Digestive Processes

1. ingestion

2. propulsion

3. mechanical digestion

4. chemical digestion

5. absorption

6. defecation

Digestive system organs

• Alimentary canal or GI tract

1. mouth

2. pharynx

3. esophagus

4. stomach

5. small intestine

6. large intestine

Digestive system organs

• Accessory glands and organs

1. teeth

2. tongue

3. salivary glands

4. gallbladder

5. pancreas

6. liver

Anatomy of the organs of the alimentary canal (GI tract)

Most digestive systems reside within the

abdominopelvic cavity. The organs are

covered by a visceral and parietal peritoneum

separated by the peritoneal cavity. The

mesentery is a piece of fused double layered

membrane connecting the peritoneal

membranes and provides a route for

conducting blood vessels lymphatics and

nerves to the digestive viscera some parts of

these membranes are giving special names

such as greater and lesser omenta.

Anatomy of the organs of the alimentary canal (GI tract)

From the esophagus to the anal canal, the walls

of every organ are made up of 4 basic tunics

from the lumen outward these layers are :

1.mucosa

2. submucosa

3. muscularis externa

4. serosa (or adventitia)

Anatomy of the organs of the alimentary canal (GI tract)

Mucosa

The mucosa consists of 3 layers

1. surface epithelium

2. lamina propia

3. muscularis mucosa

The surface epithelium, typically simple columnar

epithelia rich in mucus secreting cells goblet

cells.

Anatomy of the organs of the alimentary canal (GI tract)

Underlain the surface epithlium there is a small

amount of connective tissue made of areolar

connective tissue called the lamina propria

The lamina propia contains capillaries which

nourish the epithelia and function in

absorption. It also contains lymph nodes

important in defense against bacteria and

other pathogens.

Anatomy of the organs of the alimentary canal (GI tract)

Underlain the lamina propia there is a scant

smooth muscle layer called the muscularis

mucosae. The muscularis mucosae produces

local movements and make small folds vastly

increasing the surface area.

Anatomy of the organs of the alimentary canal (GI tract)

Submucosa

Made of dense connective contains a rich blood

supply, lymphatic, elastic fibers and nerves.

Contains the submucosal plexus which is

part of the intrinsic nerve supply of the GI

tract.

Anatomy of the organs of the alimentary canal (GI tract)

Muscularis externa

The muscularis externa is responsible for

segmentation and peristalsis typically has an

inner circular layer and outer longitudinal

layers of smooth muscle. In many places the

circular layer thickens to form sphincters that

act as valves to control food passage. The

myenteric plexuses the second intrinsic nerve

supply of the GI tract lays between the circular

and longitudinal smooth muscle layers.

Anatomy of the organs of the alimentary canal (GI tract)

Serosa-

The outermost protective is formed of aerolar

connective tissue and mesothelium a single

layer of squamouss epithelial tissue. The serosa

contains the subserous plexus the third

autonomic plexus . In the esophagus the serosa

is replaced by an adventitia which is an ordinary

fibrous connective tissue. Retroperitoneal

organs have both a serosa and an adventitia

Basic functional concepts

1. All processes in GI tract are geared to control

the lumen to maximize the digestive

processess. The lumen is actually outside the

body

How ??

By reflex arcs which involve neural and

hormonal componenets. and stimuli sensed by

sensors mechano and chemo

Basic functional concepts

2. Local control - short and long reflexes. Short

reflexes are mediated entirely by the enteric

plexuses inn response to GI tract stimuli.

Long reflexes initiated by stimuli arising

within or outside the GI tract and involve CNS

centers and extrinsic autonomic nerves.

Digestion and absorption

Carbohydrates

Broken down to monosaccharides: glucose,

fructose and galactose. Starting in mouth

with salivary amylase, a little in the stomach

before acid destroys it and finishing in the

small intestine with pancreatic amylase.

Absorbed in the first segment of small

intestine by an active transport system which

can not be saturated

Digestion and absorption

Minerals and water

Water is highly diffusable and 95% of ingested

is absorbed by passive diffusion in small

intestine. Na+, K+, active transport Na, K

ATPase pump. Other minerals such as

calcium and potassium and trace minerals

such as iron and zinc are also absorbed in the

small intestine

Digestion and absorption

Protein

First to fragments in stomach by pepsin and

small intestine by trypsin and chymotrypsin.

Further by pancreatic carboxypeptidase and

brush border aminopeptidase. Transported by

secondary active transport linked to Na+.

Small # of small intact proteins can cross the

small intestine walls- passive immunity in

infants

Digestion and absorption

Fat-

Quite complex digestion process. Fat are

insoluble in water and therefore arrived at the

small intestine as a TG fat droplet. They get

emulsify by bile salts then lipase goes to

work on their surface splitting TG into 2 free

fa and a monoglyceride and make micelles.

Digestion and absorption

Fat (continued)

Micelles get absorbed in the small intestine cell

wall and during their passage the

monoglycerides and free fa’s combine to

form TG which are wrapped in a membrane

in the rough ER called chylomicrons which

get absorbed by the lacteal not the capillaries

and end up in the general circulation

Digestion and absorption

Vitamins

Fat soluble are solubilized in chylomicrons.

Most soluble vitamins are absorbed by

diffusion or carrier mediated transport with

the exception of B12 which is a very large

molecule and requires to bind to intrinsic

factor released by parietal cells absorbed in

the ileum.