The gastrointestinal tract is responsible for the ingestion of food, the absorption of nutrients from food and the excretion of unabsorbed waste products. This process is controlled by the autonomic nervous system, together with hormones including gastrin, secretin and cholecystokinin. The musculature of the alimentary tract is composed of smooth muscle from the mid-oesophagus to the external anal sphincter. Smooth muscle has an innate tone that permits sustained and sometimes powerful contraction over long periods of time. The excretory function of the kidney regulates water and electrolyte secretion to maintain blood volume, blood pressure and plasma electrolyte composition. The 1-hydroxyl group of active vitamin D is added in the kidney, so disorders of calcium and phosphate homeostasis are a common problem in chronic kidney disease. Erythropoietin, which drives erythrocyte production in the bone marrow, is produced by fibroblasts in the renal medulla in response to hypoxia.