2. The Osmotic Anatomy of the Vertebrates
The normal osmotic concentrations of the body fluids in vertebrate species lie between 200 and 1100 m-Osmole/l of water (Table 1.1). There is sometimes a
considerable tolerance to changes in concentration without vitally serious effects.
The Australian lizard, Trachysaurus rugosus, can tolerate an increase in its plasma
sodium level from 160 to 240 m-equiv/l (BENTLEY, 1959a). Some Australian frogs
(Cyclorana) may lose water by evaporation equivalent to 50% of their body weight
and survive if they are then allowed to rehydrate, but other species (Hyla) die after
losing half as much water (MAIN and BENTLEY, 1964). Man is intolerant to such
change, and may die after losing water equivalent to 12% of his body weight, or
if his plasma sodium rises from 150 to 170 m-equiv/l (ADOLPH, 1947).
The predominant osmotically-active solutes in vertebrates are sodium,
potassium and chloride. Proteins make up only a small part of the total osmotic
concentration, but, due to their restricted movements across membranes, play an
important role in the regulation of fluid movements across capillaries and cell mem -
branes . Other solutes, such as urea and trimethylamine oxide, may, in certain osmotic circumstances, make a substantial contribution to the osmotic composition
of the animals. Significant concentrations of such organic solutes occur in marine
chondrichthyeans and the frog , Rana cancrivora, in which they contribute to
maintain hypertonicity to the sea-water. Other solutes, such as calcium, magnesium, bicarbonate and phosphate are not an important part of the osmotic
anatomy. They are, however, indirectly essential as they influence the functioning
of osmoregulatory organs like the kidney, and are necessary for the physiological
function of all living processes.
The total sodium and potassium content of the body has been measured in man,
and is 68 m-equiv/kg body weight for sodium and 55 m-equiv/kg for potassium
(THORN, 1960). The amount that is readily exchangeable, and thus substantially
active osmotically, is 42 m-equiv/kg for sodium and 46 m-equiv/kg for potassium,
so that these two cations make a similar overall osmotic contribution.
Solutes (with the exception of urea) are not distributed uniformly in the body.
The fluid inside the cells (intracellular fluid) contains mainly potassium and organic
solutes, while that outside the cells (extracellular fluid) consists principally of
sodium and chloride. This distribution is not mutually exclusive, as small amounts
of sodium and chloride may be present inside cells, while low, but essential, concentrations of potassium and protein occur in the extracellular fluid (Table 1.1).
The total body water can be estimated by measuring the dilution of certain injected solutes, or by dessication. The extracellular fluid can similarly be estimated
by measurements of the dilution of known amounts of injected inulin or sucrose.
Evans blue is confined to the blood plasma, and so can also be used to measure
this space. The volume of the intracellular fluid is derived arithmetically, as the
difference between the total volumes of body water and extracellular fluid . Using
such techniques, the intracellular fluid is found to be 45 to 55 % of a vertebrate's
body weight, while the extracellular fluid is 15 to 25% of this. The plasma varies
from about 2 % of the body weight in some bony fishes, to a more usual value of
about 5% in other vertebrates (Table 1.2). The proportions of the various fluid
3
The normal osmotic concentrations of the body fluids in vertebrate species lie between 200 and 1100 m-Osmole/l of water (Table 1.1). There is sometimes a
considerable tolerance to changes in concentration without vitally serious effects.
The Australian lizard, Trachysaurus rugosus, can tolerate an increase in its plasma
sodium level from 160 to 240 m-equiv/l (BENTLEY, 1959a). Some Australian frogs
(Cyclorana) may lose water by evaporation equivalent to 50% of their body weight
and survive if they are then allowed to rehydrate, but other species (Hyla) die after
losing half as much water (MAIN and BENTLEY, 1964). Man is intolerant to such
change, and may die after losing water equivalent to 12% of his body weight, or
if his plasma sodium rises from 150 to 170 m-equiv/l (ADOLPH, 1947).
The predominant osmotically-active solutes in vertebrates are sodium,
potassium and chloride. Proteins make up only a small part of the total osmotic
concentration, but, due to their restricted movements across membranes, play an
important role in the regulation of fluid movements across capillaries and cell mem -
branes . Other solutes, such as urea and trimethylamine oxide, may, in certain osmotic circumstances, make a substantial contribution to the osmotic composition
of the animals. Significant concentrations of such organic solutes occur in marine
chondrichthyeans and the frog , Rana cancrivora, in which they contribute to
maintain hypertonicity to the sea-water. Other solutes, such as calcium, magnesium, bicarbonate and phosphate are not an important part of the osmotic
anatomy. They are, however, indirectly essential as they influence the functioning
of osmoregulatory organs like the kidney, and are necessary for the physiological
function of all living processes.
The total sodium and potassium content of the body has been measured in man,
and is 68 m-equiv/kg body weight for sodium and 55 m-equiv/kg for potassium
(THORN, 1960). The amount that is readily exchangeable, and thus substantially
active osmotically, is 42 m-equiv/kg for sodium and 46 m-equiv/kg for potassium,
so that these two cations make a similar overall osmotic contribution.
Solutes (with the exception of urea) are not distributed uniformly in the body.
The fluid inside the cells (intracellular fluid) contains mainly potassium and organic
solutes, while that outside the cells (extracellular fluid) consists principally of
sodium and chloride. This distribution is not mutually exclusive, as small amounts
of sodium and chloride may be present inside cells, while low, but essential, concentrations of potassium and protein occur in the extracellular fluid (Table 1.1).
The total body water can be estimated by measuring the dilution of certain injected solutes, or by dessication. The extracellular fluid can similarly be estimated
by measurements of the dilution of known amounts of injected inulin or sucrose.
Evans blue is confined to the blood plasma, and so can also be used to measure
this space. The volume of the intracellular fluid is derived arithmetically, as the
difference between the total volumes of body water and extracellular fluid . Using
such techniques, the intracellular fluid is found to be 45 to 55 % of a vertebrate's
body weight, while the extracellular fluid is 15 to 25% of this. The plasma varies
from about 2 % of the body weight in some bony fishes, to a more usual value of
about 5% in other vertebrates (Table 1.2). The proportions of the various fluid
3