The kidneys filter about 180 liters of plasma a day but make only 1 to 2 liters of urine. Almost everything filtered is taken back, and only waste and extra water and salt are left behind.
Filtration
Blood pressure pushes water and small molecules out of the glomerular capillaries into Bowman's capsule. Blood cells and most proteins are too large and stay in the blood.
Reabsorption
The tubules take back what the body needs. The proximal tubule reclaims nearly all glucose and amino acids and most of the sodium, water and bicarbonate.
Secretion
The tubules also move substances from the blood into the urine. Potassium, hydrogen ions and many drugs leave the body this way.
Concentration
The loop of Henle builds a salty medulla. When ADH is present, the collecting duct lets water flow back into that salty tissue, so the urine becomes concentrated.
At the bedside
Glucose appears in the urine when blood glucose rises above about 10 mmol/L (180 mg/dL), because the tubules can no longer take it all back. Protein in the urine points to a damaged filter.
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Written by Making Synapses. Topic reference: OpenStax, Anatomy and Physiology 2e (no text adapted).