The phrase "oxygen levels in the blood" refers to the amount or concentration of oxygen present in a person's bloodstream. Oxygen is essential for life as it provides energy to cells and tissues throughout the body, including those responsible for vital functions such as breathing and maintaining heartbeat rhythm. The body constantly monitors its oxygen levels through various mechanisms, including sensors in the brain and other organs that respond to changes in blood oxygen concentrations by adjusting respiration rate or depth. Maintaining optimal oxygen levels is crucial for overall health and well-being, while low levels can lead to symptoms such as fatigue, dizziness, shortness of breath, and impaired cognitive function.