The Digestive System Flashcards
catabolic reactions
reactions the break down molecules to supply energy
anabolic reactions
synthesis of macromolecules
purpose of digestion
the breakdown of polymers into their building blocks (and also some protection from disease/immune functions)
how is the breakdown of polymers in digestion accomplished?
by enzymatic hydrolysis, also involves vitamins which serve a catalytic role as enzyme cofactors or prosthetic groups
gastrointestinal (GI) tract/digestive tract/alimentary canal/gut
where digestion is accomplished, a long, muscular tube extending from the mouth to the anus, derived from the cavity produced by gastrulation during embryogenesis
the anus is derived from the:
blastopore
GI lumen
the inside of the gut, continuous with the space outside the body, compartment where the usable components of foodstuffs are extracted while wastes are left to be excreted as feces
what are the layers from the GI tract? from lumen outwards?
lumen, mucosa (with villi/epithelial tissue), submucosa (connective tissue), circular muscularis (smooth muscle), longitudinal muscularis (smooth muscle), serosa (connective tissue)
innermost lining of the lumen is composed of:
epithelial cells, the same type of cells that line the outer surface of the body and the inner surface of the respiratory tract
basement membrane
attached to epithelial cells
apical surface
the surface of the epithelial cells which faces into the lumen, separated from the remainder of the cell surface by tight junctions
tight junctions
bands running all the way around the sides of epithelial sides, creating a barrier that separates body fluids from the extracellular environment
basolateral surface
the sides and bottom of an epithelial cell form the surface opposite the lumen
microvilli
outwards folds of plasma membrane on the apical surfaces of epithelial cells in the small intestine that increase surface area
what is responsible for most of the secretory activity of the GI tract?
specialized epithelial cells
GI muscle
a type of smooth muscle, two layers of muscle (longitudinal and circular)
GI motility
the rhythmic contraction of GI smooth muscle
GI smooth muscle is similar to cardiac muscle in that it displays:
automaticity; contracts periodically without external stimulation due to spontaneous depolarization
like cardiac muscle, GI smooth muscle is a _____ _____
functional syncytium, meaning that when one cell has an action potential and contracts, the impulse spreads to neighboring cells
enteric nervous system
the GI tract’s own nervous system that plays a major role in controlling GI motility, a branch of the autonomic nervous system that helps to control digestion via innervation of the GI tract, pancreas, and gall bladder
GI motility may be increased or decreased by:
hormonal input
what is the purpose of GI motility
mixing of food (disordered contractions of GI smooth muscle) and movement of food down the gut (orderly contractions=peristalsis)
peristalsis
an orderly form of contraction, where contraction of circular smooth muscle prevents food from moving backward while longitudinal muscle contraction causes shortening of the gut and pushes food forward, cycle repeats
bolus
a ball of food moving through the GI tract
what are the two networks of neurons that make up the enteric nervous system?
the myenteric plexus (between circular and longitudinal muscle layers and help to regulate gut motility) and submucosal plexus (found in the submucosa and regulate enzyme secretion, gut blood flow, and ion/water balance in the lumen)
exocrine glands
composed of specialized epithelial cells organized into sacs called acini, acinar cells secrete products which pass into ducts
what are the main digestive organs primarily involved in exocrine secretion?
liver, gallbladder, and pancreas, some specialized epithelial cells (miniature exocrine glands that release secretions directly into the gut lumen)
what are some exocrine epithelial cells in the wall of the gut?
gastric glands (secrete acid and pepsinogen) and goblet cells (mucus)
mucus membrane
any body surface covered with mucus (protects and lubricates the gut)
endocrine secretion is accomplished by:
specialized organs (the pancreas) and cells in the wall of the gut
what are the two types of cells in the pancreas?
1) exocrine cells: pancreatic acinar cells
2) endocrine cells: islets of Langerhans
mouth
1) fragmentation by mastication
2) lubrication
3) some enzymatic digestion
incisors
front teeth (cutting)
cuspids
canine teeth (tearing)
molars
grinding
saliva
a viscous fluid secreted by salivary glands in the mouth which perform lubrication and some digestion, contains salivary amylase , lingual lipase, lysozyme
salivary amylase (ptyalin)
hydrolyzes starch, breaking it into fragments, disaccharide
lingual lipase
fat digestion
lysozyme
attacks bacterial cell walls, performs innate immunity
pharynx
contains the opening of two tubes: trachea (cartilage-lined tube which conveys air to and from the lungs) and the esophagus (muscular tube behind the trachea which conveys food and drink to the stomach)
epiglottis
cartilaginous flap that excludes solids and liquids from the trachia during swallowing
upper esophageal sphincter
near the top of the esophagus
lower esophageal sphincter
aka cardiac sphincter, lower end of the esophagus, at the entrance to the stomach, prevents reflux from stomach into esophagus
stomach
a large, hollow, muscular organ that serves 3 purposes:
1) partial digestion of food
2) regulated release of food into the small intestine
3) destruction of microorganisms
gastric pH
approximately 2, due to secretion of HCl by parietal cells in the gastric mucosa, allows for:
destruction of microorganisms
acid-catalyzed hydrolysis of many dietary proteins
conversion of pepsinogen and pepsin
chief cells
found in the stomach wall that secretes pepsinogen (inactive precursor) that is converted to active pepsin by gastric acidity
pepsin
catalyzes proteolysis (converted by gastric acidity from inactive pepsinogen form)
zymogen
inactive form of an enzyme, most are activated by proteolysis (except for pepsinogen which is activated by gastric acidity)
chyme
food mixed with gastric secretions
lower esophageal sphincter prevents:
reflux of chyme into the esophagus
pyloric sphincter
prevents the passage of food from the stomach into the duodenum, opening is inhibited when the small intestine already has a large load of chyme
what inhibits stomach emptying past pyloric sphincter into the duodenum?
stretching or excess acidity in the duodenum, this is mediated by nerves connecting the duodenum and stomach, and by hormones
cholecystokinin
hormone secreted by epithelial cells in the wall of the duodenum into the bloodstream that mediates pyloric sphincter opening
gastrin
hormone secreted by G cells in the stomach wall, stimulates acid and pepsin secretion and gastric motility, secretion is stimulated by food in the stomach and by parasympathetic stimulation
histamine
small molecule which is secreted in response both to stomach stretching and to gastrin, binds to parietal cells which stimulate acid release
what are the segments of the small intestine
duodenum, jejunum, ileum, where digestion is completed by absorption
what allows for the large surface area of the small intestine?
length, villi, microvilli
villi
macroscopic (multicellular) projection in the wall of the small intestine
microvilli
microscopic foldings of the cell membranes of individual intestinal epithelial cells
brush border
the lumenal surface of the small intestine, brush-like appearance due to microvilli
the structure of the villus has 3 important structures
1) capillaries which absorb dietary monosaccharides and amino acids, merge into veins which merge to form the large hepatic portal vein which transports blood containing nutrients from the gut to the liver
2) small lymphatic vessels/lacteals, which absorb dietary fats and transport to thoracic duct which empties into the bloodstream
3) Peyer’s patches, part of the immune system, collections of lymphocytes dotting the villi that monitor GI contents and thus confer immunity to gut pathogens and toxins
what are the two ducts that empty into the duodenum?
pancreatic duct (delivers the exocrine secretions of the pancreas) and the common bile duct (which delivers bile)
bile
green fluid containing bile acids, which are made from cholesterol in the liver and are normally absorbed and recycled
gallbladder
where bile is stored until needed, vehicle for the disposal/excretion of waste products by the liver and is essential for the digestion of fats
sphincter of Oddi
orifice by which both the bile duct and the pancreatic duct empty into the duodenum
enterokinase/enteropeptidase
secreted by duodenal epithelial cells and activates the pancreatic zymogen trypsinogen by proteolysis
trypsinogen
zymogen form of trypsin
brush border enzymes
duodenal enzymes that are not truly secreted but rather do their work inside or on the surface of the brush border epithelial cell, hydrolyzes the smallest carbohydrates and proteins (disaccharides and dipeptides) into monosaccharides and amino acids
what are the 3 main hormones secreted by duodenal epithelial cells?
cholecystokinin (CCK), secretin, enterogastrone
cholecystokinin (CCK)
secreted in response to fats in the duodenum, causes the pancreas to secrete digestive enzymes, stimulates gallbladder contraction (bile release), and decreases gastric motility (preventing further stomach emptying to help with fat digestion)
secretin
released in response to acid in the duodenum, causes the pancreas to release large amounts of a high-pH aqueous buffer (bicarbonate in water) which neutralizes HCl released by the stomach
duodenum pH
neutral or even slightly basic (required for pancreatic digestive enzyme function)
enterogastrone
decreases stomach emptying
lower small intestine (jejunum and ileum) function
performs special absorptive processes (ex. absorption of vitamins) and absorbs substances not absorbed in the duodenum
ileocecal valve
separates the ileum from the cecum (the first part of the large intestine)
role of colon (large intestine)
a muscular tube that absorbs water and minerals, and forms and store feces until the time of defecation
appendix
finger-like appendage of the cecum, composed primarily of lymphatic tissue
rectum
last portion of the colon
anus
through which feces exit from the rectum
anal sphincter
controls defecation, consists of the internal sphincter (smooth muscle/autonomic control) and external sphincter (skeletal muscle/voluntary control)
what kind of bacteria can be found in the colon?
facultative or obligate anaerobes, metabolizes undigested materials
colonic bacteria are important for two reasons:
1) helps prevent dangerous bacteria from proliferating due to competition of space and nutrients
2) supply vitamin K, which is essential for blood clotting
what are the GI accessory organs?
plays a role in digestion, but are not part of the alimentary canal (pancreas, liver, gallbladder, salivary glands)
pancreatic amylase
hydrolyzes polysaccharides to disaccharides
pancreatic lipase
hydrolyzes triglycerides at the surface of a micelle
nucleases
hydrolyze dietary DNA and RNA
pancreatic proteases
responsible for hydrolyzing polypeptides to di- and tripeptides, secreted in their inactive zymogen forms which are activated by proteolysis
trypsinogen
zymogen which is converted to active trypsin by enterokinase, an intestinal enzyme
chymotrypsinogen
activated by trypsin into chymotrypsin
procarboxypeptidase
activated by trypsin into carboxypeptidase
procallagenase
activated by trypsin into collagenase
islets of Langerhans
regions within the pancreas that contain endocrine cells
what are the endocrine hormones secreted by the pancreas?
glucagon, insulin and somatostatin
glucagon
secreted by alpha cells in Islets of Langerhans in response to low blood sugar, functions to mobilize stored fules by stimulating the liver to hydrolyze glycogen and release glucose into the bloodstream, and by stimulating adipocytes to release fats into the bloodstream
insulin
secreted by beta cells in Islets of Langerhans in response to elevated blood sugar, stimulates the removal of glucose from the blood for storage as glycogen and fat
somatostatin
secreted by delta cells in the islets of Langerhans, inhibits many digestive processes
what are the 3 hormones responsible for raising blood glucose?
glucagon (polypeptide hormone from the pancreas), epinephrine (amino acid derivative from the adrenal medulla), and cortisol (steroid or glucocorticoid from the adrenal cortex)
what is the exocrine secretory activity of the liver?
it secretes bile
what are the principal ingredients of bile?
bile acids/bile salts, cholesterol, bilirubin
role of bile
emulsifies large fat particles in the duodenum, creating smaller cluster of fat particles called micelles
gallstone
large crystal formed from bile made with ingredients made in incorrect proportions
bile release from the gallbladder is determined by:
both the endocrine system and the nervous system
the liver receives blood from two places:
oxygenated blood from the hepatic arteries, venous blood draining the stomach and intestines through the hepatic portal vein (the nutrients in this blood is extracted by hepatocytes-liver cells)
hepatocytes
liver cells that monitor the blood and make changes to the body’s physiology based on what is and is not present
how is glycogen breakdown in skeletal muscle and liver different?
liver can release glucose into the bloodstream (dephosphorylated)
liver converts ammonia to:
urea, which is absorbed in the bloodstream and excreted by the kidney in urine
how does the liver assist with lipid metabolism?
chylomicrons are degraded by lipases into triglycerides, glycerol, and cholesterol-rich chylomicron remnants which are taken up by hepatocytes and combined with proteins to make lipoproteins that can then re-enter the blood and provide a source of cholesterol and triglycerides for other body tissues
chylomicrons
how lipids enter the lymphatic system from lipids that exited the intestine
the smooth ER in hepatocytes contains enzyme pathways that:
break down drugs and toxins into forms that are less toxic: liver is a major centre for drug and toxin detoxification
ghrelin
hormone secreted by gastric cells when the stomach is empty to stimulate appetite
peptide YY
secreted by the jejunum when the colon is full to reduce appetite
leptin
hormone produced by white adipose tissue that acts as an appetite suppressant to maintain stable lipid content in adipose tissue, secreted in response to increased triglyceride levels and works to suppress appetite until appropriate levels are restored
the apical surface of intestinal epithelial cells transport glucose into the cell by:
symport transports which allows sodium into the cell as well (concentration of sodium maintained by constant activity of Na/K ATPases), glucose reaches high concentration and driven out of cell by facilitated diffusion on basolateral surface where it is picked up by capillaries that flows toward the hepatic portal vein towards the liver
amino acid absorption
a secondary active transporter (symport) specific to each amino acid couples the uptake of an amino acid to the entrance of sodium into the cell, and a uniporter facilitates movement out of the intestinal epithelial cells into the interstitium (hepatic portal vein)
once fats are emptied into the duodenum:
CCK is released which stimulates gallbladder contraction and sends bile into duodenum, bile acids emulsify the lipids to form micelles, pancreatic lipases hydrolyzes triglycerides to monoglycerides and free fatty acids which then move into intestinal epithelial cells bysimple diffusion, once inside, they are packaged into chylomicrons which then enter lacteals by exocytosis and are later emptied into the thoracic duct and bloodstream (causing lipemia), circulate throughout the body and are gradually whittled away by removal of fat
lipoprotein lipase
found in adipose and liver tissues, hydrolyzes chylomicron triglycerides into monoglycerides and free fatty acids, diffuses into adipocytes and liver cells, are remade into triglycerides and then stored
vitamins
nutrients which must be included in the diet because they cannot be synthesized in the body, can be fat-soluble or water-soluble
fat-soluble vitamin require:
bile acids for solubilization and absorption, excess fat-soluble vitamins are stored in adipose tissue
excess water-solube vitamins are:
excreted in urine by the kidneys
vitamin A (retinol)
fat-soluble; a visual pigment that changes conformation in response to light
vitamin D
fat-soluble; stimulates calcium absorption from the gut; helps control calcium deposition in bones
vitamin E
fat-soluble; prevents oxidation of unsaturated fats
vitamin K
fat-soluble; necessary for blood coagulation factors
vitamin B1 (thiamine)
water-soluble; necessary for enzymatic decarboxylations
vitamin B2 (riboflavin)
water-soluble; made into FAD; an electron transporter
vitamin B3 (niacin)
water-soluble; made into NAD+, an electron transporter
vitamin B6 (pyridoxine)
water-soluble; a coenzyme involved in protein and amino acid metabolism
vitamin B12 (cobalamin)
water-soluble; a coenzyme involved in the reduction of nucleotides to deoxynucleotides
vitamin C (ascorbic acid)
water-soluble; necessary for collagen formation; deficiency results in scurvy
biotin
water-soluble vitamin; prosthetic group essential for transport of carbon dioxide groups
folate
water-soluble vitamin; enzyme cofactor used in the transport of methylene groups; synthesis of purines and thymine; required for normal fetal nervous system development
what are the fat-soluble vitamins?
A, D, E, K (require bile acids for solubilization and absorption, excess is stored in adipose tissue)
vitamin A (retinol)
a visual pigment that changes conformation in response to light, fat-soluble
vitamin D
stimulates calcium absorption from the gut, helps control calcium deposition in bones, fat-soluble
vitamin E
prevents oxidation of unsaturated fats, fat-soluble
vitamin K
necessary for formation of blood coagulation factors, fat-soluble
vitamin B1 (thiamine)
needed for enzymatic decarboxylations, water-soluble
vitamin B2 (riboflavin)
made into FAD, an electron transporter, water-soluble
vitamin B3 (niacin)
made into NAD+, an electron transporter, water-soluble
vitamin B6 (pyridoxine)
a coenzyme involved in protein and amino acid metabolism, water-soluble
vitamin B12 (cobalamin)
a coenzyme involved in the reduction of nucleotides to deoxynucleotides, water-soluble
vitamin C (ascorbic acid)
necessary for collagen formation; deficiency results in scurvy, water-soluble
biotin
prosthetic group essential for transport of CO2 groups, water-soluble
folate
enzyme cofactor used in the transport of methylene groups; synthesis of purines and thymine, required for normal fetal nervous system development, water-soluble
