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