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• Functions of stomach


  • Store of food
  • Mixing food with juices
  • Control emptying rate and propulsion of food to duodenum

  
• Motility


  Entering of food in stomach, receptive relaxation in stomach
• Receptive relaxation


  Relaxation of stomach wall (fundus and upper of the body) to accommodate food with little if any increase in pressure
• Peristaltic movement


  Begins after receptive relaxation in the lower of the body for propelling food into the antrum

  
• Mixing movement


  Begins in the antrum for mixing and grinding of food, allows gastric emptying of small, semiliquid portion of food to pass through pylorus into the duodenum

  
• Control of Gastric motility


  • Receptive relaxation (vagal reflexes (in esophagus and pharnix ), intrinsic reflexes (in stomach wall))
  • Peristaltic waves (controlled by BER)
  • Antral systole (each BER wave induces spike waves that cause antral contraction)
• Rates: BER and hence; antral contractions is about 3-4/min
• Regulation of gastric emptying


  Antrum, pylorus and upper duodenum all act as a unit, contraction of antrum followed by contraction of pylorus and duodenum allows entering of liquid and semiliquid mass of food into the duodenum, partial contraction of antrum prevents solid mass from entering duodenum

  
• Regurgitation from duodenum into the stomach DOES NOT occur because pyloric contraction persists slightly longer than that of duodenum, prevention of regurgitation is also due to stimulation of CCK and secretin on the pyloric sphincter

  
• Regulation of gastric contractions


  • Food in the antrum induce relaxation of antral muscles
  • BER initiated in the upper portion of the body region
  • Each BER wave induce antral systole
  • Receptive relaxation is reflexly originated in the esophagus
  • CCK induce pyloric contraction

  
• Factors affecting gastric emptying rate

  • Type of food in stomach (carbohydrates - rapid, protein - slow, fat - slowest)
  • Osmotic pressure of duodenal contents (hyperosmolarity decreases emptying rate)
  • Type of duodenal contents (fats, carbohydrates and acid inhibit gastric emptying, pepsin secretion and gastric motility)

  
• Vomiting act


  The glottis closes to prevent aspiration, breathing is held in mid inspiration, abdominal muscle wall contracts to increase intra-abdominal pressure, lower esophageal sphincter and esophagus relax and the gastric contents are ejected

  
• Vomiting Center


  Found in the reticular formation of medulla, controls the different components of the vomiting act

  
• Stimuli of vomiting center

  • Mucosal irritation (from GI mucosa)
  • Motion sickness (from vestibular nuclei in CNS)
  • Emotional changes (from diencephalon and limbic system)
  • Nauseating smell (from olfactory receptors)
  • Sickening sights (from optic receptors)
  • Blood-born emetics (from chemoreceptor trigger zone)

  
• Chemoreceptor trigger zone (CTZ)


  A group of neurons in the area postrema that sensitize to blood-born chemicals
• Lesions in the postrema abolish the vomiting induced by apomorphin injection but have no effect on other stimuli of vomiting
• Serotonin and dopamine receptors in CTZ


  CTZ contains serotonin (5-HT3) receptors and Dopamine (D2) receptors, serotonin released from enterochromaffin cells initiates impulses via 5-HT3 receptors, 5-HT3 antagonists and D2 antagonists are effective antiemetics

  
• Functions of small intestine

  • Mix the contents with pancreatic and bile secretions
  • Absorb digested nutrients, vitamins, minerals and water

  
• Intestinal motility


  BER ~ 12/min in proximal jejunum, BER ~ 8/min in distal ileum

  
• Types of muscle contractions in small intestine


  • Peristaltic waves (propel chyme toward large intestines)
  • Segmentation contractions (move chyme forward and backward, increase exposure to mucosal surface)
  • Tonic contractions (prolong contractions that isolate one segment from another)

  
• Segmentation and tonic contractions prolong the transit time in small intestine, permitting longer contact of chyme with enterocytes and fostering absorption

  
• The volume of fluids in small intestine during digestion is about 8000ml/day, the volume of fluids that delivers to colon is about 1000-2000ml/day

  
• Functions of large intestine

  • Reservoir for the residues of meals that can not be digested or absorbed
  • Move the unabsorbed contents into the rectum

  
• Colonic motility


  Slow to allow water, Na+ and other minerals absorption from colon, the colon absorb 90% of the 1000-2000ml/day fluid delivered, remaining 200-250ml in the semisolid feces

  
• Ileocecal valve


  A structure that link the ileum to the colon, normally closed and restricts flux of colonic contents into the sterile ileum, opening permits the ileal chyme to squirt into the cecum, opening/closing is regulated by colonic pressure, ileal peristaltic waves, and vagovagal reflex

  
• Colonic movements


  • Segmentation contractions (mix the colonic contents and facilitate absorption)
  • Peristaltic waves (propel the contents toward the rectum)
  • Mass action contractions (move colonic materials from one portion to another, move materials into the rectum)
• The first part of a test meal takes ~9-8hrs in the colon to reach the sigmoid, from sigmoid colon to anus the transit is much slower, 70% of the meal is recovered in 72hr in the stool but total recovery require more than a week

  
• Constipation


  Pathological decrease in bowel movement, caused by altered bowel movement and alteration in the balance between secretion and absorption in the colon, symptoms include anorexia, mild abdominal discomfort, abdominal distension, treated by promoting reflexes leading to defecation and increasing fluidity of the colonic content