Digestive PhysiologyDigestive Physiology

Physiological functions of the

small intestine , liver & pancrease

By: M.H.Dashti

Lecture 5

Anatomy of the Small IntestineAnatomy of the Small Intestine

20 feet long----1 inch in diameter Large surface area for majority of

absorption 3 parts

– duodenum---10 inches– jejunum---8 feet– ileum---12 feet

ends at ileocecal valve

Secretions of the small intestinemany villi on surface of intestine : 2 types of glands

1 -Brunner’s glands- compound mucus glands ,

Secreting Alkaline mucus-Stimulated by

parasympathetic-Inhibited by sympathetic?-stress related ulceration2 -crypts/glands of Lieberkuhn

between villi

Exocrine from Enterocyte

Endocrine: Secretin (S) , cck ( I ),GIP ( K ) ,VIP ( v ) ,Motilin ( mo )

Digestive enzymes not secreted from small intestine except enterokinase secreted from duodenal mucosa

Crypt of Lieberkuhn

Mucous cells(simple goblet cells)

Enterocyte

(absorptive cells with brush border)

Villus

Paneth cell(have Secretory granules of lysozyme)

Endocrine cells(secrete hormones for control of gut function)

under local nervous control

some minor hormonal control (VIP ,secretin,

CCK )

Secretin

Fat

HCO3-

H+

Secretin

S cells

stomach

Bile S.I.

Cholecystokinin

I cellsCCK

Peptides Amino acids ,

Enzymes

Fat

stomach

pancreas

duodenum

InsulinGlucagon

+

Stomach

Bile S.I.

Other intestinal hormonesOther intestinal hormones GIP (Gastric Inhibitory peptide ) from k cells

– Stimulated by carbohydrates in duodenum – Stimulate insulin secretion

VIP ( Vasoactive Intestinal Peptide )– Stimulated by H+ in duodenum– Inhibit gastric secretion & Stimulate intestinal secretion – VIPoma

Motilin– Stimulated by H+ in duodenum– Co-related to Migratory Myoelextric Complex ( MMC )

Small intestine motilitySmall intestine motility

Weak peristalsis in comparison to the stomach---chyme remains for 3 to 5 hours

Segmentation---local mixing of chyme with intestinal juices---sloshing back & forth

Bile ProductionBile Production

Components: water – Cholesterol , FA &

Lecithin– bile salts = Na & K salts

of bile acids – bile pigments (bilirubin)– Alkalinephosphatase

Gall bladder concentrates bile by absorbing water & Na-cl

0.5-10.5-1 Lof bile/day is Lof bile/day is secretedsecretedyellow-green in color yellow-green in color & pH 7.6 to 8.6& pH 7.6 to 8.6

Bile production & enterohepatic circulationBile production & enterohepatic circulation

Bilirubin is produced by the metabolism of Heme& is transported to the liver by albumin

Gallstones Gallstones Calcium -

bilirobinate due to

glucoronidase activity of bacteria

Cholesterol stones due to increased Cholesterol/PLratio

Pancreatic Secretions & their RegulationPancreatic Secretions & their Regulation Both endo. & exocrine Secretions are under neural

& hormonal control Regulated in 3 phases Secretin

– increased sodium bicarbonate release from ducts

CCK– increased digestive enzymes

& hormones ( I & G ) release ( EGSF )

GIP– Stimulated by fatty acids &

sugar in S.I. – causes increased insulin

release (EISF )

Regulation of pancreatic acinar secretionRegulation of pancreatic acinar secretion

Cholera toxin

Delivers Inositol tri-phosphate

Adenylatecyclase

Sites of pancreatic secretionsSites of pancreatic secretions Acinar fluid is isotonic &

its ionic composition resembles the plasma

Spontaneous secretion of interalobular ducts have greater K+ & HCO3

- than plasma Secretin stimulated

secretion from the cells of extra lobular ducts are still more rich in HCO3

-

Cellular mechanisms for HCO3- rich

secretion by pancreatic acinar cells

Cellular mechanisms for HCO3- rich

secretion by pancreatic acinar cells

K+

H2O

H2O

Cellular mechanisms for HCO3

- rich secretion by pancreatic extra lobular ductal cells

Stimulated by secretin , cck & Ach

H2O

Ionic concentration of pancreatic juice as a function of its flow rate

Ionic concentration of pancreatic juice as a function of its flow rate

Pancreatic digestive EnzymesPancreatic digestive Enzymes

Enzyme Activator Substrate Catalytic Function or

ProductsTrypsins (trypsinogens) Entero -

peptidaseProteins and polypeptides

Cleave peptide bonds adjacent to aromatic amino acids

Chymotrypsins (chmo -trypsinogens)

Trypsin Proteins and polypeptides

Cleave peptide bonds adjacent to arginine or lysine

Elastase (proelastase) Trypsin Elastine, some other proteins

Cleave bonds adjacent to aliphatic amino acids

Carboxypeptidase A (pro-carboxypeptidase A)

Trypsin Proteins and polypeptides

Cleave carboxyl terminal amino acids that have aromatic or branched aliphatic side chains

Enzyme Activator Substrate Catalytic Function or Products

Carboxypeptidase B (pro- carboxypeptidase B)

Trypsin Proteins and polypeptides

Cleave carboxyl terminal amino acids that have basic side chains

Colipase (procolipase)

Trypsin Fat droplets Binds to bile salt-triglyceride –water interface, making anchor for lipase

Pancreatic lipase … Triglycerides Monoglycerides and fatty acids

Cholesterol ester hydrolase

… Cholesteryle esters

Cholesterol and fatty acids

Pancreatic α-amylase

Cl- Starch Same as salivary α-amylase

Ribonuclease … RNA Nucleotides

Deoxyribonuclease … DNA Nucleotides

Phospholipase A2

(pro- phospholipase A2)

Trypsin Phospholipids Fatty acids, lysophospholipids

Pancreatic digestive Enzymes (Continue)