Structural Organization of Plasma Proteins as a Factor of Capillary Filtration in Pisces (Review)

Abstract

Studies of capillary filtration (CF), which supports the isotonia of extracellular fluids in a vertebrate organism, are commonly considered within the “updated” Starling hypothesis developed on terrestrial vertebrates. This hypothesis explains the homeostasis of extracellular fluid in the body by the balance of the hydrostatic and oncotic pressures of plasma and interstitial fluid (the Starling forces) and the key role of plasma proteins in providing the capillary water transfer. The model is based on the intravascular localization of plasma proteins and the dominant contribution of albumin to its oncotic pressure (up to 80%). Due to a lack of antigravity adjustment in fish, the full recognition of Starling forces is considered incorrect. The review discusses a special “albumin-free” CF model for lower aquatic vertebrates. Like the classical one, this model proceeds from the key role of plasma proteins in the water transfer, but it also takes into account specific fish features such as a high concentration of total protein in interstitium, a high content of oligomeric proteins, and the absence of albumin (or its weak effect on water homeostasis). Oligomeric proteins play an important role in maintaining the isotonia of plasma and interstitial fluid due to their reorganization by associations/dissociations, which presumably contribute to the equalization of the fluid osmolality on both sides of the capillary wall and the maintenance of their isotonia in a wide range of environmental conditions.