Download Metabolism and Digestion-Lecture 3-Physiology of The Small Intestine

Human Form & Function 2
Physiology of the Small Intestine
Dr. Neil Docherty My Teaching Objec6ves •  Describe the various patterns of intestinal motility and how
they contribute to digestion and absorption
•  Emphasise the role of both enzymatic and non-enzymatic
processes and luminal and mucosal events in the digestive
process
•  Detail the mechanisms involved in the absorption of major
food groups in the small intestine and discuss the
phenomenon of malabsorption
Intes6nal Wall Structure (Adapta6on for Diges6on and Absorp6on) SURFACE AREA
SECRETIONS
MOTILITY
DUODENUM
Add secretions
JEJUNUM
digest/absorb
ILEUM
specialised absorption
Folds, Villi and Microvilli
Structure Matches Function in The Small Intestine
Surface area for digestion/absorption masssively increased by folding of mucosa;
-Plicae Circulari
-Villous Units
-Microvillous apical membrane of absorptive enterocyte
Folding amplifies luminal surface area x600
Dietary Nutrients
Water soluble
Lipid soluble
Carbohydrates
Fats
Proteins
Vitamins (A,D,K,E)
Vitamin C
Vitamin B12
Trace Elements
(Iron and calcium are good examples for integrated
Physiology)
N.B. In order to digest and absorb the intestine wall and associated glands
must produce a large (>7 litres) volume of secretions which require
to be reabsorbed to maintain vascular volume and pH
Dietary Carbohydrate in Duodenum
(Luminal Digestion)
Salivary Gastric Intestinal
Starch
Amylose
Amylopectin
Amylase
Disaccharides
Sucrose
Lactose
Monosaccharides
Glucose
Galactose
Fructose
Insoluble material
Dietary fibre
N.B. Can be metabolised by colonic
Bacteria to produce SCFAs
Brush Border Hydrolysis
and Absorption at Jejunal Mucosal Surface
DIGESTION
Membrane bound disaccharidases
-Sucrase
-Maltase
-Lactase
ABSORPTION
-Glucose and Galactose
(Secondary Active transport)
-Fructose
(GLUT 5)
DELIVERY TO BLOOD
-GLUT 2
N.B relevant to
cholera treatment
Protein Digestion
= essential amino acids
Not all amino acids can be de novo synthesised in body
and so must be provided in the diet
Gastric and Duodenal Proteolysis
STOMACH
Chief cells /Parietal cells
Pepsin
Dietary
protein
Large peptides
Free neutral amino acids
DUODENUM
Generation of Peptides and Free AA’s
Digestion products are amino acids and di/tri-peptides
Brush border aminopeptidases derive amino acids for sodium coupled transport
Short peptides can be taken up via peptide/proton symport and
hydrolysed intracellularly
Peptide Absorption
lumen
blood
-For nutrient uptake, note the importance of the Na+ gradient
-Component of the resting membrane potential (RMP)
-N.B. Membrane potential not only important for excitable cell function!
Classes of Dietary Lipid • 
• 
• 
• 
Long chain triglycerides
Phospholipids
Sterols
Cholesterols (endogenous production 4>10 fold
higher than dietary intake!)
•  Fat soluble vitamins
Lipid Diges6on 1 Triglycerides (TAG)
(Lingual, gastric and pancreatic lipases)
Free fatty acids
(FFA)
Diglycerides
(DAG)
Monoglycerides
(MAG)
Fat Diges6on and Bile Flow Lipid droplets
TAG
Gastric
lipase
Duodenal CCK
release
FFA
+
+
+++
Gall bladder
contraction
Pancreatic lipase
and bile acid in small
Intestinal lumen
Acinar cell
secretion
More info in next
lecture
Lipase Activity In
The Small Intestine
N.B
Requires colipase co-factor activity for
function in bile acid coated lipid droplets
Role of Micelle Forma6on •  FFAs/monoglycerides are quite water soluble
•  However, some vitamins and other lipid moieties remain
insoluble
•  This prevents approximation to enterocyte for absorption
•  Carriage in mixed micelles allows solubilisation and
transport to apical membrane of enterocyte
Intracellular Processing of Lipid in the Enterocyte Absorbed lipid
Intracellular fatty acid binding proteins
sER
Re-esterification
Golgi
Apolipoprotein coating
Chylomicron exocytosis at basolateral membrane
-lymphatic system-systemic circuit-lipolysis-clearance
Water soluble Vitamin Uptake •  Vitamin C-­‐ Sodium-­‐Vitamin C co-­‐transporter •  Vitamin B12-­‐Intrinsic factor-­‐Ileal VitB12 transporter Vitamin B12 Absorption
Clinical Correlate
Pernicious
Anaemia
Destination of Absorbed
Materials
TAGs
Via lymphatics
And thoracic duct
Glucose
Amino acids
SCFAs
Soluble vitamins
Malabsorp6on & Deficiencies •  Transit )me (infec)ons, IBS) •  Exocrine pancrea)c insufficiency (CF, see next lecture) •  Gallbladder disease (gallstones) •  Inflammatory damage to mucosa (IBD) •  Resec)on induced deficiencies (Crohn’s Disease) •  Bacterial overgrowth and deple)on of nutrient
(Crohn’s Disease) Small intestinal motility in the fed state
Purpose: Mixing of chyme with intestinal secretions
Propulsion aborally
• Two steps forward,one back
• Gradual aboral propulsion of chyme
• Time permitted for absorption to occur
• Superimpose with peristaltic contractions
Segmenta6on and Peristalsis 1. Segmentation
(local back and forth mixing)
2. Peristalsis
(aboral)
of circular muscle
Peristaltic Propulsion and Muscle Dynamics
Food bolus activates afferent sensory neurones
(mechanoreceptors) which, via inter-neurones affect the output of
intrinsic enteric motor neurones. This consequently modulates
muscle contraction
PROXIMAL: CIRCULAR MUSCLE SHORTENS, LONGTIDUDINAL
MUSCLE RELAXES
DISTAL:LONGTIDUDINAL MUSCLE SHORTENS, CIRCULAR
MUSCLE RELAXES
Fasted Motility Patterns
INTESTINAL MIGRATORY MOTOR COMPLEX PROPAGATION
MOTILIN REGULATED
Ileus • 
• 
• 
• 
• 
Temporary or permanent inhibi)on of mo)lity Func)onal obstruc)on Obstruc)ve symptoms, malabsorp)on Commonly induced by abdominal surgery Nil by mouth un)l mo)lity returns N.B. Make a conceptual link between ileus and gastroparesis in terms of
neuropathy
Your Learning from Today Should focus on being able to;
•  Describe how intestinal motility patterns contribute to
digestion and absorption
•  Link enzymatic and non-enzymatic processes and luminal
and mucosal events in the digestive process
•  Detail the mechanisms involved in the absorption of major
food groups in the small intestine
•  Demonstrate an understanding of how various pathologies
can cause malabsorption and nutritional deficiency