What do the kidneys filter in the human body?

The kidneys play a vital role in maintaining homeostasis within the body, primarily through their function of filtering blood. They remove waste products, excess substances, and toxins from the bloodstream while also regulating electrolyte levels and the balance of fluids.

Blood, as the primary substance filtered by the kidneys, provides the mechanisms through which waste products such as urea, creatinine, and various ions are identified and expelled from the body. After the filtering process, the kidneys use specialized units called nephrons to ensure that essential components like glucose and certain electrolytes are reabsorbed into the bloodstream, minimizing their loss during filtration.

In contrast, the other choices represent substances that are either not filtered by the kidneys in the same direct manner (like oxygen, which is transported via hemoglobin in red blood cells) or are components that, while the kidneys help manage, are not the primary focus of their filtration function (such as water and nutrients). The filtration process specifically targets the blood, making it the most accurate answer to the question about what the kidneys filter.