Nephron

The nephron is the functional unit of the kidney, responsible for producing urine through filtration, reabsorption, and secretion. In humans, each kidney contains about one million nephrons. A nephron extends from the renal cortex into the medulla and comprises a renal corpuscle and a tubular system.

The renal corpuscle consists of a glomerulus, a tuft of capillaries, surrounded by Bowman's capsule. Blood enters the glomerulus through the afferent arteriole, is filtered across a barrier formed by fenestrated endothelium, a basement membrane, and podocyte slit diaphragms, and exits via the efferent arteriole. The filtrate is collected in Bowman's space and becomes the starting fluid for tubular processing. Glomerular filtration rate (GFR) measures this process and reflects kidney function.

The tubular system includes the proximal convoluted tubule, the loop of Henle (descending and ascending limbs), the distal convoluted tubule, and the collecting duct. The proximal tubule reabsorbs most filtered solutes and water. The loop of Henle creates and maintains a medullary osmotic gradient that concentrates urine. The distal tubule and collecting duct fine-tune reabsorption of sodium and water and participate in acid-base balance, under hormonal control by aldosterone, antidiuretic hormone (ADH), and related signals.

The nephron is supplied by the renal vasculature, with peritubular capillaries and, in the loop of Henle, vasa recta facilitating reabsorption and concentration abilities. The juxtaglomerular apparatus regulates renal blood flow and systemic pressure through the renin-angiotensin-aldosterone system, contributing to autoregulation of GFR across a range of conditions.