a) Water Loss
a) Cutaneous. Cutaneous water loss takes place through the skin by diffusion, and
as a result of secretion by the sweat glands. This water is evaporated from the body
surface, resulting in a lo ss of heat ; a process that can be used physiologically for
cooling in hot environments. In man the eccrine sweat glands are usually considered
to be associated principally with thermal cooling while the apocrine glands, which
are associated with hair follicles , respond to other stimuli. Sweating is initiated b y
exercise in the horse, a response that is stimulated by elevated levels of adrenaline
in the blood and one which contributes to heat and water losses (LOVATT EVANS,
SMITH, and W EIL-MALHERBE, 1956). The sweat glands can secrete large volumes
of fluid ; a man working in the desert loses up to 15 litres of water a da y in this
manner. Sweat glands are typically mammalian structures though their presence,
and relative importance for thermal cooling is not the same in all species. MACFARLANE (1964) found that the maximal rate of thermal sweating in merino sheep was
32 g/m
2 • h, while the camel can form 260 g/rn i -h (SCHMIDT-NIELSEN et al., 1957).
Many mammals such as the rabbit and rat, do not possess sw eat glands though water
loss can still take place across the skin. Dehydration may result in a decreased rate
of sweating. The camel after 24 h without water perspires at 60 to 70% of the
rate seen when it is normally hydrated (SCHMIDT-NIELSEN et al., 1957). Thermal
cooling response of sweat glands are under the control of the sympathetic nervous
system . In man the final transmitter is acet ylcholine. However, in bovidae, like
domestic ox and various wild species like the oryx, eland, wildebeest and buffalo
which live in East Africa, these nerves are adrenergic (FINDLAY and ROBERTSHAW,
1965 ; ROBERTSHAW and TAYLOR, 1969). The injection of adrenaline in these species
as well as in the horse, sheep and camel results in the secretion of sweat. This type
of response is generally thought to arise physiologically during exercise rather than
in response to heat stress. Noradrenaline is relatively ineffective. Thermal sweating
is abolished in th e dehydrated oryx, though sweat glands can still respond to injected adrenaline (TAYLOR, 1969) .Neurohypophysial antidiuretic hormone does not
alter the rate of sweating in man (AMATRUDA and WELT, 1953). It seems likely that
reductions in the volume of sweat secretion during dehydration are mediated by
nervous, rather than endocrine, mechanisms but changes in the level of circulating
adrenal medullary hormones could have an effect.
~) Respiratory. The respiratory tract is a major route of water loss in tetrapods and
more especially in the mammals and birds. This loss takes place by evaporation,
as an unavoidable consequence of the uptake of oxygen and excretion of carbon
dioxide, but this avenue can also be utilized physiologically for thermal cooling
by ' panting' . The normal overall obligatory losses of water from the respiratory
tract increase or decrease with parallel changes in metabolic rate and oxygen consumption. Normally mammals extract about 30% of the oxygen present in the inspired air , but in certain circumstances this may be altered, with reflected changes
in the accompanying water loss. DICK (c. R.) TAYLOR (1969) has shown that an
African antelope, the oryx, which lives in desert areas, may reduce its oxygen consumption by 30% when it is dehydrated, and, by breathing more deeply, can extract
additional oxygen from the inspired air . The net result is a considerable reduction
91
a) Cutaneous. Cutaneous water loss takes place through the skin by diffusion, and
as a result of secretion by the sweat glands. This water is evaporated from the body
surface, resulting in a lo ss of heat ; a process that can be used physiologically for
cooling in hot environments. In man the eccrine sweat glands are usually considered
to be associated principally with thermal cooling while the apocrine glands, which
are associated with hair follicles , respond to other stimuli. Sweating is initiated b y
exercise in the horse, a response that is stimulated by elevated levels of adrenaline
in the blood and one which contributes to heat and water losses (LOVATT EVANS,
SMITH, and W EIL-MALHERBE, 1956). The sweat glands can secrete large volumes
of fluid ; a man working in the desert loses up to 15 litres of water a da y in this
manner. Sweat glands are typically mammalian structures though their presence,
and relative importance for thermal cooling is not the same in all species. MACFARLANE (1964) found that the maximal rate of thermal sweating in merino sheep was
32 g/m
2 • h, while the camel can form 260 g/rn i -h (SCHMIDT-NIELSEN et al., 1957).
Many mammals such as the rabbit and rat, do not possess sw eat glands though water
loss can still take place across the skin. Dehydration may result in a decreased rate
of sweating. The camel after 24 h without water perspires at 60 to 70% of the
rate seen when it is normally hydrated (SCHMIDT-NIELSEN et al., 1957). Thermal
cooling response of sweat glands are under the control of the sympathetic nervous
system . In man the final transmitter is acet ylcholine. However, in bovidae, like
domestic ox and various wild species like the oryx, eland, wildebeest and buffalo
which live in East Africa, these nerves are adrenergic (FINDLAY and ROBERTSHAW,
1965 ; ROBERTSHAW and TAYLOR, 1969). The injection of adrenaline in these species
as well as in the horse, sheep and camel results in the secretion of sweat. This type
of response is generally thought to arise physiologically during exercise rather than
in response to heat stress. Noradrenaline is relatively ineffective. Thermal sweating
is abolished in th e dehydrated oryx, though sweat glands can still respond to injected adrenaline (TAYLOR, 1969) .Neurohypophysial antidiuretic hormone does not
alter the rate of sweating in man (AMATRUDA and WELT, 1953). It seems likely that
reductions in the volume of sweat secretion during dehydration are mediated by
nervous, rather than endocrine, mechanisms but changes in the level of circulating
adrenal medullary hormones could have an effect.
~) Respiratory. The respiratory tract is a major route of water loss in tetrapods and
more especially in the mammals and birds. This loss takes place by evaporation,
as an unavoidable consequence of the uptake of oxygen and excretion of carbon
dioxide, but this avenue can also be utilized physiologically for thermal cooling
by ' panting' . The normal overall obligatory losses of water from the respiratory
tract increase or decrease with parallel changes in metabolic rate and oxygen consumption. Normally mammals extract about 30% of the oxygen present in the inspired air , but in certain circumstances this may be altered, with reflected changes
in the accompanying water loss. DICK (c. R.) TAYLOR (1969) has shown that an
African antelope, the oryx, which lives in desert areas, may reduce its oxygen consumption by 30% when it is dehydrated, and, by breathing more deeply, can extract
additional oxygen from the inspired air . The net result is a considerable reduction
91