5. Biological Structures Participating in Fluid Exchange
Th e fluids of the body are separated from each other by the cell membrane and
capillary, and from th e external environment by the skin, the epithelia of the respiratory organs (gills and lun gs) and the gut. The integrity of the intracellular fluid
is dependent on the cell itself and its surrounding membrane, as well as the composition of its bathing (extracellular) fluids . The regulation of the components of
the extracellular fluid is due principally to the kidney, but also to the gills, various
'salt' glands , the gut and even the skin in some species.
a) The Cell Membrane
The contents of cells resist mixing with their external bathing solutions. This property has resulted in the concept that the fluid contents of cells are contained behind
a surface skin or membrane, called the cell membrane or plasma membrane, which
represents the immediate barrier separating the intracellular from the extracellular
fluid. Numerous properties, related to both the general homeostasis of the cell and
to the special functions localized in certain types of cells, have been attributed to
the cell membrane. This membrane concept has been developed from studies of
the movements of water and numerous solutes between different cells and their
immediate environment under divers conditions, including the presence and absence of metabolites, electrical stimulation and the presence of hormones. While
it is not alwa ys clear which of the properties of the cells reside precisely with the
external membrane, the presence of such a structural barrier can be demonstrated.
This is shown very elegantl y in the isolated axolemmal 'sheath' of the giant axon
of the squid Loligo. BAKER, HODGKIN, and SHAW (1961) isolated this external membrane by pressing the internal axoplasm from the nerve fibre with a squeegee, leaving the axolemma intact. This 'sheath' could then be refilled with various artificial
salt solutions and was shown under such conditions to conduct nerve impulses in
a manner similar to the intact axon . Such conduction is associated with a regular
pattern of sodium and potassium movements across the membrane. In addition,
BAKER and SHAW (1965) have shown that the isolated perfused axolemma can
breakdown ATP and contains ouabain-sensitive Na-K ATPase, the enz yme that
is closely associated with the function of the sodium 'pump'.
The behaviour of the cell with respect to movements of various solutes and water has resulted in the following general views about its structure and properties.
(i) Sieve or pore-like nature. The movement of solutes between the cell and its
environment are often related to their molecular size; large molecules move less
rapidly than smaller ones. This contributed to the concept of a porous or sieve-like
structure, which discriminates by reason of having holes of different sizes that will
admit certain molecules while excluding others. The overall permeability of the
cell will depend on the total area and length of the pores as well as their diameter.
(ii) Solvent-like nature. Lipid soluble substances such as ethers, aldeh ydes and
ketones may enter cells very rapidly compared with more water soluble solutes.
This suggests that the outer cell membrane has a lipid component, which can distinguish between materials by being able to act as a solvent for some of them.
15
Th e fluids of the body are separated from each other by the cell membrane and
capillary, and from th e external environment by the skin, the epithelia of the respiratory organs (gills and lun gs) and the gut. The integrity of the intracellular fluid
is dependent on the cell itself and its surrounding membrane, as well as the composition of its bathing (extracellular) fluids . The regulation of the components of
the extracellular fluid is due principally to the kidney, but also to the gills, various
'salt' glands , the gut and even the skin in some species.
a) The Cell Membrane
The contents of cells resist mixing with their external bathing solutions. This property has resulted in the concept that the fluid contents of cells are contained behind
a surface skin or membrane, called the cell membrane or plasma membrane, which
represents the immediate barrier separating the intracellular from the extracellular
fluid. Numerous properties, related to both the general homeostasis of the cell and
to the special functions localized in certain types of cells, have been attributed to
the cell membrane. This membrane concept has been developed from studies of
the movements of water and numerous solutes between different cells and their
immediate environment under divers conditions, including the presence and absence of metabolites, electrical stimulation and the presence of hormones. While
it is not alwa ys clear which of the properties of the cells reside precisely with the
external membrane, the presence of such a structural barrier can be demonstrated.
This is shown very elegantl y in the isolated axolemmal 'sheath' of the giant axon
of the squid Loligo. BAKER, HODGKIN, and SHAW (1961) isolated this external membrane by pressing the internal axoplasm from the nerve fibre with a squeegee, leaving the axolemma intact. This 'sheath' could then be refilled with various artificial
salt solutions and was shown under such conditions to conduct nerve impulses in
a manner similar to the intact axon . Such conduction is associated with a regular
pattern of sodium and potassium movements across the membrane. In addition,
BAKER and SHAW (1965) have shown that the isolated perfused axolemma can
breakdown ATP and contains ouabain-sensitive Na-K ATPase, the enz yme that
is closely associated with the function of the sodium 'pump'.
The behaviour of the cell with respect to movements of various solutes and water has resulted in the following general views about its structure and properties.
(i) Sieve or pore-like nature. The movement of solutes between the cell and its
environment are often related to their molecular size; large molecules move less
rapidly than smaller ones. This contributed to the concept of a porous or sieve-like
structure, which discriminates by reason of having holes of different sizes that will
admit certain molecules while excluding others. The overall permeability of the
cell will depend on the total area and length of the pores as well as their diameter.
(ii) Solvent-like nature. Lipid soluble substances such as ethers, aldeh ydes and
ketones may enter cells very rapidly compared with more water soluble solutes.
This suggests that the outer cell membrane has a lipid component, which can distinguish between materials by being able to act as a solvent for some of them.
15