Endocrine Response in Osmoregulation: Hormonal Adaptation in Aquatic Birds

Abstract

The maintenance of steady-state homeostatic conditions requires that the mean daily intakes of water and electrolytes are equivalent to the mean daily losses. Thus, the volume of water absorbed from the gastrointestinal tract (I) of a bird living in an environment where freshwater is always available must equal the sum of the volumes of water lost through excretion as urine (V) and feces (F), and through evaporation from the respiratory surfaces (R). This general relationship may be expressed as

$$I = V + F + R $$

((1))

Similarly, the daily intake of any osmotically active substance (x) must balance the sum of the amounts lost via all pathways. Since the water lost via the respiratory tract does not contain dissolved electrolytes, all of the electrolytes ingested by an organism lacking sweat glands must be excreted in the urine and feces. The osmotic balance of such an organism may therefore be represented as follows:

$$ {i_{x}}I = \left( {{u_{x}}V + {f_{x}}F} \right) $$

((2))

where i_x represents the concentration of substance x in the volume of water reabsorbed from the gut (I) and u_x and f_x represent the concentrations of x in the volumes of water excreted as urine and feces respectively.