facilitates evaporative water loss, while the latter often requires body water for the ir
excretion.
e) Feeding
Excess water and solutes may be taken in with nutrients. Animals on a succulent
herbivorous diet containing 98% water will usually have a water intake in excess
of their needs, while a diet of mar ine invertebrates will provide superfluous salt.
The magnitude of these effects varies considerably in different species . Water
exchanges in a day may be equivalent to as much as 50% of the body water in the
leopard frog, Rana pipiens, or less than 1% in a lizard like the chuckawalla, Sauromalus obesus. The daily sodium exchange of a marine fish, like the flounder, is more
than 20 times as great as the total sodium content of the body, but in man this exchange only represents about 3% of the total present. In many instances differences
in obligatory exchanges could be adaptive with respect to osmoregulation. Some
of these variations will be discussed subsequently.
4. Forces Effecting Exchanges of Water and Solutes
The osmotic composition of the vertebrates differs greatly from that of their external environments and considerable differences in solute composition exist between the different solutions present within the animal. Various physico-chemical
forces act towards the attainment of osmotic equilibrium of the animal with its
environment. In vertebrates these are mainly the processes of diffusion, osmosis
and evaporation.
a) Diffusion
Molecules in solutions, both solute and solvent, are in a continual state of motion
dictated by their kinetic energy. Collisions will take place constantly at th e borders
of the system. The number of collisions with the border will depend on the concentration of the molecules and their average velocity. Such collisions may result in
the transfer of molecules to an adjoining phase. This will be affected by the nature
of the barrier separating the two regions and the velocity or energy of the particular
molecule concerned. When two such systems are separated by a barrier through
which the molecules can pass, there will be a transfer or diffusion in both directions.
If the thermodynamic conditions on both sides of the barrier are identical, then the
rates of collision with each side will be the same and, although an exchange will
occur, there will be no net change. There will be a flux in each direction (influx
and outflux) but no net flux. If, however, due to differences in concentration, temperature or electric charge, collisions occur more often on one side of the barrier
than the other, there will be a net movement towards the solution with the lower
thermodynamic energy. This will cease when ph ysico -chemical equilibrium has
been attained. The nature of the separating barrier will playa critical role in the
8
excretion.
e) Feeding
Excess water and solutes may be taken in with nutrients. Animals on a succulent
herbivorous diet containing 98% water will usually have a water intake in excess
of their needs, while a diet of mar ine invertebrates will provide superfluous salt.
The magnitude of these effects varies considerably in different species . Water
exchanges in a day may be equivalent to as much as 50% of the body water in the
leopard frog, Rana pipiens, or less than 1% in a lizard like the chuckawalla, Sauromalus obesus. The daily sodium exchange of a marine fish, like the flounder, is more
than 20 times as great as the total sodium content of the body, but in man this exchange only represents about 3% of the total present. In many instances differences
in obligatory exchanges could be adaptive with respect to osmoregulation. Some
of these variations will be discussed subsequently.
4. Forces Effecting Exchanges of Water and Solutes
The osmotic composition of the vertebrates differs greatly from that of their external environments and considerable differences in solute composition exist between the different solutions present within the animal. Various physico-chemical
forces act towards the attainment of osmotic equilibrium of the animal with its
environment. In vertebrates these are mainly the processes of diffusion, osmosis
and evaporation.
a) Diffusion
Molecules in solutions, both solute and solvent, are in a continual state of motion
dictated by their kinetic energy. Collisions will take place constantly at th e borders
of the system. The number of collisions with the border will depend on the concentration of the molecules and their average velocity. Such collisions may result in
the transfer of molecules to an adjoining phase. This will be affected by the nature
of the barrier separating the two regions and the velocity or energy of the particular
molecule concerned. When two such systems are separated by a barrier through
which the molecules can pass, there will be a transfer or diffusion in both directions.
If the thermodynamic conditions on both sides of the barrier are identical, then the
rates of collision with each side will be the same and, although an exchange will
occur, there will be no net change. There will be a flux in each direction (influx
and outflux) but no net flux. If, however, due to differences in concentration, temperature or electric charge, collisions occur more often on one side of the barrier
than the other, there will be a net movement towards the solution with the lower
thermodynamic energy. This will cease when ph ysico -chemical equilibrium has
been attained. The nature of the separating barrier will playa critical role in the
8