The Digestive SystemSection 1, Chapter 17

• Mechanical Digestion – breaks down food into smaller pieces

Digestion is the mechanical and chemical breakdown of food into forms that cells can absorb.

INTRODUCTION

• Chemical Digestion – decomposes food into smaller molecules

The digestive system consists of:• The alimentary canal – extends from mouth to anus

• Accessory glands –secrete chemicals into the alimentary canal.

Figure 17.1 Organs of the digestive system.

• If stretched out, the alimentary canal is about 8 meters long. That’s 26 feet!

THE ALIMENTARY CANAL

• The alimentary canal (A.C.) wall has four distinct layers that differ from region to region.

STRUCTURE OF THE WALL

• Mucosa – mucous membrane• Epithelial tissues on a bed of connective tissue (lamina propria).• Makes direct contact with the lumen• This layer absorb nutrients, secrete chemicals, and protect the

underlying layers.

• Submucosa – Beneath the mucosa• Loose connective tissue• Blood vessels, nerves, and lymphatic vessels• This layer nourishes the surrounding tissues, and carries away

nutrients.

STRUCTURE OF THE WALL

• Muscular – provides movements within the A.C.• Inner coat = transverse muscles: decrease the diameter of the

A.C.• Outer coat = longitudinal muscles: shortens length of the A.C.

• Serosa – serous membrane• Composed of visceral peritoneum in most places• Secretes serous fluid, which lubricates the A.C.

Figure 17.3 The wall of the small intestine, as in other portions of the alimentary canal, consists of four layers: An inner mucosa, a submucosa, a muscular layer, and an outer serous layer.

MOVEMENTS OF THE ALIMENTARY CANAL

Muscles of the A.C. provide two basic movements:

• Segmentation – mixing movement• Smooth muscles contract and relax,

mixing foods with digestive juices.

•Peristalsis – propelling movement• Smooth muscles contract in a wave-like motion

pushing food through the alimentary canal.

Figure 17.4 (b) segmentation mixes the contents of the small intestine. (c) Peristaltic waves move the contents along the canal

INNERVATION OF THE ALIMENTARY CANAL

•Remember:• Parasympathetic impulses – increase activities of digestive system• Sympathetic impulses – inhibit certain digestive actions

• Enteric Nervous System – subdivision of the ANS that innervates the alimentary canal.

•Submucosal plexus – controls secretions

•Myenteric plexus – controls gastrointestinal motility

THE MOUTH

• The mouth:• Receives food• Mechanically breaks up solid particles using saliva• Prepares food for chemical digestion

• This action is called mastication

• The mouth also functions as an organ of speech, and sensory reception.

• The cheeks form the lateral walls of the mouth• Buccinator muscle + buccal fat pad (gives form to cheeks)

BOARDERS OF THE MOUTH

• The lips form the anterior boarder• Highly mobile structures that surround the mouth opening• Orbicularis oris skeletal muscle• Semitransparent epithelium, and a good blood supply makes the lips appear red.• Highly innervated by sensory receptors

THE TONGUE

• The tongue is a thick, muscular organ that occupies the floor of the mouth and nearly fills the oral cavity when the mouth is closed

Mucous Membrane – covers the surface of the tongue

Lingual Frenulum – membranous fold, anchors the tongue to the floor of the mouth.

Body – largely composed of skeletal muscle• Manipulate foods and aids in swallowing

Root – posterior portion of the tongue• Anchored to the hyoid bone.• Covered with lingual tonsils

Papillae – projections on the surface of the tongue.• Some provide friction, others house taste buds.

THE PALATE• The palate forms the roof of the oral cavity

Hard Palate - bony roof of the mouth• Formed by the palatine bones and portions of the maxilla

Soft Palate - Muscular arch

Uvula – cone-shaped projection

During swallowing, muscles draw the soft palate and the uvula upward preventing food from entering the nasal cavity.

Figure 17.7 Sagittal section of the mouth, nasal cavity, and pharynx.

Teeth are the hardest structures in the body

THE TEETH

Figure 17.8 This partially dissected child’s skull reveals primary and developing secondary teeth in the maxilla and the mandible.

•primary (deciduous) teeth numbering 20• Usually erupt through the gums from age of 6 months to 2 years

•secondary (permanent) teeth numbering 32• Usually begin to erupt at 6 years• 3rd molars = wisdom teeth, which may erupt between 17-25 years of age

TYPES OF TEETH

Incisors- blade shaped teeth that bite or cut off large pieces of food

Canines- cone shaped teeth that grasp and tear food

Adult – 8 incisors Child – 8 incisors

Adult – 4 canines Child – 4 canines

Premolars – flattened surface for grinding food

Adult – 8 premolars Child – 0 premolars

Molars – flattened surface for grinding food

Adult – 12 molars Child – 8 molars

TOOTH ANATOMYEnamel- Calcium salts; hardest structure in the body.

Dentin- Living cellular tissue similar to bone.

Pulp cavity- Filled with loose connective tissue, blood vessels, and nerves

Crown- portion of tooth above the gums (gingiva)Root- portion of tooth below the gums (gingiva)

Root canal – entrance into the pulp cavity

Tooth attachments to the jaw

Cementum – bone-like layer surrounding the rootPeriodontal ligaments – attaches the tooth to the jaw

Figure 17A Actinomyces bacteria (falsely colored) clinging to teeth release acids that decay tooth enamel (1,250X)

DENTAL CARIESMicrobes on the surface of teeth metabolize carbohydrates from foods left in the mouth.

• Their acidic wastes dissolves and the destroys the enamel and dentin.

• Dental caries form when the damage spreads to the underlying dentin.

• Salivary glands secrete saliva• Saliva moistens the food, and begins the digestion of carbohydrates

SALIVARY GLANDS

•Many minor glands are also scattered throughout the mucosa of the tongue, palate, and cheeks

Three pairs of major salivary glands, include:

Parotid glands – located anterior to the ear• A parotid duct enters the mouth opposite the second upper molars

Submandibular glands – located on the floor of the mouth inside the jaw.

Sublingual glands – located on the floor of the mouth, inferior to the tongue.

Figure 17.11 Locations of the major salivary glands.

• Parotid glands • Secrete clear watery, serous fluid• Rich in salivary amylase – begins the chemical digestion of carbohydrates

•Submandibular glands• Secretes a mixed saliva with both serous fluid and mucus

•Sublingual glands• Secrete primarily mucus

There are two types of secretory cells within the salivary glands: • Serous cells produce a watery fluid with digestive enzymes (salivary amylase)• Mucous cells secrete mucous

SALIVARY GLANDS

SALIVARY GLANDS

End of Section 1, Chapter 17