cellular fluid is regulated principally by the kidney, but may be assisted in different
species by the skin, gills, 'salt' glands and gut . The maintenance of the correct relationship of volume and composition between the intercellular fluid and the plasma
is due to the blood vessels, particularly the capillaries.
3. Osmotic Exchanges with the Environment
Water, solutes and nutrients continually move in and out of animals in a pattern
that results ideally in their 'steady state' with the environment. Many of the osmotic
exchanges are obligatory for the animal and not under its direct control, though
their magnitude determines the extent of the regulatory processes that are ultimately necessary in order to achieve the 'steady state'.
This obligatory osmotic 'exchange' or 'turnover' depends on several aspects
of the animal's physiology.
a) Physico-chemical Gradients with the Environment
Differences between the chemical composition of an animal and its environment
will influence water and solute movements in, or out, of the beast. Thus aquatic
species in fresh water will tend to accumulate water and lose salts , while the opposite occurs in bony fishes living in the sea. Temperature gradients will influence
evaporative water losses especiall y for cooling in homoiotherms.
b) Size and Surface Area
Exchange of water and solutes is related to surface area. Large animals have a relatively smaller surface area exposed to the environment than small animals. The gill
surface in fishes varies considerably and, apart from a relation to the animal's size,
may reflect the normal oxygen needs of the species .
c) Nature of the Integument
The barriers exposed to the environment vary in their permeability to water and
solutes. Thus, the skin of the Amphibia is usually more permeable to water than
that of reptiles, while the gills of the Chondrichthyes are less permeable than those
of the teleosts.
d) Metabolism and Oxygen Consumption
The exchanges of oxygen and carbon dioxide in terrestrial animals are usuall y accompanied by water loss, due to saturation of the expired air with water vapour.
Metabolism is accompanied by the production of heat and waste solutes; the first
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species by the skin, gills, 'salt' glands and gut . The maintenance of the correct relationship of volume and composition between the intercellular fluid and the plasma
is due to the blood vessels, particularly the capillaries.
3. Osmotic Exchanges with the Environment
Water, solutes and nutrients continually move in and out of animals in a pattern
that results ideally in their 'steady state' with the environment. Many of the osmotic
exchanges are obligatory for the animal and not under its direct control, though
their magnitude determines the extent of the regulatory processes that are ultimately necessary in order to achieve the 'steady state'.
This obligatory osmotic 'exchange' or 'turnover' depends on several aspects
of the animal's physiology.
a) Physico-chemical Gradients with the Environment
Differences between the chemical composition of an animal and its environment
will influence water and solute movements in, or out, of the beast. Thus aquatic
species in fresh water will tend to accumulate water and lose salts , while the opposite occurs in bony fishes living in the sea. Temperature gradients will influence
evaporative water losses especiall y for cooling in homoiotherms.
b) Size and Surface Area
Exchange of water and solutes is related to surface area. Large animals have a relatively smaller surface area exposed to the environment than small animals. The gill
surface in fishes varies considerably and, apart from a relation to the animal's size,
may reflect the normal oxygen needs of the species .
c) Nature of the Integument
The barriers exposed to the environment vary in their permeability to water and
solutes. Thus, the skin of the Amphibia is usually more permeable to water than
that of reptiles, while the gills of the Chondrichthyes are less permeable than those
of the teleosts.
d) Metabolism and Oxygen Consumption
The exchanges of oxygen and carbon dioxide in terrestrial animals are usuall y accompanied by water loss, due to saturation of the expired air with water vapour.
Metabolism is accompanied by the production of heat and waste solutes; the first
7