168
leatherback turtle ( Dermochelys coriacea ) (Holmes and McBean 1964 ). The salt
gland appears to be the predominant route of Na and K excretion in the marine turtle
C. mydas mydas . The kidney of this reptile is suggested not to be capable effectively
maintain a positive water balance under electrolyte loads presented by food and sea
water itself. As reported by Nicolson and Lutz ( 1989 ):
“the salt gland fl uid of the marine turtle C. midas was protein-free, and was
mainly composed of Na
+ and Cl
− , in similar relative concentrations to those in sea
water. It had substantial amounts of K
+ , Mg
2+ and HCO
3−, and negligible amounts of
glucose,” (Nicolson and Lutz 1989 ).
Seabirds According to hypothesis (Fernandez and Gasparini 2000 ), avian salt glands
evolved from the nasal glands of reptiles in the late Paleozoic. These glands were
localized “immediately under the skin in supraorbital depressions of the frontal
bone in the skull of Charadriiform birds, but in other groups they may be located
above the palate or within the orbit of the eye,” (Hughes 2003 ; see also Fernandez and
Gasparini 2000 ). Probably, such ancient birds as Ichthyornis and Hesperornis lived in
a marine habitat because their skulls possess similar depressions (Marples 1932 ).
Fish is the main diet of numerous piscivorous marine birds and contain water of
a lower salinity than seawater (see for review Goldenstein 2002 ). In contrast to this
diet, marine invertebrates such as crustaceans and mollusks are in osmotic equilibrium with seawater. Correspondingly this kind of feed requires an effective method
of salt excretion in sea birds using specialized glands (Schmidt-Nielsen 1960 ).
Jobert ( 1869 ) was the fi rst to describe the avian salt gland as being formed of two
distinct segments with separate drainage ducts. The avian salt gland is a countercurrent
system that concentrates the secreted salt solution. The capillaries are arranged
so that the fl ow of blood is in the direction opposite to the fl ow of secretory fl uid.
This fl ow maintains a minimum concentration gradient between blood and the
tubular lumen along the entire length of the tubule. The excretion of electrolytes by
the salt gland is assumed to be dependent on both cholinergic innervation and the
presence of adrenocortical hormones (Kühnel 1972 ). It was reported: “the great
number of autonomic nerves in close contact with the secretory cells indicates that
the salt gland of birds is controlled by the nervous system,” (Kühnel 1972 ; see also
Schmidt- Nielsen 1960 ) (Fig. 3.30 ).
The activity of the salt gland is an all-or-none phenomenon (Schmidt-Nielsen
et al. 1957 ). If there is an osmotic load, the gland secretes; in the absence of an
osmotic load, the gland is at complete rest. In this intermittency, the gland differs
from the kidney which produces urine continuously. The concentration of salt in the
nasal secretion is always very high, and remains fairly constant for each species. For
example, as reviewed by Schmidt- Nielsen (Schmidt-Nielsen 1960 ), in the cormorant,
which eats fi sh that is relatively low in salt content, the concentration in the secretion
from the gland is about 500–550 mEq/L. The petrel is a bird with pronounced oceanic habits. It spends most of its life at sea and comes to land only to breed. It lives
on planktonic organisms, mostly crustaceans, which it picks off the surface of the
ocean as it fl ies by. The planktonic organisms impose a considerable salt load. It is
no surprise, therefore, that in the petrel the salt concentration of the nasal fl uid is
higher than in any other bird that has been examined, up to 1,100 or 1,200 mEq/L.
3 Biocomposites and Mineralized Tissues
leatherback turtle ( Dermochelys coriacea ) (Holmes and McBean 1964 ). The salt
gland appears to be the predominant route of Na and K excretion in the marine turtle
C. mydas mydas . The kidney of this reptile is suggested not to be capable effectively
maintain a positive water balance under electrolyte loads presented by food and sea
water itself. As reported by Nicolson and Lutz ( 1989 ):
“the salt gland fl uid of the marine turtle C. midas was protein-free, and was
mainly composed of Na
+ and Cl
− , in similar relative concentrations to those in sea
water. It had substantial amounts of K
+ , Mg
2+ and HCO
3−, and negligible amounts of
glucose,” (Nicolson and Lutz 1989 ).
Seabirds According to hypothesis (Fernandez and Gasparini 2000 ), avian salt glands
evolved from the nasal glands of reptiles in the late Paleozoic. These glands were
localized “immediately under the skin in supraorbital depressions of the frontal
bone in the skull of Charadriiform birds, but in other groups they may be located
above the palate or within the orbit of the eye,” (Hughes 2003 ; see also Fernandez and
Gasparini 2000 ). Probably, such ancient birds as Ichthyornis and Hesperornis lived in
a marine habitat because their skulls possess similar depressions (Marples 1932 ).
Fish is the main diet of numerous piscivorous marine birds and contain water of
a lower salinity than seawater (see for review Goldenstein 2002 ). In contrast to this
diet, marine invertebrates such as crustaceans and mollusks are in osmotic equilibrium with seawater. Correspondingly this kind of feed requires an effective method
of salt excretion in sea birds using specialized glands (Schmidt-Nielsen 1960 ).
Jobert ( 1869 ) was the fi rst to describe the avian salt gland as being formed of two
distinct segments with separate drainage ducts. The avian salt gland is a countercurrent
system that concentrates the secreted salt solution. The capillaries are arranged
so that the fl ow of blood is in the direction opposite to the fl ow of secretory fl uid.
This fl ow maintains a minimum concentration gradient between blood and the
tubular lumen along the entire length of the tubule. The excretion of electrolytes by
the salt gland is assumed to be dependent on both cholinergic innervation and the
presence of adrenocortical hormones (Kühnel 1972 ). It was reported: “the great
number of autonomic nerves in close contact with the secretory cells indicates that
the salt gland of birds is controlled by the nervous system,” (Kühnel 1972 ; see also
Schmidt- Nielsen 1960 ) (Fig. 3.30 ).
The activity of the salt gland is an all-or-none phenomenon (Schmidt-Nielsen
et al. 1957 ). If there is an osmotic load, the gland secretes; in the absence of an
osmotic load, the gland is at complete rest. In this intermittency, the gland differs
from the kidney which produces urine continuously. The concentration of salt in the
nasal secretion is always very high, and remains fairly constant for each species. For
example, as reviewed by Schmidt- Nielsen (Schmidt-Nielsen 1960 ), in the cormorant,
which eats fi sh that is relatively low in salt content, the concentration in the secretion
from the gland is about 500–550 mEq/L. The petrel is a bird with pronounced oceanic habits. It spends most of its life at sea and comes to land only to breed. It lives
on planktonic organisms, mostly crustaceans, which it picks off the surface of the
ocean as it fl ies by. The planktonic organisms impose a considerable salt load. It is
no surprise, therefore, that in the petrel the salt concentration of the nasal fl uid is
higher than in any other bird that has been examined, up to 1,100 or 1,200 mEq/L.
3 Biocomposites and Mineralized Tissues