Digestive System

•Most animals have one of three main types of diets: they are either carnivores, herbivores, or omnivores. Most humans fall into the last category.

• In humans, herbivores are usually called vegetarians. Strict vegetarians who consume no animal products, whatsoever, are called vegans.

•Some people are lacto-ovo-vegetarians, meaning that they also eat dairy products and eggs.

•Digestion is the process of breaking down food into molecules small enough for the body to absorb. Proteins, carbohydrates, and fat in our diets must be broken down and later, reassembled in forms useful to our body.

• A. Organs

• 1. Alimentary canal

• a. Function

• i. Digests

• ii. Absorbs

• b. Organs

• i. Mouth

• ii. Pharynx

• iii. Esophagus

• iv. Stomach

• v. Small intestine

• vi. Large intestine

• 2. Accessory digestive

• a. Organs

• i. Teeth

• ii. Tongue

• iii. Gallbladder

• b. Glands

• i. Salivary

• ii. Liver

• iii. Pancreas

• The path of food through the human digestive system includes the following organs and structures:

• the mouth, which includes:

• the teeth, which grind food to increase the surface area

• the saliva, which includes mucin, a lubricant; buffers to neutralize acidic foods, antibacterial agents, and amylase, which converts starch to maltose.

• the tongue which tastes and manipulates the food

• Digestive Process

• A. Essential activities

• 1. Ingestion

• a. Taking food into the digestive system via the mouth

• 2. Propulsion

• a. Movement of food through the digestive tract

• b. Swallowing

• i. Voluntary

• c. Peristalsis

• i. Involuntary

• 3. Mechanical digestion

• a. Chewing

• b. Mixing

• i. Saliva

• c. Churning

• d. Segmentation

• i. Local constriction of the small intestine• 4. Chemical digestion

• a. Mouth through small intestine (see below)

• 5. Absorption

• a. Movement of digested end products from lumen of the GI tract into blood and lymph

• 6. Defecation

• a. Elimination of undigested materials

• the pharynx, which in domestic animals, leads to both the trachea and the esophagus.

• While food is being swallowed, the epiglottis blocks the trachea and the uvula blocks off the nose.

• the esophagus, which is the tube from the pharynx to the stomach.

• Food is moved along the esophagus by peristalsis, wave-like contractions of the muscles in the walls of the esophagus.

• The lining of the esophagus secretes mucus to lubricate the ball of food.

• There are sphincter muscles (rings of constricting muscles) at the top and bottom of the esophagus.

III. Histology of the Alimentary Canal

A. Four basic layers1. Mucosa

a. Innermost layer

b. Functions

i. Secretion

ii. Absorption

iii. Protection

c. Sub-layers

i. Lining epithelium

ii. Lamina propria

iii. Muscularis mucosae

d. Epithelium

i. Simple columnar with goblet cells

e. Lamina propria

i. Loose areolar connective tissue

ii. Capillary beds

iii. Lymph nodules

f. Muscularis mucosae

i. Thin layer of smooth muscle

2. Submucosa a. Dense connective tissue

i. Blood vessels ii. Lymphatic vessels

b. Elastic fibers

3. Muscularis externa

a. Segmentation and peristalsis b. Inner circular and outer longitudinal layer of smooth muscle c. Sphincters

i. Thickened areas of smooth muscle

4. Serosa

a. Areolar connective tissue covered with mesothelium

i. Single layer of squamous epithelium

IV. Functional Anatomy

A. Mouth (oral cavity or buccal cavity)

1. Boundaries

a. Lips

b. Cheeks

c. Tongue

d. Palate

2. Palate a. Hard palate

i. Underlain by bone

ii. Rigid

b. Soft palate i. Formed from muscle

B. Tongue

1. Bundles of skeletal muscle

2. Function

a. Mix food with saliva i. Bolus

b. Position bolus for swallowing

C. Salivary glands1. Function of saliva2. a. Cleanses mouth b. Dissolves tastants c. Moistens d. Initiates chemical breakdown2. Types a. Extrinsic i. Three pairs ii. Parotid iii. Submandibular iv. Sublingual b. Intrinsic i. Buccal glands3. Secretory cells a. Serous cells i. Watery secretion with enzymes b. Mucous cells i. Viscous

D. Teeth1. Classification a. Incisors b. Canines c. Premolars d. Molars

2. Dental formula: (2I, 1C, 2PM, 3M/2I, 1C, 2PM, 3M) X 2 = 32

• E. Pharynx• 1. Oropharynx to Laryngopharynx (see Respiration Lecture)

• F. Esophagus• 1. Laryngopharynx into esophagus• 2. Pierces diaphragm• 3. Joins stomach• a. Cardiac orifice• b. Cardiac sphincter

• 4. Four layers• a. Mucosa• b. Submucosa• c. Muscularis• i. Skeletal and smooth• ii. Distribution is regionalized

• d. Adventia (not serosa)• i. Entirely connective tissue