Kidney

The kidney is a vital organ located in the urinary system of humans and many animals. Functionally, it serves to maintain homeostasis within the body, primarily by filtering blood to remove waste products, excess substances, and toxins, which are then excreted in the urine. Each human typically has two kidneys, which are bean-shaped, approximately the size of a fist, and located retroperitoneally on either side of the vertebral column at the level of the T12 to L3 vertebrae. The kidneys regulate fluid and electrolyte balance, help control blood pressure, and are pivotal in the production of certain hormones, such as renin, erythropoietin, and the activation of vitamin D.

On a more granular level, each kidney contains about one million nephrons – the functional units that perform filtration. Blood is delivered to the kidneys via the renal arteries and is then filtered through a complex series of processes including glomerular filtration, tubular reabsorption, and secretion. The glomerulus, a network of capillaries, filters the blood, allowing water and small molecules to pass into the renal tubule, while preventing the passage of blood cells and large molecules. The precise regulation provided by the kidneys ensures the proper balance of water, salts, and minerals in the blood, and ultimately maintains overall body health.