THE DIGESTIVE SYSTEM

The digestive system is made up of: gastrointestinal tract

accessory glands of the GI tract viz:1)The salivary glands2)Liver & gall bladder3) Pancreas

THE DIGESTIVE TRACT BEGINS AT THE MOUTH AND

ENDS AT THE ANUS

It is essentially a long tube , distended at one point ( the stomach)

Entry from one part into the next is limited by the “gates” which are closed unless food /chyme has to pass through

THE DIGESTIVE TRACT

Digestive processes

Ingestion…via mouth Propulsion…peristalsis Mechanical digestion…chewing,

mixing Chemical digestion…catabolic

steps from mouth to small intestine

Absorption…mainly in small intestine

Defeacation…removal of undigested stuff

Digestive processes

Initiated Mechanical & chemical stimuli

– Organ distension– Osmolarity– pH– Substates– End product of digestion

Regulated Intrinsic & extrinsic

– Hormones, short reflexes (local plexuses), long reflexes (autonomic nerves, CNS)

Pharynx (throat) is a common channel that conducts air & food

THE PHARYNX

DEGLUTITION

Buccal phase Pharyngeal phase Oesophageal phase

BUCCAL PHASE

food rolled into bolus

tongue arches to push it backwards

PHARANGEAL PHASE

elevation of soft palate (closes nasopharynx)

Pharynx is elevated

pressure of food on pharyngeal wall stimulates receptors; activates reflexes which (a) inhibit respiration (b) raise the larynx (c) close the glottis

passage of food bolus downwards tilts the epiglottis backwards

PHARANGEAL PHASE

a wave of contractions sweep through the pharyngeal muscles; food propelled towards upper oesophageal sphincter

PHARANGEAL PHASE

OESOPHAGEAL PHASE

reflex relaxation of upper oesophageal sphincter

sphincter closes when food has passed

glottis opens; breathing resumes

peristaltic waves propel food forward

lower oesophageal sphincter (LES) relaxes due to VIP and NO ; food enters stomach

THE STOMACH

Functional anatomy

Fundus : half- sphere; less circular muscle; mucus cells

Body: curved cylinder; “pacemaker” in greater curvature, at junction with fundus; numerous cells secreting mucus + HCl + pepsinogens

Antrum :cone-shaped; muscular; gastrin and IF secreted

Pylorus: circular muscle; ? true sphincter; gatekeeper (keeps bile from stomach)

NERVE SUPPLYParasympathetic cholinergic ( ACh) stimulatory effects on GI motility and

secretion relaxes sphincters ( contraction of radial

muscles of sphincters) Vagus supplies the oesophagus, small

intestines, and colon (up to the hepatic flexure).

Sympathetic adrenergic (NA) inhibit secretion

[Peptidergic; peptides like VIP ( vasoactive intestinal peptide) influence motility and secretion]

GASTRIC SECRETION & MOTILITY

Movement: slow waves;peristalsis; churning/mixing

Secretion: gastric juice ( HCl + pepsins +IF +mucus)

Function: storage; digestion of protein (fat,C/H); erythropoiesis (IF; abs.of iron) protection (HCl)

* Not very important for absorption!

BASIC ELECTRICAL RHYTHM- GASTRIC SLOW WAVES

GD PUMP- PROPULSION

OBLIQUE MUSCLES- CHURNING/ MIXING

RECEPTIVE RELAXATION

ANTI-PERISTALSIS

MOVEMENTS IN THE STOMACH

Peristaltic waves moves towards pylorus (>vigorous) >fluid chyme is pushed into duodenum >solid chyme squeezes back towards the body for

further mixing (churning)

PYLORIC VALVECLOSED

PYLORIC VALVE SLIGHTLY OPEN

GASTRIC EMPTYING

Rate depends upon volume of food consistency of food (liquid exit 1.5-2.5 hrs,

solid longer time, totally empty 3-4 hrs) contents (fats leave slowest) nervous control humoral factors (chemicals and hormones).

“gastroduodenal pump” = antrum , pylorus and duodenum act together (pylorus = gatekeeper).

GASTRIC EMPTYING

Stimulated by the vagus GASTRIN (hormone produced by the

stomach) Inhibited by Sympathetic stimulation hormones SECRETIN, CHOLECYSTOKININ

(CCK), GASTRIC INHIBITORY PEPTIDE (GIP) fats, acid and hyperosmotic substances in

the duodenum fear and anxiety.

The stomach musculature RELAXES before and after food gets into it:

“RECEPTIVE RELAXATION” :occurs when food is still in the oesophagus; vagally- mediated

“ADAPTIVE RELAXATION”: when food is in stomach (dilates in response to gastric filling); local nerve plexus (mediated by NO)

PLASTICITY: intrinsic ability to relax ( keeps intra-gastric pressure low)

P

T

P = 2T /r

[Law of Laplace]

The stomach musculature is rarely inactive…….

..soon after the stomach is emptied, mild contractions begin, gradually increasing in intensity over a few hours ( HUNGER CONTRACTIONS/ “HUNGER PANGS”)

BEGINS FROM OESOPHAGUS AND TRAVELS ALONG THE WHOLE GI TRACT

OCCURS EVERY 60-90 MINUTES

RELATED TO MOTILIN LEVELS IN BLOOD

Gastric glands and their secretions

*n eck m u cu s ce lls(m u cu s )

p arie ta l ce lls(oxyn c tic ce lls )

H C lIn trin s ic fac to r

ch ie f ce lls(p ep tic /zym og en ce lls )

p ep s in og en s I - V II

B od y o f s tom ach *

G as tric g lan d s

*antrum + pylorus secrete GASTRIN (from G cells) and mucus

GASTRIC JUICE( 2- 3 litres/day)

H C l:*ac tiva tes p ep s in og en*a id s iron ab sorp tion

*k ills in g es ted b ac te ria

P ep s in og en* in it ia tes p ro te in

d ig es tion

IN TR IN S IC F A C TO R* req u ired fo r ab sorp tion

o f B -1 2

W ater (m ore th an 9 8 % )+ m u cu s , H C l, P E P S IN O G E N S , IF

MECHANISM OF GASTRIC SECRETION (HCl)

The Vagus stimulates HCl secretion directly and indirectly through stimulation of GASTRIN release

Distension of the stomach also stimulates HCl secretion in both ways

Stimulation of HCl secretion occurs in response to * GASTRIN ** ACh *** Hi (histamine)**** PGE (prostaglandin E)

Source: Lamb et al

Stimulation of the VAGUS results in secretion of ACID brought about by (a) direct stimulation and (b) through stimulation of GASTRIN

HCl secretion

CNS

VAGUSGASTRIN

G CELLS

Polypeptides,secretagogues

DISTENSION

GASTRIN RELEASING PEPTIDE

3 Phases of Gastric Secretion

Cephalic phase Gastric phase Intestinal phase

CEPHALIC PHASE

parasym

Ach

Stimulates secretions by parietal & chief cells

GASTRIC PHASE

parasym

(secretion)

Food chemicals

G cells

Gastrin

INTESTINAL PHASE

Inhibit gastrin sec in duo

Aa,peptides, pH>3

Fat, pH<2

X

Digestion of carbohydrate

Average diet – starch, disaccharides, monosaccharides

Broken down to monosaccharides (glucose, fructose, galactose)

Lack enzymes to breakdown polysaccharides (eg cellulose), helps peristalsis

Digestion of carbohydrates

Salivary amylase– Starch to oligosaccharides– Works best at pH 6.75-7 (saliva

buffered by bicarbonate & phosphate)– Inactivated by stomach acid

Pancreatic amylase– Conversion continues in small

intestine

CYTOPLASMMICROVILLUS

MEMBRANEINTESTINAL LUMEN

STARCHAMYLASE

MALTOSEMALTASE 80%

GLUCOSE

LACTOSELACTASE

GALACTOSE

SUCROSESUCRASE

FRUCTOSEFACILITATED DIFFUSION (GLUT5)

Na+ACTIVE TRANSPORT

LACTASE DEFICIENCY = DIARRHOEA

INSULIN

INDEPENDENT

Recreated from Rhoades& Pflanzer; Ganong

Na+

Digestion of fats

Small intestine is the only site for fat digestion

Pancreas produces lipases Emulsification with bile salts

Bile salts are not absorbed until reaching ileum, form new micelles

CHYCLOMICRON

ABSORPTION OF LIPIDS

CYTOPLASMMICROVILLUS

MEMBRANEINTESTINAL LUMEN

RESYNTHESIS

LA

CT

EA

L

PORTAL CIRCU-

LATION

F/A, MG, Chol, Bile salts

Micelle

Bile salts are absorbed only in the ileum

LONG CHAIN F/A

[ Re- esterified into TG]

SHORT CHAIN F/A (water soluble)

CHYLO-MICRON

PROTEIN + PHOSPHOLIPIDS + CHOLESTEROL

Digestion of proteins

Proteins to amino acids Begins in the stomach

– Pepsinogens → pepsin (works in pH 1.5-2.5)

– Pepsin inactivated by high pH in duodenum In the small intestine

– Pancreas: Trypsin, chymotrypsin, carboxypeptidase,

– Brushborder enzymes: carboxypeptidase, aminopeptidase, dipeptidase

CYTOPLASMMICROVILLUS

MEMBRANEINTESTINAL LUMEN

TRIPEPTIDES

DIPEPTIDES

NEUTRAL A/A; Phen; Met

ACIDIC A/A

PEPTIDASES

Na+

5 transport systems

A/A

PORTAL CIRCU-

LATION

SMALL PEPTIDES

Undigested PROTEINS ENDOCYTOSIS

BASIC A/A

Fac. Dif.

H+

THANKS FOR YOUR ATTENTION……...

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