This comprehensive textbook aims to help any learner expand their understanding of biological systems, their constituents, and how they work together to maintain homeostasis within the larger organism. You will learn about the fundamental units of biological systems (atoms and molecules) and progress to higher levels of complexity, while being given opportunities to test your knowledge and ensure a comprehensive understanding of the human body's biological functions. Labeled and detailed diagrams can be found throughout the textbook as well. This textbook is a “remix” of a variety of different Open Educational Resources (OER) with additional original text. The basic structure of the text relies heavily on life science course materials from the Open Learning Initiative (OLI) of Carnegie Mellon University. Other sources include Open Stax science textbooks. These may be downloaded free. The CK-12 Foundation uses an open-source, collaborative, and web-based model to produce science resources including CK-12 College Human Biology. All additional material and figures written or edited by Cindy Seiwert carries a Creative Commons Attribution Noncommercial-Share Alike 4.0 license. Additional detail on source materials can be found on the Acknowledgements and Attributions page of the original text.