4 The Secondary Vascular System

Abstract

This chapter examines the secondary vascular system of fishes. The arteriovenous vascular circuit of the gills may be considered as the branchial part of the secondary circulation. The secondary system originates from tiny vessels with a diameter of about 10–14 μm branching from primary arteries. They are irregularly curled, often branching, and occur in clusters. The secondary circulation has been observed in vivo in the transparent glass catfish, Kryptopterus bicirrhis . The groups of anastomoses occur along primary segmental arteries and give rise to one or two secondary arteries, which lead to the membranous part of the anal fin. Secondary arteries along both sides of the fin rays give rise to characteristic long, straight, secondary capillaries, which run across the fin membrane to join secondary veins along the opposite fin rays. The in vivo analysis of the partitioning of cardiac output in rainbow trout, based on blood sampled from the branchial vein, dorsal aorta, and sinus venosus, showed that plasma skimming occurred in the gills because hematocrit and hemoglobin concentration was only 3.5% and 1.04 g/100 ml, respectively, in the branchial vein.