GASTROINTESTINAL DISEASES --- INTRODUCTION
المحاضر: الدكتور خلدون ذنون- كلية طب نينوى- المرحله الرابعهObjectives
To know basic anatomy of the GIT.To understand functions of each part of the GIT.
To know simple facts about digestion and absorption of food elements.
OESOPHAGUS
Muscular tube 25 cm long,extends from the cricoid cartilage to the cardiac orifice.Upper sphincter (cricopharyngeus muscle) &lower sphincter.
Function : swallowing- upper sphincter relaxes,peristaltic activity is controlled by brain stem centre by autonomic nerves,the lower sphincter relaxes.
GASTRIC SECRETIONS
Parietal cells secrete HCL in response to H+,K+,ATP ase (proton pump) activity.Acid sterilizes the upper GIT & convert pepsinogen secreted by chief cells to pepsin.
Intrinsic factor is a glycoprotein secreted with the acid,needed for vitamin B12 absorption.
Gastrin secreted by G cells in the stomach stimulate acid secretion.
Somatostatin : secreted from D cells throughout the stomach , suppresses acid secretion.
Bicarbonate and mucus protect gastro duodenal mucosa against the effect of acid and pepsin.
Vagal cholinergic activity + histamine + gastrin ( acid secretion . protein + amino acids + gastric distension + thinking & smell of food , all ( acid secretin.
Vasoactive intestinal polypeptide VIP and gastric inhibitory poly- peptide GIP + somatostatin inhibit G cells & reduce acid output .
SMALL INTESTINE
Extends from the ligament of Ttreitz to the ileocaecal valve .
Fasting inhibit intestinal activity and food stimulate it.
Function : digestion,absorption,protection against pathogens& ingested toxins through immunological,enzymatic,mechanical & peristaltic function.
FAT
Long chain triglyceride , cholesterol , fat soluble vitamins A,D,K,E .
Stomach : fat emulsification,lingual lipase digest TG into diglyceride & fatty acids- limited role .
Duodenum : secretin is released in response to acid ,stimulate pancreatic bicarbonate, fat stimulate cholecystokinin secretion which stimulate gall bladder contraction & relaxes the sphincter of Oddi . Bile emulsify lipids to form chyme.
Upper jejunum : pancreatic lipase and colipase hydrolyse TG to monoglycerides and free fatty acids.phospholipids& cholesterol esters are hydrolyzed by other pancreatic enzymes. Mixed micelles are formed .
Distal small intestine : bile salts are absorbed in the terminal ileum i.e enterohepatic circulation .within enterocytes fatty acids, mono- glycerides&diglycerides are re-esterified to form TG which are coated with apoproteins,phospholipids&cholesterol in the ER to form chylomicrons which leave the cells by exocytosis and then enter the portal circulation via lymphatics.
CARBOHYDRATES
Starch , sucrose , lactose .
Starch : hydrolyzed by salivary and pancreatic amylase to 4 8 glucose molecules , disaccharide maltose &trisaccharide maltotriose
Disaccharides are digested by enzymes fixed to microvillus membrane to form monosaccharides glucose,galactose&fructose.
PROTEIN
Intragastric digestion by pepsin.CCK from proximal jejunum stimulate pancreatic trypsinogen which is activated into trypsin . Pancreatic chymotrypsin & elastase also digest proteins , small peptides & amino acids are formed .
peptidases on microvilli digest polypeptides to dipeptides & amino acids which are absorbed by enterocytes.
cytosolic peptidases in enterocytes digest dipeptides to amino acids .
WATER & ELECTROLYTES
Absorption and secretion of water& electrolytes occur through out the intestine.
Absorption predominate , transport occur through paracellular tight junctions through a gradiant & also through transcellular route by active pumps.
Vitamins & trace elements
Protective function of the small intestine
B & T lymphocytes , macrophages , mast cells are found through out GIT mucosa .
Mucosa associated lymphoid tissue constitute 25% of the total lymphatic tissue of the body.
Antigens e.g viruses are transported by M cells to Peyers patches B-lymphocytes differentiate to plasma cells which release IgA to neutralize the antigens .
T-lymphocytes localize the the plasma cells to the site of the antigen.
Macrophages are phagocytic and release cytokines.
mucosal barrier: composed of mucus secreted by goblet cells , membrane of enterocytes,tight junctions between these cells , renewed every 48 hrs.
PANCREAS
Exocrine function : digest fat , CHO & proteins.Bicarbonate rich fluid produce alkaline PH for enzymatic activity.
Enzymes : amylase , lipase , colipase , trypsinogen , chymotryps-inogen , proelastase & procarboxypeptidase.
COLON
Absorb water and electrolytes.Two types of contraction , segmentation (ring contraction) lead to mixing that enhance water&electrolytes absorption .
Propulsive (peristaltic) contraction cause mass movement .
All activities are stimulated after meal in response to motilin & CCK
Control of GIT function
Secretion,absorption,motor activity are modulated by nervous and hormonal factors.
GUT HORMONES
Gastrin : stimulate gastric acid secretion, secreted by G-cells .
Somatostatin : inhibit gastrin&insulin secretion,secreted by D-cells.
Cholecystokinin : stimulate pancreatic enzyme secretion and gall bladder contraction.
Secretin : stimulate pancreatic fluid and bicarbonate secretion.
Vasoactive intestinal polypeptide VIP : regulate blood flow.
Others : motilin , GIP , pancreatic poly peptide which inhibit pancreatic secretion, ghrelin (stomach), guanylin (small intestine).