What does the renal system primarily regulate?

The renal system, also known as the urinary system, primarily regulates fluid balance and blood filtration. Its main function involves the kidneys, which filter blood to remove waste products and excess substances, including water, electrolytes, and other compounds. By adjusting the volume and concentration of urine, the renal system plays a crucial role in maintaining homeostasis and ensuring that the body's internal environment remains stable.

Through this process, the kidneys help regulate blood pressure, acid-base balance, and electrolyte levels, contributing significantly to overall fluid balance. This regulation is essential for proper physiological functioning, as it ensures that the body has the right amount of fluids and solutes while eliminating toxins and waste materials.

Other options pertain to different physiological systems and functions. For instance, gas exchange in the lungs focuses on the respiratory system, muscle contraction and relaxation are functions of the muscular system, and nutrient absorption occurs primarily in the digestive system. These functions are critical but do not relate to the primary roles of the renal system. Therefore, fluid balance and blood filtration are the core functions regulated by the renal system.