A Brief Comparative Phylogeny

Abstract

The main function of the circulatory system is to uphold a specific inner environment, which according to the type, is essential for the life and development of the organism. This includes, but is not limited to, distribution of nutrients and removal of metabolic waste, immune defense, thermoregulation, reproduction, and transport of respiratory gases. There are two basic types of circulation. The open circulatory system of invertebrates is characterized by a system of body cavities or coeloms connected with vascular conduits, both of which lack an endothelial cell layer. This simple arrangement allows the circulating fluid, the hemolymph, to bathe the inner organs directly, thus fulfilling the dual function of tissue fluid and blood. We will see that in this respect, it functionally resembles the early embryonic circulation of vertebrates before the formation of continuous vascular endothelium. In contrast, the closed circulatory system of the vertebrates is phylogenetically younger and consists of a network of continuous vessels in which the blood does not come in direct contact with the surrounding tissues. However, a significant overlap exists at the level of local circulations and individual organs within the vertebrate and invertebrate circulations. For example, while most of the capillary beds in the mammalian circulation are of the continuous type with tight endothelial junctions, the vascular sinuses of the bone, liver, and spleen are lined with fenestrated capillaries, readily permeable to plasma and large molecules. Technically, they too belong to the open circulation. Similarly, several classes of invertebrates such as the Cephalopoda and the Oligochaeta have, in addition to coelomic spaces, continuous vascular loops which resemble the closed circulations of vertebrates.