EVAPORATIVE WATER LOSSEffective channel - heat lossTa 25 C - latent heat of vaporization = 582 cal/gm100 kg animal - with 140,000 cal/h metabolic rateCan maintain thermal equilibrium with evaporation of 240 g H2O per hour
Respiratory EvaporationDifference in moisture content - expired and inspired airExpired air collected-Open-Circuit System-Use narrow-range humidity sensing elements (hygrometers) or desiccant to absorb water vapor
Skin vaporizationAir - known moisture content passed over skin - Then change - moisture content - dependent on cutaneous vaporization Assuming constant air flowAlso assume - water vapor content - air low enough tomaintain complete and rapid evaporationImportant factors: 1. Percent Relative Humidity 2. Air Flow Rate 3. Air Temperature
Relies on water vapor pressure gradient Depends on state change - water LIQUID >>>>> GAS STATESLatent Heat Vaporization = 0.58 kcal / gmCooling - only by vaporization at skinIf water remains on skin or drips off >> NO COOLING EFFECTImpermeable garments or covering - reduces evaporation>> increased discomfort
Body Heat Loss - via evaporation -ALWAYS OCCURSEven at cold Ta there are respiratory and cutaneous evaporationsCutaneous Evaporation - both passive and activeprocessesInsensible water loss = passive evaporation Excludes excreted water (sweat, urine, feces)
Respiratory & cutaneous evap. water loss contribute equally- heat loss at restRepresents ~ 25% total heat loss - TNZ Even occurs at cold TaHuman - Ta < 30 C - evaporation is constant 12 - 15 g / m2 x hr (1/2 respiratory ; 1/2 cutaneous) -- Even dry skin has moisture loss --Ta > 30 C Evaporation increases linearily with Ta as active sweating is initiatedTakes care of decr. heat loss via radiation, convection, conduction
Cattle - 17 Watts / m2 below LCT up to 116 Watts / m2 above UCTJersey cow (400 kg) - EHL >> 18% THL (15 C) EHL >> 84% THL (35 C)Brahman cow - lower EHL rates than European cows below 32 C - due to their lower MRBrahman reach maximum EHL at 35 C European cattle reach maximum EHL at 27 CLarge species differences - water loss through skinAbility - control Tb - high Ta by cutaneous EHL - related to sweat glands.
Pigs - have no sweat glands Therefore - little incr. EHL (cutaneous) - high TaBirds - also no sweat glandsLimited water loss - skin - even during heat stress Passive water loss - not directly under thermoregulatory control.Humans - more sweat secretion than any other species 2.5 million sweat glands• Sweating - superior to panting & respiratory evap. -when no obstruction to evaporation
Sweat glands - of ancient origen - derived from glands of skin - even in amphibians & reptilesLike mammalian glands - duct surrounded by myoepithelial network (contactile epithelial cells)Sweat Gland Functions: 1. Thermoregulation 2. Lubrication 3. Secretion - noxious material 4. Defense against predators 5. Sexual attractantsSweat Gland Types: 1. Apocrine 2. Eccrine
APOCRINE - glands associated with primary hairfollicles; involves breakdown of membranes ofsecretory cells of glands - discharge cell contents.• Begin function - puberty; viscous secretionECCRINE - glands not associated with hair follicles; involves fluid secretion across intact cell membranes.Function throughout life; watery secretionHuman - all eccrine glands do not function simultaneously- or under same conditions - all parts of body.At high Ta - sweating starts - forehead - spreads >> faceand then rest of body. Finally - palms & soles increase production -BUT with nervous strain - they may start first.
Sheep - sweat glands discharge briefly - over entirebody surface. Sweating - of secondary importance to sheep. In contrast - to humans and cattleCattle - numerous apocrine glands - assoc. with hair follicles.No eccrine sweat glands - as in humans Amount of sweat per gland in cattle much less than in humans.+ total amount sweat produced per SA less than for humans.
Skin - cow - rarely appears wet - Originally thought - cows do not sweat BUT due to slow rate of secretionSweat collects - drops - on hairs(Therefore - sweat spreads >> skin surface for evap.)Sweat glands - Bos indicus - larger & morenumerous than for Bos taurus.------------------------------------------------------------------------------Apocrine glands - adrenergic control Eccrine glands - cholinergic controlhich are water repellant)
Horse - at high Ta - relies primarily on sweating - withlittle or no effective panting.Endurance trained horses at high Ta lose 10 - 15 kgsweat / hour and may become clinically dehydrated with 7-10 % decrease in body weight.For - 450 kg horse - this water loss = 30-45 L or 30-40 % EFVAlso racehorses may exhibit NaCl deficiency with excessivesweating.
Sweat differs from fluid lost via skin Contains large quantities - electrolytes + ureaSweat resembles extracellular fluid (Na and Cl being the major ionic components) BUT - hyposmotic to plasma and variable• NaCl = chief substance of sweat (0.2 - 0.4 g/ 100 ml) Concentration rises with increasing sweating rate Acclimation >> hot Ta >> decreased NaCl (sweat) This does not occur if extra salt consumed.BUT - dehydration or increased salt intake >> shiftsplasma volume and decreased sweating efficiencyand secretion rate. Due to hypertonicity - body fluids
VAPOR PRESSURE GRADIENT = Driving force for EvaporationVapor pressure = measure of absolute humidity NOT relative humidity (proportion of saturation vapor pressure represented by water vapor already present)If water vapor amount constant Increase in Ta >> decrease in % RH
If water vapor amount constant Increase in Ta >> decrease in % RH • Saturation vapor pressure higher at higher TaAt Ta 20 CSaturation vapor pressure 23.3 mbar = 100%RH 11.7 mbar = 50%RHAt Ta 30 C Saturation vapor pressure 42.0 mbar = 100%RH 23.3 mbar = 55%RH 11.7 mbar = 28%RH
Increase %RH with decrease dry bulb temperature mayeventually reach dew point temperature (Ta atwhich air is saturated and condensation occurs.)
RESPIRATORY HEAT LOSS1.Heat loss warming inspired air due to differences intemperatures of inspired and expired air• Only small part of heat balanceAir has low specific heatTherefore - heat required increase temperature is small.Heavy exercise - incr. in ventilation rate - but only in proportion to incr. HP and incr. HLProportional loss by warming air does not increase.What about very cold air? (-40°C) Heat required warm air to Tb level increased (77°C difference)
2. More heat lost - saturate inspired air with water vapor. Conditioning - inspired air - efficient processEven at -100 CAir - heated >> Tb & saturated with water vapor whenreaches alveoli.EXPIRATION - Air meets mucosa (cooled by inspiration)Heat Loss:1. Sensible heat transfer 2. Condensation - latent heat releaseMuch of respiratory exchange - seen - upper resp. tract (turbulent convection)Contrast - lungs - conditions - invariant - even under extremes
In cattle - expired air - almost saturated at TbSome water condensation - mucosa - at temp. below dew-point. Latent heat release - some heat return Air leaves at temp. below Tb - maybe below Ta.Form of counter-current exchange >> heat + water savings.Very important - desert animals Kangaroo rat - can live without water intake Instead water derived from: 1. Metabolic water 2. Respiratory counter-current exchange
At 15C & 25% RHWater recondensed on expiration=74% water added (cactus wren)83% for kangaroo ratCactus wren - 75% heat added on inspiration recoveredKangaroo rat - 88% recoveredHuman - dry air at 0 C - loss of only 20% of heat prod. Much smaller than if no counter-current exchangeDehydrated camels: upper resp. tract cools expired air - extracts waterDefense against dehydration more important for survival - when hyperthermia is allowed
PANTING - Reptiles, birds, mammals Less effective than sweatingNon-primate mammals - less than 100 kg adult BW primarilypant What is panting? Open-mouth - rapid, shallow breathing 1. Evaporative water loss 2. Convective exchange
Breathing pattern - maximizes volume - air over buccal surface.Without increasing ventilation - lungs Therefore - less CO2 blowoff & disturbance acid-base balanceOccurs in animals with lower sweating capabilities (pigs, sheep)Higher body wt. - usually correlated with lower panting freq.
Respiratory minute volume = amount air inspired/min Increases with pantingOx 10-foldSheep 12-foldRabbit 15-foldDog 23-foldIncrease in pig only 3-fold + with inability to sweat >>very poor heat tolerance.Cow pants - but less effective than sweating