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EVAPORATIVE WATER LOSS Effective channel - heat loss Ta 25 C - latent heat of vaporization = 582 cal/gm 100 kg animal - with 140,000 cal/h metabolic rate.

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Presentation on theme: "EVAPORATIVE WATER LOSS Effective channel - heat loss Ta 25 C - latent heat of vaporization = 582 cal/gm 100 kg animal - with 140,000 cal/h metabolic rate."— Presentation transcript:

1 EVAPORATIVE WATER LOSS Effective channel - heat loss Ta 25 C - latent heat of vaporization = 582 cal/gm 100 kg animal - with 140,000 cal/h metabolic rate Can maintain thermal equilibrium with evaporation of 240 g H2O per hour

2 Respiratory Evaporation Difference in moisture content - expired and inspired air Expired air collected -Open-Circuit System -Use narrow-range humidity sensing elements (hygrometers) or desiccant to absorb water vapor

3 Skin vaporization Air - known moisture content passed over skin - Then change - moisture content - dependent on cutaneous vaporization Assuming constant air flow Also assume - water vapor content - air low enough to maintain complete and rapid evaporation Important factors: 1.Percent Relative Humidity 2.Air Flow Rate 3.Air Temperature

4 Relies on water vapor pressure gradient Depends on state change - water LIQUID >>>>>GAS STATES Latent Heat Vaporization = 0.58 kcal / gm Cooling - only by vaporization at skin If water remains on skin or drips off >> NO COOLING EFFECT Impermeable garments or covering - reduces evaporation >> increased discomfort

5 Body Heat Loss - via evaporation - ALWAYS OCCURS Even at cold Ta there are respiratory and cutaneous evaporations Cutaneous Evaporation - both passive and active processes Insensible water loss = passive evaporation Excludes excreted water (sweat, urine, feces)

6 Respiratory & cutaneous evap. water loss contribute equally - heat loss at rest Represents ~ 25% total heat loss - TNZ Even occurs at cold Ta Human - Ta < 30 C - evaporation is constant 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 initiated Takes care of decr. heat loss via radiation, convection, conduction below LCT

7 Cattle - 17 Watts / m2 below LCT up to 116 Watts / m2 above UCT Jersey 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 MR Brahman reach maximum EHL at 35 C European cattle reach maximum EHL at 27 C Large species differences - water loss through skin Ability - control Tb - high Ta by cutaneous EHL - related to sweat glands.

8 Pigs - have no sweat glands Therefore - little incr. EHL (cutaneous) - high Ta Birds - also no sweat glands Limited 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

9 Sweat glands - of ancient origen - derived from glands of skin - even in amphibians & reptiles Like 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 attractants Sweat Gland Types: 1.Apocrine2.Eccrine

10 APOCRINE - glands associated with primary hair follicles; involves breakdown of membranes of secretory cells of glands - discharge cell contents. Begin function - puberty; viscous secretion ECCRINE - glands not associated with hair follicles; involves fluid secretion across intact cell membranes. Function throughout life; watery secretion Human - all eccrine glands do not function simultaneously - or under same conditions - all parts of body. At high Ta - sweating starts - forehead - spreads >> face and then rest of body. Finally - palms & soles increase production - BUT with nervous strain - they may start first.

11 Sheep - sweat glands discharge briefly - over entire body surface. Sweating - of secondary importance to sheep. In contrast - to humans and cattle Cattle - 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.

12 Skin - cow - rarely appears wet - Originally thought - cows do not sweat BUT due to slow rate of secretion Sweat collects - drops - on hairs (Therefore - sweat spreads >> skin surface for evap.) Sweat glands - Bos indicus - larger & more numerous than for Bos taurus Apocrine glands - adrenergic control Eccrine glands - cholinergic control hich are water repellant)

13 Horse - at high Ta - relies primarily on sweating - with little or no effective panting. Endurance trained horses at high Ta lose kg sweat / hour and may become clinically dehydrated with 7-10 % decrease in body weight. For kg horse - this water loss = L or % EFV Also racehorses may exhibit NaCl deficiency with excessive sweating.

14 Sweat differs from fluid lost via skin Contains large quantities - electrolytes + urea Sweat resembles extracellular fluid (Na and Cl being the major ionic components) BUT - hyposmotic to plasma and variable NaCl = chief substance of sweat ( 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 >> shifts plasma volume and decreased sweating efficiency and secretion rate. Due to hypertonicity - body fluids

15 VAPOR PRESSURE GRADIENT = Driving force for Evaporation Vapor 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

16 If water vapor amount constant Increase in Ta >> decrease in % RH Saturation vapor pressure higher at higher Ta At Ta 20°C Saturation vapor pressure 23.3 mbar = 100%RH 11.7 mbar = 50%RH At Ta 30°C Saturation vapor pressure 42.0 mbar = 100%RH 23.3 mbar = 55%RH 11.7 mbar = 28%RH

17 Increase %RH with decrease dry bulb temperature may eventually reach dew point temperature (Ta at which air is saturated and condensation occurs.)

18 RESPIRATORY HEAT LOSS 1.Heat loss warming inspired air due to differences in temperatures of inspired and expired air Only small part of heat balance Air has low specific heat Therefore - heat required increase temperature is small. Heavy exercise - incr. in ventilation rate - but only in proportion to incr. HP and incr. HL Proportional 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)

19 2.More heat lost - saturate inspired air with water vapor. Conditioning - inspired air - efficient process Even at -100 C Air - heated >> Tb & saturated with water vapor when reaches alveoli. EXPIRATION - Air meets mucosa (cooled by inspiration) Heat Loss: 1. Sensible heat transfer 2.Condensation - latent heat release Much of respiratory exchange - seen - upper resp. tract (turbulent convection) Contrast - lungs - conditions - invariant - even under extremes

20 In cattle - expired air - almost saturated at Tb Some 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

21 At 15C & 25% RH Water recondensed on expiration= 74% water added (cactus wren) 83% for kangaroo rat Cactus wren - 75% heat added on inspiration recovered Kangaroo rat - 88% recovered Human - dry air at 0 C - loss of only 20% of heat prod. Much smaller than if no counter-current exchange Dehydrated camels: upper resp. tract cools expired air - extracts water Defense against dehydration more important for survival - when hyperthermia is allowed

22 PANTING - Reptiles, birds, mammals Less effective than sweating Non-primate mammals - less than 100 kg adult BW primarily pant What is panting? Open-mouth - rapid, shallow breathing 1.Evaporative water loss 2.Convective exchange

23 Breathing pattern - maximizes volume - air over buccal surface. Without increasing ventilation - lungs Therefore - less CO 2 blowoff & disturbance acid-base balance Occurs in animals with lower sweating capabilities (pigs, sheep) Higher body wt. - usually correlated with lower panting freq.

24 Respiratory minute volume = amount air inspired/min Increases with panting Ox10-fold Sheep12-fold Rabbit15-fold Dog23-fold Increase in pig only 3-fold + with inability to sweat >> very poor heat tolerance. Cow pants - but less effective than sweating


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