b) Salt Loss
a) Th e Sw eat Glands. The secretion of the sweat glands contains dissolved solutes,
principally sodium, potassium, chloride and bicarbonate. In mammals that utilize
sweat secretion for thermal cooling the losses of these solutes, especially sodium,
may be considerable. The sodium concentration of the sweat in man varies from
5 to 60 m-equiv/l, being greatest when the rate of sweat secretion is high (THAYSEN,
1960). During sodium depletion the amount of this solute in the sweat decreases
more than 5-fold, wh ile the potassium level rises . Such changes require several days.
SCHMIDT-NIELSEN (1964 a) suggested that in herbivorous animals, like the donkey,
little sodium is lost in sweat and measurements by MACFARLANE (1964) on sheep
and camels support this view . The sweat of the camel contains 9.5 m-equiv/l sodium
and 40 m-equiv/l potassium. In man a decrease in the Na/K ratio in the sweat has
been related to the action of the adrenocortical hormones (CONN et aI. , 1947), and
it seems likely that these steroids may contribute to the acclimatization in the salt
levels of the sweat that is seen in other species.
P) The Gut. The gut is the avenue of entrance and exit of salts from the external
environment. Salts are taken into the gut with the food and water, while large vol -
umes of fluids, rich in electrolytes, are continually secreted into the lumen of the
alimentary tract in order to facilitate digestion. These fluids include saliva, various
gastric and intestinal secretions, the bile and the pancreatic juices . In order to
maintain adequate salts in the body all of the electrolytes must ultimately be reabsorbed. Inadequate intestinal absorption , which may occur, for instance, as a result
of infection, can result in death from water and electrolyte deficiency.
The composition of the secretions derived from the different salivary glands
varies, and depends on the spec ies and the physiological condition of the animal.
Salivary secretion in the sheep has been extensively studied by DENTON and his
collaborators (BLAIR WEST et aI., 1967). The total volume secreted by this species
in a day is 6 to 16 litres, principally by the parotid glands. Normally in a sheep,
with adequate dietary sodium, the parotid secretion contains 180 m-equiv sodium/l
and 5 m-equiv potassium/l (Na/K is 36). If sheep are deficient in sodium, the levels
can change to give a Na/K ratio of 10/170 m-equiv/l, or 0.06. Adrenalectomized
animals exhibit little adjustment in their response to sodium depletion. The infusion
of aldosterone, however, can restore this deficiency (Fig. 3.1) .
The rate of secretion of aldosterone from the adrenal cortex can increase 13fold in sheep depleted of 300 to 500 rn-equiv of sodium, and the levels of this steroid
are consistent with the amounts which must be infused in order to change similarly
the composition of the saliva . Cortisol and corticosterone can also alter the electrolyte level in the saliva, but the necessary concentrations are large, and probably
only of pharmacological significance. Antidiuretic hormone has no effect on salivary secretion.
The site/s) of action of the corticosteroids on the salivary glands is uncertain;
they conceivably could act on the primary secretory mechanism in the acinar cells,
or alter reabsorption from the ducts. BLAIR WEST et al. (1967) suggest that aldosterone probably acts at both sites .
The biological importance of the action of aldosterone on the electrolyte composition of saliva is more readily apparent in animals that secrete large volumes
93
a) Th e Sw eat Glands. The secretion of the sweat glands contains dissolved solutes,
principally sodium, potassium, chloride and bicarbonate. In mammals that utilize
sweat secretion for thermal cooling the losses of these solutes, especially sodium,
may be considerable. The sodium concentration of the sweat in man varies from
5 to 60 m-equiv/l, being greatest when the rate of sweat secretion is high (THAYSEN,
1960). During sodium depletion the amount of this solute in the sweat decreases
more than 5-fold, wh ile the potassium level rises . Such changes require several days.
SCHMIDT-NIELSEN (1964 a) suggested that in herbivorous animals, like the donkey,
little sodium is lost in sweat and measurements by MACFARLANE (1964) on sheep
and camels support this view . The sweat of the camel contains 9.5 m-equiv/l sodium
and 40 m-equiv/l potassium. In man a decrease in the Na/K ratio in the sweat has
been related to the action of the adrenocortical hormones (CONN et aI. , 1947), and
it seems likely that these steroids may contribute to the acclimatization in the salt
levels of the sweat that is seen in other species.
P) The Gut. The gut is the avenue of entrance and exit of salts from the external
environment. Salts are taken into the gut with the food and water, while large vol -
umes of fluids, rich in electrolytes, are continually secreted into the lumen of the
alimentary tract in order to facilitate digestion. These fluids include saliva, various
gastric and intestinal secretions, the bile and the pancreatic juices . In order to
maintain adequate salts in the body all of the electrolytes must ultimately be reabsorbed. Inadequate intestinal absorption , which may occur, for instance, as a result
of infection, can result in death from water and electrolyte deficiency.
The composition of the secretions derived from the different salivary glands
varies, and depends on the spec ies and the physiological condition of the animal.
Salivary secretion in the sheep has been extensively studied by DENTON and his
collaborators (BLAIR WEST et aI., 1967). The total volume secreted by this species
in a day is 6 to 16 litres, principally by the parotid glands. Normally in a sheep,
with adequate dietary sodium, the parotid secretion contains 180 m-equiv sodium/l
and 5 m-equiv potassium/l (Na/K is 36). If sheep are deficient in sodium, the levels
can change to give a Na/K ratio of 10/170 m-equiv/l, or 0.06. Adrenalectomized
animals exhibit little adjustment in their response to sodium depletion. The infusion
of aldosterone, however, can restore this deficiency (Fig. 3.1) .
The rate of secretion of aldosterone from the adrenal cortex can increase 13fold in sheep depleted of 300 to 500 rn-equiv of sodium, and the levels of this steroid
are consistent with the amounts which must be infused in order to change similarly
the composition of the saliva . Cortisol and corticosterone can also alter the electrolyte level in the saliva, but the necessary concentrations are large, and probably
only of pharmacological significance. Antidiuretic hormone has no effect on salivary secretion.
The site/s) of action of the corticosteroids on the salivary glands is uncertain;
they conceivably could act on the primary secretory mechanism in the acinar cells,
or alter reabsorption from the ducts. BLAIR WEST et al. (1967) suggest that aldosterone probably acts at both sites .
The biological importance of the action of aldosterone on the electrolyte composition of saliva is more readily apparent in animals that secrete large volumes
93