age plotted against the environmental temperature: the value where the animals consume the least amount of oxygen (or produces the least amount of heat) is the so-called thermoneutral zone. It is visible that this zone becomes narrower considerably as the age increases.
Heat balance:
In normal conditions animals are in heat balance. The total amount of heat of the body Ht can be divided into two components: – 1. Produced heat: metabolically produced, (Hm); and stored in chemical bonds (Hs). – 2. Exchanged heat: convection (Hc), radiation (Hr) and evaporation (He).
Heat exchange:
The exchangeable heat is dissipated from the blood via radiation and conduction, and from the skin surface via evaporation. The emission by convection can be very important even if the surrounding air is not moving: namely the air warmed by the skin becomes lighter, thus this layer leaves the neighborhood of the body and it is replaced by colder air and therefore the heat loss is continuous. The insulation of tissues plays a major role in the processes of the conductive heat-loss. In the upper part of the figure the measure of the arrowheads represents that the muscular and adipose tissue have bad thermal conductivity. Especially the controlled thickness of the adipose layer is the important element of adaptation to environmental temperature. The rate of heat-loss by radiation and convection (Q) is directly proportional to the measure of the surface area (A), the thermal gradient (t2-t1) and a proportionality constant determined by the nature of the given material (k). It is inversely proportional to the distance (l) that the heat has to cover
Heat balance:
Hypothalamic control is the basis of maintaining body temperature.
Central and peripheral cold and heat sensors accomplish the heat reception.
The center can be divided into “heating” and “cooling” areas. The simultaneous activity of these two centers adjusts the heatloss, heat conserving and heat producing processes. Forms of heat loss are conduction, radiation and evaporation ( sensible or insensible).
In cold environment heat conservation and active heat production play the major role.
At the early phase of the active heat production the heat produced by the asynchronous contractions of muscle fibers (shivering thermogenesis) is still satisfactory. In case the heat produced this way is not enough for maintaining a constant body temperature then starts an intensive heat production via chemical procedures under hormonal regulation (principally sympathetic and thyroid activation). This is the non-shivering or chemical heat production (thermogenesis).
In a well defined environmental temperature range the metabolic rate is constant. This range is wider than the thermoneutral zone. In latter zone there is no need for altering metabolic rate for maintaining a constant core temperature; the capacity of heat conserving and heat dissipation procedures is satisfactory. Animals prefer a narrower range within this thermal zone. This is the optimal zone. In the temperature range below the thermoneutral zone (below the so-called lower critical temperature) the energy production increases; the heating procedures need energy. It seems to be an antagonism but above the thermoneutral zone (upper critical temperature) the energy production grows as well. Intensive heat loss is not possible without perspiration or panting. Both procedures require much energy.
In extreme conditions the animals are not able to produce more heat (the body temperature drops) or incapable of dissipating more heat (the body temperature rises). Both extremes can lead to the death of the animal.
Defense against cold:
- Heat conservation: diminishing the previously mentioned (conduction, convection, radiation and evaporation) heat-loss. Animals living in groups cuddle up to each other diminishing the surface are of heat loss and preserve heat for each other. It is a very important way of heat preservation especially for protection of young animals (relatively high body surface area).
- Increasing the muscle activity induces heat production. This can be either an increase of tonicity (isometric) or shivering. The latter one is sequential spontaneous and desynchronized contractions of muscle fibers: there is no motion but the only heat production (see studies of muscle function) .
- If the two above-mentioned forms are not able to maintain the constant body temperature then begins the chemical (nonshivering) thermogenesis. Details of this form (brown adipose tissue and special chemical thermogenesis by skeletal muscles) will be discussed later.
The cause of uncoupling of oxidative phosphorilation in the mitochondria
The cause of uncoupling of oxidative phosphorilation in the mitochondria is the change in the proton flux which is needed for the synthesis of ATP. In the inner membrane of the mitochondria a special protein the thermogenin, uncoupling protein, UCP, makes mitochondrial membranes transparent for protons and in this instead of ATP, heat (and water) is produced (see details alter). • Contrary to the relatively rapid and short-term (1-2 day) effect of catecholamines the thyroid hormones have slower but much more prolonged effect on heat production. As a result of of thyroid hormone action the basic metabolic rate, the number of mitochondria, the amount of thermogenin and the activity of sodium/potassium pump increase in the cells. These effects cause increased energy consumption and heat production.
own adipose tissue
The brown adipose tissue is situated near the interscapular region or sometimes in the abdominal cavity. In comparison with the normal (“white”) adipose depots it is foamy, has a fine intracellular granular content, intensive blood supply and many mitochondria. The brown color is gained from the high number of cytochrome-oxydase content of the mitochondria. Fat is oxidized directly in these adipose cells, which have alpha- and betaadrenergic receptors on their surface.
Heat production in brown adipose
The permeability of the adipose cell membrane increases to sodium and potassium ions. This results in depolarization. The balance is restored by an increased functioning of the sodium potassium pump. This results in excess heat production (“futile cycle”). • cAMP increases the lipase activity, which increases the amount of FFA in the cell. A major part of this is oxidized in the mitochondria followed by considerable heat production; the rest of the FFA gets to other organs via circulation and participates in further heat production). • As a result of the above mentioned processes the brown adipose tissue gets warm strongly and because of its high perfusion it transfers heat rapidly to other parts of the body. This intensive way of heat production plays an important role in rapid warming up of animals awaking from hibernation. • A more detailed description is given under the section “Thermogenesis on the cellular level”.
Defense against heat:
By evaporation of 1 g of water 0.58 kcal (2.4 kJ) heat is released. • It is significant above the thermoneutral zone. Evaporation might account to 80 % of heat loss. This, however, might lead to extensive water and sodium loss, as well.
