First and shortest (9–11 in., or 23–28 cm) segment of the small intestine .
It curves down and then up from the pylorus of the stomach, where chyme enters it. Ducts from the pancreas and gallbladder bring in bicarbonate to neutralize stomach acid, pancreatic enzymes to further digestion, and bile salts to break up fats. Nutrient absorption begins in the lower duodenum, which has a mucous lining. Exposure to stomach acid makes the upper duodenum susceptible to peptic ulcers , the duodenum's most common problem. Compression of the lower duodenum between the liver, pancreas, and major blood vessels can require surgery.