GASTRO - INTESTINAL TRACT (1)

GASTRO - INTESTINAL TRACT (1)

1. COMPONENTS AND THEIR STRUCTURES + FUNCTIONS

   • Alimentary Canel
     • What is it?
       • This refers to the components which make up the passage along which food travels in the body
     • What components does it consist of?
       • SALIVARY GLANDS- The salivary glands include three glands and their associated ducts. Parotid, Submandibular and sublingual. They consist of two types of cells serous (which have zymogen granules) and mucous cells (which produce mucus)
         • Parotid (mucous only), Submandibular (mainly serous cells), Sublingual (mainly muscous)
       • ESOPHAGUS- This is a long muscular tube which transport food from the the mouth directly to the stomach using peristalisis
       • PHARYNX- This transports food from the mouth into the esophagus and it is involved in the process of swallowing
       • MOUTH- Contains teeth that mechanically break down food. pH around 5-7.
       • SMALL INTESTINES- Main job is the breakdown and absorption of food into the body usign enxymes from pancreatic juice, and bile There is also sodium bicarbonte which keeps the pH around 6-7.5
       • LARGE INTESTINES- Absorb some of the water from the indigestible material left after the small intestines. pH 5-7. It also contains lots of bacteria which break down foods (microbial fermentation)
   • Accessory Organs
     • What are these?
       • These are organs which provide enzymes that are secreted into the ailmentary tract to chemically breakdown food making it easier to absorb
     • PANCREAS- The pancrease produces enzymes and pancreatic jucies which it adds into the small intestines to break down food
     • GALL BLADDER- Store a bile. Bile is a yellow brown digesitve enzyme produced by the liver., it breaks down fats into fatty acids
     • LIVER- The liver is used to process nutrients absorbed by the small intestines. Also produced bile

2. Oral Cavity

   • SALIVA
     • What is saliva?
       • Saliva includes electrolytes, mucus which acts as a lubricant, anti bacterial substances and digestive enzymes (mostly salivary amylase)
     • How is the saliva secretion controllled?
       • Salivary secretions can be increased by the presence of food on the tongue. This presence is detected by chemoreceptors on the tongue. A signal is then sent to the medulla oblongata via the facial VI and glossopharyngeal IX. This signal is sent back using automatic ganglia Salivary glands secretions are increased by increase in blood to gland. This is increased by the parasympathetic nervous system.
   • Pre-Processing of food
     • This takes various stages: 1) Sensory analysis 2) Mechanical Breakdown 3) Lubrication 4) Sterilization (using lysosomes) 5) Neutralisation (bicarbonate) 6) Beginning of digestion

3. SWALLOWING REFLEX

   • What are the three stages to the swallowing relfex
     • Buccal (oral) stage
       • Voluntary bolus formation and bolus moved to the back of the mouth
     • Pharyngeal Stage
       • Bolus pushes on soft palate and initiates swallow relex activated by brain stem arc.. Uvula prevents nasal reflux. Larynx elevated and epiglottis shut down
     • Oesophageal stage
       • Bolus entering oesophagus induces oesophagael peristalsis
     • OVERALL: Pharynx -> Afferent nerves -> Brain -> Efferent nerves -> Process of swallowing

4. Peristalsis

   • What is it?
     • This is the continuous contraction and relaxation of muscles that pushes food through the digestvie system.. Occurs in the esophagus, stomach and intestines.
   • How is it initiated?
     • Initiated locally by the swallowing centre. The smooth muscle are myogenic. They have pace setter cells which depolariize more easily than other cells. This allows them to set the pace for peristalsis. The rate is controlled by the nervous and the hormonal systems
       • Nervous System
         • Intrinsic Nerves
           • There are two sets of ganglia. Auerbach's Plexus and Meissner's
         • Extrinsic Nerves
       • Hormonal System
         • SOMASTATIN - This causes decreased release ocetylcholine which leads to a decreases in peralstsis
         • SUBSTANCE P- Stimulates myogenic causing increased perastalisis
         • MOTILIN- Stimulates extended contractions causing migrating motor complex

5. STOMACH

   • ROLE
     • Produces gastric fluid to breakdown foods and has a low pH to kill bacteria
   • PARTS
     • GASTRIC GLANDS-
       • MUCOUS CELLS- Secrete mucus (protects lining, stimulated by mechanical stretch) and bicarbonate
       • PARIETAL CELLS- Secrete Hydrochloric acid
       • CHIEF CELLS- Pepsinogen (breaks down peptide bonds, activated by low pH) and gastric lipase
       • D CELL- Secrete Somatostatin (inhibits acid) This is released when pH drops below 3 and has a negative feedback effect on the G CELL
       • G CELL- Secrete gastrin (stimuates acid)
       • ENTEROCHROMAFFIN LIKE CELLS- These secrete histamine (which stimulates parietal cells)
     • AREA
       • BODY AND ANTRUM- Mixing of food takes place here
       • FUNDUS- Storage of gas and allows salva to continue food breakdown
   • GASTRIC ACID SECRETION
     • CEPHALIC PHASE- Stimulated by seeing or thought of food. The hypothalamus then stimulates the medulla oblongata which sends a signal down the parasympathetic system (vagus relex) to stimulate gastrin production
     • GASTRIC PHASE- Stomach stretches which stimulates vagus nerve. The rise in alkaninity stimulates gastrin release from G cells
     • INTESTINAL PHASE- Duodenum of the small intestines stretched causing gastrin to be released into the blood stream. If the stretching reduces or their are irritants the stimulation of enterogastric reflex occurs. or release of entergastrines
   • EXIT TO DUODENUM
     • As the mixed chyme food moves down the stomach the pyloric sphincter opens up allowing more food to pass down to the small intestines
   • GASTRIC ULCER
     • Ulcer occurs due to damage to the sub,ucosal layer. H Pylori can be one of the cause as they sythesise urease which produces ammonia and damages the stomach lining