Gas secretion in fishes lacking rete mirabile

In most fishes with a closed swimbladder the gas secretion is confined to a special organ, the so-called red gland, which consists of a secretory epithelium supported by a counter-current capillary system, the rete mirabile. All the fishes in-vestigated which possess this structure are capable of accumulating oxygen, and most of them also nitrogen, in the swimbladder against a pressure gradient. In other fishes the rete mirabile is lacking, and instead the secreting epithelium lines the swimbladder directly. Many fishes in this category apparently secrete only nitrogen. Two lines of thought have been formulated in an attempt to explain the gas secretion. According to the oldest theory the glandular epithelium secretes acid into the blood, which through the Bohr effect dissociates oxygen from the haemo-globin. The primary oxygen release would accordingly take place in the blood of the gland capillaries, and from these oxygen would diffuse through the epithelium into the swimbladder. Because of seemingly insurmountable difficulties inherent in this theory a second view has been proposed, to the effect that the gas accumulatio