Exam 4--digestion Flashcards
what are the two sets of endocrine cells embedded in the pancreas?
beta produce insulin
alpha glucagon
both are important for the metabolism of glucose
how does glucose arrive in the liver
- glucose absorbed from the intestines arrives in the liver sinusoids via the hepatic portal system
- some of the glucose continues into the venous blood and increases the amount of glucose in the blood systemically
- some of the glucose is taken up by the hepatocytes in the liver and converted to the glucose polymer glycogen
what happens when glucose from the intestine is not readily available
- glycogen stored in hepatocytes can be converted to glucose by the process of the glycogenolysis and released into the blood to increase blood glucose
- glucose may also be generated in hepatocytes by a process called gluconeogenesis where amino acids are converted to glucose and released into the blood
what happens when blood glucose levels become low
many cells (not neurons) use fatty acids mobilized from adipose tissue as an energy source which is know as lipolysis -as blood glucose levels fall alpha cells release glucagon which acts on liver cells to convert to glucose and go to the blood
glucose may be stored as…
glycogen
glucose may be taken up by…
adipocytes and converted into triglycerides
glucose may be metabolized via…
glycolysis or oxidative phosphorylation
glucose
-when a molecule of glucose is converted into two molecules of pyruvate 2 ATP molecules are generated in the cytoplasm by glycolysis
insulin and its effects on blood glucose
- decrease blood glucose concentration
- facilitates entry of glucose into most cells by stimulating transporters into the cell membrane
glucagon and its effects on blood glucose
- increase blood glucose concentration
- stimulates glycogenolysis in the liver
cortisol and its effects on blood glucose
- increase blood glucose concentration
- stimulates gluconeogenic pathway
epinephrine and its effects on blood glucose
- increase blood glucose concentration
- stimulates triglyceride breakdown into fat to provide a source for most cells
what are the 3 main categories of food?
- carbs
- fats
- proteins
carboyhdrates
- polysaccharides, such as starch and glycogen
- disaccharides such as sucrose, lactose, and maltose
- monosaccharides or simple sugars such as glucose, fructose, and galactose
proteins
- amino acids linked by peptide bonds
- broken into proteases first into peptides and finally into individual amino acids
lipids or fats
-triglycerides composed of one molecule of glycerol and 3 molecules of fatty acid each linked by an ester bond to the glycerol backbone
outcome of digestion and metabolism
- carbs to simple sugars
- proteins to amino acids
- lipids to monoglycerides, glycerol, fatty acids
- the function of digestion is to break them into one of the 4 catagories, absorb them for use in the body as a form of energy or a generation of a new molecule
secretion
the addition of enzymes or cofactors necessary for digestion
motility
agitation and movement of digesta through the system
absorbtion
the uptake of the products of digestion
secretion of the mouth…
-saliva
-mucus
-amylase
(endocrine glands)
-under the primary control of the parasympathetic nervous system
saliva
-mostly water
-the secretory cells called acinar cells are responsible for secreting saliva
-the conc of ions shortly after its produced by acinar cells is similar to the interstitial fluid
-formed by transporting sodium out of the acinar cells, potassium in, chloride out, bicarbonante in, and water follows
-most of the ions are resorbed by the duct epithelial cells so the ion conc in saliva is more dilute then in the interstitial fluid
-potassium is higher in the saliva
-with the ingestion of food saliva production increases and the concentration of salts in saliva increase
WHY ISN’T IT SALTY??
mucus
- exocrine glands produce mucus
- purpose is to lubricate the ingesta so that it can be more easily swallowed
salivary amylase
-enzyme that converts starch to dissaccharides and oligosaccharides
swallowing
-reflex with control residing in the brain stem
-upper 1/3 skeletal
>under the control of the somatic neurons in the brain stem
-lower 2/3 smooth
>under the control of the parasympathetic nervous system
-swallowing centers in the brain stem send signals to both the skeletal and smooth muscle which causes a contraction in a wave-like fashion called peristalsis
esophageal sphincter
-region of smooth muscle that controls the movement of food into the stomach from the esophagus
mucosa
- composed of a layer of epithelial cells that line the tube of the digestive system
- these are attached via basal lamina to the underlying lamina propria (connective tissue)
- under is a layer of smooth muscle
submucosa
- thick layer of connective tissue rich in elastic fibers that stretch when you’ve had a large meal
- there are two smooth muscle layers external that work in concert to churn the stomach contents into smaller pieces
chyme
-the layers of smooth muscle churn the stomach contents and help to reduce digesta to small pieces suspended into the fluid
absorbtion in the stomach
-minimal and is restricted to some lipid soluble toxins like alcohol and asprin
secretion in the stomach
- cells of the glandular epithelium in the mucosa of the stomach are responsible for secretion of acid, pepsinogen, histamine, and gastrin
- cells of the gastric gland are located at regions called the gastric pits which are extensions of the columnar epithelium of the stomach mucosa
parietal cells
- make and secrete acid
- found in gastric glands
- produce HCl contain carbonic anhydrase which converts CO2 + H2O –> H+ + HCO3-
- the H+ ion is pumped out of the cell into the lumen of the stomach–this is controlled by Ach released from the parasympathetic postganglionic neuron axon terminals, gastrin released from g-cells, and histamine released
- the HCO3- is released into the blood
- these two movements can be called the alkaline tide
- have receptors for gastrin on their basolateral surfaces
cheif cells
- make and secrete pepsinogen –>pepsin
- found in gastric glands
- the pepsinogen is inactive until it is exposed to a low pH and a portion of the enzyme is removed by pepsin
G-cells
- secrete the hormone gastrin
- found in gastric glands
enteroendocrine cells
- located near the parietal cells and secrete histamine which functions locally on the parietal cells
- found in gastric glands
gastrin
- released from G-cells and enters the blood
- when it arrives at the parietal cell it binds to receptors and induces the release of H+ ions
- stimulated by a rising pH in the stomach and the presence of amino acids and peptiddes in the lumen of the stomach
histamine
- released from the enteroendocrine cells
- binds to an H2 receptor
- on the parietal cells
cephalic response
- occurs at the sight of food when our “mouth waters”
- causes the release of acid from the parietal glands
duodenum
- when the pyloric valve opens and allows chyme in it is forced into the duodenum
- the acid in the chyme must be neutralized soon after it enters because the duodenum requires a nearly neutral pH
secretion of bicarbonate from the pancreas
-plays an important role in neutralizing the acid
s cells
- in the intestinal mucosa
- secrete the peptide hormone secretin in response to a decrease in pH in the intestinal lumen brought on by the arrival of chyme from the stomach
secretin
- binds to receptors on duct cells in the exocrine pancreas
- these cells contain carbonic anhydrase
- secrete bicarbonate into the lumen of the pancreatic duct and H+ into the blood (opposite of the stomach)
- the pancreatic duct empties into the duodenum where the HCO3- helps to neutralize the acid arriving
polypeptides and fat…
-signal the intestinal mucosa to secrete cholecystokinin (CCK)–peptide hormone which binds to acinar cells in the exocrine pancreas
acinar cells in the exocrine pancreas respond to CCK how
by releasing two enzymes trypsinogen and chymotrypsinogen–which
-pancreatic amylase and pancreatic lipase aid in the digestion of carbs and fats in the small intestine
CCK
-also signals the liver to release bile salts
motility of the small intestine
- duodenum, ileum, jejunum
- results from smooth muscle contraction
absorbtion and the final steps in carbs and protein digestion
most of the absorbtion of nutrients occurs in the small intestine
the epithelial cells of the mucosa form an absorptive epithelium
villi
-the folds of the mucosa
-serve to increase the absorptive area of the intestine
-the submucosa contain blood vessels and lymphatic vessels
>amino acids and simple sugars are destined to enter these blood vessels whereas lipids enter the lacteals
microvilli (brush border)
- sites of the final digestion and absorbtion of carbs and proteins
- the villi and microvilli lead to a large absorbtive area in the small intestine
pancreatic amylase
- was released into the duodenum by the exocrine pancreas converts the polysaccharide starch to maltose
- lactose, sucrose, maltose
lactose
- major sugar in milk
- glucose + glactose
sucrose
- refined sugar
- glucose + fructose
maltose
pancreatic amylase & salivary amylase
how is transport of monosaccharides across the epithelial cells accomplished?
Na+ cotransporter–1 molecule of monosaccharide and 1 molecule Na+ into the lumen of the epithelial cell and then diffuse down its concentration gradient
-they then diffuse into the capillaries in the submucosa of the microvilli and are transported to the liver
fate of proteins in the digestive system
- proteins are polymers of amino acids
- in the stomach protein digestion begins when pepsin digests proteins to peptides it continues in the duodenum where trypsinogen and chymotrypsinogen are secreter by acinar cells
- amino acids are transporter across the microvillar membrane by a Na+ driven cotransporter where Na+ is cotransported with the amino acid
- they then leave the cells through the basilar membrane and enter the hepatic portal system and are destined to enter the liver
entrokinase
converts trypsinogen to trypsin which then converts chymotrypsinogen to chymotrypsin
digestion and absorption of fats
- fats are mostly composed of a molecule of glycerol and three molecules of fatty acid
- begins by the break down of fats by bile produced in the liver
bile salts
- produced in the liver
- constructed from a molecule of cholesterol and choline and a molecule of glycine and taurine
- act like a detergent to increase the surface area of the fat in the aqueous environment
lipase
- secreted by the exocrine pancreas
- converts triglycerides into two molecules of fatty acid and one molecule of monoglyceride
- move easily across the endothelial cell
- once in the endothelial cell they are shuttled into the SER where they are reassembled into triglycerides and complexes with proteins, cholesterol, and phospholipids to improve solubility
- then packaged into vesicles and released as chylomicrons which then go into the lacteals
bile acids
-synthesized in the liver and stored in the gallbladder before being released in the duodenum
-release is under the control of CCK
>CCK causes muscle contraction in the gall bladder to empty into the duodenum
-produced by hepatocytes
hepatic portal system
- blood flowing through the vessels of the submucosa is delivered to the liver through this system
- the capillaries in the villi empty into the venous system that delivers blood rich in amino acids and monosaccharides to the liver
how else does the liver recieve blood
-hepatic artery
sinusoids
- the capillaries of the liver that carry the blood to the central vein
- where the blood from the hepatic portal vein and the hepatic artery come together
- lined with endothelial cells that form a barrier between the blood and the hepatocytes
- as bile is produced it flows in the other direction of the blood
kupffer cells
- speacilized macrophages
- located in the sinusoids
- monitor blood from the intestine from bacteria and other pathogens
hepatocytes
- metabolically active
- remove glucose from the blood in the sinusoids and convert it to glycogen
- remove amino acids from the blood and make proteins like albumin
- remove toxins and detoxify them
- remove ammonia and convert it to urea which is then excreted into the kidneys
