Nephron

A nephron is the microscopic functional unit of the kidney responsible for filtering blood and forming urine. Each nephron consists of a renal corpuscle (the glomerulus - a tuft of capillaries - plus Bowman's capsule) that filters blood, and a renal tubule where the filtrate is modified via reabsorption and secretion to produce urine.

• Humans have over one million nephrons in each kidney. Each nephron's tubule has several segments with specialized roles: proximal tubule (reabsorbs nutrients, electrolytes, water), loop of Henle (creates concentration gradient in the medulla for water reabsorption), distal tubule, and collecting duct (fine-tunes electrolyte and water balance, site of action for hormones like ADH and aldosterone).
• About 180 liters of fluid are filtered by all nephrons combined per day, but ~99% is reclaimed. Therefore, normal urine output is only about 1-2 liters per day.
• Name trap: Nephron (kidney unit) is sometimes confused with neuron (nerve cell) due to similar spelling. Remember that nephr- refers to kidney (as in nephrology), whereas neur- refers to nerves.

• Frequently tested in renal physiology. For example, a question might ask where in the nephron glucose is normally reabsorbed - the answer is the proximal convoluted tubule (since in healthy kidneys all filtered glucose is reabsorbed there).
• Questions may involve the effects of hormones on nephron segments: e.g., ADH acting on collecting ducts to increase water reabsorption, or aldosterone acting on the distal tubule/collecting duct to increase Na+ reabsorption and K+ secretion.
• Understanding nephron structure is key for interpreting urinalysis results. For instance, protein in the urine (proteinuria) often indicates a problem with the glomerulus (filter) because normally glomerular filtration prevents large proteins from entering filtrate.