ABSTRACT
Gastrointestinal disorders present relatively commonly to doctors, and biochemical laboratory tests have important roles in their investigation. This chapter looks at gastrointestinal pathophysiology and how chemical pathology tests may be useful in diagnosis and treatment. The major functions of the gastrointestinal tract are the digestion and absorption of nutrients. Digestion and absorption depend on food and the products of its digestion being diluted with a large volume of fluid, mostly an ultrafiltrate of extracellular fluid filtered through the ‘tight junctions’ between epithelial cells, mainly in the duodenum. The main components of gastric secretion are hydrochloric acid, pepsin and intrinsic factor, all of which are important for normal digestion and absorption. There are three phases of digestion and absorption: luminal phase, mucosal phase and post-absorptive phase. Intestinal disease or impaired mucosal surface area (as in coeliac disease) may impair xylose absorption and therefore reduces excretion.
