GI System Overview

digestive system organs are categorized into what 2 groups

alimentary canal/GI tract and accessory organs

alimentary canal (aka GI tract) includes what

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

accessory digestive organs include what

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

purpose of alimentary canal/GI tract

digest food and breaks digested fragments

purpose of accessory digestive organs

produce secretions to break down food

major functions of the GI system

1. ingestion

2. propulsion

3. mechanical breakdown

4. digestion

5. absorption

6. defecation

alimentary canal is made of what 4 layers (list from outer to inner)

mucosa, submucosa, muscularis externa, serosa

mucosa secretes what 3 things

mucous, digestive enzymes, hormoens

mucosa function

absorb end products of digestion and protect against infectious disease

submucosa contains rich supply of ______ and _______

blood; lymphatic vessels

muscularis externa is responsible for _______ of food and make up the ________ and _______ layer

motility; inner circular; outer longitudinal

serosa is formed out of _________

connective tissue

the _______ nervous system is the nerve supply for the alimentary canal

enteric

enteric nervous system is also sometimes called the “mini brain”, why?

is has as many neurons here as in our spinal cord (sensory, inter, and motor neurons)

most of the digestive system is a ______ reflex in the enteric

short

short reflexes are controlled by what

enteric neurons

long reflexes involved what

CNS and sympathetic/parasympathetic activity

enteric receptors along the GI tract detect what 3 main things

stretch, osmolarity, pH

effectors in the digestive system are smooth muscle and glands, what can these do

peristalsis and segmentation, hormone secretion

mouth is responsible for what 4 things

ingestion, mechanical breakdown, initiating propulsion, starting digestion

functions of saliva

• cleans mouth

• lubricates food

• dissolves food chemicals

• compacts food bolus

• protects against microorganisms

what are the 2 types of salivary glands

intrinsic (inside oral cavity) and extrinsic (outside oral cavity)

list the 3 extrinsic salivary glands

parotid, submandibular, sublingual

list the 3 intrinsic salivary glands

buccal, labial, palatine

what is saliva made out of

Na, K, water, IgA, amylase

deglutition means what

swallowing

what is the first phase of deglutition

the buccal phase

what happens in the buccal phase of deglutition

the mandible depresses (opens) followed by the tongue elevating to create a downward slope pushing food to the oropharynx

during the buccal phase is the upper esophageal sphincter open or closed

closed

what is the second phase of deglutition, starting at the end of the buccal phase once food has left the mouth

the pharyngeal phase

what is the goal of the pharyngeal phase

prevent the bolus from traveling down the larynx or up the nasal pharynx and opening the upper esophageal sphincter to let bolus into the esophagus

near the beginning of the pharyngeal phase of deglutition, food is pushed towards the esophagus, what is keeping food from traveling to the nasal pharynx or larynx (towards the lungs)

the soft palate and ulva prevents food from entering the nasal pharynx and the epiglottis prevents food from entering the larynx

what comes after the pharyngeal phase in deglutition (step 3)

the esophageal phase

what happens in the esophageal phase once the food bolus is in the esophagus

the upper esophageal sphincter is closed and the bolus is propelled down via peristalsis of the smooth muscle

what happens after the esophageal phase of deglutition (also explain it)

gastroesophageal opening: the lower esophageal sphincter opens allowing bolus to enter the stomach

these last 2 phases together are sometimes called teh pharyngeal-esophageal phase, what are they controlled by

the medulla and lower pons

how long does it take food to pass from the oropharynx to the stomach

8 seconds

step 1 of peristalsis

contraction of the inner circular layer behind the bolus

what happens after the initial contraction of the inner circular muscle layer behind the bolus in peristalsis (what’s step 2)

the outer longitudinal layer contracts widening the section in front of the bolus

define peristalsis

rhythmic contractions/waves of smooth muscle contraction used to move digestive material along the GI tract

in peristalsis, once the area in front of the bolus widens from outer longitudinal contraction, what happens

the contraction in that inner circular layer is still happening and it propels the bolus forward

peristalsis is controlled by _________ cells that surround the lumen of the GI tract. they respond to different ______ and ________ and trigger _______ in smooth muscle

pacesetter; actions; chemicals; action potential

4 functions of the stomach

• storage tank

• further degrades food, converting to chyme

• mixes/churns chyme

• delivers chyme to small intestine

T/F: a lot of absorption happens in the stomach

false, very little absorption happens in the stomach

what is digested in the stomach

mostly proteins because HCl breaks down dietary proteins and pepsin digests proteins into smaller peptides

But, also some lipids (via linginal lipase) and some CHOs are digested

only _____ soluble compounds are absorbed in the stomach (then give an example)

lipid (aspirin/alcohol)

T/F: the stomach undergoes peristalsis like the esophagus

true, this is to move form the fundus to the pylorus with the most vigorous peristalsis happening close to the pylorus to mix the chyme

once the cyme is mixed and churned enough in the stomach from peristalsis, what happens

the pyloric sphincter opens slightly allowing about 3mL of chyme into the duodenum of the small intestine, then the pyloric sphincter closes

why does the pyloric sphincter close after only letting in 3mL of chyme to teh small intestine

to cause retropulsion (pushing of chyme in the backwards direction to further mx it and break it into smaller and smaller peices

list the 4 gland cells of the stomach

mucous cells, parietal cells, cheif cells, and G cells

purpose and function of mucous cells in stomach

secrete alkaline mucous, produces mucous and bicarbonate (HCO3), protects epithelium against stress and acid

gastritis

inflammation caused if anything breaches mucosal barrier (gastric ulcers can form with persistent damage)

gastric secretion has 3 main phases in order to produce gastric juices, list them in order with how much gastric juice they produce

1. cephalic (1/3 of gastric juice)

2. gastric (2/3 of gastric juice)

3. intestinal

first step of the cephalic phase of gastric secretion

occurs before food even enters the stomach and is stimulated by sight and smell of food

this stimualates the vagus nerve which starts this phase

once the vagus nerve is stimulated in the cephalic phase of gastric secretion, what happens

activates parietal cells which produce intrinsic factor and HCL

activates chief cells which produce pepsinogen to help with protein breakdown

what does HCl do in the stomach

increases acidity and digests protein and plant foods

what does intrinsic factor do in the stomach

helps absorb vitamin B12

after the cephalic phase we enter the gastric phase of secretion, what is the first step of this

this starts when the food bolus enters the stomach and gets stimulated because of distension (aka stretching) of the stomach to accommodate for said food bolus

the vagus nerve is then activated again

after the vagus nerve is activated in the gastric phase of gastric secretion what happens

• this activates G cells which produce gastrin

• this gastrin then stimulates parietal cells

• the the parietal cells produce more HCl and intrinsic factor which increases acidity and digestion

• this higher acidity however has a small inhibitory effect on the G cells to prevent to much acidity, the sympathetic nervous system also inhibts G cells in this phase

after the gastric phase is over, the intestinal phase occurs in gastric secretion,,, what is the first step

this phase is largely an inhibitory phase, however there is one stimulatory part with peptones entering the small intestine from the stomach. peptones stimulate intestinal G cells which release gastrin and stimulates parietal cells

after the peptones stimulate intestinal G cells, what happens in the rest of teh intestinal phase of gastric secretion

fatty acids and glucose stimulate S cells which release secretin, this inhibits stomach G cells which then produce less gastrin and therefore also indirectly inhibits parietal cells

these same fatty acids and glucose also stimulate I cells which produce CCK which more directly inhibits parietal cells and inhibits their production of HCl and intrinsic factors

3 parts of the small intestine

duodenum, jejunum, ileum

___% of absorption happens in the small intestine

90

duodenum receives ___ from stomach and ________ from liver and pancreas

chyme; digestive enzymes

main functions of duodenum

neutralize acid, begin absorption from chyme

most nutrient absorption (sugar, amino acids, fatty acids) happens in the _______ of the small intestine

jejunum

ileum absorbs what in the small intestine

really just what was missed by the jejunum (like B12, bile salts, some sugars)

list the 4 main cells of the small intestine

enterocytes, goblet cells, enteroendocrine cells, paneth cells

enterocytes

largely absorb nutrients and electrolytes in villi and secrete intestinal juice to help absorb nutrients from chyme

goblet cells secrete _______

mucus

enteroendocrine cells secrete _____ and ______

secretin; CCK

paneth cells secrete what and kill what

secrete defensins and lysozyme, kill bacteria

segmentation happens where

the small intestine

what is the purpose of segmentation

to mix chyme with bile and enzymes

main goal tho is to enhance absorption

how does segmentation work

opposite segments of the small intestine contract to push chyme in both directions

migrating motility complex

peristalsis that occurs in the fast state (between meals when not eating) by pulling food residue, dead cells, bacteria, and indigested substances into the large intestine

stimulated by motilin

T/F: very little digestion occurs in the large intestine

true

main 3 functions of the large intestine

• form and propel feces to eliminate from body

• absorb water and electrolytes

• produce and absorb vitamins

what 3 vitamins are produced int eh large intestine by bacteria

vitamin K, biotin, and pantothenic acid (B5)

vitamin K functions

used by liver to synthesize clotting factors such as prothrombin

biotin helps with what

glucose metabolism and fat synthesis

pantothenic acid (B5) helps with what

CoA formation which then helps with fatty acid synthesis and other metabolic processes

function of intestinal bacteria

• prevents pathogens from invading

• provides protective and metabolic functions: produce and synthesize vitamins (K,B)

• ferment indigestible CHOs

_________ processes chyme delivered from the ileum of the small intestine and uses peristalsis to move the chyme

ascending colon

the ascending colon absorbs _____ and solidifies to form stool. some ______ are produced here

water; vitamins

the _______ is responsible for water and electrolyte absorption and storing solid feces

transverse colon

in the transverse colon how does feces move through it

haustration: contracting and receiving segments remain in their state for a longer period, basically is slower peristalsis with about 1 min long contractions

power propulsion also moves the feces over a longer distance which is done when we receive more chyme from the ileum

the descending colon acts as a channel between the _______ and ______ colon. it prepares _____ for ______

transverse and sigmoid; waste; elimination

the sigmoid colon acts as a reservoir for accumulating _____. this is where the final _________ of vitamins, minerals, and water occurs and we push _______ into the _________

feces; reabsorption; solid feces; rectum

movement of fecal matter into the rectum triggers what

the urge to defecate

defecation reflex is similar to micturition, what is the first step?

stretch receptors detect movement of feces into the rectum

in the defecation reflex, what happens after stretch receptors detect movement of feces into the rectum

parasympathetic motor fibers stimulate contraction of the rectum and relaxation of the internal anal sphincter

what happens after the relaxation of the internal anal sphincter in the defecation reflex

voluntary motor neurons open/relax the external anal sphincter allowing feces to pass

5 main functions of the liver

• energy metabolism (maintaining blood glucose/lipid levels)

• produces proteins

• drug metabolism (converting them to a form that kidneys can excrete them)

• storage of fat soluble vitamins and minerals

• produces bile

_________ occurs in the liver

gluconeogenesis

lipids produce ______ in the liver for energy

ketone bodies

protein synthesis inside the liver creates what proteins

• albumin (main proteins needed for blood)

• fibrinogen

• prothrombin

• transferrin

• nonessential amino acids → essential amino acids

in the liver, we form _____ to break down fates and manage cholesterol

bile

the liver converts dugs and other toxic substances to be _________ so the kidneys can excrete them

hydrophilic

look into biotransformation more

look into biotransformation more