Physiology and function of the small intestine

The small bowel is a highly adapted organ with the capacity to absorb water and nutrients far in excess of the body's requirements. Changes in absorption and secretion homeostasis can rapidly lead to diarrhoea, dehydration, electrolyte disturbance and malnutrition. This chapter begins with a discussion on the anatomy and histology of the small intestine. The small bowel includes three substructures termed the duodenum, jejunum and ileum, which extend sequentially from the gastric pylorus to the ileocaecal valve. The chapter summarises the key small bowel investigations and describes their relevance to small bowel pathology. It illustrates the differences in the ultrastructure and function of the small bowel, and also the causes of malabsorption. The disease of the small bowel should be considered in any patient presenting with systemic symptoms occurring as a result of micronutrient deficiency.