The blood vascular system is a closed system. Although the system itself consists of a continuum of ducts, all are interconnected and allow for little or no loss of contents. The circulatory system has more individual variation dm any other system, and is the first of all organ systems to become functional during development. In addition, it is highly adaptable.
Blood is a fluid tissue containing cellular elements that are derived from mesoderm.
The non-water component contains fibrinogen, which contributes to blood clotting, and globulins, which respond to the entry of foreign materials into the body. There are two types of cells in the blood. The erythrocytes are the red blood cells, which carry hemoglobin that binds oxygen to transport to the tissues. The leukocytes are the white blood cells that destroy foreign bodies through phagocytosis and are involved in the immune response. Hemopoietic tissues produce the blood cells.