Cell-Tissue-Organ-System describes the hierarchical organization of the human body. Cells specialize through cellular division to form tissues like muscle, nerve, and epithelial tissues. Tissues combine to form organs like the heart, brain, and lungs. Systems are sets of organs that work together to carry out key functions, such as the digestive system, circulatory system, and nervous system. This organization allows the human body to carry out vital activities through specialized and interconnected cells, tissues, organs, and organ systems.