NOTES: The Digestive

System (UNIT 8, part 1)

DIGESTION…

Digestion is the process of

mechanically and chemically

breaking down foods and

absorbing the breakdown products.

The digestive system consists of an

alimentary canal and several accessory

organs.

Alimentary canal includes:

● mouth

● pharynx

● esophagus

● stomach

● small intestine

● large intestine

● rectum

● anus

Accessory organs are:

● salivary glands

● liver

● gallbladder

● pancreas

GENERAL CHARACTERISTICS OF THE ALIMENTARY CANAL

*The alimentary canal is a muscular tube that passes through the body’s ventral cavity. Different regions carry out different functions, but certain structural characteristics are similar throughout its length.

Structure of the alimentary wall:

4 DISTINCT LAYERS:• MUCOSA (or mucous membrane) -surface epithelium, underlying connective

tissue, and a small amount of smooth muscle

-in some regions, folds and tiny projections increase the surface area (for absorption)

• SUBMUCOSA -loose connective tissue, glands, blood

vessels, lymphatic vessels, and nerves

4 layers of alimentary wall…● MUSCULAR LAYER-consists of two coats of smooth muscle tissue

and some nerves; produces movement of the tube

● SEROSA (or serous layer)-outer covering of the tube (also called the

visceral peritoneum)-protects underlying tissue-cells here secrete serous fluid, which moistens

and lubricates tube’s outer surface

Movements of the tube:

*motor functions include:

MIXING MOVEMENTS

and

PROPELLING MOVMENTS

(include a wavelike motion

called PERISTALSIS)

MOUTH (Oral Cavity)*receives food and

begins digestion

Tongue:

-rough projections (PAPILLAE)

on surface of tongue provide friction to handle food

-papillae also contain TASTE BUDS

-covering the root of the tongue

are masses of lymphatic tissue:

LINGUAL TONSILS

MOUTH (Oral Cavity)

Palate:

-forms the roof of the oral cavity

-hard palate (anterior) and soft palate (posterior)

-soft palate extends posteriorly

and downward as a projection

called the UVULA

MOUTH (Oral Cavity) Teeth:

-primary teeth (“baby teeth”) are replaced by the secondary teeth (permanent teeth)

-teeth are used to mechanically

break food into smaller pieces

(this increases the surface area

of food allowing digestive

enzymes to react more

effectively with the food

molecules)

SALIVARY GLANDS

*salivary glands secrete saliva, which moistens food, helps bind food particles, begins chemical digestion of carbohydrates, makes taste possible, and helps cleanse the mouth.

SALIVARY GLANDS

Serous cells: secrete a watery fluid that contains the digestive enzyme AMYLASE (starts to break down starch and glycogen into smaller disaccharides)

Mucous cells: secrete a thick fluid called mucus, which binds food particles and lubricates during swallowing

SALIVARY GLANDSThere are 3 pairs of salivary glands:

1) Parotid glands (largest); contains only serous cells

2) Submandibular glands; contains both serous and mucous cells

3) Sublingual glands (underneath the tongue; secretions are mostly mucus…thick and stringy!)

PHARYNX and ESOPHAGUS (important passageways!)

PHARYNX: connects the nasal and oral cavities with the larynx and esophagus

ESOPHAGUSESOPHAGUS: a food passageway

from the pharynx to the stomach; passes through the diaphragm

-at distal end, circular smooth muscle fibers thicken and close the entrance to the stomach (prevents regurgitation);

-if some of the acidic stomach contents are regurgitated (reflux) into the esophagus, may cause HEARTBURN

STOMACH*the stomach receives food, mixes it with

gastric juice, initiates protein digestion, carries on a limited amount of absorption, and moves food into the small intestine.

STOMACH Structure / Parts:-J-shaped, pouch-like organ that

sits inferior to diaphragm in upper left of abdominal cavity

-has a capacity of about 1 L-thick folds in the inner lining disappear when

the stomach wall stretches-separated from the small intestine by a

powerful circular muscle: PYLORIC SPHINCTER

STOMACH Gastric Secretions - produced by

GASTRIC GLANDS, which have 3 types of cells:● mucous cells: secrete mucus

● chief cells: secrete digestive enzymes

● parietal cells: secrete hydrochloric acid (HCl)

*together, these secretions form

GASTRIC JUICE

STOMACH PEPSIN: most important digestive enzyme in

gastric juice

-secreted by chief cells in an inactive form: PEPSINOGEN

-when it comes into

contact with

hydrochloric acid,

it becomes the

active PEPSIN

STOMACH

*a layer of thick mucus is produced by cells in the stomach’s inner lining…this protective coating prevents the stomach from digesting itself.

STOMACH Mixing and emptying actions:

- CHYME = a mixture

of food particles and

gastric juice produced by

mixing

-chyme is emptied into the small intestine where chemical digestion continues

-the rate of emptying depends on the fluidity of chyme and the type of food