The Secondary Heart, also called the Maintainer, is the first and least difficult of the 19 Astartes gene-seed organ implants a Space Marine Neophyte receives to transform him into a full Astartes. It resembles a smaller version of the human heart and is implanted in the chest cavity and connected to the rest of the Neophyte's circulatory and pulmonary systems near the original heart. Its function is to enhance the performance of an Astartes by supplying more oxygen and nutrients to the muscles by increasing blood flow well-beyond that capable for even the most fit normal human being. The Secondary Heart can often save an Astartes' life in combat. In the event of combat damage to or failure of the Space Marine's original heart, the Secondary Heart is usually capable of pumping enough blood through the Astartes' circulatory system to maintain survival until he can be treated by the Chapter's Apothecaries.
- Codex: Black Templars (4th Edition), pg. 10
- Codex: Space Marines (5th Edition), "The Making of a Space Marine," pg. 10
- Index Astartes I, "Rites of Initiation - The Creation of a Space Marine" by Rick Priestley & Gav Thorpe, pg. 3
- Warhammer 40,000: Compendium (2nd Edition), "The Origins of the Legiones Astartes" by Rick Priestley, pg. 7
- White Dwarf 98 (UK) (1988) "Chapter Approved: The Origins of the Legiones Astartes" by Rick Priestley, pg. 13