The kidney produces urine through glomerular filtration driven by Starling forces, generating a protein-free ultrafiltrate at ~180 L/day. Selective reabsorption in the proximal tubule recovers essential solutes (glucose, amino acids) and water, the loop of Henle creates concentration gradients for water reabsorption, and the distal tubule and collecting duct fine-tune electrolyte and water balance under hormonal control. Filtration rate is autoregulated despite changing systemic blood pressure.
Calculate glomerular filtration pressure using Starling forces. Map each nephron segment to its specific transport mechanisms (active, passive, osmotic). Use clearance equations to quantify filtration and reabsorption.
The kidney solves a logistical problem that would be impossible to manage with selective filtering at the inlet: it filters almost everything first, then carefully reclaims what the body needs. Your prior study of capillary filtration introduced the Starling forces — hydrostatic pressure pushing fluid out of capillaries, oncotic pressure pulling it back in. The glomerulus is a specialized capillary tuft where this balance is deliberately skewed toward filtration. Glomerular capillary pressure (~55 mmHg) far exceeds the oncotic pressure (~30 mmHg) and the opposing pressure in Bowman's capsule (~15 mmHg), yielding a net filtration pressure of ~10 mmHg. The result: roughly 180 liters of plasma water pass into the nephron every day — about 45 times the entire plasma volume.
That filtrate is not urine; it is a nearly perfect copy of plasma minus proteins and cells. The proximal convoluted tubule recovers the bulk of it: ~67% of filtered sodium (via Na⁺/K⁺-ATPase on the basolateral membrane creating a gradient that drives apical uptake), virtually all glucose and amino acids (via sodium-coupled cotransporters you studied in active transport), and water following by osmosis. The tubule cells are packed with mitochondria specifically to power this energy-intensive reclamation. From your work on selective permeability and membrane channels, you can recognize that each transport protein is specific to particular solutes — glucose transporters do not move amino acids; different carriers handle different substrates.
The loop of Henle creates the osmotic gradient in the medulla that enables concentrated urine. The descending limb is permeable to water but not salt — water leaves by osmosis as the medulla becomes progressively hypertonic. The ascending limb actively pumps Na⁺ and Cl⁻ out but is impermeable to water — this is the critical asymmetry that builds the gradient. The countercurrent arrangement of the two limbs means the bottom of the loop sits in the most concentrated medullary tissue, maximizing the driving force for concentration. Without this mechanism, the deepest part of the nephron would equilibrate with cortical fluid and lose the gradient.
The distal tubule and collecting duct perform fine-tuning under hormonal control. Antidiuretic hormone (ADH) inserts aquaporin water channels into the collecting duct, making it permeable to water and allowing the medullary gradient to concentrate urine when the body is dehydrated. Without ADH, the collecting duct remains water-impermeable and dilute urine is produced. Aldosterone acts on the distal tubule to upregulate Na⁺ reabsorption (retaining volume) and K⁺ secretion. These hormonal controls are what allow the same nephron architecture to produce urine ranging from very dilute (~50 mOsm) to very concentrated (~1200 mOsm) depending on hydration state.
Autoregulation keeps the glomerular filtration rate (GFR) remarkably stable despite swings in systemic blood pressure. The myogenic response constricts the afferent arteriole when pressure rises, protecting glomerular capillaries. Tubuloglomerular feedback detects changes in tubular NaCl delivery at the macula densa and adjusts afferent arteriole tone accordingly — a local feedback loop that couples filtration rate to tubular processing capacity. Together these mechanisms hold GFR near 125 mL/min across a wide range of arterial pressures, ensuring that the downstream reclamation machinery is never overwhelmed.