Lecture 1 (Homeostasis) PDF

Homeostasis and Integration: Case of Thermoregulation ANSC 3080 G. Bedecarrats Learning Objectives Explain the general concept of homeostatic control systems Using thermoregulation as an example, explain the integrated response involved in maintaining h...

Homeostasis and Integration: Case of Thermoregulation ANSC 3080 G. Bedecarrats Learning Objectives Explain the general concept of homeostatic control systems Using thermoregulation as an example, explain the integrated response involved in maintaining homeostasis What major body systems/functions are important? What controls the response? Homeostasis Definition: Maintenance of a constant (normal) internal environment Coordinated responses of organs/systems that automatically compensate for environmental changes Body temperature, blood pressure, O2/CO2 levels Coordinated response = more than 1 system Automatically: involves autonomic nervous system (reflexes) 1  Body Temperature Mammals and birds are homeotherms Warm-blooded animals - maintain body temperature within narrow limits Low and high temperature NOT tolerated Species State Temp (°C) Dog 37.5-39.0 Cat 38.0-39.5 Horse 37.5-38.5 Dairy Cow Non-Pregnant 38.0-38.5 Dairy Cow Lactation 38.3-38.9 Sheep 38.4-39.5 Sow 38.5-39.0 Broiler Chicken Rapid Growth 40.5-42 Laying Hen Egg production 40.5-42 Core body temperature: Protects vital organs Measured by rectal insertion of thermometer Influenced by environment, exercise, metabolism Thermoneutral Zone  The thermoneutral zone is the ambient temperature at which no adjustment is needed  No additional energy required to maintain body temperature = perfect husbandry conditions  Critical temperatures (hot or cold) = energy required to maintain homeothermic condition (produce or actively dissipate heat) 2   Hyperthermia (heat stress): generally fatal, cells cannot function (enzymes and proteins denatured)  Hypothermia (cold stress): cells can survive freezing. However, blood circulation and respiration fail = death.  How can a horse survive heavy exercising?  How can a cow survive outdoors in summer and winter? Balance Between Heat Production and Heat Loss  In mammals and birds, heat comes mainly from body production = endothermic (not energy efficient)  Metabolism = conversion of nutrients to heat (energy), mainly in heart, liver, kidneys and brain  Growth and production (body factory)  Exercise: about 80% of energy produced is heat  Heat transported by the blood needs to be released in the environment. Four physical processes:  Radiation  Conduction  Convection  Evaporation/Condensation How is Extra-Heat Produced? Muscle activity: exposure to cold  shivering Rhythmic (15/sec) involuntary muscle contractions Antagonistic muscles contract simultaneously Nervous input No exercise = 100% energy goes to heat production Non-shivering thermogenesis:  metabolism Sympathetic nervous system results in secretion of catecholamines (adrenalin)   metabolism (calorigenic) especially from brown fat Thyroid hormones   metabolism 3  How is Extra-Heat Lost?  Under normal conditions, metabolic heat is removed from the core by the blood through the skin  Insulation property of the skin inversely proportional to the blood flow High heat loss Low heat loss Hair, plumage Mechanisms of Heat Transport  Gradient physical laws = transfer from high to low.  Radiation: electromagnetic radiation is emitted and absorbed; relative to the surface and temperature of the object/animal  Conduction: transfer through contact; depends on the conductivity (inverse of insulation). Animal  ground  Convection: transfer via moving air or water  Evaporation: very efficient, requires energy to break water molecules apart. Through skin and airways.  Passive evaporation: normal water loss  Sweating: active process under sympathetic nervous system. Also results in loss of ions.  Panting: rapid shallow breathing (not in horses or pigs)  Bathing: water absorb energy before evaporating Most of the time, all processes occur simultaneously. Type of animal (fur, size, etc…) also important. 4 Thermoregulation: System Components  Involves participation of numerous physiological systems  Automatic = reflex = sensor + integration + effectors  Thermoreceptors = nerve cells  Integration Center = hypothalamus: information compared to internal reference set points  Effectors  Neurons connected to muscles (somatic) for shivering  Sympathetic nervous system for blood flow, sweat gland and metabolism  Hormones: Hypothalamo- pituitary axis System Components continued Voluntary component: Hypothalamus connected to the cortex = exchange of information Change of behavioral response, put your jacket on! = cognitive behavior. Response to Heat  Heat receptors:  In the skin (below epidermis), in the body core (especially hypothalamus - sensitivity of 0.1°C), blood vessels, viscera  Information transferred through warm sensory fibers  Hypothalamus: activation of the heat-loss center (anterior)  Effectors:  Sympathetic vasoconstriction fibers slow down = vasodilatation  Sympathetic activity increase for evaporative heat loss: sweating, panting  Reduction of physical activity  Behavioral response: increased exposure area to air, seeking shade, wind 5  Sensitivity to Heat Depends on the animal and the external conditions (humidity level) Cattle and sheep very good Pigs very bad = little sweat, small mouth forpanting (transport at night preferred) In birds: air sac (extension to the lung) deep in body cavity = ventilation helps cooling Response to Cold  Cold receptors:  In the skin (below epidermis). In the body core (hypothalamus, blood vessels, viscera)  Information transferred through cold sensory fibers. Much more numerous than warm fibers  Hypothalamus: activation of heat-producing center (post.)  Effectors:  Reduction of heat loss: Sympathetic vasoconstriction fibers fire; Pili muscles from hair follicle contract; “Curl-up” position  Production of heat: Muscular contraction (shivering) by somatic motor system; Sympathetic and hormonal activity  (catecholamines, thyroid hormones) = metabolism  (especially in brown fat). 6  Adaptation Mechanisms To heat: Prolonged exposure to heat = lots of sweat. Increase in aldosterone (hormone) stimulates reabsorption of Na+ and Cl- from sweat glands Shedding To cold: Change of fur coat, increase in insulation properties (subcutaneous fat) Hibernation: voluntary decrease in core body temperature, followed by a rapid raise. Pathology  Fever:  Occurs by “resetting” hypothalamic thermostat to higher level  Response is equivalent to a decrease in temperature  Caused by pyrogens released during infection and tissue damage  Hyperthermia:  Happens when temperature exceeds the regulating capability. Heat production exceeds heat loss. Convulsions, nausea, loss of consciousness, death.  Excessive exercise, exposure to high temperatures  Hypothermia:  Heat loss exceeds heat production capability. Slow nervous activity, muscular failure (cardiac, pulmonary), death  Critical with newborns 7