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Dennis Kunkel's Microscopic World 2001 Calendar

Human blood is the fluid circulated by the heart through the human vascular system. There are three cellular components of human blood: red blood cells, white blood cells, and platelets. Red blood cells transport oxygen to other cells of the body. Packed with hemoglobin (an iron-bearing protein) and shaped like plump disks with indented centers, red bloods cells are produced in bone marrow and have a life span of about 120 days. White blood cells (purple) protect the body from infection, attacking and destroying foreign particles like dust, pollen, and viruses. Platelets (blue) defend the body against excessive blood loss. Platelets flow freely in the blood in an inactive state; but when an injury is sustained, platelets become sticky to plug the injured area.