Digestive System

Objectives:

Discuss the general functions and anatomy of the digestive tract

Describe the individual organs of the system, including a discussion of the gross and microscopic anatomy.

Digestive System

Muscular, hollow tube (= “digestive tract”)

+

Various accessory organs

consists of:

Function

ingestion

mechanical digestion

chemical and enzymatic digestion

secretion

absorption

compaction

excretion and elimination

The function of the system as a

whole is processing food in such a

way that high energy molecules can

be absorbed and residues

eliminated.

Individual parts

function in:

Muscularis

externa

Histological Organization

Tube made up of

four layers.

Modifications

along its

length as

needed.

1

2

3

4

The 4 Layers of the Gut

1) Mucosa

Epithelial cells – protection, absorption

2) Submucosa – made up of loose connective tissue contains submucosal plexus and blood vessels

3) Muscularis externa – smooth muscle, usually two layers -

outer layer: longitudinal

inner layer: circular

4) Serosa

outer layer, functions in protection

Ingestion

Ingestion is the first

step in the process

of digestion.

Ingestion means

that food is taken

into the mouth,

chewed, and

swallowed.

Oral Cavity

Also called the mouth, the oral cavity is the place where ingestion happens.

Hard and soft palates - form roof of mouth

Tongue - skeletal muscle

Salivary glands - three pairs

Teeth

Oral Cavity

Digestion starts here by two

means:

Mechanical – teeth tear,

shred, and grind food.

Chemical – enzymes in

saliva break down food

molecules such as

proteins and complex

sugars.

Structure of Teeth

Crown - exposed surface of tooth

Neck - boundary between root and crown

Enamel - outer surface

Dentin – bone-like, but noncellular

Pulp cavity - hollow with blood vessels and nerves

Root canal - canal length of root

Types and Numbers of Teeth

Dental succession:

Deciduous (baby, primary) teeth - 20, replaced by

Permanent teeth - 32 teeth

Types and Numbers of Teeth

All teeth are formed

before milk teeth or

primary teeth are lost.

It is the action of adult

teeth moving into

place that causes

primary teeth to fall

out.

Three pairs of Salivary Glands

Salivary glands help with:

• digestion

• lubrication (swallowing)

• moistening (tasting)

They secrete enzyme-filled

saliva to help break down

food

Ingestion

The tongue is a

muscle that pushes

your food toward

teeth to be

mechanically

processed and

shapes your food into

small rounded pieces

to be swallowed.

Swallowing

As you swallow, your

tongue pushes the food to

the back of the mouth.

Since both air and food go

through the pharynx, a flap

of tissue called the

epiglottis covers the larynx

to prevent food from

travelling into the lungs

Lesser curvature

Greater curvature

Cardia - end under the heart

Fundus - bulge above the esophageal opening

Body - largest region

Pylorus - J curve, inferior end, terminates in

Cardiac and Pyloric sphincters (importance?)

Rugae – highly extendable interior folds Figs 25-10/11

Gross Anatomy of the Stomach

Histology of Stomach

Type of epithelium lining stomach?

Gastric pits – shallow pits, external half rapidly reproduces for replacement

Gastric glands – deep in lamina propria, 3 types of cells

1. Parietal cells (produce HCl and intrinsic factor)

2. Chief cells (produce pepsinogen)

3. Enteroendocrine cells – G cells (several hormones including gastrin which stimulates both parietal and chief cells)

Fig 25.13

Regions of Small Intestine

SI is longest part of dig. tube

Duodenum (short, 12 inches)

– fixed shape & position

Jejunum (2.5 m long)

– Most of digestion

Ileum (longest at 3.5 m) – Most of absorption, ends in

Ileocecal valve – slit valve into large intestine (colon)

Plicae circulares – circular pleats around the interior of the small intestine. Slows movement of food, increases surface area of intestine.

Villi – minute finger-like projections, contain capillaries & lacteals. Further increase surface area.

Microvilli – sub-microscopic size, projections on single cells. Aid in absorption of nutrients.

Intestinal glands (crypts)

- intestinal juice production

- Cell regeneration

Structure of Small Intestinal Wall Fig 25.15

Histology of Small Intestinal Wall

Histology of S.I. Wall (cont)

Cecum – pocket at proximal end with Appendix

Colon

Ascending colon - on right, between cecum and right colic flexure

Transverse colon - horizontal portion

Descending colon - left side, between left colic flexure and

Sigmoid colon - S bend near terminal end

Regions of Large Intestine

Fig 25-17

Rectum – terminal end is anal canal - ending at the anus -

which has internal involuntary sphincter and external voluntary

sphincter

Structure of the Large Intestine

1. Mucosa - abundant goblet cells (produce mucous), stratified squamous epithelium near anal canal

2. No villi

3. Longitudinal muscle layer incomplete, forms three bands or taenia coli

4. Circular muscle - forms pockets or haustra between bands

Histology of Large Intestine