digestive track

Mucosa. Consists of * Layer of epithelium in direct contact with contents of the GI tract * Thin layer of connective tissue * Thin layer of smooth muscle that can create folds to increase surface area. * The mucosa also contains prominent lymphatic nodules to make an immune response. * Submucosa. * Connective tissue that binds mucosa to muscularis. * Also contains blood and lymphatic vessels that receive absorbed food molecules. * Also contains networks of nerves regulated by autonomic nervous system (ANS) called enteric nervous system (ENS). ENS nerves control secretions of organs of GI tract.

Muscularis. Has a circular muscle layer and longitudinal muscle layer to help digestion and propel food. Except for pharynx, muscle layer is smooth muscle cells. ENS neurons within the muscularis control frequency and strength of its contractions. Serosa and peritoneum. The serosa, is the outermost layer around organs of GI tract. It is a membrane that secretes a slippery, watery fluid that allows the tract to glide easily against other organs. Also called the visceral peritoneum.

Main external nerve to gut is vagus nerve. It is parasympathetic causing an increase in GI secretion and motility by increasing the activity of ENS neurons. * Sympathetic nerves from vertebra in spinal cord, cause decreased GI secretion and motility by inhibiting neurons of ENS. * Emotions such as anger, fear, and anxiety may slow digestion because they stimulate the sympathetic nerves that supply the GI tract.

Mechanical digestion in the mouth from chewing, or mastication. Food is reduced to a soft, flexible, easily swallowed mass called a bolus (= lump). * Salivary amylase breaks down starches to di-and trisaccherides.

Swallowing Divided into three stages: voluntary, pharyngeal, and oesophageal. Voluntary stage; bolus forced to back of mouth into oropharynx upward and backward movement. When bolus enters oropharynx, the involuntary pharyngeal stage of swallowing begins. Oesophageal stage, food pushed down oesophagus by a process called peristalsis

Has a “tough” lining of stratified squamous epithelium

Different stimulants all signal through to the proton pump to release HCl acid. Anti-ulcer drugs affect these different receptors

When food enters stomach, waves of peristalsis churn contents * Gastric contents are mixed with gastric juice, becoming a soupy liquid called chyme and passes through pyloric sphincter into duodenum. * Main digestion in stomach is by enzyme pepsin, which breaks peptide bonds between amino acids of proteins. * Proteins become fragmented into peptides, smaller strings of amino acids. Pepsin most effective in very acidic environment of stomach, which has a pH of 2. * The mucus layer and secretion of small amounts of bicarbonate keep pH just above the stomach cells at a more neutral pH so they are protected from digestion.

Liver produces bile, it is stored in gall bladder and empties into small intestine when we eat, especially fatty food. Bile helps fat break down. * Pancreatic juice enters small intestine at same place as bile. Pancreatic enzymes have several enzymes to break down proteins e.g., trypsin. Pancreatic juice also contains bicarbonate to neutralise the acid coming from the stomach. * Enzymes in pancreatic juice include pancreatic amylase (digests starch); protein‐digesting enzymes include trypsin, and chymotrypsin, pancreatic lipase digests triglycerides (fats) and nucleic acid–digesting enzymes called ribonuclease (RNA) and deoxyribonuclease (DNA). * Protein‐digesting enzymes in pancreas produced in inactive form, when they reach the small intestine, an enzyme called enterokinase activates trypsin which then activates the other protein‐digesting enzymes.

1–2 L of intestinal juice secreted each day. * Intestinal juice contains water and mucus and is slightly alkaline (pH 7.6). * Absorptive cells of small intestine make several digestive enzymes, called brush-border enzymes, and insert them in plasma membrane of microvilli. * Some enzymatic digestion occurs at surface of absorptive cells that line the villi, rather than in the lumen. * Among the brush-border enzymes are carbohydrate-digesting enzymes including lactase that helps digest lactose from milk. * And several protein-digesting enzymes called peptidases including dipeptidase which breaks down two joined amino acids into single amino acids.

9.3 L fluid enters gut each day 7 litres produced by the gut itself Small intestine absorbs 8.3 L 0.9 L enters colon 0.8 is reabsorbed.

Gastro-oesophageal Reflux Disease (GORD) * very common * rarely life-threatening Barrett’s oesophagus * pre-malignant lesion Peptic ulcer disease * Affects stomach or duodenum * Helicobacter pylori * Non-steroidal anti-inflammatory drugs (NSAIDs)

Irritable bowel syndrome (IBS) * disorder of GI tract motility * common * no long-term damage to gut Inflammatory bowel disease (IBD)-XS inflammatory response * Crohn’s disease * Ulcerative colitis * relatively uncommon Diverticular disease-affects colon * Associated with constipation

Water and Foodborne illnesses * Viruses * Bacteria * Parasites * Clean water supply & good hygiene essential

Oesophageal cancer is relatively uncommon, prevents people swallowing food. * squamous carcinoma-smoking a risk factor * adenocarcinoma –acid reflux a risk factor Small intestinal cancer is very rare, usually genetic abnormality Colon cancer is commonest- more common in over 50s, passing blood with stool, weight loss, abdominal pains is a sign. * High fibre diets may reduce risk, high fat and meat diets increase risk.