néphrons

Néphrons are the microscopic functional units of the kidney. Each human kidney contains hundreds of thousands to millions of nephrons, which filter blood, remove waste products, and regulate the body’s fluid and electrolyte balance, acid-base status, and blood pressure.

A nephron comprises two main structures: the renal corpuscle, where filtration occurs, and the renal tubule, where filtrate is processed. The renal corpuscle includes the glomerulus, a tuft of capillaries, and Bowman's capsule, surrounding the capillaries. The renal tubule consists of the proximal tubule, loop of Henle, distal tubule, and collecting duct.

Filtration begins at the glomerulus, with blood entering via the afferent arteriole and exiting through the efferent arteriole. The filtrate collects in Bowman's capsule and passes into the proximal tubule. The filtration barrier—composed of fenestrated endothelium, a basement membrane, and podocyte slit diaphragms—restricts larger molecules.

Within the tubule, solutes and water are reabsorbed or secreted as the filtrate moves along the proximal tubule, loop of Henle, distal tubule, and collecting duct. The result is concentrated or dilute urine, guided by hormonal signals such as ADH and aldosterone, and by the countercurrent mechanism in the loop of Henle.

Nephrons receive blood via peritubular capillaries and, in certain segments, vasa recta. They are derived during embryonic development from metanephric tissue; in humans nephrogenesis largely ends before birth, and nephron number is fixed early in life, contributing to reduced renal reserve with aging.