Gi secretions

Gi secretions

• 2.
  • The salivaryglands produce a glycoprotein called haptocorrin (transcobalamin I), which binds to vitamin B12 to protect it from the acidic secretions of the stomach (Morkbak et al, 2007). The movement and absorption of vitamin B12 in the gut will be explored in part 2, part 4 and part 5. • Saliva is also produced by the serous glands of the uvula, a projection from the soft palate that prevents food entering the nasal cavity during swallowing (Fig 2). • Saliva • Saliva is an aqueous solution (Carpenter, 2013) consisting of water (99.5%), mucus, bicarbonate ions, two digestive enzymes – amylase and lipase (or lingual lipase) – and lysozyme. The pH of saliva varies between 6.2 and 7.4, with a neutral pH of 7.0 recognised as optimal for dental health (Baliga et al, 2013). The bicarbonate ions in saliva act as a chemical buffer, neutralising acid in the mouth to protect the teeth from erosion. Sugar-free chewing gum can protect the teeth as it encourages saliva production, which reduces acidity in the mouth.
• 3.
  • The digestiveenzymes start the process of chemical digestion. Salivary amylase starts digesting carbohydrates, breaking starch (long linear chains of glucose) into maltose, a disaccharide made of two glucose units (Fig 3a). This is why, after having been chewed, a piece of white bread may taste sweet. Salivary lipase starts digesting fats, breaking them down into fatty acids and glycerol (Fig 3b). It functions optimally at a pH of around 4, so it will not work at its maximal efficiency until reaching the acidic environment of the stomach
• 4.
  • Lysozyme, anenzyme found in most body fluids, is part of the non-specific immune defences and acts as a general antimicrobial, attacking and breaking down bacterial cell walls. In the mouth, it starts to kill certain types of bacteria in food, before further sterilisation occurs in the acid secretions of the stomach.
• 6.
  • Secretion ofgastric juice • The secretion of gastric juice occurs in three phases: • Cephalic phase; • Gastric phase; • Intestinal phase (Fry, 2009). • In the cephalic phase, gastric juice secretion is initiated by the sight, smell and taste of food, and is primarily mediated by the parasympathetic nervous system via the vagus nerve, which innervates the stomach wall. When a person is hungry, the mere anticipation of food is enough to stimulate gastric juice secretion (Power and Schulkin, 2008). The cephalic phase accounts for around 30% of gastric juice secretion and prepares the stomach to receive food. • The gastric phase is the longest phase of gastric juice secretion, typically lasting between two and three hours, and is responsible for around 60% of gastric secretion. It begins when food has entered the stomach and the stomach wall is stretched. This prompts the G cells in the gastric pits to release gastrin. Gastrin stimulates the parietal cells to secrete HCl, which creates a pH favourable for protein digestion and the killing of ingested pathogens. Gastrin secretion can also be stimulated by the ingestion of high-protein food, coffee, wine or beer (Papakonstantinou et al, 2016; Stermer, 2002); • Gastric juice secretion continues in the intestinal phase while food is slowly passing from the stomach into the duodenum. The duodenum and jejunum progressively release hormones such as cholecystokinin, secretin and gastric-inhibiting peptide, which gradually reduce the secretion of gastric juice (Daniels and Allum, 2005). The intestinal phase is responsible for around 10% of gastric juice secretion. • Protein digestion • The process of protein digestion starts in the stomach. HCl slowly denatures proteins (for example, actin and myosin from meat), causing structural changes that expose the peptide bonds between adjacent amino acids. This enhances subsequent chemical digestion by proteases (Goodman, 2010). Activated pepsin present in the gastric juice cleaves the initial protein molecules (which can consist of thousands of amino acids) into smaller chains called polypeptides (Fig 3). Pepsin is referred to as an endopeptidase because it acts predominantly by attacking the peptide bonds in the centre of proteins to create polypeptides, which are further broken down in subsequent regions of the GI tract (Vahdatpour et al, 2016; Daniels and Allum, 2005).