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The kidney filters ~180 liters of plasma daily at the glomerulus, reabsorbing 99% while selectively excreting wastes. Glomerular filtration is driven by Starling forces and limited by the filtration barrier's selective permeability. Proximal tubule reabsorbs glucose, amino acids, and water through active transport and osmosis. The loop of Henle multiplies osmolarity through countercurrent multiplication, allowing the kidney to produce urine more concentrated than plasma.
You have studied the kidney's gross anatomy and the selectivity of the glomerular filtration barrier. Now the question is: how does a process that filters 180 liters of plasma per day produce only 1–2 liters of urine? The answer is a precisely orchestrated sequence of reabsorption and secretion across four nephron segments: the proximal convoluted tubule, the loop of Henle, the distal convoluted tubule, and the collecting duct.
Glomerular filtration is governed by the same Starling forces that drive fluid movement across any capillary — the balance of hydrostatic pressure (pushing fluid out) against oncotic pressure from plasma proteins (pulling fluid back in). In the glomerular capillaries, hydrostatic pressure is unusually high (~55 mmHg) because the afferent arteriole is wider than the efferent arteriole, creating a high-resistance downstream bottleneck. The glomerular filtration barrier — fenestrated endothelium, the glomerular basement membrane, and podocyte slit diaphragms — is size- and charge-selective. Small molecules (water, ions, glucose, amino acids, urea, creatinine) freely cross; large proteins and blood cells do not. The resulting filtrate is essentially protein-free plasma.
The proximal tubule performs the bulk of reabsorption: approximately 65–70% of filtered sodium, water, and chloride, plus nearly 100% of filtered glucose and amino acids. Glucose reabsorption uses sodium-glucose cotransporters (SGLT2) on the luminal membrane — the same cotransport mechanism used in intestinal absorption. The proximal tubule also secretes organic acids, drugs, and metabolic waste products into the lumen. Crucially, water reabsorption here is obligatory and isosmotic — water follows solute proportionally — so the filtrate leaving the PCT has the same osmolarity as plasma (~300 mOsm/kg) but only one-third the original volume.
The loop of Henle does something qualitatively different: it creates a concentration gradient in the medullary interstitium without which concentrated urine is impossible. This is countercurrent multiplication. The descending limb is water-permeable but poorly permeable to solutes; the ascending limb actively pumps out sodium and chloride but is impermeable to water. As filtrate descends into the progressively hypertonic medullary interstitium, water exits and solute enters, concentrating the tubular fluid. As the fluid then ascends, the sodium-potassium-chloride cotransporter (NKCC2) pumps solute out without water following, diluting the luminal fluid. The net effect: filtrate leaving the ascending limb at the cortex is actually hypotonic (~100 mOsm/kg), but the medullary interstitium has been loaded to ~1200 mOsm/kg at the papillary tip. This medullary gradient is the "potential energy" stored for water reabsorption in the collecting duct when ADH is present — the mechanism you will study next in fluid and electrolyte regulation.