Renal System Foundations

The kidneys do more than produce urine. Each nephron filters plasma through the glomerulus, then selectively reabsorbs water and needed solutes while secreting additional waste into the tubular fluid. This allows the kidneys to regulate circulating volume, sodium, potassium, calcium, phosphate, and acid-base balance while excreting metabolic waste and many medications. The kidneys also release renin, which contributes to blood-pressure regulation; produce erythropoietin, which supports red blood cell production; and activate vitamin D, which helps maintain calcium balance and bone health. Reduced kidney function can therefore present as hypertension, oedema, hyperkalaemia, metabolic acidosis, anaemia, or mineral-bone abnormalities—not only as a change in urination. Glomerular filtration rate (GFR) describes how much plasma the functioning kidneys filter. Urine output is related to filtration but is not interchangeable with it. A person can have chronic kidney disease with a relatively normal urine volume, while a sudden reduction in urine may reflect poor perfusion, tubular injury, or urinary obstruction.