Digestive systemDigestive systemDigestive system

Dr. Carmen E. RexachDr. Carmen E. RexachPhysiologyPhysiology

Mt San Antonio CollegeMt San Antonio College

Functions

• Motility– ingestion– mastication– deglutition– peristalsis

• Secretion– exocrine– endocrine

• Digestion• Absorption

Tunics

Innervation• Autonomic nervous system• Parasympathetic (vagus nerve)

– motility– secretion

• Sympathetic– decreases peristalsis– secretion– contraction of sphincters

Esophagus• Collapsible tube• first 1/3

– Skeletal muscle• last 2/3

– Smooth muscle• gastroesophageal

sphincter

Stomach• Function

– food storage– initiates protein

digestion– kills bacteria– moves food to SI– intrinsic factor– Digestion and

absorption• proteins• alcohol/aspirin

Gastric Glands

• Goblet cells– mucus

• Parietal cells– HCl

• Chief cells– pepsinogen

• Argentaffin cells– Seratonin (correct)

• G cells– gastrin

pH• 3 functions of pH in

stomach– denature ingested

protein– convert pepsinogen to

pepsin– destroy bacteria

Protective mechanisms• Impermeability of gastric mucosa to

CO2 and NH3

• alkaline mucus• tight junctions• rapid cell division and replacement• protective effects of prostaglandins• In doudenum

– bicarbonate from pancreatic juice – Brunner’s glands

• Secrete mucinous alkaline solution

Peptic ulcers– Gastric ulcers

• Weakened defense mechanisms– Duodenal ulcers

• Increased acid and pepsin production– Helicobacter pylori

• Major factor in both types• Present in 100% of gastric ulcer patients and 95% of

duodenal ulcer patients– Exceptions: Those whose ulcers are the result of overuse of

NSAIDS = inhibits prostaglandin production• Produces NH3 and urease

– damages epithelium and allows H+ to permeate• Also linked to increased acid secretion

Small intestines

• Length• 3 divisions

– duodenum– jejunum– ileum

• absorption– doudenum + jejunum– ileum

• system of folds– microvilli, villi, plicae circularis

Villi

microvillivillus• Columnar epithelium

• goblet cells• crypts of Lieberkuhn

– Glands that secrete intestinal juice at base of the villi

• brush border– surface area– enzymes– enterokinase

capillaries

nerve

lactealarteriole

arteryMuscularismucosa

veinLymphduct

Intestinal motility• Major types of

contractions– peristalsis– segmentation

• Pacesetter potential– smooth muscle cells– can lead to a/p– parasympathetic and

sympathetic influences

• Relaxation– NO

Large Intestine

• Structure• Haustra• Fluid and electrolyte absorption

– 90% in SI– passive osmosis– water secretion

• Defecation

Liver

Liver lobules: functional unit

Functions of liver• Exocrine• Endocrine• Clotting functions• Plasma proteins• Organic metabolism• Cholesterol metabolism• Excretory and degradative functions

Exocrine & Endocrine• Synthesis and

secretion of bile salts– 250-1500ml/day

• Adds bicarbonate rich solution to bile

• Secretes IGF-1 = promotes cell division

• Forms T3 from T4• Secretes

Angiotensinogen• Metabolizes hormones• Secretes immune

cytokines

Clotting and Plasma proteins• Produces

prothrombin & fibrinogen

• Produces bile salts needed for vitamin K absorption

• Produces plasma albumin, acute phase proteins, binding proteins, and lipoproteins

Organic metabolism• Converts plasma glucose to glycogen and

triglycerides• Converts amino acids to fatty acids• Produces triglycerides and secretes them

as lipoproteins• Gluconeogeneisis and glycogenolysis• Converts fatty acids into ketones• Produces urea

Cholesterol metabolism/excretory & degradative functions

• Synthesizes cholesterol

• Secretes plasma cholesterol into bile

• Converts plasma cholesterol into bile salts

• Secretes bile pigments

• Excretes toxins via bile

• Destroys old erythrocytes

• …and lots, lots more!

Gallbladder & pancreas

Pancreaticduct

Pancreas

Doudenum

Duodenalpapillae

gallbladder

Commonbile duct

Hepatic ducts

Pancreas

• Endocrine = islets of Langerhans– insulin and glucagon, etc.

• Exocrine = pancreatic acini– pancreatic juice– composition

• water• bicarb• digestive enzymes

– role of enterokinase

Neural and endocrine regulation• Long reflex

– preconditioning• Short reflex

– more local• GI hormones• Extrinsic controls

– 3 phases (cephalic, gastric, intestinal)• some events without neural or

endocrine control

GI hormones• Gastrin• Secretin• CCK• glucose-dependent insolinotropic

peptide (GIP)• Glucagon-like peptide 1 (GLP-1)• Guanylin

Gastrin• endocrine cell location

– antrum of stomach• stimulus for release

– amino acids, peptides in stomachparasympathetic nervous system

• actions– stimulates secretion of HCl and

pepsinogen

Secretin• Endocrine cell location

– small intestine• stimulus for release

– acid in small intestine• action

– stimulate pancreatic bicarbonate secretion

– potentiate CCK-stimulated pancreatic enzyme secretion

Cholecystokinin (CCK)• Endocrine cell location

– small intestine• Stimulus for release

– amino acids, fatty acids in small intestine

• Action– contraction and emptying of the gall

bladder– stimulates release of pancreatic

enzymes– (in the brain: acts as satiety hormone)

Glucose-dependent insulinotropic peptide (GIP)

• Endocrine cell location– small intestine

• Stimulus for release– glucose, fat in the small intestine

• Action– inhibits gastric emptying– potentiates insulin release

Glucagon-like peptide 1 (GLP-1)

• Endocrine cell location– ileum and colon

• Stimulus for release– oral ingestion of nutrients– glucose, fats, amino acids (mixed meals)

• Action– inhibits gastric motility– stimulates insulin secretion

Guanylin

• Endocrine gland location– ileum and colon

• Action– stimulates intestinal Cl- release– result: increased NaCl and water in

feces

Cephalic phase• First 30 minutes

– Smell, sight, and taste of food stimulates vagus nuclei of brain (long reflex)

• vagus nerve stimulates:– parietal and chief cells– Gastrin secretion by G cells

• Also stimulates parietal and chief cells

Gastric phase• Begins when food arrives in stomach• Stimulus

– distension– chemical composition of chyme

• short polypeptides and amino acids in stomach– Positive feedback, cells secrete gastrin– HCl and pepsinogen are released

• Glucose– No effect

• Lipids– Inhibition of gastrin secretion– pH<2.5 negative feedback inhibition of gastrin

Intestinal phase• Begins when chyme enters small intestines• neural inhibition of gastric emptying and acid

secretion due to:– increased osmolality– stretching

• enterogastrone– stimulus: fat in chyme– inhibits gastric acid secretion

• GIP = insulin secretion if glucose is present• CCK = inhibits gastric emptying when chyme

in duodenum

Intestinal reflexes

• Gastroileal reflex– increased gastric activity = increased

motility of ileum = increased chymethrough sphincter

• ileogastric reflex– ileal distension = decreased gastric

motility• intestino-intestinal reflex

– one segment overdistends = rest relax

Regulation pancreatic juice & bile• Neural & hormonal• Pancreatic juice

– secretin• pH <4.5 in duodenum• stimulates bicarbonate production by pancreas• cAMP as a second messenger

– CCK• fat content in chyme• production of pancreatic enzymes• Ca++ as second messenger

• Secretion of bile– continuous secretion– gallbladder contraction under influence of CCK and

secretin

Digestion and absorption of carbohydrates

• Mouth– salivary amylase

• Stomach– enzymes denatured

• Small intestines– pancreatic amylase = maltose, maltriose– brush border enzymes =

monosaccharides– secondary active transport with Na+

– Into capillaries in villus

Mouth:Salivary amylase

Stomach:Enzymes denatured

Duodenum:Pancreatic amylaseBrush border enzymes

Digestion of Carbohydrates

Absorption of carbohydrates

Digestion and absorption of proteins

• Mouth– nothing

• Stomach– Pepsin = short chain polypeptides

• Small Intestines– trypsin, chymotrypsin, elastase– carboxypeptidase– aminopeptidase = from brushborder– across into blood– Whole proteins

• babies• botulinum toxin

Free amino acidsDipeptidestripeptides

Digestion of proteins

Stomach:Pepsin and HCl

Small intestines:Trypsin, chymotrypsinCarboxypeptidaseBrush border:aminopeptidases

Protein absorption

-Amino acids move into enterocytes by counter-transport, in exchange for two Na+. -They are absorbed into a capillary bed on the basal side of the cell and taken to the liver via the hepatic portal system.

aa

Na+

Digestion and absorption of lipids

• Emulsification• pancreatic lipase• Mixed micelles to brushborder• transport

– across into epithelial cells– can be moved inside cell– chylomicron into lacteal

Digestion of Fat

Small intestines:Pancreatic lipase

lacteal

Lipid transport• From lymphatics to thoracic duct• Triglycerides removed by lipoprotein lipase

on endothelial membranes• Free fatty acids and glycerol into tissues• Leftovers to liver

– Remnant particles contain cholesterol– Combined with apoproteins to produce VLDL’s– Deliver triglycerides to other organs– Later converted to LDL’s when triglyceride has

been removed

Lipoproteins• What are they?

– Lipid & protein complexes– Transport cholesterol & triglycerides in blood– Protein allows hydrophobic lipids to remain in

suspension • Five classes: Based on density, molecular weight, size,

chemical composition• Chylomicrons• VLDL• IDL• LDL

– High levels associated with increased risk CVD• HDL

– Low levels associated with increased risk of CVD– Best profile = high HDL, low LDL

Treatment for morbid obesity

Lapband

Roux-en-Y Gastric Bypass