digestive system Flashcards
digestion breaks large molecules into their monomers by _______ reactions
hydrolysis
What is motility?
the movement of food through the digestive tract
What are the four layers of the alimentary tract?
- Mucosa
- Submucosa
- Muscularis
- Serosa
The digestive system is regulated EXTRINSICALLY by the ________ nervous system and ________ system
autonomic; endocrine
The digestive system is regulated INTRINSICALLY by the ______ nervous system and _______ system
enteric; paracrine
what are enteric neurons?
- in the alimentary tract
- interneurons and intrinsic sensory neurons that have their cell bodies in the gut wall
- provide local regulation of the alimentary tract
Which digestive enzyme is present in the oral phase of digestion, and what type of molecule does it catalyze the digestion of?
salivary amylase; digestion of starches
What is peristalsis?
contraction and relaxation of longitudinal and circular muscles throughout the digestive tract, allowing for propulsion of materials
What is the esophagus lined with?
non-stratified squamous epithelium
Each third from top to bottom has different muscles:
- top - only skeletal
- middle - mix of skeletal and smooth
- bottom - only smooth
Circular muscle in the esophagus _____ behind and ______ in front of the bolus
contracts; relaxes
circular muscle contraction is accompanied by simultaneous ______ muscle contraction
longitudinal
What is the purpose of contraction of the lower esophageal sphincter?
prevent regurgitation of digested material out of stomach
what is the most distensible part of the GI tract?
stomach
what are the folds of the stomach called?
rugae
What is the purpose of stomach contractions?
churning of chyme to mix it with gastric juices
What are gastric pits?
the openings of rugae
What is the function of gastric glands?
- line the gastric pits
- secretion of things into stomach
Gastric pits are found in the _____ layer of the stomach
mucosa
What are seven types of gastric gland cells in the stomach?
- mucous cell
- Parietal cell
- Chief cell (zymogenic)
- Enterochromaffin-like
- G cells
- D cells
- P/D1 cells
What do parietal cells in the digestive system secrete?
- HCl
- intrinsic factor
What do chief cells in the digestive system secrete?
pepsinogen (inactive form of pepsin)
What do enterochromaffin cells in the digestive system secrete?
- histamine
- serotonin
What do G cells in the digestive system secrete?
gastrin
What do D cells in the digestive system secrete?
somatostatin
What do P/D1 cells in the digestive system secrete?
ghrelin
Parietal cells secrete protons into the gastric lumen primarily by what type of transport?
active
How does the transport of H+ and K+ by parietal cells in the digestive system work?
- H+/K+ ATPase pumps
- H+ transported out of parietal cell into stomach lumen against a very large conc. gradient
- K+ transported into the parietal cell
What is potassium recycling in the parietal cells of the digestive system?
-K+ that is transported into the parietal cell through the H+/K+ ATPase pumps leaks back into the lumen of the stomach through K+ channels to prevent the depletion of K+ in the stomach lumen
Parietal cells can secrete ____ and _____ into the gastric juice while secreting _____ into the blood
Cl-; H+; bicarbonate (H2CO3)
In the parietal cells of the digestive system, how is Cl- taken into the cell from the blood, and bicarbonate created and secreted into the blood?
- bicarbonate produced by dissociation of carbonic acid (CO2 + H2O with carbonic anhydrase)
- Cl- is taken in from the blood against the electrochemical gradient by coupling with downhill movement of HCO3- out of the cell into the blood
What are the three functions of the strong acidity of the stomach?
- ingested proteins become denatured and thus more digestible
- pepsinogen enzymes partially digest e/o freeing fully active pepsin
- pepsin is more active at low pH
What are the four functions of the adherent layer of mucus in the stomach?
- protects stomach lining from pepsin and the low pH (contains HCO3- which is alkaline, to neutralize the epithelial surface
- slows the diffusion of pepsin to protect epithelial cells from interacting with it
- tight junctions between the epithelial cells prevent leakage of pepsin and acid
- promotes rapid epithelial cell division
What is the only type of macromolecule that is partially digested in the stomach?
proteins
Are alcohol and aspirin absorbed in the stomach?
Yes
Where does most absorption in the digestive system occur?
small intestine
How do peptic ulcers occur?
erosion of the stomach or duodenum MUCOSA that penetrate through the MUSCULARIS MUCOSA layer
What are the two main causes of peptic ulcers?
Helicobacter pylori and NSAIDS
How does acute gastritis occur?
- gastric barriers to self-digestion of the stomach broken down
- acid leaks through the MUCOSA to the SUBMUCOSA
What are the three portions of the small intestine starting from the stomach?
Duodenum -> Jejunum -> Ileum
What does the duodenum primarily absorb?
- carbohydrates
- amino acids
- lipids
What does the Jejunum primarily absorb?
- Ca2+
- Fe
What does the Ileum primarily absorb?
- bile salts
- vitamin B12
- electrolytes
- water
What are villi?
microscopic folds of the mucosa that increase the surface area of the small intestine
The ________ cover the villi
simple columnar epithelium (enterocytes)
What do the villi project into in the small intestine?
Intestinal lumen
What do goblet cells in the small intestine do?
secrete mucus
What is the function of the intestinal crypt in the small intestine?
where new epithelial cells are produced by mitosis in the small intestine
What happens to epithelial cells made in the intestinal crypt?
They travel up the villi until they undergo apoptosis at the top and are shed
How are microvilli formed?
folding of epithelial cells
What is the name for intestinal enzymes?
brush border enzymes
where are brush border enzymes located?
- in plasma membranes of the microvilli
- Not secreted into the lumen
- active sites are exposed to chyme
what is the function of brush border enzymes?
digestive enzymes that hydrolyze disaccharides and polypeptides
what does enterokinase do?
activation of trypsin (digestion of proteins)
What are the two types of intestinal contraction?
- Peristalsis
- Segmentation
The major contractile activity of the small intestine is:
Segmentation
Peristalsis is much weaker in the _____ compared to the _____ and ______
intestine; stomach; esophagus
Where are is intestinal segmentation contraction more frequent, the proximal or distal region?
proximal (pressure difference helps to move the chyme)
How is the rhythm of the contractions in the small intestine paced?
graded depolarizations called slow waves
What are slow waves produced by?
gap junctions in interstitial cells of cajal that produce depolarizations
What are the 6 parts of the large intestine?
- ascending colon
- transverse colon
- descending colon
- sigmoid colon
- rectum
- anal canal
does the large intestine have villi?
no
What are the pouches of the small intestine called?
Haustra
How are the pouches of the small intestine formed?
- the external layer of muscle in the large intestine (taenia coil) is shorter than the length of the colon
- this causes bunching of the colon
Describe the digestive functions of the large intestine
- little to no digestive function
- absorbs remaining water, B-vitamins, electrolytes and vitamin K
Are the majority of the intestinal microbiota bacterium anaerobic or aerobic?
anaerobic
what is the function of the intestinal microbiota?
- digestion of fiber into short-chain fatty acids that we cannot digest on our own
- SCFAs promote motility and secretion, help retain H2O, nutrients and electrolytes and stimulate active Na+/Cl- absorption
What is the effect of antibiotics on the intestinal microbiota
- bad for microbiota
- can destroy bacteria
What is the defecation reflex?
sensory signals sent to brain urging defecation
During defecation, _______ muscles contract and _______ relax
longitudinal muscles; anal sphincters (internal / external)
What are hepatocytes
liver cells
What are sinusoids?
- separate hepatic plates
- very porous and permeable
What happens to hepatocytes in a damaged liver
they convert to stem cell-like cells which can divide and convert back to hepatocytes
Where do digested products absorbed into intestinal blood capillaries go?
the hepatic portal system; they are not absorbed into the circulation right away
What are the steps of blood travel after it enters the hepatic portal system?
- capillaries in the GI tract drain into the hepatic portal vein
- blood passes through a second hepatic capillary bed
- blood enters the circulation through the hepatic vein
capillaries -> hepatic portal vein -> capillaries -> hepatic vein
What organs does the hepatic portal vein drain?
- intestine
- pancreas
- gallbladder
- omentum
- spleen
What delivers nutrients to the liver?
the hepatic portal vein
What is enterohepatic circulation?
recirculation of compounds between the liver and intestine
What happens to exogenous compounds in the liver?
- exogenous compounds secreted by the liver into the ductules
- compounds are “cleared” from the blood by the liver into the intestine with bile
- some molecules are reabsorbed and secreted again with bile (enterohepatic circulation), others are excreted during defecation
What are the steps of metabolism of heme and bilirubin? (describe the molecules that are transformed)
red blood cells -> hemoglobin -> heme -> bilverdin -> bilirubin -> conjugated bilirubin (occurs in liver) -> elimination in bile in feces
What is emulsification?
conversion of large fat globules to smaller fat globules creating a larger surface area for digestion
what is the major cholesterol breakdown pathway?
bile acids
What are the three types of cells with pathogen recognition receptors?
- hepatocytes
- Kupffer cells
- dendritic cells
The _____ can add or remove glucose from the blood through glycogenesis, gluconeogenesis and glycogenolysis
liver
what is glycogenesis and under what circumstances does it occur?
- glucose removed from the blood and stored as glycogen
- might occur after a meal
What is gluconeogenesis and under what circumstances does it occur?
- conversion of non-carb molecules such as amino acids to glucose
- might occur during fasting
What is glycogenolysis and under what circumstances does it occur?
- breakdown of glycogen to glucose
- might occur during fasting
What is ketogenesis and under what circumstances does it occur?
- conversion of free fatty acids to ketones secreted into blood
- might occur during fasting
What does the gallbladder do?
stores and concentrates bile
the pancreas has both _____ and ______ functions
endocrine; exocrine
where is the gallbladder situated?
attached to liver
through what ducts does the gallbladder drain?
- bile ducts
- hepatic ducts
- cystic duct
Which enzymes does the pancreatic juice contain?
- amylase
- trypsin
- lipase
what is trypsin activated by?
brush border enzymes
what enzyme(s) break(s) down polysaccharides into disaccharides?
amylases
what brush border enzyme(s) break(s) down disaccharides to monosaccharides?
- lactase
- maltase
- sucrase
- (dextrinase and glucoamylase)
salivary amylase is inactivated by _________
pH of the stomach
complex carbs that are not digested by the salivary amylase are digested by ______
pancreatic amylase in the duodenum
what is lactose broken into
galactose and glucose
what is maltose broken into
2X glucose
what is sucrase broken into
glucose and fructose
what happens to monosaccharides in the intestine?
- internalized by the absorptive cell and diffuse out into caps of the intestinal villi
- water-soluble; immediately absorbed into blood stream
what is the final product of CHO hydrolysis
monosaccharides
_____ helps bring glucose into cells
insulin
what are the major lipid groups in our diet?
- triglycerides
- phospholipids
- sterols
why is lipid digestion more difficult than other macromolecule digestion?
- not water soluble
- enzymes can’t get to them
what helps with the digestion of lipids other than lipases?
bile salts
where are the majority of lipids digested and by what?
in the small intestine, by lipases in pancreatic juices following emulsification by bile salts
what does pancreatic lipase do to triglycerides
splits them into free fatty acids and monoglycerides
what happens to fatty acids and monoglycerides that are taken up into absorptive cells
they are converted back into triglycerides and exocytosed into lacteals of the villi
what do chylomicrons do?
add ApoE protein to the blood in the lacteals of the villi to help binding to capillaries in target muscles and adipose tissues
what are the major steps in digestion of proteins?
- protein broken down by PROTEASES into:
- Large polypeptides broken down by PEPTIDASES into:
- small polypeptides broken down by BRUSH BORDER PEPTIDASES into:
- amino acids
where are proteins broken down and by what enzyme?
- stomach - protease (pepsin)
- pancreas - pancreatic proteases and peptidases
- intestine - brush border enzymes
how are amino acids absorbed?
- transported into absorptive cells by transport proteins
- transport proteins carry AAs out of absorptive cells to capillaries of villi
Can amino acids be stored?
no
what happens to excess amino acids?
metabolized by liver cells into urea and lipids. urea is excreted in urine
