The GIT

    Cards (33)

    • The GITs function is to breakdown food into smaller fragments through digestion, and absorb the nutrients, water and electrolytes from this from the lumen through the GIT wall into the bloodstream.
    • Epithelial cells lines the cavities, surfaces of blood vessels and organs throughout the body.
      They secrete, selectively absorb, protect, transport transcellular and sense.
    • The GIT is a collection of organs divided into the GIT and accessory organs.
      The accessory organs contribute to the GIT but the food doesn't pass through.
      They include the salivary glands, the liver, the pancreas and the gall bladder
    • The mouth (buccal cavity) contributes to digestion through mechanical digestion which consists of mastication (chewing) in which the tongue manipulates the food and the teeth grind it.
      Mastication turns the food into a bolus and breaks the food into smaller pieces so that there is more contact area for digestive enzymes.
    • The teeth are secured in sockets of the alveolar processes of the maxillae and the mandible.
      They are covered in enamel which helps prevent them from being worn down when we chew, and it helps keep out acids which could easily dissolve the interior of a tooth.
    • Humans have 2 sets of teeth - 20 deciduous (baby) teeth and these are replaced by 32 permanent teeth.
    • There are 3 salivary glands
      • Sublingual (below the tongue) - secrete mostly mucous cells through lesser sublingual ducts (thick saliva)
      • Parotid (infront of the ear) - secrete salivary amylase into the mouth through the parotid duct (watery saliva)
      • Submandibular (floor of the mouth/by the jaw) - secrete mucous secreting cells into the mouth through the submandibular ducts
    • The intestinal lumen is composed of 4 tissue layers:
      • Mucosa - epithelium, lamina propria (loose connective tissue), muscularis mucosae (thin smooth muscle)
      • Submucosa - loose collagenous tissue, containing blood vessels, lymphatics and nerves
      • Muscularis propria - 2 layers of smooth muscle - inner circular and outer longitudinal which mediate peristalsis
      • Adventitia/Serosa - outer loose tissue, vessels, nerves and adipose
    • The oesophagus links the stomach to the mouth and divides from the trachea with the epiglottis.
    • Both voluntary skeletal muscles and involuntary smooth muscles are involved in propelling food material through the digestive system.
      Voluntary - tongue involved in swallowing bolus and anus in defecation
      Involuntary
      • pharynx and epiglottis involved in swallowing bolus
      • food is moved from mouth through GIT via peristalsis
    • The smooth muscle of the GIT is arranged in 2 layers - longitudinal along the length of the tube and circular around the diameter of the tube
      Coordinated contraction and relaxation of these 2 layers creates a wave like propulsion of the food called peristalsis
    • Sphincter muscles are specialised circularly arranged skeletal and smooth muscles
      The majority of sphincters along the length of the GIT are involuntary smooth muscle structures contracted and relax only to allow food ti move from one organ to the next
    • Upper/oesophageal sphincter - separates the oesophagus from the stomach
      Pyloric sphincter - separates the stomach from the small intestine
      Hepatopancreatic sphincter - controls the release of accessory secretions from the liver and pancreas into the beginning of the small intestine
      Ileocecal sphincter - controls movement into the large intestine/colon
      Anal sphincter - is skeletal muscle controlled consciously to relax during defecation
    • The stomach has 3 mechanical tasks
      1. Stores the swallowed food and liquid
      2. Mix up the food, liquid, and digestive juice produced by the stomach
      3. Empty its contents slowly into the small intestine
    • The stomach is made of 4 parts
      1. Cardia - where the oesophagus meets the stomach. Passage of food through this area is regulated by the lower oesophageal sphincter
      2. Fundus - stores undigested food and gas
      3. Body - the largest part of the stomach
      4. Pylorus - where gastric contents flow out of the stomach into the duodenum (regulated by the pyloric sphincter)
    • The stomach contains an extra layer of smooth muscle that allows it to churn, mix, and pummel the food, physically reducing it to smaller fragments (oblique muscle layer)
      The rugae are folds in the stomach walls which form ridges to increase the surface area
      The rugae mucosa is marked by depressions called gastric pits lined with epithelial cells, containing parietal cells which produce HCl and chief cells which produce pepsinogen
    • The small intestine:
      • neutralises chyme pH
      • mixes chyme with digestive juices
      • digests
      • absorbs
    • The duodenum forms a C shaped loop inferior (below) to the stomach and receives acidic chyme from the stomach and pancreatic juice from the pancreatic ducts.
      Pancreatic juice is rich in digestive enzymes, bicarbonate ions and phosphate buffers (neutralise stomach acid)
    • The ileum is where absorption occurs.
      Circular folds within the mucosa increase the surface area and maximise its absorption
      Nutrients from the interstitial space move into an intestinal villus and are absorbed through a lymphatic lacteal
      Simple columnar epithelium cells with microvilli face the lumen for absorption of digested material
      Intestinal crypts are analogous to the rugae of the stomach and are formed by invaginations of the epithelium
    • The intestinal villi are lined with microvilli (brush border) which are so small and tightly packed.
      They contain digestive enzymes (mesh of carbohydrates) for the final part of digestion
      Absorption happens at the brush border
    • The small intestine contains epithelial cells:
      • Microvilli - digestion and absorption
      • Goblet cells - produce alkaline mucus
      • Endocrine cells - produce regulatory hormones
      • Paneth - secrete lysozomes and other defensive proteins
    • Within the intestinal glands there are 3 types of cell:
      • S cells - produce hormone secretin to buffer intestinal pH (stimulates production of bicarbonates)
      • I cells (CCK cells) - secrete hormone cholecystokinin (CCK) which stimulates the release of digestive enzymes from the pancreas and gall bladder into the intestine
      • K cells - secrete hormone gastric inhibitory peptide (GIP) which influences insulin levels
    • The large intestine:
      • Enteric bacteria digest some food residue and vitamins
      • Absorb most residual water, electrolytes, and vitamins produced by enteric bacteria
      • Propels faeces toward rectum
      • Defecation reflex eliminates faeces
    • The large intestine is composed of simple columnar epithelium cells in the mucosa
      It also has crypts which contain both epithelial cells and goblet cells
      The distal anal canal is an exception which is composed of non keratinized stratified squamous epithelial cells (it's more durable to the abrasion that occurs when faeces moves)
      Since most digestion and absorption occurs before the large intestine, the only significant secretion is mucus which lubricates the passage of digestive residue
    • The peritoneum is a membrane that lines the abdominal cavity and covers the organs.
      It acts as a conduit for their blood vessels, lymphatic vessels and nerves.
    • Exocrine glands release chemical substances through ducts to outside the body or onto another surface within the body e.g. gastric, salivary, sweat
      Endocrine glands release chemical substances directly into the bloodstream or tissues of the body. The chemical substances released are known as hormones e.g. thyroid, pituitary, adrenal
    • Gastric activity is regulated in 3 phases:
      • cephalic
      • gastric
      • intestinal
    • The cephalic phase is controlled by the brain and begins when you see, smell, taste or think of food
      It's mediated by the CNS via the vagus nerve and gastrin secretion
      It stimulates the stomach to produce mucus (goblet cells), pepsinogen (chief cells), HCl (parietal cells)
      Approximately 20% of gastric secretion occurs during the cephalic phase
    • The gastric phase builds on the stimulation of the cephalic phase when food arrives in the stomach
      It is stimulated by
      1. Distension of the stomach - stretching of stomach
      2. Decreased pH of the stomach - secretion of acid
      3. Appearance of undigested food in stomach
    • The gastric phase has 3 responses:
      1. Local - distension stimulates parietal cells to produce HCl and histamine (which stimulates the parietal cells to produce more HCl)
      2. Neural - detection of food stimulates stretch and chemo receptors which triggers the vagus nerve to trigger gastric secretion and contractions of stomach muscles
      3. Hormonal - neural stimulation stimulates secretion of gastrin which stimulates the increase of gastric secretion and thus the lowering of pH, and it stimulates gastric motility
    • The intestinal phase begins when the chyme first enters the duodenum
      It has an inhibitory effect which controls the rate of gastric emptying to ensure efficient digestion and absorption in the small intestine.
      It prevents the duodenum from being overloaded with more chyme than it can handle
    • The intestinal phase has a neural response stimuli
      1. Reduced gastric distension as the chyme leaves the stomach
      2. Increased duodenum distension
      This stimulates the enterogastric phase which is a negative feedback mechanism that reduces acid secretion and prevents chyme from becoming too acidic. It inhibits gastrin secretion and gastric motility while increasing duodenal secretions of mucus
    • Negative feedback of enterogastrones
      Acidic chyme in the duodenum causes secretions of secretin - inhibits parietal and chief cells and stimulates secretion of pancreatic digestive enzymes
      Lipids and amino acids induce secretion of GIP and CCK - inhibit gastric secretion