How the Kidneys Filter Blood

01A filter with active control

Each kidney contains roughly a million nephrons. They are not passive strainers: the renal corpuscle filters plasma, while a long tubule retrieves valuable water and solutes and adds selected wastes.

02The glomerulus sets the first gate

Blood enters a tuft of capillaries under pressure. Fenestrated endothelium, a basement membrane, and podocyte slit diaphragms favor water and small dissolved molecules while retaining blood cells and most large proteins.

03Filtration is measured as GFR

The glomerular filtration rate estimates how much filtrate both kidneys produce per minute. It depends on net filtration pressure, surface area, permeability, and blood flow. Clinicians interpret estimated GFR alongside age, body size, trends, and other tests.

04The tubule takes back the essentials

The proximal tubule reabsorbs most filtered sodium and water, plus nearly all glucose in healthy conditions. The loop of Henle establishes a medullary gradient; the distal nephron and collecting duct fine-tune salt, potassium, acid, and water balance.

05Hormones adjust the final mixture

Aldosterone promotes sodium retention and potassium secretion. Antidiuretic hormone increases collecting-duct water permeability, concentrating urine when the body needs to conserve water. The kidney also helps regulate blood pressure through renin.

06Urine is the edited result

The final fluid is what remains after filtration, retrieval, and secretion. Protein or blood in urine, a falling GFR, or persistent electrolyte changes can signal injury. Hydration advice cannot substitute for evaluation when kidney disease is suspected.