Normal kidney structure and function

The kidneys maintain the homeostasis of electrolyte, fluid, and acid–base balance; eliminate waste products; and have an endocrine-metabolic function. They secrete hormones such as erythropoietin, Klotho, and 1,25-(OH)2-vitamin D and clear other hormones and cytokines. Each kidney contains 1 million basic functional units, or nephrons. Each nephron is composed of a glomerulus and a renal tubule. The glomerulus is a tightly woven, highly permeable capillary bed, surrounded by differentiated, very specialized cells, the podocytes. The mesangium contains mesangial cells and holds the capillaries together. Every day, 180 L of plasma is filtered through the glomeruli. Podocytes prevent the filtration of proteins, and their injury will lead to pathological urinary protein excretion (proteinuria). Podocytes do not divide, and podocyte loss causes podocytopenia, an early event in progressive glomerular scarring. Tubular cells reabsorb most of the filtered fluid and nutrients, and only 1 to 2 L of urine is excreted. Proximal tubular cells are responsible for the bulk of reabsorption. They are rich in mitochondria, consume high amounts of energy, and express a variety of transporters that favor the uptake of nephrotoxins. Thus they are prime targets in toxic and ischemic renal injury.