chapter 25

digestive system

organ system that processes food, extracts nutrients, and eliminates residue

5 stages of the digestive system

-ingestion

-digestion

-absorption

-compaction

-defecation

ingestion

selective intake of food

digestion

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

absorption

uptake of nutrient molecules into the epithelial cells of the digestive tract and then into the blood and lymph

compaction

absorbing water and consolidating the indigestible residue into feces

defecation

elimination of feces

What is mechanical digestion?

The physical breakdown of food into smaller particles.

What role do teeth play in mechanical digestion?

They perform cutting and grinding actions.

What actions occur in the stomach and small intestines during mechanical digestion?

Churning action.

Why is mechanical digestion important for enzymes?

It exposes more food surface to digestive enzymes.

What is chemical digestion?

A series of hydrolysis reactions that breaks dietary macromolecules into their monomers.

What carries out chemical digestion?

Digestive enzymes produced by salivary glands, stomach, pancreas, and small intestine.

What do polysaccharides break down into during chemical digestion?

Monosaccharides.

What do proteins break down into during chemical digestion?

Amino acids.

What do fats break down into during chemical digestion?

Monoglycerides and fatty acids.

What do nucleic acids break down into during chemical digestion?

Nucleotides.

What nutrients can be directly absorbed from ingested food?

Vitamins, amino acids, minerals, cholesterol, and water.

what are the 2 subdivisions of the digestive system?

-digestive tract

-accessory organs

digestive tract (alimentary canal)

-30 ft long muscular tube extending from mouth to anus

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

-gastrointestinal (GI) tract

GI tract

the stomach and intestines

accessory organs

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

What is the status of material in the digestive tract before absorption?

It is considered external to the body until absorbed by epithelial cells.

How is the digestive tract open to the environment?

It is open at both ends.

What happens to food residue after defecation?

It was never in the body.

digestive tract basic structural plan

-mucosa: epithelium, lamina propria, muscularis mucosae

-submucosa

-muscularis externa: inner circular layer, outer longitudinal layer

-serosa: areolar tissue, mesothelium

mucosa (mucous membrane) and makeup

-lines the lumen

-inner epithelium

-lamina propria

-muscularis mucosa

-mucosa-associated lymphatic tissue (MALT)

inner epithelium

-simple columnar in most of digestive tract

-stratified squamous from mouth through esophagus, and in lower anal canal

lamina propria

loose connective tissue layer

muscularis mucosa

-thin layer of smooth muscle

-tenses mucosa creating grooves and ridges that enhance surface area and contact with food

-improves efficiency of digestion and nutrient absorption

mucosa associated lymphatic tissue (MALT)

the mucosa exhibits an abundance of lymphocytes and lymphatic nodules

What is the submucosa?

A thicker layer of loose connective tissue.

What does the submucosa contain?

Blood vessels, lymphatic vessels, a nerve plexus, and mucus-secreting glands.

What is the function of mucus-secreting glands in the submucosa?

They dump lubricating mucus into the lumen.

What extends into the submucosa in some parts of the GI tract?

MALT (Mucosa-Associated Lymphoid Tissue).

muscularis externa

-consists of usually two layers of muscle near the outer surface

-inner circular layer

-outer longitudinal layer

inner circular layer

in some places, this layer thickens to form valves (sphincters) that regulate the passage of material through the tract

outer longitudinal layer

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

serosa

-composed of a thin layer of areolar tissue

topped by simple squamous mesothelium

-begins in the lower 3 to 4 cm of the esophagus

-ends just before the rectum

-adventitia

adventitia

fibrous connective tissue layer that binds and blends the pharynx, most of the esophagus, and the rectum into adjacent connective tissue of other organs

What is the enteric nervous system?

A nervous network in the esophagus, stomach, and intestines that regulates digestive tract motility, secretion, and blood flow.

How many neurons are thought to be in the enteric nervous system?

Over 100 million neurons.

Can the enteric nervous system function independently of the central nervous system?

Yes, it can function independently.

Does the central nervous system influence the enteric nervous system?

Yes, the CNS usually exerts influence on its action.

What system is the enteric nervous system often considered part of?

The autonomic nervous system.

what are the 2 networks of neurons that makeup the enteric nervous system?

-submucosal (Meissner) plexus

-myenteric (Auerbach) plexus

submucosal (meissner) plexus

-in submucosa

-controls glandular secretions of mucosa

-controls movements of muscularis mucosae

myenteric (auerbach) plexus

-parasympathetic ganglia and nerve fibers between the two layers of the muscularis externa

-controls peristalsis and other contractions of muscularis externa

What are mesenteries?

Connective tissue sheets that suspend the stomach and intestines from the abdominal wall.

What is the function of mesenteries in the abdominal cavity?

They allow the stomach and intestines to undergo strenuous contractions with freedom of movement.

How do mesenteries help maintain the position of abdominal viscera?

They hold abdominal viscera in proper relationship to each other.

What do mesenteries prevent in the intestines?

They prevent intestines from becoming twisted and tangled by changes in body position and by its own contractions.

What do mesenteries provide passage for?

They provide passage for blood vessels and nerves that supply the digestive tract.

What do mesenteries contain that is important for the immune system?

They contain many lymph nodes and lymphatic vessels.

What is the parietal peritoneum?

A serous membrane that lines the wall of the abdominal cavity.

Where does the parietal peritoneum turn inward?

Along the posterior midline.

What is the dorsal mesentery?

A translucent two-layered membrane extending to the digestive tract formed by the parietal peritoneum.

What happens to the two layers of the mesentery around an organ?

They separate and pass around opposite sides of the organ, forming the serosa.

What is the anterior (ventral) mesentery?

A sheet of tissue that the two layers of the mesentery come together to form on the far side of the organ.

How may the parietal peritoneum be positioned in the abdominal cavity?

It may hang freely or attach to the anterior abdominal wall or other organs.

lesser omentum

a ventral mesentery that extends from the lesser curvature of the stomach to the liver

Where does the greater omentum hang from?

The greater curvature of the stomach.

What does the greater omentum cover?

The small intestine, like an apron.

What happens to the inferior margin of the greater omentum?

It turns back on itself and passes upward.

What does the greater omentum form between its deep and superficial layers?

A deep pouch.

What does the inner superior margin of the greater omentum form around the spleen and transverse colon?

Serous membranes, known as mesocolon.

mesocolon

extension of the mesentery that anchors

the colon to the abdominal wall

intraperitoneal

-when an organ is enclosed by mesentery on both sides

-considered within the peritoneal cavity

-stomach, liver, and parts of small and large intestine

retroperitoneal

-when an organ lies against the posterior body wall and is covered by peritoneum on its anterior side only

-considered to be outside the peritoneal cavity

-duodenum, pancreas, and parts of the large intestine

oral or buccal cavity

the mouth

buccal cavity functions

-ingestion (food intake)

-taste and other sensory responses to food

-chewing (mechanical/physical breakdown) and some chemical digestion

-swallowing, speech, and respiration

what type of epithelium lines the mouth?

stratified squamous epithelium

keratinized

-in areas subject to food abrasion

-gums and hard palate

nonkeratinized

-in other areas

-floor of mouth, soft palate, and insides of cheeks and lips

dentition

the teeth

masticate

-chew food into smaller pieces

-makes food easier to swallow

-exposes more surface area for action of digestive enzymes, speeding chemical digestion

-first step in mechanical digestion

saliva functions

-moistens mouth

-begins starch and fat digestion

-cleanses teeth

-inhibits bacterial growth

-dissolves molecules so they can stimulate the taste buds

-moistens food and binds it together into bolus to aid in swallowing

what type of solution is saliva?

a hypotonic solution of 97.0% to 99.5%

water and solutes

what solutes are contained in saliva?

-salivary amylase

-lingual lipase

-mucus

-lysozyme

-immunoglobulin A (IgA)

-electrolytes

salivary amylase

enzyme that begins starch digestion in the mouth

lingual lipase

enzyme that is activated by stomach acid

and digests fat after food is swallowed

mucus

binds and lubricates a mass of food and aids in swallowing

lysozyme

enzyme that kills bacteria

immunoglobulin A (IgA)

an antibody that inhibits bacterial growth

electrolytes

Na+, K+, Cl−, phosphate, and bicarbonate

what is the pH of saliva?

6.8-7.0 (slightly acidic)

extrinsic salivary glands

-three pairs connected to oral cavity by ducts

-parotid

-submandibular gland

-sublingual gland

parotid

-located beneath the skin anterior to the earlobe

-mumps is an inflammation and swelling of the parotid gland caused by a virus

submandibular gland

-located halfway along the body of the mandible

-its duct empties at the side of the lingual frenulum, near the lower central incisors

sublingual gland

-located in the floor of the mouth

-has multiple ducts that empty posterior to the papilla of the submandibular duct

swallowing (deglutition)

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

swallowing center

-pair of nuclei in medulla oblongata that coordinates swallowing

-communicates with muscles of the pharynx and esophagus by way of trigeminal, facial, glossopharyngeal, and hypoglossal nerves

What is the length of the esophagus?

25 - 30 cm long

At what level does the esophagus begin?

Between C6 and the cricoid cartilage

Where does the esophagus extend from and to?

From the pharynx to the cardiac orifice of the stomach

What structure does the esophagus pass through to reach the stomach?

Esophageal hiatus in the diaphragm

What is the function of the lower esophageal sphincter?

Food pauses here because of constriction

What does the lower esophageal sphincter prevent?

Stomach contents from regurgitating into the esophagus

How does the lower esophageal sphincter protect the esophagus?

It protects esophageal mucosa from erosive stomach acid

What sensation is produced by acid reflux into the esophagus?

Heartburn