Anatomy and physiology of GIT

Foregut

Midgut

Hindgut

Coeliac artery

Superior mesenteric artery

Inferior mesenteric artery

5m

Pharynx to duodenum

Duodenum to first 2/3 of transverse colon

Last 1/3 of transverse colon to upper half of anal canal

Accessory digestive organs

• Teeth• Tongue• Salivary glands• Liver• Gallbladder• Pancreas

Esophagus

25cm

Pharynx

Stomach

A: L gastric artery (from celiac trunk)V: Portocaval anatomososes

Lymph: Lt gastric nodesDrain mainly to celiac lymph nodes

Nerve: Ant + post gastric nerves (vagi) , sympathetic branches of thoracic trunk.

Internal circular and external longitudinal layers of muscle1/3: voluntary1/3: mix1/3: smooth muscle

stratified squamous non-keratinized epithelium

Function: Oral cavity and esophagus

• Mechanical: Chew swallow peristalsis to stomach

• Secretion: Saliva (lysozyme, defensins, andIgA ab), amylase, lipase

• Digestion: Carbohydrates and fat (minimal)• Absorption: None

Fundus

Greater curvature

Lesser curvature

Body

AntrumPylorus

Cardiac orifice

Lt of midline, T11

Rt of midline, L1 (Transpyloric plane) Can hold up to

2-3L

Simple columnarCovered by mucous layer

Lymph: follows arteries celiac nodes

Nerves: Celiac plexus – both sympathetic and parasympathetic

Celiac trunk

Portal vein

Pain – poorly localisedReferred – gastric ulcer – T7,T8 sensory ganglia

Glands• The stomach is divided into three histological regions based on the nature of the glands.• Cardiac region: near the opening of the oesophagus. Mucus-secreting cells. Protects the

oesophagus against gastric reflux.• Fundic region: long glands, narrow neck and a short, wider base.

– Cell types found– Mucous neck cells– Parietal (oxyntic) cells: HCL and intrinsic factor (B12). – Chief cells: pepsinogen and a weak lipase– Enteroendocrine cells: more prevalent near the base. Secrete products into lamina

propria where it is taken up by blood vessels. Secretes gastrin – stimulates production of HCL.

• Pyloric region: mucous

Function: Stomach

• Mechanical: mixing and propulsion• Secretion:– Parietal cells: HCl– Chief cells: Pepsinogen and lipase– Surface mucus cells: Mucus and HCO-3

– G cells: Gastrin– ECL cells: Histamine

• Digestion: Proteins and fats• Absorption: Lipid soluble (alcohol, aspirin etc)

Coeliac art

Sup mesenteric art

Through mesentry, forming arcades

Lymph: Coeliac + Sup mesenteric nodes

Nerve: Coeliac + sup mesenteric plexus

Small intestine epithelium• Villi covered by simple columnar epithelium• Intestinal glands• Enterocytes (absorptive cell)• Goblet cells: mucus secreting• Paneth cells: regulate intestinal flora• Enteroendocrine cells: CCK, secretin (bicarb), GIP (gastric

inhibitory peptide- inhibits gastric acid)

Function: Small intestine

• M: Mixing – enzymes from pancreas and liver; propulsion – segmentation.

• S:– Goblet cells: Mucus– Hormones: CCK, Secretin, GIP

• D: Carbohydrates, fats, protein and nucleic acids.• A: Peptides by active transport; amino acids,

glucose and fructose by secondary active transport; fats by simple diffusion; water by osmosis; ions, minerals and vitamins by active transport

sup mesenteric nodes.

Sup mesenteric nerve plexus

inf mesenteric nodes.

Inf mesenteric plexus:Sympathetic (lumbar splanchnic nerves)Parasympathetic S2-S4

Function: Large intestine

• M: Segmental mixing; propulsion – mass movement.

• S: mucus by goblet cells.• D: None.• A: Ions, water, minerals, vitamins produced by

bacteria.

Physiology of absorption: Carbohydrate

• Glucose rapidly absorbed before terminal part of ileum.

• Transport affected by Na+ in intestinal lumen sodium-dependent glucose cotransporter.– Secondary active transport– Congenital defective – glucose/galactose malabsorption

(severe diarrhoea)• Fructose different mech, independent of Na+.• Insulin little effect on sugar absorption in intestine

not depressed during DM.

Physiology of absorption: Protein• 7 diff syst for amino acids: 3 Na+ dependent, 2 Na+ & Cl-

dependent.• Di/tripeptides H + dependent.• Hartnup disease: defect in AA absorption from intestine and

tubules in the kidneys.• Cystinuria: inadequate reabsorption of cystine in PCT of kidneys.• Infants: undigested proteins absorbed maternal IgA by

transcytosis.– Adults: causes allergies.

• Absorption of antigen by microfold (M) cells transport to Peyer’s patches, lymphocytes activated.

Physiology of absorption: Lipid

• Passive diffusion esterified.• Uptake of bile salts by jejunal mucosa low form

new micelles.• Process not fully matured in infants fail to absorb

10-15% of ingested fat.– More susceptible to fat malabsorption diseases.

• Cholesterol: needs bile, fatty acids and pancreatic juice.– Sterols of plant origin poorly absorbed compete with

cholesterol and reduce cholesterol absorption.

Physiology of absorption: water and electrolytes.

• 98% of fluid reabsorbed,~200mL excreted in stool.– Mainly in small and large intestine.

• Na+ diffuses across small intestine through gradient; basolateral surface has Na+-K+ ATPase actively absorbed.

• Cl- enterocytes via Na+-K+ -2Cl- cotransporters secreted via channels.– Cholera bacillus: increased Cl- secretion, reduced Na+

absorption.• Glucose / cereal containing carbs (tx of diarrhoea).

• Jejunum – osmolality of content close to that of plasma absorption of osmotically active particles.

• Saline cathartics (Mg2+ sulfates) poorly absorbed salts, increase intestinal volume laxatives.

• K+ secreted into intestinal lumen as mucus. H+-K+ ATPase in distal colon reabsorbs.– Loss of ileal or colonic fluid (diarrhoea) can lead to

severe hypokalaemia.

Physiology of absorption: water and electrolytes.