chapter 25: digestion

30 ft long muscular tube extending from the mouth to the anus. its an organ system that processes food, extracts nutrients, and eliminates residue

digestive tract

what structures make up that digestive tract

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

what structures make up the accessory organs digestive tract

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

list the 5 stages of digestion

1. ingestion: selective intake of food

2. diagestion: mechanical & chemical breakdown of food in a form usable to the body

3. absorption: uptake of nutrient molecules into digestive tract & and then into blood/lymph

4. compaction: absorbing water and consolding the indigestible residue into feces

5. defecation: elimination of feces

the physical breakdown of food to expose more food surface to digestive enzymes. ex: the cutting/grinding action of teeth.

mechanical digestion

a series of hydrolysis reactions that breaks dietary macromolecules into their monomers. done by digestive enzymes made by salivary glands, stomach, pancreas, and small intestine

chemical digestion

list the lining of the digestive tract from inner to outer

mucosa, submucosa, muscularis externa, and serosa

internal epithelium lining of the digestive tract that lines the lumen. contains lamina propria (connective tissue) & muscularis mucosae (smooth muscle)

mucosa

glands, blood vessels, lymphatic vessels, and the nerve plexus reside in this layer of the digestive tract

submucosa

contains the inner, circular layer (segment & churning) and outer longitudinal (pushes food through tract) layer of the digestive tract

muscularis externa

the outermost layer of the digestive tract containing areolar tissue (connective tissue that cells absorb/release nutrients and waste from) and lined with simple squamous mesothelium (lower esophagus to sigmoid colon)

serosa

Tenses mucosa creating grooves and ridges that enhance surface area and contact with food. Improves efficiency of digestion and nutrient absorption

muscularis mucosae

what is the pharynx, upper esophagus, and rectum lined with?

adventitia

2 names for the mouth

oral cavity or buccal cavity

functions of the mouth

ingestion, mechanical/chemical digestion, swallowing, speech, and respiration

muscles contained within the tongue, produce subtle tongue movements of speech

intrinsic muscles

attachments on the outside of the tongue. produce stronger movements of food manipulation

extrinsic muscles

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

palate

hangs down from the soft palate at the back of the oral cavity. prevents food from going up into the nasal cavity during swallowing and helps with speech

uvula

how many adult teeth do we have

32

chisel like cutting teeth used to bite off a piece of food

incisors (2)

pointed and act to puncture and shred food

canine (1)

have two rounded bumps = bicuspid. responsible for crushing, shredding, and grinding

premolars (2)

have 4-5 cusps. responsible for crushing, shredding, and grinding

molars (3)

First step in mechanical digestion. breaks down food into smaller pieces, which stimulates oral receptors that trigger an involuntary chewing reflex

mastication

small glands among oral tissues; secrete saliva at constant rate (whether we are eating or not), lingual lipase and lysozyme

intrinsic (minor) salivary glands

the intrinsic salivary glands & their locations

lingual (tongue), labial (lips), palatine (palate), & buccal (cheek)

salivary glands connected by ducts

Extrinsic (major) salivary glands

the extrinsic salivary glands & their locations

parotid (front of earlobe), submandibular (beneath mandible), & sublingual (floor of mouth) gland

components of saliva

97-99.5% water, and the following solutes: mucus (binds and aids in swallowing), electrolytes (Na, K, Cl, etc), lysozymes (kills bacteria), IgA (antimicrobial), salivary amylase (starch digestion), and lingual lipase (fat breakdown)

circular skeletal muscles that form pharyngeal constrictors (superior, middle, and inferior). force food downward during swallowing and the inferior constrictor keeps air out of the esophagus

pharynx

Prevents stomach contents from regurgitating into the esophagus. Protects esophageal mucosa from erosive stomach acid

lower esophageal sphincter (LES)

a complex action involving over 22 muscles in the mouth, pharynx, and esophagus. its coordinated by swallowing center—a pair of nuclei in medulla oblongata

deglutition (swallowing)

the first phase of swallowing: the oral phase

the tongue forms a food bolus and mushes it into the laryngopharynx

the second phase of swallowing: the pharyngeal phase

the palate, tongue, vocal cords, and epiglottis block the oral canal, nasal canal, & airway while pharyngeal constrictors push the bolus into the esophagus

the third phase of swallowing: the esophageal phase

peristalsis drives the bolus downward and relaxation of the lower esophageal sphincter admits it into the stomach

primarily functions as a food storage organ, mechanical digestion, chemical digestion of proteins and some fat. results in chyme

stomach

acidic, soupy mixture of semi-digested food that passes on to the small intestine

chyme

the mucosa & submucosa are flat when the stomach is __ bc they are streched

full

when the stomach is __, it is wrinkled (rugae)

empty

Secrete mucus in the stomach (cardiac and pyloric glands)

mucous cells

produce new cells to replace cells that die in the stomach

regenerative (stem) cells

Secrete hydrochloric acid (activates pepsin & lingual lipase to liquidize food), intrinsic factor (B12 absorption) in the stomach

parietal cells

Secrete gastric lipase (fat in milk) and pepsinogen (converted into pepsin by HCL) in the stomach

chief cells

Secrete hormones and paracrine messengers that regulate digestion in the stomach. also produces gastrin (stimulates gastric juice production)

enteroendocrine cells

the site of nearly all chemical digestion and nutrient absorption

small intestine

Receives secretions from the pancreas liver and gallbladder, neutralizes stomach acid, inactivates pepsin, reactivates amylase, amd pancreatic enzymes take over chemical digestions at this point

duodenum

Functions to absorb water and salts from indigested food before it is eliminated. starts with cecum & appendix attached to cecum

large intestine

densely populated with lymphocytes and is a significant source of immune cells

appendix

a portion of large intestine between ileocecal junction and rectum (Ascending colon, Right colic (hepatic) flexure, Transverse colon, Left colic (splenic) flexure, Descending colon, Sigmoid colon, Rectum, Anal canal, & Anus)

colon

The motility and secretion of the digestive tract are controlled by what 3 things?

hormonal, paracrine, and neural controls

2 hormones that regulate the motility and secretion of the digestive tract

Gastrin (stimulates the release of gastric juice) and Secretin (stimulates the pancreas to produce digestive enzymes)

cell to cell signaling; autocrine-same cell, paracrine-next cell or into tissue fluids. produces Histamine (increases HCl) and Prostaglandins (inhibits gastric secretion)

paracrine controls

contain both short and long reflexes to regulate motility and secretion in the digestive tract

neural controls

what are short reflexes (myenteric nerve plexus)?

stretching or chemical stimulation of the digestive tract that causes peristaltic contractions in nearby regions of the muscularis externa

what long-reflexes (vagovagal)?

receptors involving the CNS triggered by sight, smell, and taste to regulate motility and secretion

regulates the digestive tract motility, secretion, and blood flow

enteric nervous system

in submucosa (CNS via the vagus nerve). it controls movements in the muscularis mucosae and the glandular secretions of the mucosa

Submucosa plexus (Meissner)

in between the layers of the muscularis externa. it controls movements (peristalsis) and contractions of muscularis externa

Myenteric plexus (Auerbach)

when does gastric secretion increase?

when food is eaten

3 phases of gastric activity

1. cephalic phase: stomach being controlled by the brain

2. gastric phase: stomach controlling itself

3. intestinal phase: stomach being controlled by the small intestine

first phase of gastric activity: cephalic phase

vagus nerve stimulates gastric secretions even before food is swallowed

second phase of gastric activity: gastric phase

stomach stretches, activating myenteric/vasovagal reflexes to stimulate gastric secretion. histamine/gastrin stimulate stomach acid and enzyme secretion

third phase of gastric activity: intestinal phase

intestinal gases briefly stimulates the stomach but then secretin, CCK, and the enterogastric reflex inhibit gastric secretion/motility while duodenum processes chyme. sympathetic nerve fibers suppress gastric activity, while parasympathetic stimulation of the stomach is inhibited

After a meal, they absorb nutrients from the blood for metabolism or storage. Remove and degrade: hormones, toxins, bile pigments, and drugs. Between meals, they break down stored glycogen and release glucose into the blood. Secrete into the blood: albumin, lipoproteins, clotting factors, angiotensinogen, and other products

hepatocytes (liver)

___ ___ are reabsorbed (80%) by the ileum and then sent back to the liver to be reused

bile acids

hard masses in either the gallbladder or bile ducts. Composed of cholesterol, calcium carbonate, and bilirubin. Most common in obese women over 40. Can occur when cholesterol becomes too concentrated, precipitates as crystals that grow in size and results in jaundice

gallstones

pancreatic islets that secrete insulin and glucagon

endocrine portion of the pancreas

secretes pancreatic juice: alkaline mixture of water, bicarbonate, and enzymes

exocrine portion of pancreas

Most digestion and nutrient absorption occurs in this part of the small intestine. Has large circular folds for increased absorption

jejunum

Contains Peyer’s Patches. Contents move by segmentation (circular constrictions along the intestine to churn and mix material)

ileum

the ___ ___ ___ activates peristalsis when the stomach empties and stops segmentation

Migrating motor complex

what makes up fecal content?

water, bacteria, fiber, fat, and proteins

what do bacterial flora in the small intestine do?

digest cellulose, and produces vitamins B & K

how is water absorbed in the digestive system?

osmosis following the absorption of salt and organic nutrients

occurs when large intestine absorbs too little water, causes fluid to pass to quickly due to the intestine being irritated

diarrhea

occurs when fecal movement is slow, too much water gets reabsorbed, and feces become hardened

constipation

pouches in the walls of the colon that contract every 30 minutes to promote water and salt absorption

haustra

produces stronger contractions that occur one to three times a day. Moves further distance than haustral. Filling of the stomach and duodenum stimulates motility of the colon

mass movements

Stretching of rectum yields two defecation reflexes:

contraction of the rectum (myenteric plexus) and relaxation of the internal anal sphincter (parasympathetic)

lines external anal sphincter to give voluntary control of the rectum

skeletal muscle