The sweat glands in the skin of man y mammals playa role in thermoregulation,
the rate of secretion increasing in response to increased bod y temperature. Solutes,
especially sodium chloride, are simultaneously secreted, the amounts being variable
and subject to ph ysiolo gical regulation th at is at least partl y under hormonal
control.
d) Water Loss from the Tetrapod Respiratory Tract
Water loss takes place from the tetrapod respiratory tract as a result of evaporation
that accompanies both respiratory gas exchange and , in some birds and mammals,
a regulatory thermolysis.
The amount of water lost depends on the amount of air inspired and on the
difference between the water content of this and the expired air. The latter is saturated with water vapour, usually near the animal's body temperature.
The evaporative water loss, that accompanies gas exchange, depends on the oxygen requirement and the efficiency of extraction of this gas from the inspired air.
In mammals this is usuall y reduced from its normal atmospheric concentration of
21% to 13 or 14% , but in reptiles it may diminish onl y slightly, to 20 % , or to
as low as 5% (calc. from BENTLEY and SCHMIDT-NIELSEN, 1966; SCHMIDT-NIELSEN, CRAWFORD , and BENTLEY, 1966). Thus, a high rate of oxygen consumption
will result in an increased minute volume and evaporative water loss, but th e magnitude of the latter w ill depend on the amount of oxygen that can be extracted from
the inspired air. The rate of oxygen consumption is related to the body temperature
and in cold blooded verte brates this ma y normally show considerable variation.
In addition, homoiotherm y places a heavy burden on the water metabolism of th e
animal , especiall y at temperatures above the th ermoneutrallevel of about 30 0 C.
Large amounts of water may then be evaporated in order to maintain body temperature within a ph ysiologically acceptable range .
Endocrines can influence general metabolism and so may indirectly influence
its water metabolism, due to reflected changes in respiratory gas exchange.
e) Gills
Gills are primarily tissues for respiratory gas exchanges; among the vertebrates,
they are present in the 'fishes' and larval and neotenous Amphibia. The permeability of the gills to oxygen and carbon dioxide is accompanied by a diffusivity
to water and some solutes. This will affect the osmotic equilibrium of the animal
in various ways depending on the properties of the gills, the species and the difference between the composition of the animals' body fluids and the external solution. In addition to transfers down ph ysico-chemical gradients, the gills of some
species have an ability to transport sodium and chloride actively against such
gradients in a manner that contributes to the animals' osmoregulation.
i. Diffusion and osmosis across gills. Freshwater fishes are hyperosmotic to thei r
environment so that water will tend to move osmotically into the animal. A gradient
identical in direction, but smaller in magnitude, also exists between sea-water and
21
the rate of secretion increasing in response to increased bod y temperature. Solutes,
especially sodium chloride, are simultaneously secreted, the amounts being variable
and subject to ph ysiolo gical regulation th at is at least partl y under hormonal
control.
d) Water Loss from the Tetrapod Respiratory Tract
Water loss takes place from the tetrapod respiratory tract as a result of evaporation
that accompanies both respiratory gas exchange and , in some birds and mammals,
a regulatory thermolysis.
The amount of water lost depends on the amount of air inspired and on the
difference between the water content of this and the expired air. The latter is saturated with water vapour, usually near the animal's body temperature.
The evaporative water loss, that accompanies gas exchange, depends on the oxygen requirement and the efficiency of extraction of this gas from the inspired air.
In mammals this is usuall y reduced from its normal atmospheric concentration of
21% to 13 or 14% , but in reptiles it may diminish onl y slightly, to 20 % , or to
as low as 5% (calc. from BENTLEY and SCHMIDT-NIELSEN, 1966; SCHMIDT-NIELSEN, CRAWFORD , and BENTLEY, 1966). Thus, a high rate of oxygen consumption
will result in an increased minute volume and evaporative water loss, but th e magnitude of the latter w ill depend on the amount of oxygen that can be extracted from
the inspired air. The rate of oxygen consumption is related to the body temperature
and in cold blooded verte brates this ma y normally show considerable variation.
In addition, homoiotherm y places a heavy burden on the water metabolism of th e
animal , especiall y at temperatures above the th ermoneutrallevel of about 30 0 C.
Large amounts of water may then be evaporated in order to maintain body temperature within a ph ysiologically acceptable range .
Endocrines can influence general metabolism and so may indirectly influence
its water metabolism, due to reflected changes in respiratory gas exchange.
e) Gills
Gills are primarily tissues for respiratory gas exchanges; among the vertebrates,
they are present in the 'fishes' and larval and neotenous Amphibia. The permeability of the gills to oxygen and carbon dioxide is accompanied by a diffusivity
to water and some solutes. This will affect the osmotic equilibrium of the animal
in various ways depending on the properties of the gills, the species and the difference between the composition of the animals' body fluids and the external solution. In addition to transfers down ph ysico-chemical gradients, the gills of some
species have an ability to transport sodium and chloride actively against such
gradients in a manner that contributes to the animals' osmoregulation.
i. Diffusion and osmosis across gills. Freshwater fishes are hyperosmotic to thei r
environment so that water will tend to move osmotically into the animal. A gradient
identical in direction, but smaller in magnitude, also exists between sea-water and
21