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 Rate3. 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 skinIf 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 - TNZEven occurs at cold Ta

Human - Ta < 30 C - evaporation is constant12 - 15 g / m2 x hr

(1/2 respiratory ; 1/2 cutaneous)-- Even dry skin has moisture loss --

Ta > 30 C Evaporation increases linearily with Taas active sweating is initiated

Takes care of decr. heat loss via radiation, convection, conduction

below LCT

Cattle - 17 Watts / m2 below LCTup 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 CEuropean 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 glandsTherefore - 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. Lubrication3. 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 humansAmount 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 controlEccrine 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 skinContains 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 humidityNOT relative humidity (proportion of saturation

vapor pressure represented by water vapor already present)

If water vapor amount constantIncrease in Ta >> decrease in % RH

If water vapor amount constantIncrease in Ta >> decrease in % RH

• Saturation vapor pressure higher at higher Ta

At 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 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 returnAir 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 water2. Respiratory counter-current

exchange

At 15C & 25% RHWater recondensed on expiration=74% water added (cactus wren)83% for kangaroo rat

Cactus wren - 75% heat added on inspiration recoveredKangaroo 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 loss2. Convective exchange

Breathing pattern - maximizes volume - air over buccal surface.

Without increasing ventilation - lungsTherefore - 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/minIncreases with panting

Ox 10-foldSheep 12-foldRabbit 15-foldDog 23-fold

Increase in pig only 3-fold + with inability to sweat >> very poor heat tolerance.

Cow pants - but less effective than sweating