NOTES: The Digestive

System (Ch 14, 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 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 MOVEMENTSand 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 ofcarbohydrates, 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 GLANDS

There are 3 pairs of salivary glands:

1) Parotid glands (largest)•releases salivary amylase: breaks down starch•makes up about 25% of

saliva

SALIVARY GLANDS

2) Submandibular glands•on the floor of the mouth•produces mucin

(glycoprotein) •makes up 70% of the saliva

and responsible lubrication of food

SALIVARY GLANDS

3) Sublingual glands (underneath the tongue)•produces secretions that act as a buffer and lubricant• secretions are mostly mucus…thick and stringy•makes up only 5% of saliva

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 (rugae) in the inner lining

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

powerful circular muscle: PYLORIC SPHINCTER

Rugae

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

Next slide, some folks might find gross

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