Ch. 9: The Digestive System Flashcards
defn: intracellular digestion (+ what process is this a part of)
part of metabolism
involves the oxidation of glucose and fatty acids for energy
defn + loc: extracellular digestion
the process by which nutrients such as glucose and fatty acids are obtained from food
occurs within the lumen of the alimentary canal
why is the lumen of the alimentary canal technically outside of the body?
because the lumen of the GI tract communicates directly with the outside world
def: alimentary canal
runs from the mouth to the anus, sectioned off by sphincters
defn: sphincter
circular smooth muscles around the canal that can contract to allow compartmentalization of function
defn: digestion vs. absorption
DIGESTION = the breakdown of food into its constituent organic molecules
ABSORPTION = the transport of products of digestion from the digestive tract into the circulatory system for distribution to the body’s tissues and cells
what are the constituent organic molecules that digestion can break food into? + what specific part of food do they come from? (3)
- starches and other carbs into monosaccharides
- lipids (fats) into free fatty acids and glycerol
- proteins into amino acids
defn: mechanical digestion
the physical breakdown of large food particles into smaller food particles (no breaking of chemical bonds)
defn: chemical digestion
the enzymatic cleavage of chemical bonds (i.e. the peptide bonds of protein or the glycosidic bonds of starches)
pathway: the digestive tract (7) + what are the 4 other body parts that help to provide the enzymes and lubrication necessary to aid in digestion?
- oral cavity (mouth)
- pharynx
- esophagus
- stomach
- small intestine
- large intestine
- rectum
FOUR OTHERS
1. salivary glands
2. pancreas
3. liver
4. gallbladder
defn: pharynx
a shared pathway for both food entering the digestive system and air entering the respiratory system
func: rectum
waste products of digestion enter here
where feces are stored until an appropriate time of release
defn: enteric nervous system
a collection of one hundred million neurons that govern the function of the GI system
func + loc: neurons of the enteric nervous system
LOC: walls of the digestive tract
FUNC: trigger peristalsis
defn: peristalsis
rhythmic contractions of the gut tube in order to move materials through the system
can the enteric nervous system function independently of the brain and spinal cord?
yes, but its heavily regulated by the autonomic nervous system
what is the impact of the parasympathetic nervous system on the digestive system? (2)
- stimulation of digestive activities, increasing secretions from exocrine glands and promoting peristalsis
- sleepy and lethargic after eating a big meal
what is the impact of the sympathetic nervous system on the digestive system (2)?
- inhibition of digestion, peristalsis
- bloodflow decreased to digestive tract
what impact do ADH and aldosterone have on ingestion?
they trigger the sensation of thirst –> encourages the behavior of fluid consumption
what impact do glucagon and ghrelin have on ingestion?
stimulate feelings of hunger
what sections of the digestive system does digestion occur through?
- oral cavity
- stomach
- duodenum
does the oral cavity play a role in mechanical or chemical digestion of food?
both!
defn: mastication
chewing
mechanical digestion in the mouth involving the breaking up of large food particles into smaller particles using the teeth, tongue, and liips
how does chewing increase enzymatic digestion?
chewing helps to increase the surface area-to-volume ratio of the food, creating more surface area for enzymatic digestion
what is the role of saliva in mechanical and chemical digestion?
CHEMICALLY –> the breakdown of chemical bonds in the macromolecules relies on enzymes from saliva
MECHANICALLY –> adds mechanical digestion by moistening and lubricating food
how is saliva production stimulated (2)?
- presence of food in the oral cavity triggers a neural circuit that ultimately leads to increased parasympathetic stimulation of the salivary glands
- signals that food is near (smell, sight)
func + aka + where is this: salivary amylase
aka: ptyalin
where is this: saliva
hydrolyzes starch into smaller sugars (maltose and dextrins)
func + where is this: lipase
where is this: saliva
catalyzes the hydrolysis of lipids
defn: bolus
food forms into a bolus by our muscular tongue
forced back to the pharynx and swallowed
defn: pharynx
the cavity that leads from the mouth and posterior nasal cavity to the esophagus and also connects to the larynx
what are the 3 parts of the pharynx + where are they located?
- nasopharynx (behind the nasal cavity)
- oropharynx (at the back of the mouth)
- laryngopharynx (above the vocal cords)
defn + func + what happens if it doesnt work: epiglottis
a cartilaginous stricture that folds down to cover the laryngeal inlet
prevents food from entering the larynx during swallowing
failure: aspiration of food and choking
defn: esophagus
a muscular tube that connects the pharynx to the stomach
what type of muscle is the esophagus composed of?
top third: skeletal muscle
bottom third: smooth muscle
middle third: mix of both
how does the types of muscle that the esophagus is composed of impact nervous control?
top of esophagus: under somatic (voluntary) motor control
bottom: under autonomic (involuntary) nervous control
defn: peristalsis
the rhythmic contraction of smooth muscle that propels food toward the stomach
what is vomiting in relation to peristalsi?
the opposite! reversal of peristalsis
what are 4 things that can lead to vomiting?
- exposure to chemicals
- infectious agents
- physical stimulation in the posterior pharynx
- cognitive stimulation
where is swallowing initiated?
the upper esophageal sphincter (the muscles of the oropharynx)
func: lower esophageal sphincter
as the bolus approaches a stomach, this muscular ring relaxes and opens to allow the passage of the food
where is the chemical digestion of carbohydrates and fats initiated?
in the mouth
does any digestion take place in the esophagus?
not really! except for the continued enzymatic activity initiated in the mouth by salivary enzymes
defn: stomach
a highly muscular organ with a capacity of approx. 2 liters that uses hydrochloric acid and enzymes to digest food (creating a harsh environment)
why is the mucosa of the stomach thick?
to prevent autodigestion
what are the four main anatomical divisions of the stomach and what kind of glands do they contain?
contain gastric glands: 1. fundus 2. body
contain pyloric glands: 3. antrum 4. pylorus
which is the lesser and greater curvature of the stomach?
internal: lesser
external: greater
defn: rugae
folds of the stomach that holds the lining of the stomach
func: gastric glands
respond to signals from the vagus nerve of the parasympathetic nervous system which is activated by the brain in response to the sight, taste, and smell of food
what are the three different cell types that gastric glands have?
- mucous cells
- chief cells
- parietal cells
func: mucous cells
produce the bicarbonate-rich mucus that protects the muscular wall from the harshly acidic and proteolytic environment of the stomach
defn: gastric juice
a combination of secretions from two cell types in the gastric glands: chief cells and parietal cells
func: chief cells
secrete pepsinogen
defn: pepsinogen
the inactive, zymogen form of pepsin (a proteolytic enzyme)
func (2): parietal cells
secrete hydrogen ions in the form of hydrochloric acid which cleave pepsinogen into pepsin
secrete intrinsic factor
func: pepsin
digests proteins by cleaving peptide bonds near aromatic amino acids, resulting in short peptide segments
what pH are most human enzymes activated at? what pH is pepsin activated at?
most human enzymes: physiological pH
pepsin: low pH (activated by an acidic environment)
what is the one harmful bacteria that stomach acid does NOT kill? and what are the effects of this bacteria?
H. pylori
usually asymptomatic, can cause inflammation, ulcers, and certain gastric cancers
defn: intrinsic factor
a glycoprotein involved in the proper absorption of vitamin B12
what are the six products that the stomach secretes and what are their main functions?
- HCl –> kills microbes, denatures proteins, and converts pepsinogen into pepsin
- Pepsinogen –> cleaved in the stomach to pepsin; an enzyme that partially digests proteins
- mucus –> protects mucosa
- bicarbonate –> protects mucosa
- water –> dissolves and dilutes ingested material
- intrinsic factor –> required for normal absorption of vitamin B12
func + loc: G-cells
loc: pyloric glands
func: secrete gastrin
defn + func: gastrin
a peptide hormone
induces the parietal cells in the stomach to secrete more HCl and signals the stomach to contract, mixing its contents
defn: chyme
an acidic, semifluid mixture that results from the digestion of solid food in the stomach
what allows for maximal absorption of nutrients from chyme in the small intestine?
the combined mechanical and chemical digestive activities of the stomach results in a significant increase in the surface area of the food particles
char (2): small intestine
- 3 segments (duodenum, jejunum, ileum)
- long (up to 7 m)
where does the majority of chemical digestion occur in the small intestine? where does the majority of absorption occur in the small intestine?
DIGESTION: duodenum
ABSORPTION: jejunum and ileum
where does the food go after it leaves the stomach?
it leaves the stomach through the pyloric sphincter and enters the duodenum
what causes the release of brush-border enzymes in the duodenum?
the presence of chyme in the duodenum
what are 2 groups + 5 specific brush-border enzymes?
- disaccharides (maltase, isomaltase, lactase, sucrase)
- peptidases (dipeptidase)
func + location: brush-border enzymes
present on the luminal surface of cells lining the duodenum
break down dimers and trimers of biomolecules into absorbable monomers
what 2 other things does the duodenum secrete + what do they do?
- enteropeptidase (involved in the activation of other digestive enzymes from the accessory organs of digestion)
- hormones like secretin and cholecystokinin (CCK) into the bloodstream
func + 4 specifics: disaccharidases
DIGEST disaccharides
maltase digests maltose
isomaltase digests isomaltose
lactase digests lactose
sucrase digests sucrose
what happens after disaccharides are broken down?
bacteria in the intestines are able to hydrolyze that disaccharide, producing methane gas as a byproduct
what impact do undigested disaccharides have?
an osmotic effect, pulling water into the stool and causing diarrhea
this is why people who are lactose intolerant have symptoms of bloating, flatulence, and diarrhea after ingesting dairy products
func: peptidases
break down proteins (or peptides)
defn + func: aminopeptidase
a peptidase secreted by glands in the duodenum that removes the N-terminal amino acid from a peptide
func: dipeptidases
cleave the peptide bonds of dipeptides to release free amino acids
what is a difference in how carbohydrates and proteins are broken down?
carbs must be broken down into MONOsaccharides for absorption
proteins can be broken down into di and tripeptides and can be absorbed
defn + secondary func: enteropeptidase
an enzyme critical for the activation of trypsinogen, a pancreatic protease, to trypsin
can also activate procarboxypeptidases A and B to their active forms
func: tryspin
initiates an activation cascade
defn + secondary func: secretin
a peptide hormone that causes pancreatic enzymes to be released into the duodenum
also regulates the pH of the digestive tract by reducing HCl secretion from parietal cells and increasing bicarbonate secretion from the pancrease
defn + example: enterogastrone
a hormone that slows motility through the digestive tract
secretin is an example
what is the benefit of slowing motility through the digestive tract?
allows increased time for the digestive enzymes to act on chyme (especially fats)
why is CCK secreted + func (2): cholecystokinin (CCK)
secreted in response to the entry of chyme (amino acids and fat in the chyme) into the duodenum
stimulates the release of bile and pancreatic juices and also acts in the brain, promoting satiety
defn: bile
a complex fluid composed of bile salts, pigments, and cholesterol
defn + origin + func: bile salts
derived from cholesterol
they are NOT enzymes, but amphipathic molecules that can emulsify fat in the digestive system
serve an important role in the mechanical digestion of fats and ultimately facilitate the chemical digestion of lipids
why are bile salts able to serve as a bridge between aqueous and lipid environments?
their hydrophobic and hydrophilic regions
why are bile salts similar to soaps?
in the small intestine, bile salts emulsify fats and cholesterol into micelles
what would happen without bile?
fats would spontaneously separate out of the aqueous mixture in the duodenum and would not be accessible to pancreatic lipase, which is water-soluble
why do micelles improve the work of lipase?
micelles increase the surface area of the fats, increasing the rate at which lipase can act
summary: how proper fat digestion depends on both bile and lipase
BILE gets the fats into the solution and increases their surface area by placing them in micelles (mechanical digestion)
then LIPASE can come in to hydrolyze the ester bonds holding the lipids together (chemical digestion)
defn + func: pancreatic juices
a complex mixture of several enzymes in a bicarbonate-rich alkaline solution
func: 1. this bicarb helps to neutralize acidic chyme and 2. provides an ideal working environment for digestive enzymes (which are most active around pH 8.5)
3. contain enzymes that can digest all 3 types of nutrients (carbs, fats, and proteins)
what are the 3 accessory organs of digestion and where do they all derive from?
- pancreas
- liver
- gallbladder
all originate as outgrowths of endoderm from the gut tube during development
endocrine func: pancreas
release of insulin, glucagon, and somatostatin (peptide hormones necessary for the maintenance of proper blood sugar levels)
where is the hormonal function of the pancreas limited to?
cells residing in islets of Langerhans scattered throughout the organ
defn: acinar cells
exocrine cells that make up the bulk of the pancreas and produce pancreatic juices
func: pancreatic amylase
breaks down large polysaccharides into small disaccharides and is thus responsible for carbohydrate digestion
func + 4 names: pancreatic peptidases
released in their zymogen form, but once activated are responsible for protein digestion
- trypsinogen
- chymotrypsinogen
- carboxypeptidases A and 4. B
why is enteropeptidase the master switch?
it converts trypsinogen to trypsin, which can then activate the other zymogens and also activates procarboxypeptidases A and B
func: pancreatic lipase
capable of breaking down fats into free fatty acids and glycerol
where do all exocrine cells secrete their products into?
ducts!
where to pancreatic juices go?
they are transferred to the duodenum via a duct system that runs along the middle of the pancreas
these ducts then empty into the duodenum through the major and minor duodenal papillae
diagram: pancreas anatomy
summary (5): liver functions
- processing and synthesis of nutrients (glycogenesis and glycogenolysis, storage and mobilization of fats, gluconeogenesis)
- production of urea
- detoxification of chemicals
- production of bile
- synthesis of albumin and clotting factors
what are the 2 unique structures that the liver has for communicating with the digestive system?
- bile ducts
- hepatic portal vein
func: bile ducts
- connect the liver with the gallbladder and the small intestine
- bile is produced in the liver and travels down these bile ducts where it may be stored in the gallbladder or secreted into the duodenum
func: hepatic portal vein
the liver receives all blood draining from the abdominal portion of the digestive tract through the hepatic portal vein
explain how nutrient-rich blood can be processed by the liver before draining into the inferior vena cava
- the liver takes up excess sugar to create glycogen, the storage form of glucose
- the liver stores fats as triacylglycerols
- the liver can also reverse these processes, producing glucose for the rest of the body through glycogenolysis and gluconeogenesis and mobilizing fats in lipoproteins
what does the liver detoxify (2 categories + examples of each)
- endogenous compounds (those made in the body)
- ex: liver converts ammonia (a toxic waste product of amino acid metabolism) to urea (which can be excreted by the kidneys) - exogenous compounds (those brought in from the environment)
- ex: alcohol and medications
defn + origin: bilirubin
the major pigment in bile
a byproduct of the breakdown of hemoglobin
what happens to bilirubin when it travels to the liver?
it is conjugated (attached to a protein) and secreted into the bile for excretion
what happens if the liver is not able to process or excrete bilirubin?
jaundice may occur
what 2 important proteins does the liver synthesize + what are the funcs of these proteins?
ALBUMIN = maintains plasma oncotic pressure and serves as a carrier for many drugs and hormones
CLOTTING FACTORS = used during blood coagulation
location + func: gallbladder
located just beneath the liver
both stores and concentrates liver
what happens to the gallbladder upon release of CCK? (2)
- the gallbladder contracts and pushes bile out into the biliary tree
- the bile duct system merges with the pancreatic duct before emptying into the duodenum
defn: stone formation
precipitation of a solid once its concentration has reached the Ksp of the compound
why is stone formation praricularly common in the gallbladder?
because bile is concentrated there
char (3): stone formation in the gallbladder
- usually cholesterol or bilirubin stones
- causes gallbladder inflammation
- the stones may travel into the ducts and get stuck in the biliary tree
cause: pancreatitis
stones from the gallbladder can get caught just before entering the duodenum, resulting in blockage of the biliary tree and the pancreatic duct
diagram: summary of digestive processes
table: function of various digestive enzymes and bile (p. 358)
defn: villi
small, finger-like projections from the epithelial lining that line the small intestine
func (3) + diagram: microvilli
on each villus
- drastically increases the surface area available for absorption
- capillary bed at the middle of each villus for absorption of water-soluble nutrients
- lacteal in the middle of each villus (a lymphatic channel that takes up fats for transport into the lymphatic system)
how are simple sugars (glucose, fructose, galactose, and amino acids) absorbed?
- by secondary active transport and facilitated diffusion into the epithelial cells lining the small intestine
- they then move across the epithelial cell membrane into the intestinal capillaries
- blood constantly passing by the epithelial cells carries the carbs and amino acids away
- carbs and amino acids diffuse from the epithelial cells into the capillaries
- the absorbed molecules then go to the liver via the hepatic portal circulation
what effect does the following have: blood constantly passing by the epithelial cells carries the carbs and amino acids away
this creates a concentration gradient such that the blood always has a lower concentration of monosaccharides and amino acids than inside the epithelial cells
how are short-chain fatty acids absorbed?
they follow the same process as carbs and amino acids by diffusing directly into the intestinal capillaries
they do not require transporters because they are nonpolar, so they can easily traverse the cellular membrane
how are larger fats, glycerol, and cholesterol absorbed? (4 steps)
- they move separately into the intestinal cells but then reform into triglycerides
- the triglycerides and esterified cholesterol molecules are packaged into chylomicrons
- chylomicrons enter the lymphatic circulation through lacteals (small vessels that form the beginning of the lymphatic system) instead of entering the bloodstream
- these lacteals converge and enter the venous circulation at the thoracic duct in the base of the neck, which empties into the left subclavian vein
what part of the digestive tract are vitamins absorbed by?
the small intestine
what are the 4 fat-soluble vitamins? what are the water-soluble vitamins?
fat-soluble: A, D, E and K
water-soluble: all other (B complex and C)
how are fat-soluble vitamins absorbed?
they dissolve directly into chylomicrons to enter the lymphatic circulation
how are water-soluble vitamins absorbed?
they are taken up, along with water, amino acids, and carbohydrates, across the endothelial cells of the small intestine, passing directly into the plasma
what is the last thing other than fats, carbs, amino acids, and vitamins that the small intestine absorbs?
water!
char (4): secretions (water)
- much of the water in chyme is a result of these
- secretions into the upper GI tract may total up to 7 liters of fluid per day
- much of this must be reabsorbed by osmosis in order to maintain proper fluid levels within the body
- as solutes are absorbed int the bloodstream, water is drawn with them, eventually reaching the capillaries
what are the 2 ways that water passes to reach the blood?
transcellularly (across the cell membrane)
paracellularly (squeezing between the cells)
main func: large intestine
water absorption
in what ways is the large intestine larger than the small intestine? in what ways is it not?
larger diameter
shorter length
what are the 3 major sections of the large intestine?
- cecum
- colon
- rectum
defn: cecum
an outpocketing that accepts fluid exiting the small intestine through the ileocecal valve
defn + func: appendix
a small finger-like projection that was once thought to be vestigial and is attached to the cecum
may have a role in warding off certain bacterial infections and repopulating the large intestine with normal flora after diarrhea
main func: colon
to absorb water and salts from the undigested material left over from the small intestine
concentrates the remaining material to form feces
what causes diarrhea or constipation?
too little or too much water absorption, respectively
function: rectum
storage site for feces
what is feces composed of?
indigestible material, water, bacteria, and certain digestive secretions that are not reabsorbed (enzymes, bile)
defn: anus
the opening through which wastes are eliminated and consists of the internal and external anal sphincters
func: external vs. internal anal sphincter
external: under voluntary control (somatic)
internal: under involuntary control (autonomic)
are most bacteria in the large intestine anaerobes or aerobes?
most are anaerobes, but there are many aerobes in the cecum
explain: the symbiotic relationship of the presence of bacteria in the colon
the bacteria are provided with a steady food source and the byproducts of bacteria are beneficial to humans
