PHAY2004: PHYSIOLOGY AND PHARMACOLOGY OF

THE GASTROINTESTINAL TRACT

Lecture 1: GI Anatomy

Dr Afia B Ali

Room:524

afia.b.ali@ucl.ac.uk

A: Anatomy and function of GI system.

The digestive system and associated exocrine glands

Oral cavity (mouth)

What are the sequential events involved in swallowing?

The oral cavity contains the teeth and tongue, is closed by the lips and

surrounded by the cheeks.

The main anatomical features of the stomach.

The diagonal line shows the approximate division of the stomach into the

two secretory regions: the oxyntic secretory area consisting of the fundus

and the body, and the pyloric secretory area consisting of the pyloric

antrum. (B) The structural features of the pylorus.

The stomach stores food and controls its release into the duodenum. It also

churns the food so that it mixes with the gastric secretions to make chyme.

Left: The anatomical arrangement of the small intestine and associated

structures.

Right: Structure of the submucosa of the small intestine showing the relationship

of the glands to the wall.

Small intestine

In an adult, the small intestine is about 6 metres long and is the site of most digestion

and absorption. The first 25 cm comprises the duodenum, the next 2.5 metres is the

jejunum and the remainder is termed the ileum.

Structural features of the villus

The surface of the small intestine is covered with small finger-like projections called

villi which serve to increase the surface area for nutrient absorption. Each villus has

a caplillary network and is covered by a layer of columnar epithelial cells, many of

which possess microvilli on their luminal surfaces.

Absorptive columnar cell. (B)

Structure of microvilli. (C)

Paneth cell. (D) Endocrine

cell. (E) Goblet cell. (F)

Undifferentiated columnar

cell. (G) Localisation of the

different cell types in the

epithelium of the crypts and

the villi.

Cell types in the intestinal epithelium.

Large intestine

Structure of the wall of the colon. Top: Layers. Bottom: Cell types in the epithelium

•The large intestine (colon) comprises the

last 1.5 metres of the gastrointestinal tract.

•Its main function is to store faecal material

and regulate its release to the external

environment.

•It is also involved in water and electrolyte

reabsorption, and provides an environment

for bacteria that are important in

synthesising certain vitamins, particularly

those of the B complex and vitamin K.

•The bacteria are also involved in digesting

bile acids.

Illustration of the relationship between the liver, gall bladder, stomach, pancreas, duodenum and the

common bile duct. The gall bladder is shown surrounded by smooth muscle. Notice also

the two sphincters controlling the food and fluid entry into the duodenum.

B. Organs of Gastrointestinal Tract

Liver

Involved in providing bile acids and alkaline fluid for the absorption of fats and

neutralisation of gastric acids, the conjugation, degradation and excretion of

waste products of metabolism of metabolism and detoxification of poisonous

substances

Liver

Diagrammatic structure of the Liver. Left: Section showing the canaliculus formed between two

hepatocytes,

Gall bladder

The gall bladder stores bile. It is situated below the

liver and is connected to the duodenum by the bile

duct. The entry of food into the duodenum causes

the gall bladder to contract and bile to be released

into the duodenum via the sphincter of Oddi. The

stimulus for bile secretion is cholecystokinin

(CCK) and gastrin, hormones released from the

duodenum and stomach, respectively.

Common bile

duct

Cystic duct

Body

Pancreatic duct

Common hepatic duct

Delta cells (~10%), secrete growth hormone inhibiting hormone (GHIH), or

somatostatin, which acts as a paracrine to inhibit the secretion of insulin and

glucagons.

Alpha-cells (~20% of the islet population), secrete the hormone glucagon;

Beta cells (~70%), secrete the hormone, insulin;

Liver

Stomach

Pancreas

Duodenum

Exocrine cells

Islets of Langerhans

Endocrine tissue

Pancreas