KAAP310 Digestive System Exam 4

digestive system

organ system that processes food, extracts nutrients from it, and eliminates the residue

Five stages of the digestive function

1. Ingestion

2. Digestion

3. Absorption

4. Compaction

5. Defecation

ingestion

selective intake of food

digestion

mechanical and chemical breakdown of food into a usable form by the body

absorption

uptake of nutrient molecules into the body’s own tissues

compaction

absorbing water and consolidating the indigestible residue into feces

defecation

elimination of feces

what are the two facets of digestion

mechanical and chemical

mechanical digestion

physical breakdown of food into smaller particles

• achieved by cutting and grinding action of teeth + churning of stomach

• exposes more food surface to the action of digestive enzymes

Chemical digestion

series of hydrolysis reactions that break dietary macromolecules into their monomers

• carried out by digestive enzymes produced by salivary glands, stomach, pancreas and small intestine

what are nutrients that are already preset in usable form in ingested food and are absorbed without being digested

vitamins, free amino acids, minerals, cholesterol, and water

what are the anatomical subdivisions

digestive tract and the accessory organs

digestive tract (alimentary canal)

muscular tube extending from mouth to anus

includes:

• mouth, pharynx, esophagus, stomach, small intestine, and large intestine

what makes up the gastrointestinal tract?

stomach and intestines

accessory organs

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

arrangement of digestive tract walls (inner to outer)

1. mucosa

   1. epithelium, lamina propria, muscularis mucosae

2. submucosa

3. muscularis externa

4. serosa

   1. areolar tissue

   2. mesothelium

mucosa (mucous membrane)

inner lining of tract with:

• epithelium

• lamina propria- loose connective tissue layer

• muscularis mucosae- thin layer of smooth muscle

how does epithelium change throughout the tract?

simple columnar in most of digestive tract

stratified squamous from oral cavity through esophagus and in the lower anal canal (where there is more abrasion)

what does the muscularis mucosae do

tenses the mucosa, creating grooves and ridges that enhance its surface and contact with food

• improves efficiency of digestion and nutrient absorption

MALT

mucosa associated lymphoid tissue

• abundance of lymphocytes and lymphoid nodules

***sometimes extends into submucosa

submucosa

thicker layer of loose connective tissue containing blood vessels and lymphatics, a nerve plexus, and sometimes glands that secrete lubricating mucus into the lumen

muscularis externa

consists of two layers of muscle near outer surface

• cells of inner layer encircle the tract while those of the outer layer run longitudinally

• responsible for motility that propels food and residue through the digestive tract

sphincters

thickened parts of the circular layer that regulate the passage of material through the tract

serosa

thin layer of areolar tissue topped by simple squamous mesothelium

• not found in pharynx, most of the esophagus, or rectum

adventitia

fibrous connective tissue layer that blends into the adjacent connective tissue of other organs

enteric plexus

regulates the digestive tract motility, secretion, and blood flow

• found on the esophagus, stomach and intestines

can function independently of CNS but it exerts significant influence on its action

• part of ANS

has 2 networks of neurons (submucosal and myenteric plexus)

• includes sensory neurons

submucosal plexus

found in submucosa

• controls movements of muscularis mucosae and glandular secretion of mucosa

myenteric plexus

ganglia and nerve fibers between layers of the muscularis externa

• controls peristalsis and other contractions of the muscularis externa

what do sensory neurons in the enteric plexus do

monitor tension in the gut wall and conditions in lumen

mesentery

single, complexly folded serous membrane

• holds abdominal viscera in proper relationship to each other

• provides passage for blood vessels and nerves that supply the digestive organs

• contain many lymph nodes and lymphatic vessels

• nearly every nervous and hormonal signal between lower GI and rest of body goes thru here

mesentery and small intestine’s relationship

small intestine is very mobile as it moves a lot during digestion - mesentery prevents it from becoming tangles by its own contractions

• mesentery’s peritoneal fluid eases intestinal movements

parietal peritoneum

serous membrane that lines the wall of the abdominal cavity

posterior mesentery

transluscent two layered membrane extending to the digestive tract

• when they reach an organ the two layers separate and pass round opposite sides of the organ, forming the serosa

• come together on far side of organ to create anterior mesentery

anterior mesentery

may hang freely in abdominal cavity or attach to the anterior abdominal wall or other organs

what are two anterior extensions of the mesentery

lesser and greater omentum - associated with the stomach

lesser omentum

extends short distance from the liver to the lesser curvature of the stomach

• bile duct embedded in lesser omentum along its course between the liver and duodenum, as well as major blood vessels leading to liver

greater omentum

much larger and fatty extension that hangs like an apron from the greater curvature of the stomach, loosely covering the small intestine

mesocolon

portion of the mesentery that supports the colon

what is the job of omentas?

adhere to perforations or inflamed areas of the stomach or intestine to contribute immune cells to the site, and isolate infections that might otherwise give rise to peritonitis

• greater omentum is part of the body’s first line of defense against toxins and infections

what are milky spots

clusters of lymphoid tissue in the omentum that police around

• collect bacteria/othercells/antigens from peritoneal fluid

• initiate an immune response if a threat is detected

intraperitoneal organs

enclosed by mesentery (serosa) on all sides

• stomach

• liver

• parts of small and large intestines

retroperitoneal organs

lie against the posterior body wall and are covered by peritoneum only on anterior side

• duodenum

• most of pancreas

• parts of large intestine

what controls the motility and secretion of digestive tract

nervous, hormonal, and paracrine mechanisms

short (myenteric) reflexes

stretching or chemical stimulation of digestive tract acts through myenteric plexus to stimulate contractions in nearby regions of the muscularis externa (peristalsis while swallowing)

long (vasovagal) reflexes

act through autonomic nerve fibers that carry sensory signals from the digestive tract to the brainstem and motor commands back to the digestive tract

• vagal parasympathetic fibers important

important hormones to stimulate digestive functions

gastrin and secretin

important paracrine secretions to stimulate digestive functions

histamine and prostaglandins

what is another name for the mouth

oral or buccal cavity

functions of the oral cavity

1. ingestion

2. taste

3. sensory responses to food

4. mastication

5. chemical digestion (partial starch digestion)

6. swallowing

7. speech

8. respiration

oral fissure

anterior opening between lips

fauces

posterior opening of oral cavity into the throat

what tissue epithelium does the mouth have

stratified squamous

• keratinized in areas subject to greatest food abrasion (gums and hard palate)

• nonkeratinized (floor of mouth, soft palate, inside of cheeks and lips)

what are the cheeks and lips important for

• retaining food and pushing it between teeth for chewing

• articulate speech and sucking/lowing actions

labial frenulum

median folled that attaches each lip to the gum, anteriorly between the incisors

vestibule

space between the cheeks or lips and the teeth

lip areas

1. cutaneous

2. red area

3. labial mucosa

cutaneous area of the lip

colored like the rest of hte face and has hair follicles and sebaceous glands

red area (vermilion) of the lip

hairless region where lips meet

• unusually tall dermal papillae, allow blood capillaries and nerve endings to come closer to the epidermal surface

  • redder and more sensitive than cutaneous

labial mucosa

inner surface of lip

• contains mucous labial glands that contribute to the saliva

the tongue

• aids in food intake

• very sensitive

  • receptors for: taste, texture, and temperature

• important in acceptance/rejection of food

• compresses and breaks up food

• maneuvers food between teeth

• dislodges food particles from teeth

• secretes mucus and enzymes

• compresses the chewed food into a soft mass

• initiates swallowing

• necessary for articulate speech

• surface is nonkeratinized stratified squamous epithelium

lingual papillae

bumps and projections that are the site of most taste buds

body of tongue

anterior two thirds of tongue that occupy oral cavity

root of tongue

posterior one third that occupies the oropharynx

vallate papillae

v shaped boundary of papillae between root and body

terminal sulcus

groove behind vallate papillae

lingual frenulum

median fold that attaches the body of tongue to the floor of the mouth

intrinsic muscles

contained entirely within the tongue and produce the relatively subtle tongue movements of speech

extrinsic muscles

arise from attachments outside the otngue and produce most of the stronger tongue movements of food manipulation

lingual glands

serious and mucous glands that secrete a portion of the saliva

lingual tonsils

contained in root of tongue

Palate

separates the oral cavity from the nasal cavity, making it possible to breathe while chewing food

hard palate

bony anterior portion of the palate

• has palatine rugae

palatine rugae

transverse ridges on hard palate that aid the tongue in holding and manipulating food

soft palate

has a more spongy texture and no bone

• has uvula

uvula

helps to retain food in the mouth until one is ready to swallow

Mastication

breaks food into pieces small enough to be swallowed and exposes more surface to the action of digestive enzymes

• first step in mechanical digestion

• requires little thought because food stimulates oral receptors that trigger an automatic chewing reflex

what does saliva do?

• moistens and cleanses the mouth

• inhibits bacterial growth

• dissolves molecules

• stimulates taste buds

• digests starch and fat

• makes swallowing easier by binding food particles into a bolus

• lubricate bolus with mucus

Saliva qualities

• hypotonic solution

• 97-99.5%

• pH of 6.8-7

what solutes are in saliva

• mucus

• electrolytes

• lysozyme

• immunoglobulin

• salivary amylase

• lingual lipase

mucus

binds and lubricates food bolus

electrolytes

salts of Na⁺, K⁺, Cl^-, phosphate, and bicarbonate

lysozyme

enzyme that kills bacteria

immunoglobulin A

antibacterial antibody

salivary amylase

begins starch digestion in the moith

lingual lipase

enzyme that begins fat digestion in mouth (but mainly after swallowing food)

how is salivation controlled?

parasympathetic nerve fibers originating in a group of salivatory nuclei (in brainstem near junctino of pons and medulla oblongata)

role of sympathetic nerve fibers on salivary glands

produce slight increase in salivation, may inhibit during fear or nervous responses (dry mouth)

what do salivatory nuclei respond to?

• feel of food in mouth

• flavor of food sensed by taste buds, oflactory receptors and free nerve endings in oral mucosa

• aroma, sight, or thought of food

• Irritants in pharynx, esophagus, stomach, and upper duodenum (to wash away)

How is salivation increased?

1. parasympathetic stimulation dilates the blood vessels to the salivary glands to increases blood flow

2. cells filter water and electrolytes from blood and add own secretory products to filtrate (salivary solutes)

Pharynx

muscular funnel that connects the oral cavity to the esophagus and the nasal cavity to the larynx

• point where the digestive and respiratory tracts intersects

pharyngeal constrictors

circular muscle in pharynx that force food downward during swallowing

upper esophageal sphincter

inferior pharyngeal constrictors remains contracted to exclude air from the esophagus

• not anatomical, but physiological sphincter

Esophagus

straight muscular tube that penetrates the diaphragm at the esophageal hiatus into the cardiac orifice.

Lower esophageal sphincter

prevents stomach contents from regurgitating into the esophagus, protecting esophageal mucosa from erosive effect of stomach acid

Wall of esophagus layers

• Adventitia (merges into the adventitias of the trachea and thoracic aorta in the mediastinum)

• Muscularis externa (skeletal muscle in upper 1/3, mix of skeletal and smooth in middle 1/3, just smooth in lower 1/3)

• Mucosa (nonkeratinized stratified squamous)

• Submucosa (esophageal glands - secrete lubricating mucus into lumen)

• serosa (partially covers short segment below diaphragm)

deglutition

complex action involving over 22 muscles in the mouth, pharynx, and esophagus

• coordinated by swallowing enter (nuclei in medulla oblongata)

• occurs in 3 phases

3 phases of swallowing

1. Oral

2. Pharyngeal

3. Esophageal

Oral phase

under voluntary control

• during chewing, the tongue collects food, presses it against palate to form a bolus and pushes it posteriorly

• food accumulates in oropharynx in front of blade of epiglottis

• bolus reaches critical size

• epiglottis tips posteriorly and bolus slides around it into the laryngopharynx

pharyngeal phase

involuntary

• soft palate and root of tongue block food and drink from entering the nasal cavity or reentering the mouth

• breathing automatically suspended

• widens upper esophagus to receive food

• pharyngeal constrictors contract in order from superior to middle to inferior

  • drive bolus down esophagus

esophageal phase

peristalsis

• bolus stimulates stretch receptors that feed into the plexus, transmits signals to muscularis externa above and below bolus

• circular muscle layer above the bolus constricts and pushes the food downward

• below bolus circular muscle relaxed while longitudinal muscle contracts

• when bolus reaches lower end of esophagus, lower esophageal sphincter relaxes to allow it to pass into the stomach