Biomedical Engineering Reference
In-Depth Information
700
600
500
400
Crossbred
Indigenous
300
200
100
0
Evaporative loss
through pulmonary
surface
Evaporative loss
through skin
Fig. 4
Evaporative loss through skin and pulmonary surface in indigenous and crossbred cattle (Aggarwal and
Upadhyay
1997
)
Table 2
Relative contributions (%) of sweating and panting to evaporative heat loss in various domestic animals in a
hot-dry environmen
t
Relative contribution (%) to evaporative heat loss
Donkey
Camel
Cow
Sheep/goat
Buffalo
Pig
Sweating
100
95
85
40
12
0
Panting
0
5
15
60
88
100
Source: Modi fi ed from Jenkinson (
1972
)
on cold mucosa upon exhalation. The passive
transfer of water through the skin depends partly
on the surface temperature of the epidermis and
the vapour pressure of the air.
Thermoregulatory sweating is elicited in two
ways:
First by a rise of the CNS temperature and
responsible for thermal sweating, but apocrine
sweat glands are important for evaporative heat
loss. The relative importance of sweating as a
heat-dissipation mechanism varies among species
(Table
2
). In cows, maximum evaporation from
the skin surface amounts to about 150 g/m
2
/h at
an external temperature of 40°C. The respiratory
evaporation under the same condition is only
about one-third of that amount. In sheep, sweat
secretion is less important than in the cow. Sweat
secretion in shorn sheep during heat stress is
32 g/m
2
/h, which means that sheep may dissipate
about 20 kcal/h by sweating. Consequently, evap-
orative heat loss via respiratory passages is more
important in sheep than in the cow. In humans,
heat loss from sweating may be as high as
1,000 kcal/h (Andersson and Jonasson
1992
) .
Panting appears to be the more efficient of the
two methods of evaporative cooling, that is,
sweating and panting. Both methods use latent
heat from the body core, but sweating can also
use solar radiation on the body surface. Panting
re fl exively
Second by stimulation of skin warm receptors
and other parts of the body outside the CNS,
that is, blood vessels, abdomen and viscera
Although reflex sweating may occur in absence
of an increased central temperature, a high skin
temperature cannot elicit full-scale sweating
without simultaneous CNS facilitation.
Regarding sweating, the eccrine sweat glands
(in humans) are innervated by cholinergic sympa-
thetic nerve fibres, and the apocrine sweat glands
(in many domestic animals) develop from hair
follicles. The latter glands do not receive nerve
supply, but are sensitive to epinephrine carried in
the blood stream. The eccrine sweat glands are
