LECTURE 1 - Overview of anatomy and physiology of the human gastrointestinal system

Upper GIT

Mouth

Esophagus

Stomach

Lower GIT

Small intestine

• Duodenum

• Jejunum

• Ileum

Duodenum (25 cm)

digestion, secretion

Large intestine

Colon

• Cecum, A/T/D/S

Rectum

Anus

Jejunum (2/5th of SI)

further digestion, secretion and absorption

Ileum (3/5th of SI)

absorption and some secretion

Colon (cecum, A/T/D/S)

absorption , secretion, colonic microbiota breakdown of undigested fiber, storage of solid waste

Rectum

temporary storage of solid waste

Anus

elimination

Accessory organs

• Salivary glands

• Liver

• Pancreas

• Gallbladder

what is the common structure tha Liver and Pancreas come to?

comes into the duodenum

Liver

Hepatic Triad

Liver lobules

hepatocytes

little thin cells

glucose and store glycogen → bile

Hepatic Triad

Hepatic artery

Portal vein

Bile duct

hepatic artery

bring oxygenated blood

portal vein

deoxygenated blood

brings in nutrient rich blood from digestive

bile duct

sent to the gall bladder

Liver Functions

Stores blood, minerals, and vitamins

Removes bacterial and foreign particle

Synthesizes clotting factors, glucose, and lipids

Produces bile to absorb fat-soluble vitamins

Metabolizes fats, proteins, and carbohydrates

Removes metabolites of drugs and xenobiotics

Gallbladder

Stores and concentrates bile between meals (holds ~90 ml of bile)

What does bile contain?

Bile salts

Cholesterol / Lecithin

Bilirubin

Electrolytes / Water

EXOcrine Pancreas structure

Acini

Pancreatic duct

Major duodenal papilla

Major duodenal papilla

bile / pancreatic juice empty into duodenum via Ampulla of Vater

Exocrine Pancreas Functions

Produces alkaline fluids / enzymes, secretes → duodenum

Exocrine Pancreas Alkaline fluids

Na+ , K+ , Cl- , HCO3 - , Mg+2, Ca+2

Exocrine Pancreas Pancreatic enzymes:

Trypsinogen → trypsin → amino acids

Amylase : starch → disaccharides → monosaccharides

Lipase : triglycerides → monoglycerides / FFA

what does CCK hormone stimulate?

acinar cells to release pancreatic fluids into the duodenum

gallbladder to release bile into the duodenum

GIT function

regulate motility, secretion, digestion, and absorption

GIT controls

Neural control

Hormonal control

Paracrine control

Neural control

Extrinsic innervation

Intrinsic innervation

Neural control - Extrinsic innervation

Parasympathetic (PNS) : controls secretion / movement

Sympathetic (SNS) nerves : stops movement

Neural control - Intrinsic innervation

Myenteric plexus

Submucosal plexus

Myenteric plexus

GI motility

muscularis layer

PNS / SNS

Submucosal plexus

secretions

only parasympathetic input

GIT Histology

Serosa

Muscularis

Submucosa

Mucosa

NE → α1-adrenergic receptors on GI smooth muscle cells

vasoconstriction → decreased blood flow to GIT

NE → α2-adrenergic receptors

results in decreased NE release / motility

what does the stimulation of M2&3R help do?

increased motility and secretions

Gastrin stimulates

secretion of HCl / pepsinogen

what does gastrin do ?

enhances gastric motility

Secretin

inhibits gastric secretions / emptying acid → duodenum

what does secretin stimulate?

stimulates secretion of HCO3 - (pancreas) and bile (liver)

Cholecystokinin inhibits

gastric secretions / emptying when lipid-rich chyme reaches the duodenum

what is cholecystokinin secreted by ?

endocrine cells in duodenum

Cholecystokinin stimulates

secretion of digestive enzymes and bile

Glucose-independent insulinotropic peptide inhibits

gastric secretions / emptying when glucose, lipids, acid, and hyperosmotic chyme reaches the duodenum

GIP stimulates

pancreatic β cells to secrete insulin required for sugar utilization

what kind of signaling do paracrine molecules engage in ?

intercellular signaling - short distance

what are Paracrine molecules?

Histamine

Somatostatin

Prostaglandins

Histamine

secreted by enterochromaffin-like cells

what are somatostatin secreted by?

D cells in the stomach

what are prostaglandins secreted by?

nucleated cells

what does histamine stimulate?

stimulates acid secretion from parietal cells

what does somatostatin inhibits ?

gastrin secretion bc HCl levels are high in the stomach

what does prostaglandins inhibit?

acid secretion from parietal cells

what does prostaglandins stimulate?

mucus and bicarbonate secretion

what is the prostaglandins protective role?

promote healing, but increase pain

Prostaglandins - NSAIDs and corticosteroids action

decrease mucus production

causes GI bleeding

antrum

secretion of mucus, pepsinogen, and gastrin

fundus

secretion of mucus, pepsinogen and HCl & intrinsic factor

Gastric juices

Mucus

Hydrochloric acid (HCl)

Intrinsic factor (IF)

Enzymes (pepsin)

Hormones (gastrin)

Hydrochloric Acid (HCl)

Secreted by

Parietal cells

Hydrochloric Acid (HCl)

In response to

Ach, gastrin, histamine

Hydrochloric Acid (HCl)

Function of enzyme

optimal pH for enzymes

Protection against pathogens

Gastrin

Secreted by

G cells (stomach)

Gastrin responds to

Food, stress, coffee, alcohol

Gastrin

Function of enzyme

stimulates CCKb receptor on parietal cells → acid secretion

Pepsin

Secreted by

Chief cell of the stomach

Pepsin

In response to

Anything that stimulates acid secretion

Pepsin

Function of enzyme

Protein digestion (protein → peptides → AA)

Secretin

Secreted by

S cells of duodenum

Secretin

In response to

High acidity in duodenum

Secretin

Function of enzyme

gastric acid secretion

production of pancreatic enzymes

increase pH & bile (liver)

Cholecystokinin (CCK)

Secreted by

I cells / enteric neurons

Cholecystokinin (CCK)

In response to

Partially digested fats and proteins in duodenum

Cholecystokinin (CCK)

Function of enzyme

bile → digestion of chyme

Histamine

Secreted by

Enterochromaffin-like cells (ECL)

what does Histamine respond to ?

Vagal stimulation of G cells

Histamine

Function of enzyme

Stimulate H2 receptor on parietal cells → acid secretion

Somatostatin (SST)

Secreted by

D cells in GIT

Somatostatin (SST)

In response to

High acidity in GIT

Somatostatin (SST)

Function of enzyme

Inhibits release of glucagon, insulin , cholecystokinin and enzymes: pepsin, secretin and gastrin

Decreases rate of gastric emptying

what is bile?

an alkaline, bitter-tasting, yellowish green fluid that contains

break down fats into fatty acids