The microcirculation of the respiratory organ of water and air breathing vertebrates is similar and can be described as sheet flow. The gross morphologies of the systems, however, are very different and reflect the properties of the medium. The fish heart has a single ventricle that forces blood first through the gills and then through the body. The pressure in the gills is higher than in the systemic circulation, the reverse of the situation seen in mammals. The gill epithelium is thicker than that in the lung and is involved in ionic and acid-base functions carried out in the kidney of mammals. Gills stick together in air. Therefore, fish breathe air using some other structure, such as the gut or mouth, the swimbladder, or the skin. The gills are retained for carbon dioxide excretion and ion and acid-base regulation. This results in a separation of oxygen uptake and carbon dioxide excretion. The gills are often modified in air-breathing fish such that venous blood flows to well developed gills for carbon dioxide and acid excretion, whereas oxygenated blood flow bypasses the gills. This is the beginning of a separation of flows in the heart which is more highly developed in amphibians and reptiles and complete in mammals. The loss of gills requires transfer of ionic and acid base regulation processes to the skin in amphibia and to the kidney in reptiles and mammals, allowing a completely terrestrial existence. The organization of the venous system is influenced by the degree of support offered by the medium.(ABSTRACT TRUNCATED AT 250 WORDS)