Evaporation - ThermalNet

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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
Can maintain thermal equilibrium with evaporation
of 240 g H2O per hour
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
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
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
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)
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
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 initiated
Takes care of decr. heat loss via radiation, convection,
conduction
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.
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
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.
Apocrine
2.
Eccrine
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.
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.
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)
Horse - at high Ta - relies primarily on sweating - with
little or no effective panting.
Endurance trained horses at high Ta lose 10 - 15 kg
sweat / 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 % EFV
Also racehorses may exhibit NaCl deficiency with excessive
sweating.
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 (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 >> shifts
plasma volume and decreased sweating efficiency
and secretion rate.
Due to hypertonicity - body fluids
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
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
Increase %RH with decrease dry bulb temperature may
eventually reach dew point temperature (Ta at
which air is saturated and condensation occurs.)
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)
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
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
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
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
Breathing pattern - maximizes volume air over buccal surface.
Without increasing ventilation - lungs
Therefore - less CO2 blowoff &
disturbance acid-base balance
Occurs 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 panting
Ox
Sheep
Rabbit
Dog
10-fold
12-fold
15-fold
23-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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