The Gastro- Intestinal System An Introduction: Prof Sethuraman.

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1 The Gastro- Intestinal System An Introduction: Prof Sethuraman.

2 2  The GI tract (gastrointestinal tract) The muscular alimentary canal  Mouth  Pharynx  Esophagus  Stomach  Small intestine  Large intestine  Anus  The accessory digestive organs Supply secretions contributing to the breakdown of food  Teeth & tongue  Salivary glands  Gallbladder  Liver  Pancreas

3 3 The Digestive Process  Ingestion  Taking in food through the mouth  Propulsion (movement of food)  Swallowing  Peristalsis – propulsion by alternate contraction &relaxation  Mechanical digestion  Chewing  Churning in stomach  Mixing by segmentation  Chemical digestion  By secreted enzymes: see later  Absorption  Transport of digested end products into blood and lymph in wall of canal  Defecation  Elimination of indigestible substances from body as feces Food Bolus Chyme Nutrients Faeces

4 Chemical digestion  Complex food molecules  (carbohydrates, proteins and lipids)  broken down into chemical building blocks  (simple sugars, amino acids, and fatty acids and glycerol)  Carried out by enzymes secreted by digestive glands into lumen of the alimentary canal 4

5 5 Digestive Process Disorders  APPLY MURPHY’S LAW  “if anything can go wrong, it will…”  Ingestion - Anorexia – no appetite  Propulsion - (Dysphagia)  Mechanical digestion  Chewing (no teeth, poor jaw muscle power)  Churning & mixing (gastroparesis)  HCl acid - (achlorhydria)  Chemical digestion - Maldigestion  Absorption – Malabsorption  Defecation - Constipation / Diarrhoea

6 Mal-digestion & Mal-absorption a simile…

7 Pneumonoultramicroscopicsilicovolcanoconiosis ? Mal-digestion ! (BTW – Looooongest word!)

8 Pneumono-ultramicroscopic-silico-volcano- coniosis  Pneumo = a lung disease  Ultramicroscopic = very fine  Silico = silica dust  Volcano = volcanic origin,  Pneumo-coniosis = inflammation of the lungs due to dust inhalation Absorbed after Digestion !

9 Pneumono-ultramicroscopic-silico-volcano- coniosis  Pneumo = a lung disease (due to)  Ultramicroscopic = the inhalation of very fine  Silico = silica dust  Volcano = of volcanic origin,  Pneumo-coniosis = (causing) inflammation in the lungs. ! If still not absorbed, it is “Malabsorption” !

10 10 The Alimentary canal wall Same 4 layers: from esophagus to anal canal 1.Mucosa 2.Submucosa 3.Muscularis externa 4.Serosa from lumen (inside) out

11 11 Inner Layer: the mucosa  Epithelium:  absorbs nutrients,  secretes mucus  Muscularis mucosae  Thin layer of muscle producing only local movements

12 12 Second layer: the submucosa*  Connective tissue containing major blood and lymphatic vessels and nerves  Many elastic fibers so gut can regain shape after food passes

13 13 Layer-3: the muscularis externa* (AKA just “muscularis”) Two layers of smooth muscle responsible for peristalsis and segmentation  Inner circular layer (circumferential)  Squeezes  In some places forms sphincters (act as valves)  Outer longitudinal layer:  shortens gut

14 14 Last (outer layer), the serosa* (the visceral peritoneum)  Simple squamous epithelium (mesothelium)  Thin layer of areolar connective tissue underneath  Exceptions:  Parts not in peritoneal cavity have adventitia, lack serosa  Some have both, e.g. retroperitoneal organs

15 15 Nerves  Enteric nervous system: the gut’s own  Visceral plexuses within gut wall controlling the muscles, glands and having sensory info  100 million neurons! (as many as the spinal cord)  Autonomic input: speeds or slows the system  Parasympathetic  Stimulates digestive functions  Sympathetic  Inhibits digestion  Largely automatic

16 The Peritoneum  Peritoneum:  serous membranes of the abdominopelvic cavity  Visceral peritoneum:  covers external surfaces of most digestive organs  Parietal peritoneum:  lines body wall  Peritoneal cavity:  slit-like potential space between visceral and parietal peritoneum  Serous fluid – lubricating 16

17 17 Some organs are “retroperitoneal”  Are “behind the peritoneum”  (as opposed to the organs which are intraperitoneal, or just “peritoneal”)  Fused to posterior (dorsal) abdominal wall  Lack a mesentery  Include:  Most of duodenum (1 st part of small intestine)  Ascending colon  Descending colon  Rectum  Pancreas  Clinical point: Tend to cause back pain, instead of abdominal pain (eg., Steve Jobs’ missed Diagnosis)

18 18 Esophagus  Continuation of pharynx in mid neck  Muscular tube collapsed when lumen empty  Descends through thorax  On anterior surface of vertebral column  Behind (posterior to) trachea Esophagus___________ *

19 19 Esophagus continued  Passes through “esophageal hiatus” in the diaphragm to enter the abdomen  Abdominal part only 2 cm long  Joins stomach at cardiac orifice*  Cardiac sphincter at cardiac orifice to prevent regurgitation (food coming back up into esophagus)  Gastroesophageal junction and GERD ___________________esophageal hiatus (hiatus means opening) *

20 20 Stomach  J-shaped; widest part of alimentary canal  Temporary storage and mixing – 4 hours  Into “chyme”  Starts food breakdown  Pepsin (protein-digesting enzyme needing acid environment)  HCl (hydrochloric acid) helps kill bacteria  Stomach tolerates high acid content but esophagus doesn’t – why it hurts so much when stomach contents refluxes into esophagus (heartburn; GERD)  Most nutrients wait until get to small intestine to be absorbed; exceptions are:  Water, electrolytes, some drugs like aspirin and alcohol (absorbed through stomach)

21 21 Stomach  Lies mostly in LUQ  But pain can be epigastric or lower  Just inferior to (below) diaphragm  Anterior (in front of) spleen and pancreas  Tucked under left lower margin of liver  Anchored at both ends but mobile in between  Main regions in drawing to right--------------------------------  Capacity: 1.5 L food; max capacity 4L (1 gallon) epigastrium junction with esophagus funnel shaped contains pyloric sphincter dome

22 22 Small intestine  Longest part of alimentary canal (2.7-5 m)  Most enzymatic digestion occurs here  Most enzymes secreted by pancreas, not small intestine  Almost all absorption of nutrients  3-6 hour process  Runs from pyloric sphincter to RLQ Small intestine___________

23 23  Small intestine has 3 subdivisions  Duodenum – 5% of length  Jejunum – almost 40%  Ileum – almost 60% Blood supply: superior mesenteric artery; Veins drain into hepatic portal vein Duodenum is retroperitoneal (stuck down under peritoneum); others are loose Duodenum receives bile from liver and gallbladder via bile duct * enzymes from pancreas via main pancreatic duct * * *

24 24 Large intestine Subdivisions Cecum Appendix Colon Rectum Anal canal Digested residue reaches it Main function: to absorb water and electrolytes

25 25  Defecation 1.Triggered by stretching of wall, mediated by spinal cord parasympathetic reflex 2.Stimulates contraction of smooth muscle in wall and relaxation of internal anal sphincter 3.If convenient to defecate voluntary motor neurons stimulate relaxation of external anal sphincter (aided by diaphragm and abdominal wall muscles - called Valsalva maneuver)

26 26 The Liver  Largest gland in the body (about 3 pounds)  Over 500 functions  Inferior to diaphragm in RUQ and epigastric area protected by ribs  R and L lobes  Plus 2 smaller lobes  Falciform ligament  Mesentery binding liver to anterior abdominal wall  2 surfaces  Diaphragmatic  Visceral  Covered by peritoneum  Except “bare area” fused to diaphragm

27 27 Just some of the liver’s repertoire  Produces bile  Picks up glucose from blood  Stores glucose as glycogen  Processes fats and amino acids  Stores some vitamins  Detoxifies poisons and drugs  Makes the blood proteins

28 28 Gallbladder *  Bile is produced in the liver  Bile is stored in the gallbladder  Bile is excreted into the duodenum when needed (fatty meal)  Bile helps dissolve fat and cholesterol  If bile salts crystallize, gall stones are formed  Intermittent pain: ball valve effect causing intermittent obstruction  Or infection and a lot of pain, fever, vomiting, etc. *

29 29 Pancreas ( exocrine and endocrine) Lies in LUQ kind of behind stomach Is retroperitoneal Has a head, body and tail Head is in C-shaped curve of duodenum Tail extends left to touch spleen Main pancreatic duct runs the length of the pancreas, joins bile duct

30 Much more to Come… 30