Kidneys

About the Kidneys

The kidneys are paired bean shaped organs approximately 10 to 12 cm long, located retroperitoneally at the level of T12 to L3 vertebrae. Each kidney contains approximately 1 million nephrons, the functional units. Blood flow is enormous at 20 percent of cardiac output. Filtration occurs in the glomerulus, a tuft of capillaries within Bowman capsule, producing 180 liters of filtrate daily, of which 99 percent is reabsorbed, producing 1 to 2 liters of urine. The proximal convoluted tubule reabsorbs 65 percent of filtrate including glucose, amino acids, and bicarbonate. The loop of Henle creates the concentration gradient via countercurrent multiplication. The distal convoluted tubule and collecting duct are regulated by aldosterone sodium reabsorption, ADH water reabsorption, and ANP sodium excretion. The kidneys produce erythropoietin stimulating red blood cell production, renin activating RAAS for blood pressure control, and active vitamin D 1,25 dihydroxycholecalciferol. Acid base balance is maintained by excreting hydrogen ions and reabsorbing bicarbonate. Serum creatinine and estimated GFR are key clinical markers of kidney function.