Fluid volume, fluid hydraulic pressure and osmolarity are intimately related; in fact they are probably different attributes of a more general physiological phenomenon. Although osmoregulation and blood pressure regulation have been intensively examined in fish, fluid compartments and their control have received surprisingly little attention. Blood and extracellular fluid volume are governed by a variety of physical and physiological processes. Factors that affect water and salt movement across the gills, skin, gut, and kidney determine overall fluid balance. Fluid distribution between intravascular and extravascular compartments is further subjugated to hydraulic, osmotic, and mechanical forces in the microcirculation. The remarkable constancy of compartmental volumes, even in the face of adverse circumstances, emphasizes that homeostatic reflexes, especially in bony fish, are highly developed and efficient. Appreciation of volume regulatory mechanisms in fish has suffered from a lack of information on the nature and disposition of the fluid compartments themselves, the relative importance of various factors that affect intercompartmental fluid movements, and the regulatory processes involved. The afferent limb of volume regulatory reflexes in fish, or for that matter vertebrates in general, has not been characterized. Efferent control of appropriate effectors of fluid balance is under investigation. Two systems, the renin-angiotensin system and the atrial natriuretic peptides, have been shown to affect a variety of vascular and extravascular effectors and these systems may play an important role in volume homeostasis. The kallikrein-kinin system may also prove to have important systemic as well as intraorgan cardiovascular functions. Undoubtedly, other central and peripheral systems are also involved. Of all vertebrates, fish are probably the most versatile and manipulative model, from both phylogenetic and physiological perspectives, in which fluid compartments and volume regulation can be examined. I hope this chapter provides a convenient starting point for such studies.
ASJC Scopus subject areas
- Animal Science and Zoology