Chapter 23: The Digestive System

digestive system

functions to take in food and break it down into nutrient molecules that can be absorbed into the blood stream, ridding the body of anything indigestible that remains

alimentary canal

“GI tract”; continuous muscular tube where food is broken down into fragments (digested) mechanically and chemically, and absorbed

mechanical digestion

physical breakdown of food which includes chewing and swallowing food, mixing food with saliva, and the mixing and churning of food with stomach acid in the stomach

chemical digestion

enzymatic breakdown of food that occurs in the mouth, pharynx, esophagus, stomach, and intestines

accessory digestive organs

teeth, tongue, gallbladder, digestive glands (liver, salivary, and pancreas glands) aid the digestive organs in the breakdown of food physically or by producing necessary enzymes

ingestion

the taking in of food

propulsion

the movement of food, which includes swallowing and peristalsis

peristalsis

between meals; coordinated contractions that force food down the alimentary canal (major means of propulsion); contraction/relaxation coupling of rest and digest process

segmentation

local, coordinated constriction in the intestines to mix chyme with digestive juices

digestion

a series of catabolic steps where enzymes break down complex molecules (macromolecules) into monomers that are small enough for absorption

defacation

the passing of undigested materials as feces

peritoneum

serous membrane of the abdominal cavity (visceral and parietal)

visceral peritoneum

the part of the serous membrane of the abdominal cavity that exists on the surface of most digestive organs

parietal peritoneum

the part of the serous membrane of the abdominal cavity that lines the cavity wall

peritoneal cavity

prevents friction between the visceral and parietal peritonea

mesentery

double-layered peritoneum that provides routes for blood vessels to innervate the digestive system; stores fat and secures digestive organs in place

intraperitoneal organs

organs inside the peritoneum

retroperitoneal organs

organs outside the peritoneum

peritonitis

inflammation of the peritoneum that can be caused by abdominal wounds, chemical damage to the abdominal wall (ulcers), or appendicitis that results in pain due to the dense innervation of the digestive system and chemoreceptors

mucosa

lines the lumen of the alimentary canal to secrete mucus, digestive enzymes, and some hormones, absorbs the end-products of digestion, and can help to protect against infections and diseases

epithelium

secretes mucus to protect the alimentary canal from strong digestive enzymes

lamina propria

the primary capillary location of the alimentary canal that contains lymphoid follicles (part of mucosa)

muscular mucosa

(“muscularis”) smooth muscle of the alimentary canal that produces localized movements of the mucosa

submucosa

layer of the alimentary canal that contains blood, lymphatic vessels, and lots of elastic fibers to allow the canal to recoil and retain its shape after stretch

muscularis external

areas and prevent layer of the alimentary canal that is thick in places with sphincters to close food from moving from one layer to the next

serosa

the outermost layer of the alimentary canal that is continuous with the visceral peritoneum; single layer of simple squamous epithelium

eccentric nervous system

innervates the GI tract with more than the spinal cord

submucosal nerve plexus

helps regulate glands and smooth muscle with the mucosa of the digestive system

myenteric nerve plexus

helps control GI tract motility

short reflexes

mediated by the enteric nervous system and responds to stimuli within the GI tract (intrinsic)

long reflexes

external factors (smell, taste, etc. - extrinsic) are affected by the sympathetic and parasympathetic nervous system

regulation

provoked by mechanical and chemical digestion stimulation; receptors detect changes in osmolarity, pH, bi-products of digestion for reabsorption, etc.

effectors

smooth muscle and glands whose reflexes are stimulated by some receptors to move contents of the lumen to specific areas and can reflexively inhibit or release enzymes and/or hormone

mouth

“buccal/oral cavity”, made up of lips, the palates, tongue, and cheeks

lips

(labia) fleshy reticuoral vestilaris muscles that anteriorly bind the mouth

palate

superiorly binds the mouth; hard and soft

soft palate

helps to close off the nasopharynx during swallowing and can contribute to snoring

tongue

complex skeletal muscle that posteriorly binds the mouth and positions/molds food, mixes food and saliva, and is key for swallowing, taste, and speech

oral vestibule

the area between the teeth and the lips

labial frenulum

the sharp, small piece of tissue between the teeth and the lips; superior and inferior

linguil frenulum

connects the tongue to the base of the mouth

filiform papillae

bumps on the tongue that contribute to its roughness but do not contain any tastebuds

fungiform papillae

mushroom-shaped taste buds that are scattered over the tongue and are highly vascularized, leading to the tongue’s pinkish hue

vallate papillae

taste buds that create a V-shape on the back of the tongue

foliate papillae

taste buds located on the lateral aspects of the tongue

salivary glands

produce saliva and are controlled by chemo- and mechano-receptors that detect the presence of food and stretch of stimuli to notify the parasympathetic nervous system; parotid, submandibular, and sublinguil

saliva

primarily water, slightly acidic (pH of 6.8), with lots of electrolytes and digestive enzymes (linguil and salivary amylase), as well as proteins and wastes; helps to cleans the mouth, chemically dissolves food creating “tastes”, moistens foods, and is where starch breakdown begins (with salivary amylase)

xerostomia

“dry mouth”; a lack of saliva that is caused by THC use, HIV/AIDS, and certain medications and can lead to dental carries and tooth decay

teeth

dentitions; key starters of the mechanical digestive process; shape-determines-funuction

incisors

teeth that are chisel-shaped for cutting (4 upper and 2 lower)

canines

teeth that are pointy and fang-like for piercing and tearing food (2 upper and 2 lower)

premolars

teeth also known as “bicuspids”

molars

large teeth for grinding and crushing; key for mechanical digestion of food in oral cavity (4 upper and 4 lower)

crown

the exposed part of a tooth (above the gums) that is the hardest substance in the body

enamel

a heavily mineralized substance made of calcium that makes up a layer on the crown of teeth

root

the part of the tooth under the gums; some teeth have 1, but molars have 2, 3, or 1 extra-large one

cement

holds teeth in place with dental ligaments that surround the root

impacted teeth

teeth that are stuck in the jaw bone that can cause pain and/or structural damage

dental carries

“cavities”; where bacteria wears away enamel

gingivitis

inflammation of the gums that occurs when plaque builds up and calcifies into tartar, disrupting the boundary between teeth and gums, leading to inflammation and bleeding

peridontitis

bad gingivitis that leads to tooth decay

pharynx and esophagus

(oropharynx, laryngopharynx, and esophagus) passage for food, air, and fluids that is made up of stratified squamous epithelium to resist stress from stretch and abrasions with mucus-producing cells to coat the tube for protection

esophagus

organ whose main function is the movement of food (peristalsis); no digestion occurs here

gastroesophageal sphincter

esophagus-stomach junction that surrounds the cardio orifice to keep it closed when not swallowing food to prevent acid from escaping the stomach (controls what enters the stomach)

mucus cells

protect bodily tissues from stomach acid

heartburn

“acid reflux”; when the sphincter opens due to damage or it malfunctions, allowing acid to escape from the stomach, leading to burning sensations

deglutination

swallowing; begins with the voluntary contraction of the tongue to force food bolus from the mouth to the oropharynx

bolus

a ball of food and saliva

pharyngoesophageal phase

when the tongue blocks off the mouth to prevent back flow and peristalsis occurs, but no digestion

stomach

extremely expansive tank for food storage where chemical breakdown begins

chyme

paste-like substance that food bolus is transformed into from churning aspect of the stomach

cardia

surrounds the cardio orifice of the stomach

fundus

dome of the stomach that sits just inferior to the diaphragm

pyloric sphincter

separates the stomach from the duodenum to control what leaves the stomach

greater omentum

(greater curvature) helps anchor the spleen, stomach, and pancreas to the abdominal wall and contains fat-deposits and lymph nodes

modified muscularis externa

circulate, longitudinal, oblique layers of the stomach that allow three different directions of contractions, leading to the churning aspect and mixing of stomach acids and chyme

pummelling

mixing that occurs due to the churning of the stomach, packing the chyme into the small intestine

modified mucosa

made of simple columnar epithelial cells and mucus-producing cells that produce about 3 L a day of double-layered secretions for protection (alkaline and bicarbonate buffer)

gastric pits

lead into gastric glands which produce gastric juice

secretory cells

rapidly dividing cells that completely regenerate every 3 to 6 days; mucus neck cells, parietal cells, chief cells, enteroendocrine cells, and the mucosal barrier

mucus neck cells

secrete mucus

parietal cells

secrete HC (low pH) to denature proteins, break down plant cell walls, kill bacteria, and help to secrete intrinsic factor

intrinsic factor

glycoprotein that helps absorb vitamin B12

pepsinogen

chief cells produce this inactive form of pepsin, which is activated by HCl

chief cells

secrete pepsinogen and lipases (lipid digestive enzymes)

enteroendocrine cells

secrete chemical messengers into surrounding tunics of the stomach (lamina propria); some act like paracrines, others produce serotonin, histamine, gastrin, and somatotropin

mucosal barrier

part of bicarbonate mucus layer created by tight junctions between epithelial cells that prevent caustic secretions from damaging tissues

stimulation of stomach

causes pH levels to rise, creating more gastrin, so more HCL is released; cephalic/reflex phase, gastric phase, and G cells

cephalic/reflex phase

a conditioned, stimulating response to seeing/thinking/smelling/tasting food that directly leads into the gastric phase

gastric phase

stretch receptors are primed so that stretch stimulates telling the brain how full someone is

G cells

secrete gastrin for the stimulation of HCl release from parietal cells, causing enteroendocrine cells to release histamine

distinction of duodenum

stretch and expansion from over-filling tell the stomach to reduce gastric secretions to slow down chemical digestion

HCl formation

parietal cells pump hydrogen from the breakdown of carbonic acid into the lumen of the stomach through the movement of a proton pump (stopping the movement of hydrogen ions)

stomach filling

operates according to pressure with reception relaxation and gastric accommodation

reception relaxation

reflex mediated by the brain stem when swallowing food, telling the stomach to relax and not contract

gastric accommodation

“pliability”; the stomach’s innate ability to stretch and accommodate more food

peristaltic wave

gastric contractile activity that occurs 3 times every minute and propels food from one location towards the pyloric sphincter along with grinding and pummeling contractions

liver

the largest gland in the body (secondary digestive organ) that has different ligaments to separate its 4 lobes whose main function is the production of bile; regenerative

bile ducts

a combination of ducts in the liver that produce bile for fat breakdown

hepatocytes

liver cells that filter and process nutrient rich foods