Chapter 24: Digestive system

composed of two groups of organs: the gastrointestinal (GI) tract and the accessory digestive organs
- break down food into smaller molecules that can be used by cells

digestive system

a continuous tube that extends from the mouth to the anus through the thoracic and abdominopelvic cavities
*Organs:*
- mouth, pharynx, esophagus, stomach, small intestine, and large intestine

gastrointestinal tract

include the teeth, tongue, salivary glands, liver, gallbladder, and pancreas

accesory digestive organs

6 Functions of the Digestive System

*1. Ingestion:*
- eating
*2. Secretion:*
- water, acid, buffers, and enzymes into the lumen of GI tract
*3. Mixing and Propulsion:*
- contraction and relaxation of GI tract smooth muscle (motility)
*4. Digestion:*
- mechanical digestion
- chemical digestion
*5. Absorption:*
- passage of digested products through epithelial cells of GI tract into blood and lymph
*6. Defacation:*
- elimination of waste through the anus

4 Layers of the GI Tract

(1) Mucosa
(2) Submucosa
(3) Muscularis
(4) Serosa/ Adventitia

innermost layer of GI tract
- mucous membrane
*Composed of:*
- epithelial layer
- lamina propria (areolar c.t.)
- muscularis mucosae

mucosa

*Nonkeratinized stratified squamous epithelium:*
- mouth, pharynx, esophagus, and anal canal
*Simple columnar epithelium:*
- stomach and intestines

Epithelium of the GI Tract

endocrine cells that secrete hormones into GI tract

enteroendocrine cells

an areolar connective tissue layer containing many blood and lymphatic vessels
- carry the nutrients
- contains MALTs
- supports epithelium and binds it to muscularis mucosae

lamina propria

mount immune responses against microbes in GI tract

mucosa-associated lymphatic tissue (MALT)

a thin layer of smooth muscle fibers
- forms folds for increased surface area
- small movements to expose all absorptive cells to GI tract contents
- helps move lymph in lacteals (lipid transport)

muscularis mucosae

a thin meshwork that contains collagen fibers, nerves, and blood vessels
- areolar c.t. that binds mucosa to muscularis
- contains submucosal plexus of enteric nervous system

submucosa

*Skeletal Muscle:*
- mouth, pharynx, superior/middle esophagus
*2 Sheets of Smooth Muscle:*
(circular and longitudinal)
- rest of GI tract
*3rd sheet of smooth Muscle:*
(oblique, innermost layer)
- stomach

Muscularis of the GI Tract

controls GI tract motility (movement)
- found in muscularis

myenteric plexus

a serous membrane composed of areolar connective tissue and simple squamous epithelium (mesothelium)
- epithelial portion of the serosa is also called the *visceral peritoneum*

serosa

double-layered membrane surrounding the abdominal organs
- consists of a layer of simple squamous epithelium (mesothelium) with an underlying supporting layer of connective tissue

peritoneum

2 Layers of Peritoneum

(1) visceral peritoneum (serosa)
(2) parietal peritoneum

lines the wall of the abdominal cavity

parietal peritoneum

also *serosa*;
covers /some/ organs in the abdominal cavity

visceral peritoneum

contains lubricating serous fluid

peritoneal cavity

retroperitoneal organs

organs outside the peritoneum
*Examples:*
- ascending/ descending colon
- duodenum
- pancreas
- kidneys/ adrenal glands

(1) greater omentum
(2) falciform ligament
(3) lesser omentum
(4) mesentery
(5) transverse mesocolon
(6) sigmoid mesocolon
6 Major Peritoneal Folds

(1) greater omentum
(2) falciform ligament
(3) lesser omentum
(4) mesentery
(5) transverse mesocolon
(6) sigmoid mesocolon

part of the peritoneum attached to the *greater curvature of the stomach* and to the transverse colon
- fatty apron covering the intestines

greater omentum

attaches the liver to the anterior abdominal wall and diaphragm
- remnant of umbilical vein

falciform ligament

attached to the *lesser curvature of the stomach*
- connects the stomach and duodenum *to the liver*

lesser omentum

binds the jejunum and ileum of the small intestine to the posterior abdominal wall

mesentery

binds transverse colon and sigmoid colon to the posterior abdominal wall

mesocolon

also "oral cavity";
formed by the cheeks, hard and soft palates, and tongue
- digestion of starch and triglycerides begins

mouth

form the lateral walls of the oral cavity
- external skin
- internal mucous membrane (nonkeratinized stratified squamous epithelium)

cheeks

fleshy folds surrounding the opening of the mouth
- external skin
- internal mucous membrane

lips

median fold of mucous membrane that joins the internal surface of each lip to the gum

labial frenulum

space between teeth and cheeks

oral vestibule

the space that extends from the gums and teeth to the fauces;
area contained by the teeeth

oral cavity proper

the opening between the oral cavity and the oropharynx

fauces

a wall or septum that separates the oral cavity from the nasal cavity
- forms the roof of the mouth

palate

bony anterior of the palate formed by the maxillae and palatine bones
- covered by a mucous membrane

hard palate

muscular posterior portion of the palate
- partition between the oropharynx and nasopharynx
- lined with mucous membrane

soft palate

small muscular projection hanging from the back of the soft palate

uvula

What is the function of the soft palate and uvula?

The soft palate and uvula prevent food from entering the nasal cavity by closing off the nasopharynx during swallowing.

muscular fold that extends from the lateral side of the soft palate to the base of the tongue

palatoglossal arch

muscular fold that extends from the lateral side of the soft palate to the side of the pharynx

palatopharyngeal arch

a gland that releases saliva into the oral cavity

salivary gland

3 Major Salivary Glands

(1) parotid glands
(2) submandibular glands
(3) sublingual glands

large salivary glands at the angle of the jaw which secrete saliva into the oral cavity via a parotid duct
- open into vestibule
- ONLY serous acini

parotid glands

found in the floor of the oral cavity beneath the base of the tongue
- open into oral cavity proper
- MOSTLY serous acini

submandibular glands

superior to the submandibular glands
- open into oral cavity proper
- MOSTLY mucous acini

sublingual glands

digestive juice produced by salivary glands
- contains *salivary amylas*

saliva

enzyme that begins starch digestion in the mouth

salivary amylase

secretion of saliva
- controlled by ANS (sympathetic and parasympathetic)

salivation

What are the two salivary nuclei in the brain stem?

Impulses from taste buds are sent to *superior and inferior salivatory nuclei* in brain stem which stimulates salivary gland secretion.

an accessory digestive organ composed of skeletal muscle covered with mucous membrane
- manipulates food for chewing and swallowing
- provides taste sensation
- assists speech

tongue

- maneuver food for chewing and swallowing
- form the floor of the mouth
- hold the tongue in position
- assist in speech

extrinsic muscles of the tongue

alter the shape and size of the tongue for speech and swallowing

intrinsic muscles of the tongue

anchors tongue to floor of mouth
- limits movement of the tongue posteriorly

lingual frenulum

"bumps" on the tongue;
projections of the lamina propria covered with stratified squamous epithelium
- contain taste buds (gustatory receptors)

papillae

mushroom-like elevations on dorsum and margin of tongue
- most contain taste buds

fungiform papillae

papillae arranged in an inverted V-shape on the posterior surface of the tongue
- ALL contain taste buds

vallate papillae

located in small trenches on the lateral margins of the tongue
- *Adult: DO NOT contain taste buds*

foliate papillae

threadlike elevations that cover most of the anterior tongue
- DO NOT contain taste buds
- *receptors for touch*
- increase friction of food against tongue

filiform papillae

enzyme secreted by lingual glands that breaks down lipids; activated in the stomach

lingual lipase

accessory digestive organs located in sockets of the alveolar processes of the mandible and maxillae

teeth

cover the alveolar processes of the upper and lower jaws and surrounds the necks of the teeth

gingivae (gums)

space between tooth and gum

gingival sulcus

dense fibrous connective tissue that lines sockets of teeth
- anchors the teeth in position
- acts as a shock absorber during chewing

periodontal ligament

3 Regions of a Tooth

(1) crown
(2) root
(3) neck

the visible portion above the level of the gums

crown

2 or 3 extensions of the tooth embedded in the socket

root

the constricted junction of the crown and root near the gum line

neck

internal calcified connective tissue that gives the tooth its basic shape and rigidity
- beneath the enamel at the crown
- makes up majority of tooth

dentin

consists primarily of calcium phosphate and calcium carbonate
- covers dentin at the crown
- protects tooth from wear and tear

enamel

attaches the root of the tooth to the periodontal ligament

cementum

enclosed *space within the dentin* of the tooth lying within *the crown of the tooth*

pulp cavity

a connective tissue in teeth containing blood vessels, nerves, and lymphatic vessels

pulp

narrow *extensions of the pulp cavity* that run through the root of the tooth

root canals

opening at the base of the root canal through which blood and lymphatic vessels and nerves pass

apical foramen

sets of teeth

dentitions

How many sets of teeth do humans have?

Two sets of teeth:
(1) deciduous teeth
(2) permanent teeth

20 baby teeth that begin to erupt around the age of 6 months
- eventually pushed out by the permanent teeth

deciduous teeth

teeth closest to the midline between the canines

incisors

2 Types of Incisors

(1) central incisors
(2) lateral incisors

front teeth at the midline

central incisors

teeth adjacent to the central incisors

lateral incisors

teeth next to the incisors with a pointed cusp

canines

teeth posterior to the canines having four cusps each
-ONLY in "baby" deciduous teeth
- (replaced by premolars in adults)

first and second deciduous molars

the 32 teeth of the adult that erupt between age 6 and adulthood

permanent teeth

replace first and second deciduous teeth

first and second premolars

do not replace any deciduous teeth
- next to second premolars
- erupt at age 6

first permanent molars

do not replace any deciduous teeth
- erupt at age 12

second permanent molars

also "wisdom teeth";
erupt at age 17 or not at all

third permanent molars

the third molars remain embedded in the alveolar bone and cause pain
- removed surgically

impacted (teeth)

chewing of food;
tongue manipulates food, teeth grind it, food mixes with saliva

mastication

soft flexible mass of food;
term used to describe food after it has been chewed and mixed with saliva

bolus

tube that extends from internal nares to the esophagus (posteriorly) and larynx (anteriorly)

pharynx

also "deglutition";
a mechanism that moves food from mouth to the stomach

swallowing

A muscular tube that connects the mouth to the stomach
- lies posterior to the trachea

esophagus

opening in the diaphragm where esophagus passes through

esophageal hiatus (diaphragm)

Histology of the Esophagus

(1) Mucosa:
- nonkeratinized stratitified squamous epithelium
- lamina propria (areolar c.t.)
- muscularis mucosae (smooth muscle)
(2) Submucosa:
- areolar connective tissue
(3) Muscularis:
- Superior 1/3: skeletal muscle
- Middle 1/3: skeletal AND smooth muscle
- Inferior 1/3: smooth muscle
(4) Adventitia
- areolar c.t.
- NO mesothelium*

regulates passage of food from laryngopharynx into the esophagus
- *skeletal muscle*

upper esophageal sphincter (UES)

waves of muscle contractions that propel food along the digestive tract
- circular muscles contract
- longitudinal muscles contract
- repeat

paristalsis

ring of muscles between the esophagus and the stomach
- *smooth muscle*

lower esophageal sphincter (LES)