Biomedical Engineering Reference
In-Depth Information
of tongue. Panting is often accompanied by
increased salivary secretion and may cause a
considerable increase in respiratory evaporative
cooling. Panting may be initiated in response to a
raised environmental temperature and an increase
in the temperature of the blood supplying the
brain. Panting in cattle and buffalo may be
induced due to work and high external tempera-
ture by a rise in body temperature or by local
warming of the anterior hypothalamus. This indi-
cates that the panting, like sweating, may be initi-
ated both reflexively and centrally. Birds, which
have no sweat glands, increase evaporation not
only by panting but also by a mechanism named
regular flutter. It consists of rapid oscillations of
the thin floor of the mouth and the upper part of
the throat (Andersson and Jonasson
1992
) .
The proportion of metabolic heat that is dissi-
pated from the cow by evaporation increases with
rising environmental temperatures and a decreas-
ing temperature gradient between the animal and
air. The morphology and functioning of the apo-
crine sweat glands of cattle during hot climatic
conditions have been extensively investigated
(Montgomery et al.
1984
) . Blazquez et al. (
1994
)
reported that increased blood flow to the skin is
positively correlated to the sweating rate. The
correlation between rectal temperature and water
loss from skin was observed to be negative in buf-
faloes which could be due to higher rectal tem-
perature in relation to skin water loss (Aggarwal
and Upadhyay
1998
) .
is sufficient to maintain a thermal steady state.
But, above and below this thermoneutral zone,
circulatory adjustments are no longer enough for
maintenance of heat balance or body temperature.
Heat production (HP) is a measure of the sum
total of energy transformations happening in
the animal per unit time (Yousef
1985
) . Heat
production is directly under control of the nervous
system (Hammel
1968
), by the endocrine system,
through modification of appetite and digestive
processes, and indirectly by alterations of the
activity of respiratory enzymes and protein
synthesis (Yousef
1985
) . The in fl uence of envi-
ronmental temperature on feed intake, production
and thermoregulation in the animal greatly
affects the rate of heat production (Brody
1945
) .
Because of their effect on the rate of metabo-
lism, the concentrations of hormones such as
thyroxine, triiodothyronine, growth hormone
and glucocorticoids are closely related to heat
production (Yousef and Johnson
1966
) . Other
factors influencing heat production in mammals
include the following: body size (Brody
1945
) ,
the environment (Salem et al.
1982
) animal species
and breed and the availability of feed and water
(Graham et al.
1959
) . Breed differences between
Jersey and Holstein cows in the rate of heat
production and dissipation have been reported,
which may be attributed to differences in body
size. The temperature gradient between internal
organs and external environment is steeper in the
smaller Jerseys than in the larger Holstein cows
(Kibler and Brody
1952
) .
7
Heat Production
8
Heat Increment
A thermal steady state in an animal exists, when
the heat gain and the heat loss are in equilibrium.
In livestock species similar to other mammalian
homeotherms, the thermoregulatory mechanisms
consist of a series of physiological adjustments
help to establish a thermal equilibrium under heat
stress to maintain equality in heat gain and heat
loss. These physiological adjustments are highly
dependent on the external temperature and physi-
ological state of animal. Normally the variable
insulation, mainly due to circulatory adjustments
in the thermoneutral zone of constant metabolism,
Under warm to hot (ambient temperatures of 15
and >25°C, respectively) conditions, the cow
gains heat from solar radiation and from the usual
metabolic processes. If the heat gain exceeds heat
loss from radiation, convection, evaporation and
conduction, heat is stored in the body, and core
temperature rises (Finch
1986
). At night or at low
ambient temperatures, heat stored is dissipated
to the environment, and the body temperature
may reach an equilibrium or normal body set
point. Webster et al. (
1976
) discussed other heat
