luble materials cross the capillary boundary very rapidly and, as this property is
also exhibited by the plasma membrane, it probably takes place through the cells
themselves.
Changes in the filtration coefficient of capillaries do not seem to occur in normal
circumstances (but note changes in renal glomerular filtration in lower vertebrates),
but their permeability may be altered under extreme conditions associated with
mechanical, bacterial, and chemical injury. Conceivably, capillary filtration could
be affected by: (1) alteration in capillary pressure, (2) changes in the permeability
of the wall to proteins and (3) changes in concentrations of proteins in the body
fluids. The latter may occur in severe starvation and malnutrition resulting from
a dietary protein deficiency, and result in a characteristic accumulation of fluid in
int ercellular spaces (oedema fluid). Infections and other injury may increase the
permeability of the capillaries to protein, thus disturbing the balance of colloid osmotic pressure. Changes in the capillary hydrostatic pressure may result from
changes in the tone of the smooth muscle surrounding the arterioles and venules
at either end of the capillaries; thus noradrenaline constricts the arterioles and lowers the capillary pressure.
It is doubtful that hormones normally influence the permeability of capillaries.
RENKIN and ZAUN (1955) were unable to show any such changes in perfused tissue
from adrenalectomized rats.
c) Skin
Skin is a generic term used to describe the barrier separating most (but note the
area of the gills in many species) of the external surface of the animal from the outside environment. It has conservative and protective functions related to the osmoregulation of animals . Such roles are fulfilled in various ways associated with
its considerable structural divers ity.
In its simplest form the skin consists of several layers of simple epithelial cells
overlaying the dermis (or corium). Such simplicity is probably never achieved, as
some degree of cellular modification is invariably present. This includes calcified
and keratinized structures such s scales, feathers and hairs, which apart from having architectural and decorative significance, may affect the exchanges of water and
solutes across this barrier. Glands, if present, produce a variety of secretions, including the mucins that occur commonly in the fishes and the Amphibia and the
sweat of mammals. Among the Amphibia the skin has the unique ability to take
up sod ium from very dilute external solutions, a process which, along with water
uptake, may be regulated by the activity of hormones.
There is considerable diversity in the rate of cutaneous water transfer among
the vertebrates (Table 1.3), and in the importance of such exchanges relative to the
total water metabolism of the animal. Such differences may have adaptive significance; evaporative water losses from the skin of the lizard, Sauromalus obesus, and
the tortoise, Gopherus agassizii, which live in hot desert areas, are one-tenth to
one-twentieth that of the lizard, Iguana iguana , which lives in tropical forests
(Table 1.3). Such cutaneous water losses at 25
0 C make up about two-thirds of
the total evaporative loss in such reptiles. Osmotic water transfer across the skin
19
also exhibited by the plasma membrane, it probably takes place through the cells
themselves.
Changes in the filtration coefficient of capillaries do not seem to occur in normal
circumstances (but note changes in renal glomerular filtration in lower vertebrates),
but their permeability may be altered under extreme conditions associated with
mechanical, bacterial, and chemical injury. Conceivably, capillary filtration could
be affected by: (1) alteration in capillary pressure, (2) changes in the permeability
of the wall to proteins and (3) changes in concentrations of proteins in the body
fluids. The latter may occur in severe starvation and malnutrition resulting from
a dietary protein deficiency, and result in a characteristic accumulation of fluid in
int ercellular spaces (oedema fluid). Infections and other injury may increase the
permeability of the capillaries to protein, thus disturbing the balance of colloid osmotic pressure. Changes in the capillary hydrostatic pressure may result from
changes in the tone of the smooth muscle surrounding the arterioles and venules
at either end of the capillaries; thus noradrenaline constricts the arterioles and lowers the capillary pressure.
It is doubtful that hormones normally influence the permeability of capillaries.
RENKIN and ZAUN (1955) were unable to show any such changes in perfused tissue
from adrenalectomized rats.
c) Skin
Skin is a generic term used to describe the barrier separating most (but note the
area of the gills in many species) of the external surface of the animal from the outside environment. It has conservative and protective functions related to the osmoregulation of animals . Such roles are fulfilled in various ways associated with
its considerable structural divers ity.
In its simplest form the skin consists of several layers of simple epithelial cells
overlaying the dermis (or corium). Such simplicity is probably never achieved, as
some degree of cellular modification is invariably present. This includes calcified
and keratinized structures such s scales, feathers and hairs, which apart from having architectural and decorative significance, may affect the exchanges of water and
solutes across this barrier. Glands, if present, produce a variety of secretions, including the mucins that occur commonly in the fishes and the Amphibia and the
sweat of mammals. Among the Amphibia the skin has the unique ability to take
up sod ium from very dilute external solutions, a process which, along with water
uptake, may be regulated by the activity of hormones.
There is considerable diversity in the rate of cutaneous water transfer among
the vertebrates (Table 1.3), and in the importance of such exchanges relative to the
total water metabolism of the animal. Such differences may have adaptive significance; evaporative water losses from the skin of the lizard, Sauromalus obesus, and
the tortoise, Gopherus agassizii, which live in hot desert areas, are one-tenth to
one-twentieth that of the lizard, Iguana iguana , which lives in tropical forests
(Table 1.3). Such cutaneous water losses at 25
0 C make up about two-thirds of
the total evaporative loss in such reptiles. Osmotic water transfer across the skin
19