Nieren

The nieren are a pair of bean-shaped organs located in the retroperitoneal space on either side of the spine. They filter the blood to remove waste products and excess fluids, producing urine. In addition to waste removal, the kidneys regulate fluids, electrolytes, acid-base balance, blood pressure, and production of certain hormones.

Each kidney contains about one million nephrons, the functional units. Blood enters through the renal artery and is filtered at the glomerulus in Bowman's capsule. The filtrate then passes through the tubules where most water, salts, and nutrients are reabsorbed, while waste is secreted into the urine. The final urine is collected in the renal pelvis and transported via the ureter to the bladder.

The kidneys regulate fluid balance via hormonal systems including the renin-angiotensin-aldosterone system and antidiuretic hormone. They produce erythropoietin to stimulate red blood cell production. They convert vitamin D to its active form, helping calcium metabolism and bone health.

Anatomically, the kidneys lie retroperitoneally at roughly the level of the T12 to L3 vertebrae, protected by the lower ribs and surrounding fat. Each kidney has a hilum, through which vessels, nerves, and the ureter enter or exit. The nephron, comprising the glomerulus, Bowman's capsule, proximal tubule, loop of Henle, distal tubule, and collecting duct, is the basic functional unit.

Common conditions affecting the kidneys include kidney stones (nephrolithiasis), infections such as pyelonephritis, chronic kidney disease, and acute kidney injury. Diagnosis relies on blood tests (creatinine and eGFR), urinalysis, and imaging. Treatments range from lifestyle and medication to dialysis or kidney transplantation.