140
Marine Mammal Physiology: Requisites for Ocean Living
physiological mechanisms to conserve water and electrolytes. Subsequently, when these
mammals radiated into a marine environment they had to evolve mechanisms to allow
them to tolerate and thrive in an external milieu of high salinity with limited availability
to freshwater. A number of morphological and physiological adaptations have allowed
them to cope with these osmotic challenges. Additionally, other life-history traits such as
mating, molting, and migration are associated with protracted periods of fasting, which
place an additional osmoregulatory burden on marine mammals. The evolved physiological mechanisms have allowed marine mammals to conserve salts and water during prolonged fasting as well with virtually no consequences. In this chapter, the osmoregulatory
mechanisms in marine mammals are reviewed.
7.2 Knowledge
7.2.1 Constancy of internal environment
When solutions with different solute concentrations are separated by a semi-permeable
membrane, water molecules will move down their concentration gradient (from the compartment with the lower concentration of dissolved particles to the compartment with the
greater concentration) through the membrane until the concentrations of the two compartments are equal, or reach an osmotic equilibrium. The process is defined as osmosis, and
the power of the solution to draw water through the membrane is referred to as osmotic
pressure. The osmolality of the plasma, therefore, is determined by the sum total of all the
dissolved particles in a solution (water) such as electrolytes, glucose, and urea (blood urea
nitrogen, BUN) and can be approximated by the following formula:
Plasma osmolality Na mEq/l 2
glucose mg/dl BUN (mg/dl)
=
* +
+
(
)
(
)
.
18
2 8 8
Because the concentrations of electrolytes, especially sodium, potassiums, and chloride,
in plasma are rigidly controlled so is the osmolality. The maintenance of plasma constituents within defined ranges suitable for cellular activity represents the homeostatic control
afforded by adaptable osmoregulatory mechanisms. Aside from the potential impacts of
the environment on plasma osmolality, diet and prolonged fasting can also contribute to
alterations in osmolality. For example, protein loading from protein-rich diets can influence plasma urea concentration, which would be reflected by an increase in plasma osmolality. Plasma osmolality in humans ranges from 275 to 290 mOsm/kg; however, those
in marine mammals are comparatively higher (Table 7.1). If the osmotic gap between the
internal fluid and the surrounding medium (seawater) is reduced, the energetic costs for
maintaining plasma osmolality in seawater may be comparatively reduced. Therefore,
the presence of higher plasma osmolality might be advantageous for marine mammals
that expend more energy for thermoregulation and locomotion in the water (Williams
et al. 2001; Williams and Worthy 2002). The higher concentrations of plasma electrolytes
and urea in marine mammals may contribute to their relatively higher plasma osmolality
(Table 7.1) (reviewed in Ortiz 2001). The comparatively higher plasma urea concentrations
in marine mammals, especially in cetaceans and sea otter, may confer an additional beneficial in the conservation of water (see Section 7.2.2).
The regulation of water and salt balance in the internal milieu is dependent on the
physiological responses to the changes in input and losses from the kidney (Eaton and
Pooler 2009). Under the condition of excessive salt-loading and limited water, it is necessary
Marine Mammal Physiology: Requisites for Ocean Living
physiological mechanisms to conserve water and electrolytes. Subsequently, when these
mammals radiated into a marine environment they had to evolve mechanisms to allow
them to tolerate and thrive in an external milieu of high salinity with limited availability
to freshwater. A number of morphological and physiological adaptations have allowed
them to cope with these osmotic challenges. Additionally, other life-history traits such as
mating, molting, and migration are associated with protracted periods of fasting, which
place an additional osmoregulatory burden on marine mammals. The evolved physiological mechanisms have allowed marine mammals to conserve salts and water during prolonged fasting as well with virtually no consequences. In this chapter, the osmoregulatory
mechanisms in marine mammals are reviewed.
7.2 Knowledge
7.2.1 Constancy of internal environment
When solutions with different solute concentrations are separated by a semi-permeable
membrane, water molecules will move down their concentration gradient (from the compartment with the lower concentration of dissolved particles to the compartment with the
greater concentration) through the membrane until the concentrations of the two compartments are equal, or reach an osmotic equilibrium. The process is defined as osmosis, and
the power of the solution to draw water through the membrane is referred to as osmotic
pressure. The osmolality of the plasma, therefore, is determined by the sum total of all the
dissolved particles in a solution (water) such as electrolytes, glucose, and urea (blood urea
nitrogen, BUN) and can be approximated by the following formula:
Plasma osmolality Na mEq/l 2
glucose mg/dl BUN (mg/dl)
=
* +
+
(
)
(
)
.
18
2 8 8
Because the concentrations of electrolytes, especially sodium, potassiums, and chloride,
in plasma are rigidly controlled so is the osmolality. The maintenance of plasma constituents within defined ranges suitable for cellular activity represents the homeostatic control
afforded by adaptable osmoregulatory mechanisms. Aside from the potential impacts of
the environment on plasma osmolality, diet and prolonged fasting can also contribute to
alterations in osmolality. For example, protein loading from protein-rich diets can influence plasma urea concentration, which would be reflected by an increase in plasma osmolality. Plasma osmolality in humans ranges from 275 to 290 mOsm/kg; however, those
in marine mammals are comparatively higher (Table 7.1). If the osmotic gap between the
internal fluid and the surrounding medium (seawater) is reduced, the energetic costs for
maintaining plasma osmolality in seawater may be comparatively reduced. Therefore,
the presence of higher plasma osmolality might be advantageous for marine mammals
that expend more energy for thermoregulation and locomotion in the water (Williams
et al. 2001; Williams and Worthy 2002). The higher concentrations of plasma electrolytes
and urea in marine mammals may contribute to their relatively higher plasma osmolality
(Table 7.1) (reviewed in Ortiz 2001). The comparatively higher plasma urea concentrations
in marine mammals, especially in cetaceans and sea otter, may confer an additional beneficial in the conservation of water (see Section 7.2.2).
The regulation of water and salt balance in the internal milieu is dependent on the
physiological responses to the changes in input and losses from the kidney (Eaton and
Pooler 2009). Under the condition of excessive salt-loading and limited water, it is necessary