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Filtering blood and producing urine to maintain fluid and electrolyte balance is what the kidneys primarily do. Each nephron filters plasma, reabsorbs necessary water and ions, and secretes wastes into urine. By adjusting how much water and ions are excreted, the kidneys control fluid volume and the concentrations of electrolytes like sodium, potassium, and bicarbonate, as well as the body's acid-base status. They also influence blood pressure through the renin-angiotensin-aldosterone system and contribute to red blood cell production and calcium metabolism via hormones they produce, but their main function is urine formation and keeping fluid and electrolyte balance stable. The other descriptions correspond to functions of the liver, thyroid gland, or bones, which are not the primary roles of the renal system.