The primary functions of the kidneys are crucial for maintaining overall fitness and health. They regulate fluid balance and electrolytes, ensuring that the body maintains proper hydration levels, which is essential for physical performance. Kidneys also filter waste products from the blood, contributing to metabolic and detoxification processes. Additionally, they play a role in the regulation of blood pressure and the production of hormones such as erythropoietin, which stimulates red blood cell production. Overall, healthy kidney function supports optimal fitness and bodily function.