digestive Flashcards
digestive system includes
gastrointestinal (GI or alimentary) tract (mouth, pharynx, esophagus, stomach, small intestine, large intestine), plus accessory organs that secrete via ducts into the GI tract (salivary glands, liver, gallblader, exocrine pancrease).
digestion
breaking down of food into smaller particles and of macromolecules in food into smaller, absorbable molecules (accomplished mechanically and by the actios of digestive enzymes, acid, and bile)
secretion
release of substances into the lumen of the GI tract
absorption
movement of the molecules resulting from digestion from the GI tract lumen across a layer of epithelial cells and into the blood or lymph
motility
contractions of the smooth muscles in the GI tract wall that mix the luminal contents and propel them from mouth to anus
salivary glands (3 pair) secete saliva, which
is antibacterial
contains muscus for lubricating food particles before swallowing
contains anylase - enzyme that begins polysaccharide digestion
dissolves molecules that can then interact with chemoreceptors to give rise to taste sensations
pharynx and esophagus
provide pathway from oral cavity to stomach
are muscular walls that control swallowing
stomach stores food, produces the hormone gastrin, and produces exocrine secretions including:
- HCl which kills bacteria, dissolves the particulate matter in food and activates pepsinognes into pepsins
- pepsins - which begin the digestion of proteins
- mucus - for lubrication and protection
- intrinsic factor for vitamin B12 absoprtion in the small intestine
chyme
the solution of partially digested protein and polysaccharide fragments, fat droplets, salt, water, and other small molecules in the stomach, small intestine, and large intestine
no absorption of carbs, fats, or proteins in the _______
stomach
stomach regulates the rate at which its contents empty into the
small intestine
small intestine is divided into
a duodenum, jejunum, and ileum
small intestine performs
most digestion (via hydrolytic enzymes) and absorption of food molecules
small intestine enzymes for carbs, fats, and proteins are located where
embedded in the luminal surfaces of cells lining the small intestine or are secreted into the small intestine lumen by the pancreas
small intestine secretes:
liver/gallbladder secrete:
salts, water, and mucus (from goblet cells) into its own lumen
bile into its lumen
in addition to absorbing monosaccharides (from carbs), fatty acids (from fats), and amino acids (from proteins), the small intestine also absorbs
vitamins, mineals, and water
small intestine mixes
its luminal contents with the various secretions, brings the contents into contact with the epithelial surface of its wall where things can be absorbed, and slowly propels its contents toward the large intestine
the pancreas has an exocrine and endocrine portion; the exocrine portion secretes
pancreatic juice into the small intestine
pancreatic juice contains bicarbonate (HCO3-) for:
pancreatic juice also contains enzymes specific for:
neutralizing stomach acid (necessary to prevent inactivation of pancreatic enzymes)
digesting carbs, proteins, fats, and nucleic acids
the liver secretes
bile salts (via the bile) into the small intestine - not enyzmes but break fats into smaller fat droplets
bicarbonate into to small intestine for acid neutralization
the liver puts some organic waste products and trace metals into
the feces (via the bile) for elmination from the body
the liver performs many metabolic activities that help
the body handle carbohydrates (especilly glucose and glycogen), amino acids/proteins, and fatty acids/lipids (including cholesterol)
the gallbladder
stores and concentrates bile between meals and contracts to secrete it into the duodenum when needed
large intestine functions
secretes mucus, concentrates and temporarily stores undigested matter, absorbs water and salts from it, and mixes and propels its contents
functions of the final segment of the large intestine (the rectum)
performs defecation in which feces are expelled by muscular contractions and sphincter relaxation
the luminal (inside) surface of the wall of the GI tract is
convoluted to increase surface area
from stomach on, the luminal surface of the GI tract is lined by
a single layer of epithelial cells across which absorption occurs
in the single layer of epithelial cells that line the GI tract from the stomach on, there exist
exocrine cells that secrete (e.g. mucus) into the lumen and endocrine cells that secrete homrones into the blood
the single layer of epithelial cells that line the luminal surface of the GI tract from the stomach on sometimes invaginates
into the underlying tissue to form small exocrine glands that secrete water, acid, enzymes, mucus, and ions into the lumen
4 tunics (layers) of the GI wall
mucosa
submucosa
muscularis externa
serosa
mucosa
the mucosal epithelial (thin layer of epithelial cells lining the luminal surface of the GI tract from stomach on), plus a layer of connective tissue containing blood and lymphatic vessels and nerve fibers (the lamina propria), plus a thin layer of smooth muscle (th muscularis mucosa)
submucosa
connective tissue layer just beneath the mucosa that contains a network of neurons (the submucosal plexus, some neurons of which project to the mucosa) plus blood and lymphatic vessels
muscularis externa
2 layers of smooth muscle (plus a network of neurons between them, the myenteric plexus) that contract to provide mixing and propulsion of the GI contents
2 layers of the muscularis externa:
circular muscle: the fibers of which encircle the tube and thus narorw its lumen when they contract
longitudinal muscle: the fibers of which run along the tube and thus shorten the tube when they contract
the neurons of the myenteric plexus are innervated by:
and are connected with neurons of the:
neurons from the autonomic nervous system
submucosal plexus
the muscularis externa of the stomach contains
an additional layer of muscle
serosa
thin layer of connective tissue forming outer surface of the GI tube
the outer tunic in the esophagus is called the _______ rather than the serosa
adventitia
in the abdominal cavity, sheets of connective tissue connect to serosa to
the abdominal wall
projections, villi, extend from the lumial surface of the small intestine into
its lumen
the surface of each villus is lined by a layer of musosal epithelial cells, the plasma membranes of which form much smaller projections _________ , that are collectively called the ________
microvilli
brush border
villi and microvilli increase
the small intestine surface area
in the center of each villus are
capillaries and a single blind-ended lymphatic vessel called a lacteal
most absorbed fat enters the lymph in the:
most other absorbed materials enters:
lacteal (lymph is eventually emptied into the blood)
the blood capillaries
veins draining the small intestine (and LI, pancrease, and part of the stomach) do not empty directly into the inferior vena cava. instead, these veins merge to form the
heptatic portal vein which travels to and gives rise to a second capillary netowrk within the liver
products of digestion are absorbed into the SI blood ( but not the lacteal) are processed (metabolized, detoxified, stored, etc) by
liver enzymes before entering the general circulation
peritoneum
a serous membrane similar to the pleural and pericardial membranes that covers the external surfaces of many digestive organs and also lines the inner surface of the abdominopelvic cavity wall
the peritoneum encloses a cavity, the
peritoneal cavity, that is filled with a thin layer of lubricating serous fluid
a fused, double layer of the peritoneal membrane called _________ extends from the body wall to:
mesentery
some digestive organs
functions of the mesentery
holds organs in place and provides routes for blood vessels, lymphatics, and nerves to reach the organs
carbs are ingested mostly as
starch (a plant polysaccharide), sucrose (table sugar, a disaccharide), and lactose (milk sugar, a disaccharide)
only small amounts of monosacchardies such as ______ and _______ are normally ingested
glucose and fructose
cellulose (fiber), a plant polysaccharide, is not metabolized by _______ enzymes and thus travels to the _______ , where it is partly borken down by ______ enzymes
human-produced
Large intestine
bacterial
amylase in saliva
provides a small amount of starch digestion in the oral cavity, but it is inactivated by acid in the stomach
amylase in pancreatic juice secreted into the SI performs the vast majority of
carb digestion in the SI, resulting in a mix of maltose (a disaccharide) and short chains of glucose molecules
several enzymes ( ______ , _______ , _______ , and _______ ) are embedded in the luminal membranes (that is the brush border) of:
these enzymes protrude into the:
lactase, sucrase, maltase, and alpha-dextrinase
SI mucosal epithelial cells
SI lumen wehre they can be contacted by chyme
the actions of the brush border enzymes and of pancreatic amylase result in water-soluble monosaccharides - ______ , _______ , and especially _______ - that are absorbed across the
fructose, galactose, and especially glucose
SI mucosal epithelium into blood capillaries, and the moved to the liver via the hepatic portal vein
fructose is absorbed into SI mucosal epithelial cells via
facilitated diffusion (non ATP using carrier) and thus requires a membrane-bound carrier protein (GLUT) and a fructose concentration gradient, but does not require ATP expenditure
glucose and galactose enter SI mucosal epithelial cells via
secondary active transport that uses a sodium-glucose cotransport protein called SGLT
fructose, glucose, and galactose move from teh epithelial cells into the interstitial fluid via
facilitated diffusion (using a GLUT carrier in the basolateral membranes of the cells) and then into the blood via simple diffusion through pores in capillary walls
the majority of absorptiono occurs in
the duodenum and first part of jejunum
cause of lactose intolerance
absence of brush border lactase
fructose:
transport into enterocyte:
transport out of enterocyte:
GLUT (facilitated diffusion)
GLUT (facilitated diffusion)
glucose and galactose:
transport into enterocyte:
transport out of enterocyte:
SGLT (secondary active transport)
GLUT (facilitated diffusion)
protein digestion and absorption is required in the diet to provide
essential amino acids and to replace the nitrogen contained in protein/amino acids that are converted to urea
in addition to dietary protein, much protein is
secreted into the GI tract lumen as enzymes and mucus; these are also digested and absorbed
pepsin (secreted as pepsinogen)
enzyme created by stomach and secreted into the stomach lumen, breaks proteins into peptide fragments
trypsin and chymotrypsin
pancreatic enzymes that act in the SI and assist with digesting proteins into peptide fragments
peptide fragments are borken into free amino acids in the SI by the pacreatic enzyme ________ and by ________ (and several othe peptidases, such asdipeptidase) embedded in:
carboxypeptidase and by aminopeptidase
SI mucosal epithelial cell membranes
absorption of amino acids
free amino acids enter epithelial cells by
sodium cotransport. this involves many transporters with different amino acid specificities
some chains of 2 or 3 amino acids move into the cells by
secondary active transport coupled to H+ (rather than Na+) ; these chains are hydrolyzed into free amino acids within the cells
amino acids move from the cells to the interstitial fluid by
facilitated diffusion involving many carrier molecules and then enter the blood by diffusin through capillary pores
occasionally very small amounts of intact proteins can be absorbed without being _______ . these proteins are engulfed by the SI mucosal epithelial cells via _______
digested
endocytosis
the ability to absorb proteins without being digested is especially great in _______ and allows them to get ______ from teh mother’s milk
infants
antibodies
most lipids are ingested as
triglycerides
most digestion of fats/lipids occurs in
the SI by the pancreatic enzyme lipase
a triglyceride molecule is
3 fatty acids bound to a single glycerol
digestion of a triglyceride molecule results in
2 free fatty acids and one monoglyceride molecule (2-monoglyceride)
most ingested lips are not _______ and thus form _______
water soluble
large lipid droplets
because lipase is water soluble, its digestive action can only occur
at the surface of the large lipid droplets
emulsification
process of breaking large lipid droplets into many smaller droplets which serves to speed up digestion of fats/lipids by increasing the surface area accessible to lipase
emulsification is achieved by
by mechanical disruption of the droplets due to churning activity in the stomach and SI, and by emulsifyin agents contained in bile (bile salts and the phospholipid lecithin)
a bile salt or lecithin molecule has a nonpolar part which is:
a bile salt or lecithin molecule also has a polar part which is:
soluble in fat and thus dissolves in the surface layer of a fat droplet
water soluble and is left exposed at the water surface of the small fat droplet where it repels other lipid droplets
fatty acids and monoglycerides are also not _______ so absorbing them would be slow, except that bile salts form ______
water soluble
micelles
micelles
are similar to emulsifed small fat droplets but MUCH smaller, and consist of fatty acids, monoglycerides, bile salts, and phospholipids with the polar ends of each moelcule facing the surface of the micelle
a fatty acid or monoglyceride up against a SI mucosal epithelial cell can diffuse across
the lipid bilayer portion of the cell’s plasma membrane
micelles (by breaking down and reforming) slowly release
fatty acids and monoglycerides where they can undergo diffusion across the lipid bilayer portion of a SI mucosal epithelial cell’s plasma membrane
in the smooth ER of epithelial cells, enzymes reassemble the fatty acids and monoglycerides into ______ which
triglycerides
aggregate into small fat droplets (coated by amphiphatic proteins)
vesicles containing small fat droplets pinch off from the ER, move to the _______ and fuse with the cell’s _______ . this releases fat droplets into
Glogi
plasma membrane
the ECF where they are called chylomicrons
basement membranes associated with blood capillaries prevent
chylomicrons from entering capillaries, instead, chylomicrons pass into lacteals
the lymph from the lacteal (as with all lymph) eventually empties
into the blood contained within systemic veins
the fat-soluble vitamins are
A, D, E, and K
fat-soluble vitamins undergo the same absorption pathway as
fats
some water soluble vitamins are absrobed by _______ and some by _______
diffusion and some by carrier-mediated transport
B12 , is water soluble, is large and charged and cannot be absorbed unless it is first combined with
intrinsic factor (protein) that is secreted by the stomach
intrinsic factor bound B12 binds to
epithelial cells in the ileum, where B12 is absorbed by endocytosis into mucosal epithelial cells
B12 is required for
red blood cell production
a deficiency in B12 can result if
pernicious anemia
small amounts of water are absorbed in
the stomach
about 80 percent of the ingested and secreted water entering the SI is absorbed
there in the SI
SI epithelial membranes are very water permeable, and water absorption occurs whenever
a water concentration gradient exists
water concentration gradient are established by the
absorption of solutes, especially sodium
Na+ is moved via the sodium-potassium pump _______ the intestinal epithelial cells and into the ______. This ______ the Na+ concentration within the cell to below the value typically found in the chyme
out of
ECF
lowers
once out of the intestinal epithelial cells and into the ECF, Na+ then moves down its electrochemical gradient from
the chyme into the epithelial cells by moving through ion channels or by co-transport mechanisms that also bring amino acids, glucose, galactose, or chloride ions into the cells
potassium, magnesium, calcium, zinc, iodide, iron, and other minerals are absorbed via various mechanims, some that are ______ and others that are ______
active and others that are passive
when the entire lenghts of the SI and LI are considered, some ions such as bicarbonate, are ________ from the lumen in some locations and are _______ into it in other locations
absorbed
excreted
