Small, simple animals rely on diffusion to move solutes throughout their body. However, for larger and more complex animals the rate of diffusion is too slow so a circulatory system is needed for rapid transport of gases, nutrients, chemicals, and waste products. Circulatory systems may be open, where the circulating fluid is not always contained in vessels and is at times in direct contact with tissue cells (e.g., arthropods), or closed, where the circulating fluid is always contained inside vessels (e.g., vertebrates; Figure 7). The circulating fluid is known as hemolymph i = e a e a v v

Figure 7 Vascular arrangement for a closed circulatory system, showing arterial blood supply, capillary bed, and venous drainage, in the vascular supply to the eye of a marine toad (Bufo marinus). Photograph courtesy of T. Stewart, J. O'Shea, and S. Dunlop.

(open systems) or blood (closed systems). It consists of plasma, a fluid containing water, ions and organic molecules, and various blood cells. These cells can be involved in transport of 02 (erythrocytes), defense (leukocytes), or hemostasis (thromobcytes).

Blood and hemolymph flow is maintained by positive pressure created by the contraction of muscles in the body wall, or by the pumping of one or more hearts. Animal hearts are classified as neurogenic if they require innervation for contraction (e.g., arthropods), or myo-genic if the contraction is spontaneous (e.g., mollusks and vertebrates). The complexity of animal hearts varies from the simple tubular hearts of insects that push blood by peristaltic contractions of the muscular wall, to the multichambered hearts of mollusks and vertebrates. Chambered hearts have a varying number of muscular-walled compartments, which contract in a coordinated manner to circulate blood. Generally circulatory systems transport oxygenated blood from the respiratory sur-face(s) to the tissues and deoxygenated blood from the tissues to the respiratory surfaces. They can also be important in supplying nutrients to the tissues from the digestive system, transporting hormones from sites of synthesis to target cells, circulating cells of the immune system throughout the body, transporting heat, and generating a hydrostatic pressure.

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