Digestive System

Question 1

Identify the components of the digestive system, and relate their structure to their function.

Answer 1

• Oral cavity (and surrounds) consists of teeth and muscles of mastation which mechanically digest food. It also consists of the tongue with taste buds and the openings of the salivary glands (parotid, submandibular and sublingual). The salivary glands produce saliva, which is an exocrine secretion which begins the digestion of carbohydrates and has various antimicrobial proteins.
• Pharynx consists of three parts: the nasopharynx, the oropharynx and the laryngopharynx. Only the oropharynx is part of the digestive system.
• Oesophagus is located in the neck, thorax and abdominal cavity. It is a tube that conveys ingested material to the stomach.
• Stomach secretes HCl, pepsinogen and mucous from the gastric epithelium. The HCl has a low pH and thus denatures protein and has an antibacterial effect. The mucous protects the epithelium from acidic chime. The stomach has gastric motility, so it is able to fill, mix and empty.
• Small intestine is the site of most digestion and absorption. It consists of three parts: the duodenum, the jejunum and the ileum.
• Liver produces bile, an alkaline secretion that neutralises acidic chime from the stomach and aids in the emulsification of lipids. The liver has many other functions as well.
• Pancreas secretes pancreatic juice (which contains HCO3-) and digestive enzymes (trypsin, chymotrypsin, amylase, lipase, RNase, DNase) which it secretes into the duodenum, and which performs much of the chemical digestion. The pancreas also has endocrine roles.
• The gall bladder stores bile, which is releases in the duodenum.
• Small intestine has structural specialisation at gross, light microscopic and electron microscopic levels (e.g. circular fold of mucous and submucosa, villi and microvilli on apical surface of epithelial cells) which increase surface area for increased digestion. Mucous helps the movement of matter through the small intestine. Smooth muscle contractions mix content (segmentation) and move content along its length (peristalsis). In the lumen of the small intestine, enzymes from the pancreas break covalent bonds in macromolecules to form smaller macromolecules (proteases, amylase, lipase). At the brush border, intestinal epithelial enzymes (peptides, various disaccharides, etc.) complete digestion prior to absorption. Most nutrients are absorbed from the small intestine into blood, although fats and fat-soluble material are absorbed into the lymph. Venous blood from the gut contains most absorbed nutrients and travels to the hepatic portal vein, where it then perfuses the liver before returning to the heart and distributed around the body by the cardiovascular system.
• Large intestine is composed of the caecum, appendix, ascending colon, transverse colon, descending colon, sigmoid colon, rectum and anal canal. It secretes mucous and absorbs most of the remaining ions and water, as well as some nutrients. Bacteria in the colon performs some digestion and peristalsis moves the faeces along before defecation occurs.

Question 2

Illustrate the components of the digestive system

Answer 2