152
Marine Mammal Physiology: Requisites for Ocean Living
In addition, the excretion of osmotic solutes following infusion of mannitol required 2.5%
of total body water in these seals suggesting that they regulate plasma osmolality by rapid
excretion of diluted urine (Skog and Folkow 1994).
7.2.4.2 Loading of isotonic water
Gelatin is proteinaceous matrix void of electrolytes that makes colloid particles in a solution, thus, its infusion can be used to expand extracellular volume. Excessive colloid particles in the blood will increase plasma osmolality resulting in an osmotic gradient that
favors the movement of water from the intracellular into the extracellular space. The infusion of gelatin in harbor seal increased urine volume and osmolality, excretion, resulting
from an increase in GFR without changing plasma osmolality (Murdaugh et al. 1961a).
This observation is significant because it suggests that plasma volume is regulated, at least
in part, by volume receptors (likely vasopressinergic) that lead to a diuresis (as would be
expected by suppression of vasopressin).
7.2.4.3 Loading of freshwater
All mammals require access to freshwater in some form to survive; however, this can be
especially challenging for animals living in a strictly marine habitat or arid environment.
Paradoxically, the excessive loading of solute-free water can also be intoxicating, resulting
in hemodilution, hyponatremia, pulmonary edema, coma, and ultimately death (Gardner
2002). Thus, the intake of freshwater needs to be properly regulated as with the ingestion
of electrolytes. From an experimental perspective, freshwater loading causes diuresis to
maintain the appropriate plasma osmolality.
In bottlenose dolphin, the effects of freshwater ingestion seem to be incongruent.
Malvin and Rayner (1968) reported no change in urine osmolality and urine flow following the loading of 4 L of freshwater. Whereas Ridgway and Venn-Watson (2010) described
that 2–4 L of freshwater ingestion lead to a diuresis, associated with decreases in plasma
and urine osmolalities and electrolyte concentrations (sodium and chloride), indicative
of a state of hypocholoremia, hyponatremia, and hemodilution. In pinnipeds, freshwater ingestion also induces diuresis (Albrecht 1950; Ladd et al. 1951; Bradley et al. 1954;
Tarasoff and Toews 1972; Hong et al. 1982; Skog and Folkow 1994). The infusion of freshwater in pinnipeds induced: (1) an increase in urine flow and/or volume (Bradley et al. 1954;
Tarasoff and Toews 1972; Ortiz et al. 2002); (2) a decrease in urinary osmolality (Ladd et al.
1951; Tarasoff and Toews 1972; Hong et al. 1982; Ortiz et al. 2002); (3) a decrease in excreted
Na + and K + (Bradley et al. 1954); and (4) an increase in the excretion of Na + , K + , Cl − , and/or
urea (Hong et al. 1982; Ortiz et al. 2002). Changes in GFR in response to freshwater loading
are also inconsistent among studies in seals. For example, in the harbor seal, no changes
were reported (Bradley et al. 1954; Murdaugh et al. 1961a), whereas in the Baikal seal (Pusa
sibirica), harbor seal, and ringed seal (Pusa hispida) (Ladd et al. 1951; Hong et al. 1982) and
fasting northern elephant seal pups (Ortiz et al. 2002), GFR increased with freshwater loading. These data indicate that freshwater loading results in increase in urine volume and
fractional clearance of water and electrolytes, and decrease in water reabsorption in the
renal tubule. As the consequences of excessive freshwater ingestion (infusion) were previously mentioned, it is of note that harbor seals died from water intoxication induced by
excessive loading (Ladd et al. 1951).
Similarly, in manatees, which inhabit both freshwater and marine habitats, freshwater exposure is associated with reduced plasma and urine Na + , Cl − , and osmolality (Ortiz
et al. 1998) while water flux is nearly twice that for animals in fresh water versus saltwater
(Ortiz et al. 1999).
Marine Mammal Physiology: Requisites for Ocean Living
In addition, the excretion of osmotic solutes following infusion of mannitol required 2.5%
of total body water in these seals suggesting that they regulate plasma osmolality by rapid
excretion of diluted urine (Skog and Folkow 1994).
7.2.4.2 Loading of isotonic water
Gelatin is proteinaceous matrix void of electrolytes that makes colloid particles in a solution, thus, its infusion can be used to expand extracellular volume. Excessive colloid particles in the blood will increase plasma osmolality resulting in an osmotic gradient that
favors the movement of water from the intracellular into the extracellular space. The infusion of gelatin in harbor seal increased urine volume and osmolality, excretion, resulting
from an increase in GFR without changing plasma osmolality (Murdaugh et al. 1961a).
This observation is significant because it suggests that plasma volume is regulated, at least
in part, by volume receptors (likely vasopressinergic) that lead to a diuresis (as would be
expected by suppression of vasopressin).
7.2.4.3 Loading of freshwater
All mammals require access to freshwater in some form to survive; however, this can be
especially challenging for animals living in a strictly marine habitat or arid environment.
Paradoxically, the excessive loading of solute-free water can also be intoxicating, resulting
in hemodilution, hyponatremia, pulmonary edema, coma, and ultimately death (Gardner
2002). Thus, the intake of freshwater needs to be properly regulated as with the ingestion
of electrolytes. From an experimental perspective, freshwater loading causes diuresis to
maintain the appropriate plasma osmolality.
In bottlenose dolphin, the effects of freshwater ingestion seem to be incongruent.
Malvin and Rayner (1968) reported no change in urine osmolality and urine flow following the loading of 4 L of freshwater. Whereas Ridgway and Venn-Watson (2010) described
that 2–4 L of freshwater ingestion lead to a diuresis, associated with decreases in plasma
and urine osmolalities and electrolyte concentrations (sodium and chloride), indicative
of a state of hypocholoremia, hyponatremia, and hemodilution. In pinnipeds, freshwater ingestion also induces diuresis (Albrecht 1950; Ladd et al. 1951; Bradley et al. 1954;
Tarasoff and Toews 1972; Hong et al. 1982; Skog and Folkow 1994). The infusion of freshwater in pinnipeds induced: (1) an increase in urine flow and/or volume (Bradley et al. 1954;
Tarasoff and Toews 1972; Ortiz et al. 2002); (2) a decrease in urinary osmolality (Ladd et al.
1951; Tarasoff and Toews 1972; Hong et al. 1982; Ortiz et al. 2002); (3) a decrease in excreted
Na + and K + (Bradley et al. 1954); and (4) an increase in the excretion of Na + , K + , Cl − , and/or
urea (Hong et al. 1982; Ortiz et al. 2002). Changes in GFR in response to freshwater loading
are also inconsistent among studies in seals. For example, in the harbor seal, no changes
were reported (Bradley et al. 1954; Murdaugh et al. 1961a), whereas in the Baikal seal (Pusa
sibirica), harbor seal, and ringed seal (Pusa hispida) (Ladd et al. 1951; Hong et al. 1982) and
fasting northern elephant seal pups (Ortiz et al. 2002), GFR increased with freshwater loading. These data indicate that freshwater loading results in increase in urine volume and
fractional clearance of water and electrolytes, and decrease in water reabsorption in the
renal tubule. As the consequences of excessive freshwater ingestion (infusion) were previously mentioned, it is of note that harbor seals died from water intoxication induced by
excessive loading (Ladd et al. 1951).
Similarly, in manatees, which inhabit both freshwater and marine habitats, freshwater exposure is associated with reduced plasma and urine Na + , Cl − , and osmolality (Ortiz
et al. 1998) while water flux is nearly twice that for animals in fresh water versus saltwater
(Ortiz et al. 1999).