The most abundant element in the human body is oxygen, which accounts for about 65% of body mass. Oxygen is a crucial component of water (H2O) and organic molecules, making it essential for life. It plays a fundamental role in cellular respiration, where it is used by cells to produce energy (ATP) from glucose, helping to power various biological processes. Additionally, oxygen is vital for metabolic functions, the formation of tissues, and maintaining cellular integrity. Its abundance reflects the significance of water and organic compounds in the biological framework of human physiology.