What is one of the primary substances that kidneys regulate?

The kidneys play a crucial role in maintaining the body's homeostasis, and one of their primary functions is to regulate ions in the bloodstream. This includes the control of essential electrolytes such as sodium, potassium, calcium, and chloride. By filtering blood, the kidneys can adjust the concentration of these ions, maintaining proper balance and preventing imbalances that can lead to health issues. The regulation of ion levels is vital for processes such as nerve function and muscle contraction.

While substances like oxygen, glucose, and fatty acids are essential to the body, they are not primarily regulated by the kidneys. Oxygen is primarily transported by hemoglobin in red blood cells, glucose levels are mainly regulated by hormones like insulin and glucagon, and fatty acids are metabolized by liver cells. Therefore, the accurate answer highlights the kidneys' specific function in maintaining ionic balance within the body.