A&P II - chapter 25 (digestive system)

digestive system

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

what are the 5 stages of digestion?

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

- occurs in small intestine

compaction

absorbing water and consolidating the indigestible residue into feces

defecation

elimination of feces

mechanical digestion

the physical breakdown of food into smaller particles

- increases surface area (of food)

- exposes more food surface to digestive enzymes

what are examples of mechanical digestion?

- cutting and grinding action of the teeth

- churning action of stomach and small intestines

chemical digestion

a series of hydrolysis reactions that breaks dietary macromolecules into their monomers (residues)

- carried out by digestive enzymes

where are the digestive enzymes that carry out chemical digestion produced?

salivary glands, stomach, pancreas, and small intestine

what are the results of chemical digestion?

- polysaccharides into monosaccharides

- proteins into amino acids

- fats into monoglycerides and fatty acids

- nucleic acids into nucleotides

what nutrients can be directly absorbed?

vitamins, amino acids, minerals, cholesterol, and water

what are the 2 subregions of the digestive system?

digestive tract and accessory organs

digestive tract (alimentary canal)

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

- 30 ft long muscular tube extending from mouth to anus

gastrointestinal (GI) tract

stomach and intestines

what are the accessory organs of the digestive system?

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

what are the layers of the digestive tract?

mucosa

- epithelium

- lamina propria

- muscularis mucosae

submucosa

muscularis externa

- inner circular layer

- outer longitudinal layer

serosa

- areolar tissue

- mesothelium

what is the function of the muscularis mucosae in the digestive tract?

helps increase mucous secretion

- increases pressure onto mucous glands

enteric nervous system

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

- over 100 million neurons

- part of autonomic nervous system

can enteric nervous system function independently of CNS?

yes, it can function independently

- CNS usually externs influence on its action

what 2 networks of neurons is the enteric nervous system composed of?

submucosal plexus and myenteric plexus

submucosal plexus

in submucosa

- controls glandular secretions of mucosa

- controls movements of muscularis mucosae

- controls contraction of submucosa

myenteric plexus

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

- controls peristalsis and other contractions of muscularis externa

mesenteries

connective tissue sheets that suspend stomach and intestines from abdominal wall (anterior and posterior)

- hold abdominal viscera in proper relationship to each other

- provide passage of blood vessels and nerves that supply digestive tract

how does the mesenteries prevent intestines from becoming twisted?

by changes in body position and by its own contractions

what does looseness in the mesenteries effect?

allows stomach and intestines to undergo strenuous contractions with freedom of movement in the abdominal cavity

what do mesenteries contain?

lymph nodes and lymphatic vessels (lots of blood vessels)

- want to monitor concentration of blood to prevent toxins entering bloodstream

parietal peritoneum

serous membrane that lines the wall of the abdominal cavity

- turns inward along posterior midline

- 2 layers of mesentery separate and pass around opposite sides of organ forming serosa

what does the parietal peritoneum form?

dorsal mesentery

dorsal mesentery

a translucent two-layered membrane extending to the digestive tract

how is the anterior (ventral) mesentery formed?

when parietal peritoneum comes together on the far side of the organ and continue as another sheet of tissue

- may hang freely in abdominal cavity

- may attach to anterior abdominal wall or other organs

lesser omentum

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

greater omentum

hangs from the greater curvature of the stomach (its left inferior margin)

- covers small intestine like an apron

- inferior margin turns back on itself and passes upward

what does the greater omentum form?

a deep pouch between its deep and superficial layers

what does the superior margin form?

serous membranes around the spleen and transverse colon (mesocolon)

omentum

fat storage membrane, where we store deep adipose tissue

mesocolon

extension of the mesentery that anchors the colon to the posterior 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

what controls motility and secretion of the digestive tract?

neural, hormonal, and paracrine mechanisms

neural control of digestive tract

short (myenteric) reflexes and long (vagovagal) reflexes

short (myenteric) reflexes

stretch or chemical stimulation acts through myenteric plexus

- stimulates peristaltic contractions of swallowing

long (vagovagal) reflexes

parasympathetic stimulation of digestive motility and secretion

- mass movements of digestive system

hormonal control of digestive tract

-chemical messengers secreted into bloodstream, and stimulate distant parts of the digestive tract

-gastrin and secretin

paracrine secretions of digestive tract

chemical messengers that diffuse through the tissue fluids to stimulate nearby target cells

- go through tissues directly

leptin

hunger hormone that is secreted by adipose tissue (not largely understood)

- if not a lot in the body, hunger occurs and causes menstrual cycle to be absent

oral or buccal cavity

mouth

functions of the mouth

- ingestion (food intake)

- taste and other sensory responses to food

- chewing and chemical digestion

- swallowing, speech, and respiration

what is the mouth enclosed?

cheeks, lips, palate, and tongue

what lines the mouth?

stratified squamous epithelium

- keratinized and nonkeratinized

keratinized stratified squamous epithelium of mouth

in areas subject to food abrasion

- gums and hard palate

nonkeratinized stratified squamous epithelium of mouth

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

tongue

muscular, bulky, but remarkably agile and sensitive organ

- manipulates food between teeth

- senses taste and texture of food

- can extract food particles from the teeth after a meal

what is the surface of tongue the made up of?

nonkeratinized stratified squamous epithelium

lingual papillae of tongue

bumps and projections that are the sites of most taste buds

- fleshy extension of tongue

body of the tongue

anterior two-thirds of tongue

- it occupies oral cavity

root of tongue

posterior one-third of the tongue

- occupies the oropharynx

palate

separates the nasal cavity from the oral cavity

- makes it possible to breathe while chewing food

hard (bony) palate

anterior portion that is supported by the palatine processes of the maxillae and the palatine bones

soft palate

posterior with a more spongy texture

- composed of skeletal muscle and glandular tissue

- no bone

- uvula

uvula

conical medial projection visible at the rear of the mouth

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

regions of the tooth

crown, root, neck, gingival sulcus

crown

portion above the gum

root

the portion below the gum, embedded in alveolar bone

neck

the point where crown, root, and gum meet

gingival sulcus

space between the tooth and the gum

- hygiene in the sulcus is important to dental health

dentin

hard yellowish tissue that makes up most of the tooth

enamel

covers crown and neck

- noncellular secretion that cannot regenerate

- hardest substance of the body

cementum

covers root

- cementum and dentin are living tissue and can regenerate

root canal

space in a root leading to pulp cavity in the crown

- nerves and blood vessels

- apical foramen

- anchored by cementum

apical foramen

pore at the basal end of each root canal

occlusion

meeting of the teeth with the mouth closed

plaque

sticky residue on the teeth made up of bacteria and sugars

- calculus (calcified plaque)

- bacteria metabolize sugars and release acids that dissolve the minerals of enamel and dentin to form dental caries (cavities)

when does root canal therapy become necessary?

when cavity reaches pulp

mastication (chewing)

breaks food into smaller pieces to be swallowed and exposes more surface to digestive enzymes

- first step in mechanical digestion

- food stimulates oral receptors that trigger an involuntary chewing reflex

saliva

moistens mouth, begins starch and fat digestion, cleanses teeth, and inhibits bacterial growth

how does saliva stimulate taste buds?

by dissolving molecules that stimulate them

how does saliva aid in swallowing?

moistens food and binds it together into bolus

bolus

mass swallowed as a result of saliva binding food particles into a soft, slippery, easily swallowed mass

pharynx

muscular funnel connecting oral cavity to esophagus and nasal cavity to larynx

- deep layer of longitudinal skeletal muscle

what does the superficial layer of circular skeletal muscle of the pharynx form?

pharyngeal constrictors (superior, middle, and inferior) that force food downward during swallowing

what remains constricted when not swallowing?

the inferior constrictor (upper esophageal sphincter) remains contracted to exclude air from the esophagus

esophagus

straight muscular tube 25 - 30 cm long

- extends from pharynx to cardial orifice of stomach

- nonkeratinized stratified squamous epithelium

lower esophageal sphincter

-food pauses here because of constriction

-prevents stomach contents from regurgitating into the esophagus

-protects esophageal mucosa from erosive stomach acid

- heartburn

heartburn

burning sensation produced by acid reflux into the esophagus

swallowing (deglutition)

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

- once it hits the uvula, becomes automatic

stomach

a muscular sac in upper left abdominal cavity immediately inferior to the diaphragm

- primarily functions as a food storage organ

- mechanically breaks up food, liquefies it, begins chemical digestion of protein and fat

what is the internal volume of stomach when empty?

about 50mL

what is the volume of the stomach after a typical meal?

1.0 to 1.5 L

what is the volume of the stomach when extremely full?

up to 4L

- can extend nearly as far as the pelvis

chyme

soupy or pasty mixture of semi-digested food in the stomach

what type of mucosa is in the stomach?

simple columnar epithelium mucosa

when are mucosa and submucosa flat?

when stomach is full

when are gastric rugae formed?

when stomach is empty

what are the 3 layers of the muscularis externa?

outer longitudinal, middle circular, inner oblique

gastric pits

depressions in gastric mucosa

- lined with simple columnar epithelium

- 2 or 3 tubular glands open into bottom of each gastric pit

what are the 3 types of glands in gastric pits?

- cardiac glands (cardial part)

- pyloric glands (pyloric parts)

- gastric glands (rest of stomach)

what types of cells are located in the stomach?

mucous cells, parietal cells, chief cells, enteroendocrine (G) cells

mucous cells (stomach)

secrete mucous

- predominate in cardiac and pyloric glands

- in gastric glands, called mucous neck cells since they are concentrated at the neck of the gland