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Comprehensive and Detailed Explanation:

Erythrocytes (red blood cells) carry oxygen from the lungs to tissues via hemoglobin, a protein with iron-containing heme groups that bind oxygen molecules. Each hemoglobin can carry up to four oxygen molecules, making RBCs essential for maintaining oxygen delivery. Leukocytes (white blood cells) are immune defense cells, while platelets are fragments critical for clotting. Plasma cells are differentiated B-lymphocytes producing antibodies, not involved in oxygen transport. Understanding cellular function in the circulatory system is critical for phlebotomists, as the quality of blood collection and handling directly influences test results that assess hematologic and metabolic health. For example, errors that cause hemolysis may release hemoglobin, complicating oxygen-related assays. ASCP content includes circulatory system knowledge as a foundation to understanding why proper collection, specimen integrity, and patient physiology impact diagnostic outcomes.