Digestive System
Dr Raghuveer Choudhary
Food is vital to life because:
INTRODUCTION TO DIGESTION
provides energyprovides building blocks for growth and maintenance
Why do we need a digestive system?food comes to us as complex molecules
complex molecules are too large to be absorbed into the blood
What are the complex molecules and what do we do with them?
carbohydrates monosaccharides (simple sugars)proteins amino acidsfats fatty acids, glycerols, glycerides
MAJOR FOOD COMPONENTS
•Carbohydrates•Proteins•Fats•Water•Ions/Minerals/Fibers/Enzymes
Digestive SystemDIGESTIVE TRACT – tube ~ 9 m longGastrointestinal tract or Alimentary canalDIGESTIVE ORGANS - Oral cavity,
pharynx, esophagus, stomach, small intestine, large intestine, anal canal
ACCESSORY ORGANS - teeth, tongue, salivary glands, liver, gallbladder, pancreas
Gastrointestinal tract
ORGANIZATION
AKA = alimentary canal
Accessory organsTeethTongueSalivary glandsLivergallbladderpancreas
mouthpharynxesophagusstomachsmall intestinelarge intestine
mouth (oral cavity) pharynx
esophagus
stomach
anus
large intestine
small intestinerectum
appendix
liver
pancreas
salivary glands
gallbladder
Digestive functions & processes
FOUR FUNCTIONS1. Ingestion – selective intake of nutrients2. Digestion – breakdown of molecules;
mechanical & chemical3. Absorption – uptake of nutrients by
digestive tract, blood & lymph4. Defecation – elimination of undigested
residue
Digestive Processes1. Motility – muscle contractions
– Break up food – mastication, churning of stomach
– Propel thru canal - peristalsis– Mix with digestive enzymes - segmentation
2. Secretion – enzymes, hormones to carry out or regulate digestion
3. Membrane transport – absorbing nutrients to blood and lymph
Stages of DigestionMECHANICAL DIGESTIONPhysical breakdown Teeth – cutting and grinding - mastication –
(chewing)Churning – stomach & small intestineAction of digestive enzymes – food to small
particles
CHEMICAL DIGESTION Macromolecules to monomers Polysaccharides to monosaccharides Proteins to amino acids Fats to glycerol and fatty acids Nucleic acids to nucleotides Done by digestive enzymes produced by: Salivary glands, stomach, pancreas, small intestine Absorbed – no digestion – vitamins, free amino
acids, minerals, cholesterol, water
Forms of Nutrients
In Food Absorbable by Intestine
Protein Amino acidsCarbohydrates Monosaccharides (glucose)Fat Fatty acids, glycerol DNA, RNA Bases + monosaccharides Vit B12 B12+intrinsic factor Other vitamins Original formCholesterol Original formElectrolytes Original form Water Original form
Major Physiologic Processes of the Gut
Motility Secretion Digestion Absorption Elimination
Secretion Delivery of enzymes, mucus, ions and the like into the lumen, and
hormones into blood.
Absorption
Transport of water, ions and nutrients from the lumen, across the epithelium and into blood.
MotilityContractions of smooth muscle in the wall of the tube that crush,
mix and propel its contents.
Motility:– Movement of of food through the GI tract.
Ingestion:– Taking food into the mouth.
Mastication:– Chewing the food and mixing it with saliva.
Deglutition:– Swallowing the food.
Peristalsis:– Rhythmic wave-like contractions that move food through
GI tract.
Functions of the GI Tract
Secretion:– Includes both exocrine and endocrine secretions.
Exocrine:– HCl, H20, HC03
-, bile, lipase, pepsin, amylase, trypsin, elastase, and histamine are secreted into the lumen of the GI tract.
Endocrine:– Stomach and small intestine secrete hormones to help regulate
the GI system.• Gastrin, secretin, CCK, GIP, GLP-1, guanylin, VIP, and
somatostatin.
Functions of the GI Tract (continued)
Digestion:– Breakdown of food particles into subunits
(chemical structure change).Absorption:
– Process of the passage of digestion (chemical subunits) into the blood or lymph.
Storage and elimination:– Temporary storage and elimination of
indigestible food.
Functions of the GI Tract (continued)
Digestive System (GI)
GI tract divided into:– Alimentary
canal.– Accessory
digestive organs.
GI tract is 30 ft long and extends from mouth to anus.
Insert fig. 18.2
MouthFoodstuffs are broken down mechanically by chewing and saliva is added as a
lubricant.
EsophagusA simple conduit between the mouth and stomach.
StomachEnzymatic digestion of proteins initiated and foodstuffs reduced to liquid form.
Liver
The center of metabolic activity in the body - provide bile salts to the small intestine, critical for digestion and absorption of fats.
PancreasProvides a potent mixture of digestive enzymes to the small intestine which are
critical for digestion of fats, carbohydrates and protein.
Small IntestineThis is where the final stages of chemical enzymatic digestion occur and where
almost all nutrients are absorbed.
Large IntestineWater is absorbed, bacterial fermentation takes place and feces are formed.
Layers of GI Tract
Composed of 4 tunics:– Mucosa.– Submucosa.
Muscularis. Serosa.
Lines the lumen of GI tract.– Consists of simple columnar epithelium.
Lamina propria: – Thin layer of connective tissue containing lymph
nodules. Muscularis mucosae:
– Thin layer of smooth muscle responsible for the folds. Folds increase surface area for absorption.
Goblet cells:– Secrete mucus.
Mucosa
Thick, highly vascular layer of connective tissue.
Absorbed molecules enter the blood and lymphatic vessels.
Submucosal plexus (Meissner’s plexus):– Provide autonomic nerve supply to the
muscularis mucosae.
Submucosa
Responsible for segmental contractions and peristaltic movement through the GI tract.– Inner circular layer of smooth muscle.– Outer longitudinal layer of smooth muscle.
Contractions of these layers move food through the tract; pulverize and mix the food.
Myenteric plexus located between the 2 muscle layers.– Major nerve supply to GI tract.
Fibers and ganglia from both sympathetic and parasympathetic nervous systems.
Muscularis
Serosa
Binding and protective outer layer. Consists of areolar connective tissue covered with
simple squamous epithelium.
Extrinsic innervation:– Parasympathetic nervous system:
Vagus and spinal nerves:– Stimulate motility and GI secretions.
– Sympathetic nervous system: Postganglionic sympathetic fibers that pass through
submucosal and myenteric plexuses and innervate GI tract:
– Reduce peristalsis and secretory activity.
Regulation of the GI Tract
Enteric nervous system:– Sites where parasympathetic fibers synapse with
postganglionic neurons that innervate smooth muscle. Submucosal and myenteric plexuses:
– Local regulation of the GI tract. Paracrine secretion:
– Molecules acting locally. Hormonal secretion:
– Secreted by the mucosa.
Regulation of the GI Tract (continued)
Parasympathetic n.s. Sympathetic n.s.
Vagal nuclei CNS Preganglionic Fibres
Preganglionic fibres
Sacral spinal Sympathetic cord ganglia
Postganglionic fibres
Enteric nervous System Myenteric Submucosal plexus plexus
Smooth Secretory Endocrine BloodMuscle cells cells vessels
Effect of different stimuli onmuscle contraction
Stimulus Effect on muscle
more depolarised smooth
1. Stretch of GI tract wall muscle, more excitable
2. Acetylcholine release Leads toaction potential
3. Parasympathetic stimulation generation and smooth
muscle contraction.
4. Noradrenaline release more hyperpolarised smooth
5. Sympathetic stimulation muscle, Less excitable and
fewer contractions .
Nervous regulation
Enteric nervous system (ENS)
Extrinsic nervous system
Enteric nervous system (ENS)
myenteric plexus (Auerbach’s plexus)
submucosal plexus (Meissner’s plexus)
myenteric plexus (Auerbach’s plexus)
Excitatory effects
Inhibitory effects
Increase the intensity of contraction
Increase the rate of the rhythm of contraction
Increase the velocity of conduction of contraction along the tract
Sphincter
Pyloric sphincter
submucosal plexus (Meissner’s plexus)
secretion
absorption
contraction
Extrinsic nervous system
parasympathetic nervous
sympathetic nervous
parasympathetic nervous
Enteric nervous system
increase
sympathetic nervous
Activity of gastrointestinal tract
Decrease
norepinephrine
ENSSmooth muscle
Afferent sensory nerve fibers from the gut
Irritation of the gut mucosa
Distension of the gut
Chemical substances
ENS
Sympathetic ganglia
Spinal cord or brain stem
Gastrointestinal reflexes
ENSSympathetic ganglia
Spinal cord or brain stem
stimulation
Activity of gastrointestinal tract