Gastroenterology is the branch of medicine focused on the digestive system and its disorders. Diseases affecting the gastrointestinal tract, which include the organs from mouth into anus, along the alimentary canal, are the focus of this speciality. Physicians practicing in this field are called gastroenterologists. They have usually completed about eight years of pre-medical and medical education, a year-long internship (if this is not a part of the residency), three years of an internal medicine residency, and two to three years in the gastroenterology fellowship. Gastroenterologists perform a number of diagnostic and therapeutic procedures including colonoscopy, endoscopy, endoscopic retrograde cholangiopancreatography (ERCP), endoscopic ultrasound and liver biopsy. Some gastroenterology trainees will complete a "fourth-year" (although this is often their seventh year of graduate medical education) in transplant hepatology, advanced endoscopy, inflammatory bowel disease, motility or other topics. Hepatology, or hepatobiliary medicine, encompasses the study of the liver, pancreas, and biliary tree, while proctology encompasses the fields of anus and rectum diseases. They are traditionally considered sub-specialties of gastroenterology.