Biomedical Engineering Reference
In-Depth Information
also provides its own airflow over moist surfaces,
thus facilitating water evaporation. Salt and elec-
trolytes are not lost, as in sweating, unless the
saliva drips out of the mouth. The panting cools
the nasal and oral passages as the cool blood
flows into the venous sinus bathing the carotid
plexus. Thus, the blood supply to the brain can be
kept cool, even when the body temperature is ris-
ing (Taylor and Lyman
1972
) . The disadvantages
of panting include a risk of respiratory alkalosis,
particularly in the goat (Jenkinson
1972
) , sheep
and buffaloes, and the increase in work and
therefore heat production by the respiratory
muscles. However, much of this work is reduced
by the elastic property of the respiratory system,
which has its own natural frequency of oscilla-
tion. The high respiratory rate associated with
panting has the effect of keeping the system
oscillating at its own resonant frequency with the
minimum of muscular effort. Thus, the thermo-
regulatory efficiency of panting is high in such
species as sheep, which show no increase in total
body heat production above normal levels (Hales
and Brown
1974
) .
The evaporation is a very significant process
for loss of body water. A fully hydrated camel
weighing 260 kg lost 91 of water a day through
sweat when standing in the desert sun. This quan-
tity represented a loss of 4% of total body weight,
and a loss much in excess of 25% would probably
be fatal. Assuming that heat load and therefore
evaporation are proportional to body surface,
then water loss under hot, desert conditions
increases exponentially with decreasing size.
There is very little difference in water loss per
hour in the camel at 1.0%, and man at 1.5%, but
the rate in animals weighing 2.5 kg is nearly 5%.
Many animals also have lower lethal limits than
the camel (Schmidt-Nielsen
1965
) . The need to
preserve vital functions, as an animal becomes
dehydrated, results in a reduction in the rate of
evaporative cooling. The sequel to this reduction
is either a rise in body temperature or a depres-
sion of heat production.
Kibler and Brody (
1952
) observed similar
sweating rates for
B
.
taurus
and
B
.
indicus
breeds;
however, Allen (
1962
) showed that
B
.
indicus
or
zebu cattle had significantly higher sweating rates
than breeds from temperate regions and ascribed
elevated sweating rates of
B
.
indicus
zebu cattle
to their higher density of sweat glands. Schmidt-
Nielsen (
1964
) reported that as the environmental
temperature rose,
B
.
taurus
cattle showed an
appreciable increase in evaporation between 15
and 20°C, with a maximum rate of evaporation
being reached before 30°C. On the contrary,
Brahman cows (
B
.
indicus
) had initially lower
evaporation rates, but rapid evaporation rates
occurred when temperatures were between 25 and
30°C, and continued rising up to 40°C. Cattle of
temperate and tropical regions possess the same
type of sweat glands, one to each hair follicle
( Findlay and Yang
1950
) . However, Dowling
(
1955
) found that tropical breeds have a higher
density of hair follicles (1,698/cm
2
for zebu) than
is the case in
B
.
taurus
breeds (1,064/cm
2
for
Shorthorn), and in zebu cattle, sweat glands are
located much closer to the skin surface than is the
case in temperate breeds of cattle. Blazquez et al.
(
1994
) attributed more significance to the pro-
duct that these sweat glands produce than to the
number of sweat glands per unit area or to their
individual size. They measured a fivefold increase
in the rate of skin moisture loss (up to 279 g/m
2
/h)
from the scrotum at 36.2°C in bulls.
The measurement of sweating rate in large
animals is difficult due to equipment availability
and placement of device. Robertshaw and Vercoe
(
1980
) reported a twofold increase in the rate of
skin moisture loss (up to 77 g/m
2
/h) from the
scrotum after exposure to a temperature of 40°C
in bulls (
Bos
sp.). Finch (
1986
) found that the
sweating rates of
B
.
indicus
increased exponen-
tially with rises in body temperature, whereas in
B
.
taurus
, the sweating rates tended to plateau
after an initial increase. Within
B
.
taurus
breeds,
Singh and Newton (
1978
) found higher (
P
< 0.05)
sweating rates in Ayrshire calves than in Guernsey
calves and suggested that Ayrshire calves were
more capable of acclimation to hot weather than
Guernsey calves.
Polypnea or a rapid breathing in cattle, buf-
falo, dog, sheep, goat, etc. is evoked by heat load
and lack of oxygen. Polypneic panting is breath-
ing occurring at a frequency between 200 and
400 breaths/min with open mouth and protrusion
