The gastrointestinal (GI) tract consists of the mouth, part of the pharynx, oesophagus, stomach, small and large intestines and the rectum. Accessory organs are the liver, gallbladder and pancreas (Figure 39.1). Together, these areas of the alimentary system are responsible for ingesting, digesting, absorbing food and then eliminating the waste products.

Once food has been ingested, mastication with the aid of salivary gland secretions (Figure 39.2a) will propel the bolus into the hollow, muscular oesophagus. Peristaltic action here will move it on through the cardiac sphincter into the stomach where it will mix with gastric juice (Figure 39.2b) to form a soup-like substance called chyme. Very little absorption takes place in the stomach because the epithelial cells found here are impermeable to most substances. However, its mucous cells can absorb some water, fatty acids and significantly, drugs like aspirin and 20% of alcohol. From the stomach, the chyme will pass through the pyloric sphincter to the duodenum, the first part of the small intestine. In the duodenum, intestinal juices, water and mucus are joined by pancreatic secretions (Figure 39.2d). Together these facilitate the intestinal absorption of the constituents in the chyme, by the villi and microvilli. Microvilli are known as the ‘brush border’ due to their fine, brush-like appearance. In the jejunum, the second part of the small intestine, glucose, amino acids, water and fat soluble vitamins and 80% of any alcohol is absorbed. In the ileum, the third part of the small intestine, Vitamin B12 and bile salts are soaked up. In total, the small intestine is 3 m in length and is the major site of digestion. The large intestine is 1.5 m long and is composed of the appendix, the caecum