What substance do the kidneys primarily filter from the blood?

The kidneys play a crucial role in maintaining homeostasis by filtering blood and regulating various substances. The primary function of the kidneys is to filter out waste products and excess substances from the bloodstream. Among these, the end products of metabolism, such as urea, creatinine, and uric acid, are key constituents that the kidneys remove from the blood to facilitate their excretion through urine.

When proteins are metabolized, they generate ammonia, which is converted to urea in the liver and subsequently filtered out by the kidneys. This process is vital for preventing the accumulation of harmful metabolic byproducts in the body. The kidneys also regulate electrolytes, pH levels, and fluid balance, but their primary role in relation to the question is filtering the end products of metabolism.

Other substances listed do not serve as the primary focus of kidney filtration. Oxygen is transported by the blood and is not filtered out by the kidneys, carbohydrates are primarily metabolized and utilized by the body rather than filtered, and vitamins, while they can be processed by the kidneys, are not the main substances that are filtered out. Hence, the correct answer reflects the kidneys' primary filtration duty regarding metabolic waste.