Lecture#27 Thermoregulation PDF

Outline Learning Objectives: Introduction: temperature dependence of metabolism rate ✔ Temperature classification of animals ✔ Mechanisms of thermoregulation Ectotherms Heterotherms Endotherms ✔ Thermoregulation 28 Temperature Regulation Heterothermy – combines characteristics of ectothermy and endo...

Outline Learning Objectives: Introduction: temperature dependence of metabolism rate ✔ Temperature classification of animals ✔ Mechanisms of thermoregulation Ectotherms Heterotherms Endotherms ✔ Thermoregulation 28 Temperature Regulation Heterothermy – combines characteristics of ectothermy and endothermy 1. Temporal heterothermy (e.g. small birds, mammals and some insects in cool environment) 2. Regional heterothermy (e.g. certain fishes and reptiles) Endothermy Strategies of thermoregulation a) Cold environments b) Warm environments Thermoregulation 29 Temporal heterothermy Hibernation - Smaller sized animals only (≤3kg), where Tb drops to near Ta - Bears winter sleep (not hibernation) with Tb dropping only 5℃ below normal Daily torpor - state of decreased physiological activity in an animal by reduced body temperature and metabolic rate (e.g. some rodents, marsupials, hummingbirds) Thermoregulation 30 Regional heterothermy 2) Regional heterothermy: localized areas of the body that operate at a higher temperature than the rest of the body able to elevate their core body temperature while extremities remain at ambient temperature some fish can increase core temperature through intense metabolic activity (e.g. Tuna, Mako Shark) special vascular system arrangement, the rete mirabile Thermoregulation 31 Regional heterothermy Rete mirabile countercurrent exchange system ▪ The major blood vessels are located under the skin ▪ Red (aerobic) swimming muscles are located deep in the body core ▪ Parallel arrangement of incoming arterioles (cold) and outgoing venules (warm) allows heat retention in swimming muscles Thermoregulation Fig. 15-22b, p751 32 Regional heterothermy Vascular anatomy of the rete mirabile Eckert Fig. 17-20 Thermoregulation 33 Thermoregulation in endotherms Maintain constant body temperature within narrow range Balance between heat gain and heat loss: Heat input + Heat production = Heat loss Regulation may involve changes in: skin blood flow (i.e. vasoconstriction/vasodilation) heat dissipation (e.g. sweating, heat windows) metabolic heat production Thermoregulation 34 Thermoregulation in endotherms Key terms: Upper critical temperature (UCT) Zone of active heat dissipation Lower critical temperature (LCT) Zone of metabolic regulation Thermoregulation Similar to Fig. 15-21, p749 LCT UCT Ambient temperature (Ta) 35 Thermoregulation in endotherms Key terms: Thermal neutral zone (TNZ) Basal rate of heat production matches heat loss to the environment Endotherms adjust their rate of heat loss through metabolically inexpensive mechanisms Vasomotor responses Postural changes Insulation adjustments Thermoregulation Similar to Fig. 15-21, p749 LCT UCT Ambient temperature (Ta) 36 Thermoregulation in endotherms Metabolically inexpensive strategies are used by endotherms in the TNZ response to cold Ta response to warm Ta 1) Vasomotor response Selective vasoconstriction or vasodilation of blood vessels to the periphery Thermoregulation Shunt vessels (metarterioles) Fig. 15-17, p744 37 Thermoregulation in endotherms Metabolically inexpensive strategies are used by endotherms in the TNZ 2) Postural changes Change in body shape or body orientation relative to the sun or shade Sherwood et al., p743 Thermoregulation 38 Thermoregulation in endotherms Metabolically inexpensive strategies are used by endotherms in the TNZ 3) Insulation adjustments Pilomotor muscles of birds and mammals can raise or lower feathers/hair to alter the thickness of the insulating layer Thermoregulation Eckert Fig. 17-8a 39 Thermoregulation in endotherms Heat production/distributions by endotherms in cold environments 1) Shivering thermogenesis Chemical energy of muscle used for heat production Groups of antagonistic muscles produce shivering No physical work generated (energy used for heat) Thermoregulation 40 Thermoregulation in endotherms Heat production/distributions by endotherms in cold environments 2) Non-shivering thermogenesis Brown adipose tissue (BAT, brown fat) mitochondrial-rich and highly vascularized capable of substantial and rapid heat generation abundant in hibernating mammals lipid oxidation takes place within adipose tissue activated by the sympathetic nervous system, epinephrine and thyroid hormones Thermoregulation 41 Thermoregulation in endotherms 2) Non-shivering thermogenesis Production of heat in brown fat ⇧ ATP hydrolysis by epinephrine and thyroid hormones and sympathetic NS energy liberated is used for heat uncouples proton flow into the mitochondria from ATP synthesis protons enter mitochondria through special channels, thermogenin (UCP-1) lipid oxidation energy captured as heat Thermoregulation Fig. 15-19, p746 Brown adipose tissue (BAT) in a bat 42 Thermoregulation in endotherms Heat production/distribution by endotherms in cold environments Blood flow without countercurrent heat exchange 3) Countercurrent heat exchange Found in poorly insulated flippers (dolphines) & flukes (whales) Blood flow with countercurrent heat exchange Central outgoing artery (warm) is surrounded by incoming veins (cold) = ‘central rete’ Arterial blood gives up heat to the incoming venous blood Thermoregulation Fig. 15-18b-d, p745 43 Thermoregulation in endotherms Heat production/distribution by endotherms in cold environments 3) Regional heterothermy Countercurrent heat exchange Birds and arctic land mammals use countercurrent heat exchange to minimize heat loss at extremities Fig. 15-18a, p745 Thermoregulation Eckert Fig. 17-24 extremities of cold climate endotherms are often well below core Tb and instead approach ambient temperature (Ta) 44 Thermoregulation in endotherms Heat production/distribution by endotherms in cold environments 4) Fur and blubber Vasoconstriction of arterioles leading to the skin lowers conductance of peripheral tissues (better insulation) Blubber is more easily regulated compared to fur Thermoregulation Eckert Fig. 17-8 45 Thermoregulation in endotherms Heat production/distribution by endotherms in cold environments 4) Fur and blubber Eckert Fig. 17-25 metabolic rate increase that must be sustained by an endotherm as Ta falls depends on the degree of insulation animal with more insulation has greater range of TNZ Thermoregulation 46 Thermoregulation in endotherms Endotherms in warm environments 1) Evaporative cooling Sweating Panting Gular fluttering Saliva spreading Thermoregulation 47 Thermoregulation in endotherms Endotherms in warm environments 1) Evaporative cooling Sweating increases as Tb approaches/exceeds 37℃ due to increases in Ta or onset of exercise Sweating is less effective for heat dissipation under high humidity conditions Thermoregulation Eckert Fig. 17-32 48 Thermoregulation in endotherms Endotherms in warm environments Eckert Fig. 14-10a 2) Limited heterothermy Occurs in very hot, dry climates in relatively large endotherms Camels, with large body mass and thick pelage, can insulate from the external heat Dehydrated camel increases its heat absorbing capacity by tolerating greater range in its core Tb Thermoregulation 49 Thermoregulation in endotherms Endotherms in warm environments 3) Heat windows Enhanced heat dissipation over a body surface by radiation or convection 50 Thermoregulation Eckert Fig. 17-28 50 Thermoregulation in endotherms Endotherms in warm environments 4) Carotid rete (or rete mirabile epidurale) present in ruminants (e.g. sheep, goats, deer) and some carnivores (e.g. dogs, cats) prevents overheating in the brain countercurrent exchange system uses cool venous blood from the nasal airways to remove heat from the arterial blood destined for the brain Thermoregulation 51 Mechanisms of temperature control The hypothalamus: an endogenous thermostat Temperature sensitive neurons exist in the mammalian brain along with nerve endings associated with spinal cord, skin and other sites in the body core Most important thermoregulatory center is located in the hypothalamus – not surprisingly since even small variations in brain temperature can seriously affect neuronal function Thermoregulation 52 Mechanisms of temperature control At least three unique subsets of neurons are temperature sensitive and yield different responses: 1. 2. 3. Sharp increase in activity with increased temperature (red) Less activity as temperature increases (purple) Enhanced activity when temperature drops (green) Neuron firing frequency high Stimulates thermogenesis, vasoconstriction and piloerection Stimulates vasodilation and sweating Hypothalamic set point temperature Decreases thermogenesis, vasoconstriction and piloerection low Neuron temperature (ºC) Thermoregulation 53