The maintenance of homeostasis in their blood is essential for vertebrates to live in various circumstances. To maintain blood homeostasis, various osmoregulatory organs, such as intestine, gills and kidney, must collaborate. Using eels as a model system, we will describe in this article how the osmoregulatory organs work and how they are regulated.

Seawater eels drink sea water, desalt in the esophagus [1, 2], and absorb NaCl and water in the intestine from the diluted sea water, approximately isotonic to the eel  plasma. During desalination through the esophagus, highly concentrated NaCl enters into the blood. In fact, plasma osmolality in the bulbus arteriosus is higher than that in caudal vessel [3]. The high osmolality enhances atrial contraction [3], and stimulates secretion of atrial natriuretic peptide (ANP) into the blood [4]. The enhanced contraction is expected to increase the cardiac outflow into the gill. Since ANP stimulates Cl^- secretion across the opercular epithelium, a model system for fish gills [5], it is likely that the desalination in the esophagus stimulates NaCI extrusion from the gill, then maintains homeostasis of their blood. If desalination is not sufficient, then high concentration of luminal Cl^- ([Cl^-]₁) will arrive at the intestine. The high [Cl^-]₁ depresses the drinking rate, and the lowered drinking enhances desalination efficiently in the esophagus [6]. During this feedback control, eel intestinal pentapeptide (EIPP) might be involved as a humoral mediator, since intravenous EIPP reduces drinking rate [7]. If excess desalination occurs, highly concentrated NaCI will enter into the heart, which will stimulate ANP release into the blood. Since intravenous ANP depresses drinking [7], this mechanism might act to inhibit excess salt uptake from sea water.