Homeostasis and Body Heat (2) PDF

Thermoregulation How do animals cope with changing environmental temperatures? Many animals inhabit hostile environments We are going to replace the imprecise terms "warm blooded" and "coldblooded" with ac...

Thermoregulation How do animals cope with changing environmental temperatures? Many animals inhabit hostile environments We are going to replace the imprecise terms "warm blooded" and "coldblooded" with accurate physiological terms. These are based on 1) how an animal obtains its body heat:  ectotherm - main heat source is the external environment (from the Greek ecto meaning "outside")  endotherm - main heat source is internal metabolic reactions (from the Greek endo meaning "inside")  mesotherm – a strategy intermediate to ectotherms and endotherms (from the Greek mesos meaning "intermediate") AND 2) how an animal regulates its body temperature:  poikilotherm - temperature regulated primarily by environment (from the Greek poikilo meaning "variable")  homeotherm - temperature regulated primarily by metabolism (from the Greek homeo meaning "constant") How an animal obtains its body heat i.e. source of body heat: i) endotherms, ii) ectotherms, iii) mesotherms Endotherms Ectotherms Heat obtained mostly Mostly self- generated heat (internally from from environment (external source) metabolic processes) lower metabolic rates compared  high metabolic rates to endotherms Mesotherms Mesotherms A strategy intermediate between endotherms and ectotherms. If temperature drops, a mesotherm will have lower body temperature and metabolic rates than endotherms. Think of them as being able to turn on the “heating” but not having thermostats Use metabolism to raise body temperature above that of the environment but do not maintain a constant body temperature 1  Previously thought dinosaurs were ectotherms but did you know many were probably mesotherms Looking at bone growth of dinosaur skeletons (growth rates were quite high), able to calculate metabolic rates (metabolic rates drive growth rates and so endotherms tend to grow faster than ectotherms) and dinosaur growth rates sit between endotherms and ectotherms = mesotherms. Animals are defined based on 2) temperature regulation: i) homeotherms, ii) poikilotherms, iii) heterotherms (ectotherm) (endotherm) i) Homeotherms ii) Poikilotherms Homeothermy is when a constant internal Poikilothermy is where body temperature is maintained regardless body temperatures fluctuate of the environmental temperature. depending on the environmental We would term temperature. We would term homeotherms “regulators” poikilotherms “conformers”. Note: Poikilotherms can regulate their temperatures behaviourally though Animals are defined based on 2) temperature regulation: i) homeotherms, ii) poikilotherms, iii) heterotherms Heterothermy is a physiological term for animals that vary between self-regulating their body temperature, and allowing the surrounding environment to affect it. iii) Heterotherms regional temporal Regional heterothermy describes Temporal heterothermy describes organisms that are able to maintain organisms that have different body different temperature "zones" in temperatures in the same animal different regions of the body. over time. eg. Tuna maintain elevated eg. Hummingbirds show daily torpor swimming muscle temperatures of lowered body temperatures compared to other body regions. and metabolic rates. (See slide on regional heterothermy in tuna) 2  Now that you have been introduced to the following terms: Source of body heat: ectotherm, endotherm, mesotherm Temperature regulation: homeomotherm, poikilotherm and heterotherm How would you describe your pet reptile’s (lizard) thermophysiology? Spend a few minutes thinking about this Additional Reading (SUNLearn): Rohrig B. 2013. Chilling out warming up. How Animals survive temperature extremes. ChemMatters 6-9. Regional heterothermy in tuna Some fish are heterothermic (eg. Tuna, sharks) and have evolved a counter-current heat exchange mechanism that allows a part of the body (red swimming muscle) to be kept above ambient temperature. Arterial blood passing through gills is cooled. This cooled arterial blood is routed near the surface, so core body heat isn't conducted to it. When this blood is routed into red muscle deep in body, it passes through a system of veins and arteries running in opposite directions in contact with each other, called a counter-current heat exchanger — Cold arterial blood is warmed by passing venous blood, which has been heated up by metabolic activity in muscle. Consequently, red muscles in the tuna's core can be kept up to 15°C above water temperature. Body temperature of ectotherms and endotherms depends simply on the difference between heat input and heat output. Heat input occurs from the external environment (dominates in ectotherms) and from metabolic heat production (important for endotherms). Heat output occurs through heat loss from exposed body surfaces to the external environment in both ectotherms and endotherms. This heat exchange relies on four mechanisms (see next slide) and heat always moves down a thermal gradient from warmer to cooler regions. Internal heat production Heat input and output Core temperature Heat input Total body Heat output heat content Heat Heat gain External environment loss 3  Heat Only occurs if there is a temperature differential transfer Rate of transfer depends on the route and medium eg. Water has a high heat capacity  easily lose heat to H2O CONVECTION CONDUCTION Transfer of heat due to the mass Direct transfer of thermal energy movement of a gas or liquid (heat) between two objects in contact with one another having a temperature differential. Conduction is always from high to low Basic mechanisms of heat transfer between animal and environment RADIATION EVAPORATION Transfer of heat by electromagnetic energy without direct contact between objects. All objects at a temp Converting liquid to gas requires energy above absolute zero emit electromagnetic radiation. and animals dissipate heat allowing water A dark body radiates and absorbs more strongly to evaporate from their surfaces. This is than a reflective body affected by the relative humidity of the air Rate of heat transfer Heat balance depends on: 1. Surface area  small animals have a high heat flux per unit of body weight (i.e. they lose heat quicker) 2. Temperature difference  the closer body temperature (Tb) is to environmental/ambient temperature (Ta), the less heat will flow into and out of the body Heat always moves from warmer to cooler objects, as described in the 2nd Law of Thermodynamics Thermal homeostasis requires a balance 3. Thermal conductance  ectotherms between heat gained from have high heat conductance (heat metabolism and from external environmental easily transferred between 2 objects) factors with the heat lost & Tb similar to Ta. Homeotherms = fur, feathers which decrease conductance by trapping air with low thermal conductivity Thermal balance: Pathways of heat gain and loss Recall that the total heat of an animal = metabolic heat production and the thermal flux between the animal and the environment The equation would look like this: Hs = Hm ± Hr ± Hc - He Hs = stored body heat Hm = metabolic heat produced Hr = heat exchange via radiation Hc = heat exchange via conductance He = evaporative heat loss Hm important for endotherms Hr and Hc more important for ectotherms He important avenue for heat loss – major route in mammals Hs constant in mammals and birds Hs rises during heating and drops during cooling in ectotherms (when no opportunity for behavioural thermoregulation) 4  Endotherms vs ectotherms mouse The graph on the left shows how body temperature changes with environmental temperature. The graph on the right shows the metabolic rate of the lizard (blue curve) and mouse (red curve) across a range of environmental temperatures. Note that the lizard has a very much lower metabolic rate (the left axis units) compared to that of the mouse (right axis). The metabolic rate of the lizard decreases as temp drops from 40°C to 10°C (and vice versa). The metabolic rate of the mouse is constant within the thermal neutral zone (TNZ) but as environmental temperatures drop, the mouse needs to generate heat to maintain Tb and so metabolic rate increases (shivering) and pants (energy requiring)– when temp increases) Let us take a closer look at the thermal neutral zone (TNZ) TNZ: Range of ambient temperature (Ta) in which an endotherm does not need to alter metabolic rate to maintain constant Tb (body temperature). Upper critical temperature (UCT) – Ta above which energy-requiring heat loss mechanisms are used - sweating, panting. Lower critical temperature (LCT) - energy-requiring heat production mechanisms are used - shivering, non-shivering thermogenesis. Below LCT Shivering thermogenesis Metabolic rate Muscle contractions liberate heat Non-shivering thermogenesis Fats are broken down and oxidised TNZ to produce heat Some mammals have specialised fat BMR stores = brown fat Above UCT Energy-requiring heat loss mechanisms LCT UCT are used i.e. sweating, panting. Ambient temperature (Ta) LCT = lower critical temperature Shivering and non-shivering thermogenesis UCT = upper critical temperature and active heat dissipation are adaptive thermoregulatory control mechanisms Let us see how the thermal balance equation corresponds to the graph below Above UCT the He component (evaporative heat loss) is important > UCT Hs = Hm ± (Hr ± Hc – He) < LCT Hs = Hm ± (Hr ± Hc – He) Below LCT the Hm component (metabolic heat production) TNZ is important Hs = Hm ± (Hr ± Hc – He) In the TNZ, heat is gained or lost via radiation (Hr) or conduction (Hc) - non energy-requiring mechanisms 5  Endothermy comes with costs. Why? Think about it. Costs: 1) High energy requirements (eat frequently) 2) Increased metabolic rates = increased evaporative water loss due to high respiratory rates 3) Susceptible to thermal stress  enzymes evolved to function within a narrow temperature range Endothermy does come with benefits though: 1) Can live in hostile environments 2) Can sustain high levels of activity Three basic methods of thermoregulation 1. Adaptive control Heat production (adjusting metabolic rate) Brown fat  shivering and non-shivering thermogenesis Evaporative heat loss (most important cooling mechanism) Non-shivering  sweating, panting, hyperventilation, thermogenesis 2. Changes in peripheral circulation Controlling the amount of blood flowing to the skin changes the “insulative” capacity (conductance) of skin. Excess heat is lost via vasodilation (increased blood flow through the skin), while heat is retained through Vasoconstriction (decreased blood flow to skin). Vasoconstriction Vasodilation Low conductance High conductance Insulating shell Vasodilation/constriction adjust relative thickness of insulating shell Let us look at this mechanism in a marine ectotherm Regulation of blood flow to periphery Basking before a dive Marine iguana basks on black larval rocks before diving into cold ocean to feed. Vasodilation = During basking it vasodilates High conductance (increased conductance) and increases heart rate During dive (increased blood flow during warming). During dive, vasoconstricts (reducing heat loss to water) and reduces heart rate. Vasoconstriction = Heat retention Vasoconstriction conserves heat Vasodilation increases heat loss (or conductance) Normal Vasoconstriction Vasodilation 6  Marine iguana Heart rate and thermoregulation in the marine iguana As soon as the iguana enters The rate of warming is greater ) Body temperature (C the ocean, it begins to cool than the rate of cooling At the same Tb, heart rate is lower during cooling than warming Heart rate (beats/min The iguana’s heart rate drops rapidly when it is The heart rate rises cooling rapidly when it leaves the ocean to bask on the shore ) Time (min) When the iguana’s body temperature is 30°C in the ocean, its heart rate is 40 beats per minute, and when its body temperature is 30°C after coming out on land, its heart rate has risen to 70 beats per minute. This faster heart rate moves blood more quickly to the skin and through the body, allowing faster distribution of the heat being absorbed from the hot rocks (also vasodilates), increasing the rate of warming of the iguana’s body. What other strategies could this Take a moment marine iguana use to thermoregulate? to think about it 3. Behavioural thermoregulation When melanin is aggregated in the centre of the melanophore (cell)  skin is light. When melanin is dispersed throughout cell  skin is dark. Thermal gaping is a means of evaporative cooling. Selects micro-climate close to optimal Tb orientation “shuttling” (between sun and shade) Important to behavioural thermoregulation is the high thermal conductance of ectotherms Behavioural thermoregulation in an ectothermic lizard Note that the daily body temperature of the lizard remains relatively constant using behavioural thermoregulation. Only at night does the Midday Tb of the lizard drop to Mid-morning that of the burrow Late afternoon temperature. What Early morning strategy do we call this? 7  Not only ectotherms use behavioural thermoregulation Endotherms also use behavioural mechanisms (within the TNZ) to maintain optimal Tb. From your own behaviour, in winter you dress warmly, sit in the sun etc. while in summer, we look for shade, may take cold showers etc. Penguins huddle For example penguins huddle (to reduce the surface area available to heat loss) This introduces the concept of social thermoregulation Thermoregulation in elephants and penguins: Video 2 in the Video repository on SUNLearn Imagine standing on the ice all day. Their feet are not insulated because they must have some means of heat loss. How do you think they cope? They use a countercurrent heat exchange Thermal gradients between arteries and veins results in heat transfer down the gradient (introduced this concept in heterothermic tuna) The cold blood returning in veins to the body (from their feet) is warmed from heat transferred from the warm blood travelling in the arteries (coming from the heart to the feet) that pass the veins Social thermoregulation in honey bees Group behaviour regulates fairly constant hive environments Cold environment Hot environment foraging on concentrated nectar forage on dilute nectar clustering  dynamic increase water collecting adjust joint metabolic heat production fanning (see photo above) Honey bees use colony-level thermoregulation and this determines their survival during cold winters and extreme heat. They also have individual-level thermoregulation. Honey bees generate metabolic heat by contracting and relaxing their flight muscles (having uncoupled the wings from them) and this allows them to forage under a wide range of Ta and they have exploited this function to thermoregulate their hives. Heinrich, B. 1993. Comfort in a hive: Heads you're hot, tails you're cold. Natural History. 102 (8): 52. http://search.ebscohost.com.ez.sun.ac.za/login.aspx?direct=true&db=aph&AN=9307230152&site=ehost-live&scope=site Additional Reading (SUNLearn): Heinrich, B. 1993. Comfort in a hive: Heads you're hot, tails you're cold. Natural History. 102 (8): 52. Example of bees using social thermoregulation as a defence strategy Video 4 in the Video repository on SUNLearn 8  While moths as solitary insects rely on regional heterothermy Usually the moth will have a Tb close to that of the Ta until it prepares for flight at which “shivering” time it “warms up” insects Metabolic heat production Fight muscles contract and produce heat prior to flight (“shivering”), unlike honey bees that heat up silently. Heat generated keeps Tb (specifically thorax temperature) elevated Heat producing ability allows extended periods of activity J. Crespo Infrared photos showing elevated thorax temperature of moth from preflight through to flight General endothermic strategies Endotherms in hot environments Endotherms in cold environments Passive evaporation occurs across 1. Subcutaneous fat / blubber 1. Body surface 2. Effective insulation – thick fur/ plumage 2. Respiratory epithelia 3. Increased body size Regulated evaporation 4. Cardiovascular adjustments 1. Sweating (vasoconstriction) 2. Panting Humans have 5. Countercurrent heat exchangers 3. Hyperventilation ~3 million sweat glands 6. Metabolic heat production 4. Spreading of urine and saliva Core body temperature over body surface 36C Other 1. Cardiovascular adjustments (vasodilation) 2. Countercurrent neat exchange 5C Temperature (cool brains) of environment Will now look at adaptations of endotherms in hot, dry environments The terrestrial habitat lacks both water and salts in the surrounding medium, thus, terrestrial animals often face the problem of both water and salt losses. Osmoregulation is vital in maintaining an appropriate intracellular environment for biochemical processes as well as cell functioning. Osmoconformers vs osmoregulators Water loss 1. Through integument 2. During air breathing 3. During excretion Kidney plays major role in H2O conservation. Kidneys originally evolved to excrete H2O, now in birds + mammals they do the opposite - conserve H2O 9  Water conservation strategies of the kangaroo rat Nocturnal Humidity in burrow 2-5x that of environment due to breathing Don’t sweat Smears saliva on throat + chin = evaporation - Emergency adaptation The Oryx Cooling the Brain The carotid rete or rete mirabile Found in camels, sheep, goats, number of ungulates and some carnivores Baker, MA. 1993. A wonderful safety net for mammals. Natural History. 102 (8): 63. http://search.ebscohost.com.ez.sun.ac.za/login.aspx?direct=true&db=aph&AN=9307230258&site=ehost-live&scope=site Additional Reading (SUNLearn): Baker, MA. 1993. A wonderful safety net for mammals. Natural History. 102 (8): 63. Carotid Rete Cooled blood to brain Cool blood from muzzle (nasal mucosa) Blood to brain is bathed by cool venous blood from muzzle on its way to heart 10  Honeybees also keep a cool head Overheating honeybees regurgitate a drop of nectar onto tongue + use evaporative cooling Cool head Evaporative cooling  by tongue “wagging” + smearing liquid onto thorax How the camel survives water shortages Adaptations to a life in the desert A. Surviving dehydration 1. Able to survive 15 days without water at 30°- 35°C 2. Lose 25-30% of their body water (but blood volume only  10%) 3. Oval-shaped RBC do not shrivel during dehydration nor does blood flow cease 4. Rehydrates by drinking 130l in less than 10 minutes (RBC unaffected under osmotic stress) 5. Large bodies = high thermal inertia (store heat rather than lose water through evaporation) Yagil, R. 1993. From its blood to its hump, the camel adapts to the desert. Natural History. 102 (8): 30. http://search.ebscohost.com.ez.sun.ac.za/login.aspx?direct=true&db=f5h&AN=9307230035&site=ehost-live&scope=site Additional Reading (SUNLearn): Yagil, R. 1993. From its blood to its hump, the camel adapts to the desert. Natural History. 102 (8): 30 B. Water conserving mechanisms Produce concentrated urine Produce dry faeces Moist nasal passages recover water from expired air Carotid rete High body temperatures (adaptive hyperthermia – saves 5l water a day) Adjusts metabolic rate ( thyroxine - breathe slowly) Insulation via thick light-coloured fur Fat concentrated in hump = heat loss unimpeded C. Behavioural adaptations Face the sun – reduces area exposed to heat radiation and fat in the hump insulates the exposed back Stands to increase heat loss and position body where temperatures are cooler Lies down reducing muscle activity Urinates on its legs – evaporative cooling Squat close together using each others shade Avoids overgrazing (sustainable food source) 11