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Physiology of Digestion and Absorption 1. Professor John Peters E-mail: j.a.peters@dundee.ac.uk. Learning Objectives. After this lecture students should be able to: Describe the nature of the chyme List the hormones released from the duodenal mucosa - PowerPoint PPT Presentation
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Physiology of Digestion and Absorption 1
Professor John PetersE-mail: j.a.peters@dundee.ac.uk
After this lecture students should be able to:
Describe the nature of the chyme List the hormones released from the duodenal mucosa Relate function to basic structure/cell type on drawings of the pancreas Compare salivary and pancreatic secretion - primary and modified
secondary secretion List the composition of pancreatic secretion Indicate briefly how pancreatic enzymes are activated Describe the importance of bicarbonate State the importance of bile in fat digestion Describe briefly the hormonal controls of biliary and pancreatic
secretions in the cephalic, gastric and intestinal phases of digestion
Learning Objectives
The Small Intestine Major site for digestion and absorption
Approx. 6 m long, 3.5 cm diameter (longer when relaxed - after death)3 parts Duodenum – approx. 25 cm Jejunum – approx. 2.5 m Ileum – approx. 3 m
Receives chyme from stomach pancreatic juice from pancreas bile from gall bladder
Secretes intestinal juice
Moves remaining residues to the large intestine
Duodenum secretes (into the blood) various peptide hormones from endocrine cells within the mucosa:
Gastrin – from D cells of gastric antrum (mainly) and duodenum Cholecystokinin (CCK) – from I cells of duodenum and jejunum Secretin – from S cells of duodenum
Motilin – from M cells of duodenum and jejunum Glucose-dependent (or glucagon-like) insulinotropic peptide (GIP)
– an incretin from K cells of duodenum and jejunum [note activity is potentiated by gliptins (e.g. sitagliptin) used in treatment of type 2 diabetes mellitus]
Secretions of the Small Intestine (1)
All act on G-protein coupled receptors
Glucagon-like peptide-1 (GLP-1) – an incretin from L cells of small and large intestine [note activity mimicked by extenatide and potentiated by gliptins (e.g. sitagliptin), both used in treatment of type 2 diabetes mellitus]
Succus (juice) entericus (of the intestine) – approximately 2 litre secreted per day - composition varies throughout small intestine
Secretions of the Small Intestine (2)
Control mechanisms include Distension/irritation, gastrin, CCK, secretin, parasympathetic nerve
activity (all enhance), sympathetic nerve activity (decreases) Secretion contains
mucus – for protection/lubrication (from goblet cells) aqueous salt - for enzymatic digestion (mostly from the crypts) no digestive enzymes
Secretion involves
Na+/K+ ATPase
Na+/K+/2Cl- co-tranporterChloride channel (CFTR)
Nb. Excessive activity causes secretory diarrhoea (as in cholera)
Mixing and Propulsion of Chyme (1)1. Segmentation (mixing)
“chopping” moves chyme back and forth – very vigorous after a meal (little / none between meals)
Alternating contraction and relaxation of segments of circular muscle Initiated by small intestine pacemaker cells causing the BER which is
continuous. At threshold activates segmentation which in the duodenum is primarily due to distension by entering chyme
Duodenum has frequent segmentation contractions (12 per min), ileum has fewer (9 per min) net movement is aboral
Segmentation in the empty ileum is triggered by gastrin from the stomach (gastroileal reflex)
Mixing and Propulsion of Chyme (2) Movement is slow - takes 3-5 hrs - allows time for absorption Strength of segmentation enhanced and decreased by parasympathetic
and sympathetic activity, respectively
2. PeristalsisTwo activities occur in the interdigestive, or fasting, state
A few localised contractions The migrating motor complex (MMC)
Occurs between meals every 90 – 120 minutes Strong peristaltic contraction passing length of the intestine
(stomach ileocaecal valve) Clears small intestine of undigested debris, mucus and sloughed
epithelial cells between meals – ‘housekeeper function’ Inhibited by feeding and vagal activity Triggered by motilin, suppressed by gastrin and CCK
Nb. Macrolide antibiotics (e.g. erythromycin) mimic the action of motilin and may cause unpleasant G.I. disturbances
Pancreatic Secretions Endocrine – insulin and glucagon – secreted to blood Exocrine – digestive enzymes (acinar cells), aqueous NaHCO3
- solution (duct cells) – secreted to the duodenum collectively as pancreatic juice
Pancreatic Enzymes
Trypsinogen
Acinar cellsEnzymes stored in
zymogen granules Duodenum
Enterokinase (mucosal cells)
Trypsinogen Trypsin+
Chymotrypsinogen
Procarboxypeptidase
Autocatalysis
Chymotrypsinogen
Procarboxypeptidase
Chymotrypsin
Carboxypeptidase
+
+
Proteases
AmylasesPancreatic amylase Pancreatic amylase
LipasesPancreatic lipase Pancreatic lipase
Inactive enzyme
Active enzyme
Can completely digest food in the absence of all other of enzymes
Secretion of the Pancreatic Duct Cells Duct cells secrete 1 – 2 litre of alkaline (HCO3
- - rich) fluid into the duodenum per day
Neutralises acidic chyme entering the duodenum Provides optimum pH for pancreatic enzyme function Protects the mucosa from erosion by acid
K+
Na+
Na+
H+
K+
Cl-Cl-
HCO3-HCO3
-
CO2
CO2 + H2O
Carbonic anhydrase
H2CO3
Na+, H2O Na+, H2O
Duc
tule
Secretion involves
Na+/K+ ATPase
Na+/H+ exchanger
K+/H+ ATPase
Cl-/HCO3-
exchanger
Chloride channel (CFTR)*
*Nb. Patients with cystic fibrosis have reduced fluid secretion
Control of Pancreatic Secretion
Acid in duodenal lumen
Secretin release from S cells
Pancreatic duct cells
Secretion of aqueous NaHCO3
solution into duodenal lumen
Secretin carried by blood Neutralizes
Fat and protein in duodenal lumen
CCK release from I cells
Pancreatic acinar cells
Secretion of digestive enzymes
into duodenal lumen
CCK carried by blood Digests
Three phases: Cephalic – mediated by the vagal stimulation of mainly the acinar cells (20% total
secretion) Gastric – gastric distension evokes a vagovagal reflex resulting in parasympathetic
stimulation of acinar and duct cells (5-10% total secretion) Intestinal (see below; 70-80% of total secretion)
The Biliary SystemComprises liver, gall bladder and associated ducts
Bile Produced continuously by the liver 0.6 – 1.2 litre per day
Between meals Stored and concentrated in gall bladder (sphincter of Oddi closed)
During a meal Chyme in duodenum stimulates gall bladder smooth muscle to contract Sphincter of Oddi opens Bile spurts into duodenum via cystic and common bile ducts
Bile composition Secretion of bile duct cells Secretion of hepatocytes
Secretions mix in the ducts
Bile participates in the digestion and absorption of fats
Acid in duodenal lumen
Secretin release from S cells
Pancreatic duct cells
Secretion of aqueous NaHCO3
solution into duodenal lumen
Secretin carried by blood Neutralizes
Hepatocytes and duct cells
Secretion NaHCO3 – rich bile into
duodenal lumen
Gastric secretion Gastric emptying
Integrative Activity of Secretin
Fat and protein in duodenal lumen
CCK release from I cells
Pancreatic acinar cells
Secretion of digestive enzymes
into duodenal lumen
CCK carried by blood Efficient Digestion
Gall bladder and sphincter of Oddi
Contraction of Gall bladder
Relaxation of sphincer of Oddi
Gastric secretion Gastric emptying
Integrative Activity of CCK
Recommended