nephrons

Nephrons are the microscopic structural and functional units of the kidney, responsible for filtering blood and forming urine. In adult humans, each kidney contains about one million nephrons, which consist of a renal corpuscle and a renal tubule. Nephrons are located primarily in the renal cortex, with some extending into the outer medulla; two main types are cortical nephrons and juxtamedullary nephrons, differing in glomerular location and loop of Henle length.

The renal corpuscle comprises the glomerulus, a tuft of capillaries, and Bowman's capsule, which collects the filtrate produced by filtration across the glomerular membrane. From Bowman's capsule, filtrate enters the renal tubule, including the proximal tubule, loop of Henle, distal tubule, and collecting duct.

Functions: The tubule segments reabsorb water, ions, and nutrients back into the bloodstream and secrete waste products into the filtrate, producing urine with regulated volume and composition. The proximal tubule reabsorbs the majority of filtered solutes; the loop of Henle establishes an osmotic gradient that concentrates urine; the distal tubule and collecting duct adjust sodium, water, and acid-base balance under hormonal control, notably aldosterone and antidiuretic hormone.

Blood supply: Afferent arteriole brings blood to the glomerulus; efferent arteriole drains it. Peritubular capillaries and, in juxtamedullary nephrons, vasa recta, surround the tubules and mediate reabsorption and secretion. The juxtaglomerular apparatus helps regulate glomerular filtration rate and blood pressure.

Clinical relevance: Nephrons can be damaged in kidney disease; loss of nephrons reduces filtration rate and urinary concentrating capacity. Chronic kidney disease often reflects progressive nephron loss from conditions such as diabetic nephropathy and hypertension.