Functional anatomy and physiology of GIT

Prof Dr Amira Shubbar MRCP, FRCP

Upper esoph. sphincter
Lower esop. Sphincter
Pylorus
Ileocecal Valve
Anal sphincter
Receptive relaxation Regulated emptying
Migrating motor complex
Segmenatation Propulsive peristaltic contraction

Anatomy & Function (The swallowing wave)

Gastric secretion
Parietal cells :HCL + Intrinsic factor. Chief cells :Pepsinogen +mucus. G cells :Gastrin. D cells : Somatostatin. Protective factors: Mucus + Bicarbonate Oxyntic gland: ghrelin

Gastric distension & food stimulate the release of Gastrin from G cells which will act on CCk-2R on ECL cells releasing Histamine which will act on H2 receptors in the Parietal cells. Gastrin act direct on the Parietal cells too. Vagal stimulation through anticipation or smell of food act on the Ach-R M3 receptors on the parietal cells Hydrogen + Chloride are secreted in response to H/K ATPase (Proton pump ) from the apical membrane of the Parietal cell.


What switches off the parietal cell?
Somatostatin D cell CCK I cell Secretin S cell GIP K cell glucose-depedent insulinotrophic polypeptide

Functions of the small intestine

Digestion Absorption Protection against ingested toxins & immune regulation.

Carbohydrates

Starch Alpha-limit dextrins containing 4-8 glucose molecules. Disaccharide Maltose Trisaccharide Maltotriose Disaccharides are digested by enz. in microvilli to monosaccharides glucose , fructose & galactose which diffuse through enterocytes

Protein

Protein in the stomach converted by pepsin to A.A & polypeptide which stimulate secretion of pancreatic proenzymes trypsinogen , chymotrypsinogen , proelastases & procarboxypeptidase. Trypsinogen converted to trypsin on enterocyte brush border . Protein digested by trypsin to oligopeptides & A.A. Oligopeptides hydrolyzed to dipeptides , tripeptide & A.A then actively transported into the enterocytes & then to the portal circulation.

Protective Function of the small intestine

Immunology & T Lymphocytes , Macrophages & Mast cells are found through out GIT. MALT :constitute 25% of total lymphatic tissue of the body. Luminal Ag stimulate B cells to Plasma cell in peyers patches to mesenteric LN then thoracic duct & blood stream & lamina propria & secret IgA. T lymphocyte localize the plasma cell at the site of Ag. Macrophages: Phagocyte foreign material & secret cytokines.

Mucosal Barrier

Mucus. Enterocytes membranes & tight junctions between them. Renewal of the intestinal cells every 48 hours.

Pancreas

Exocrine pancreas is necessary for the digestion of protein , fat & carbohydrate. Pancreatic enzymes: Amylase: Starch & glycogen Lipase: TG Colipase: TG Proteolytic enzymes: Protein & polypeptide.

Colon

For absorption of water & electrolytes & storage organ. Contractile activity : Segmentation. Peristaltic contraction

Gut hormones and peptides

Gut hormones and peptides (2)

Gut hormones and peptides (3)

Investigations of GIT diseases
Tests of structures
Tests of infection
Tests of function
Imaging
Histology
US, CT MRI
Endoscopy
Contrast studies
Plain Radiograph
Bacterial culture
Serology
Breath Tests
Pancreatic Exocrine function
Mucosal Inflammation/ permeability
Absorption
GIT Motility
Radioisotope Tests



It is useful in diagnosis of intestinal obstruction or paralytic ileus The outlines of soft tissues e.g. liver, spleen kidneys may be visible Calcification in the abdominal structures as well as calculi can be detected Abdominal radiographs are not useful in GIT bleeding CXR shows the diaphragm and erect films may detect sub-diaphragmatic free air in cases of perforation

Normal Plain Abdominal Radiograph

Normal Plain Abdominal Radiograph showing the identification of transverse colon

Air under the diaphragm (perforated DU)

Small Intestinal obstruction (multiple fluid levels)

Hiatus hernia (fluid levels behind the heart)

Calcification of the pancreas (chronic pancreatitis)



Toxic megacolon