Meridian and Evolution of Circulation System

Larger body size is a major trend in animal evolution. To achieve further 3dimensional increases in size, it is necessary to employ internal transport and exchange systems (i.e. circulatory systems) to provide bulk flow delivery of substances. Previously, we discuss the meridian and water transportation in living organism and emphasis the meridian provides important function for fluid circulation. Meridian increases the efficiency also solves the circulation problem. The most important evolution of transportation in living organism is the circulation system, as known as from open system for insects and to close for mammal, one internal circulatory systems for aquatic animal and to two for land animals, irregular contraction of myoepithelial cells to periodic pulsatile heartbeat, vessel spaces are lined only by matrix to a secondary cell lining, termed endothelium in Vertebrates, without lymphatic system to with, simple diffusion through the skin to metal ioncontaining respiratory pigments, etc.. However, these concepts could not explain the difference of mammal from rat to human and the division between closed and open system is not always clearcut. Conversely, the closed system of vertebrates contains vascular beds, such as the sinusoids of the liver, spleen, and bone marrow, where there is direct contact between blood and the interstitial space. In hemochorial placentation (Ex: In primates), the maternal spiral arteries become openended, and blood is released into a placental labyrinth where it bathes the chorionic villi and is drained by the spiral veins. Such an arrangement is highly reminiscent of an open system. Meanwhile, the gap between plant and animal is even huge beyond these concepts.