BIOL 0800 Reading- Chapter 15 Flashcards
What acid is in the stomach?
HCl
What is pepsin?
Protein-digesting enzymes in the stomach
What two molecule types are typically not digested well by acid in the stomach?
Polysaccharides and fats
How much absorption of organic nutrients occurs in the stomach?
Virtually none
Where does most absorption of nutrients occur?
In the small intestine
What is the duodenum?
The initial segment of the small intestine
What is the jejunum?
The second segment of the small intestine
What is the ileum?
The third, longest segment of the small intestine
What are the three segments of the small intestine?
Duodenum, jejunum, ileum
What two organs secrete substances into the duodenum?
Pancreas and liver
How does the pancreas contribute to digestion?
Secretes digestive enzymes and a fluid rich in HCO3- to neutralize the acidic chyme
What does the liver do?
Secretion of bile to break down fast
What does the gallbladder do?
Stores secreted bile
What are the four layers of the GI wall?
Mucosa, submucosa, muscularis externa, serosa
What are the three layers of the mucosa?
Epithelium, lamina propria, muscularis mucosa
What is the submucosa?
Major blood and lymphatic vessels, submucosal plexus
What is the submucosal plexus?
Network of neyrons
What is the muscularis externa?
Circular muscle and myenteric plexus
How do circular muscles work?
Fibers oriented in circulate pattern around the tube so that contraction produces a narrowing of the lumen
What is the myenteric plexus?
Second network of neurons: innervated by nerves from the autonomic NS and has neurons that project to the submucosal plexus
What kinds of nerves innverate the myenteric plexus?
Autonomic nervous system
What is the serosa?
Thin layer of connective tissue to connect outer surface of tube to abdominal wall
What is a lacteal?
Single, bind-ended lymphatic vessel that is at the center of each villi
Most of the fat absorbed in the small intestine goes where?
Through the lacteals, to be emptied through the lymphatic systme into the thoracic duct
What is the hepatic portal vein?
The vein through which absorbed nutrients drain from the small intestine to the vena cava
What does the hepatic portal vein allow?
Material absorbed into the intestinal capillaries to be processed by the liver before entering general circulation: detox
What biological accounts for the liver’s function as detox?
Hepatic portal vein: carries intestinally absorbed material through liver before returning it to capillary circulation
What are Peyer’s patches?
Patches in the SI to secrete inflammatory mediators (cytokines) to alter motility of harmful substances not destroyed by acidity
What happens to amylase in the stomach?
It gets inactivated! Oh no!
What are the products of salivary and pancreatic amylase?
Maltose and short, branched glucose
What happens to carbs after salivary and pancreatic amylase converts them into maltose/short glucose?
Broken down, along with sucrose and lactose, into the three monosaccharides (glucose, fructose, galactose) in the brush border
Where are carbs completely broken into monosaccharides?
In the brush border of the small intestine
How does fructose get reabsorbed from the brush border?
Facilitated diffusion visual glucose transporter GLUT
How do glucose and galactose get reabsorbed from the brush border?
Secondary active transport coupled to Na via sodium-glucose cotransporter SGLT
How do all three monsaccharides get transported from the epithelium to the interstitial fluid?
GLUT transporters’ facilitated diffusion
Where does protein digestion begin?
Stomach with pepsin
What substances digest protein in stomach vs SI?
Pepsin in stomach, trypsin and chymotrypsin in SI
What organ secretes trypsin and chymotrypsin into the SI for protein digestion?
Pancreas
What happens after pepsin/trypsin/chymotrypsin partially fragment proteins in the stomach/SI?
Fragments broken down by carboxypeptidase from pancreas and aminopeptidase from SI epithelium
Where are carboxypeptidase and aminopeptidase produced?
In the pancreas and the epithelium of the SI
What do carboxypeptidase and aminopeptidase do?
Break down protein fragments in the SI into AAs
How do free aas enter the epithelial cells of the SI?
Secondary active transport coupled to Na+ cotransporters: different transporters for different aas
Can short chains of aas be absorbed to the SI epithelium?
Yes: couples to hydrogen ion gradient
What happens to di and tripeptides that were transported into the epithelial cells of the SI coupled to hydrogen ion gradient?
Broken down into single amino acids and leave through facilitated diffusion
What enzyme is responsible for fat digestion?
Lipase
Where is lipase produced?
In the pancreas
What does lipase do?
Breaks a triglyceride into a monoglyceride and two fatty acids
What are the main emulsifying agents in digestion?
Phospholipids ingested or secreted in bile, and bile salts
What is colipase?
Protein secreted by pancreas to lodge on lipid droplets to bind the lipase enzyme
What are micelles?
Very small emulsion droplets: bile salts, monoglycerides and fatty acids: with polar side out and nonpolar side in
How do micelles increase absorption?
Continuously break down and reform, keep the lipids emulsified, while also allowing small amounts of broken-down lipids into the fluid to cross the membrane into the epithelial cells
What happens to monoglycerides and fatty acids in the epithelial cells?
Resynthesized into triglycerides for passage through to the interstitial fluids
Why is it advantageous that lipids leave the epithelial cells for the interstitium as triglycerides rather than fatty acids and monoglycerides?
Maintains the concentration gradient so that monoglycerides and fatty acids keep diffusing into the epithelium
What are chylomicrons?
Small extracellular fat droplets released by the smooth ER when the triglycerides were reformed
Where do chylomicrons go after formation?
To the lacteals! Because those have big pores and let the chylomicrons out easily
How are fat soluble vitamins absorbed? (ADEK?)
Same way as fats: broken into micelles
How are water soluble vitamins absorbed?
Through diffusion or mediated transport, EXCEPT for B12
Why is B12 tricky for absorption?
It’s a large charged molecule
How is B12 absorbed?
Binds to an intrinsic factor protein, which binds to sites on epithelial ileum for endocytosis
Where is intrinsic factor for B12 produced?
Acid-secreting cells of the stomach
What is the most abundant solute in chyme?
Na+
How does water absorption occur in the small intestine?
Wherever there’s a water concentration gradient created by actively transported solutes (Na+)
What solutes are absorbed with Na+ to cause water diffusion?
Cl- and HCO3-
What happens to iron in the small intestine?
Acitvely transported into epithelial cells, mostly bound into ferritin (intracellular iron storage) to be released to the blood side onto transferrin
How does the body control storage of iron?
Increased transcription of gene for ferritin: increased binding of iron in SI epithelial cells, so increased iron storage and reduced release into the bloodstream
What are four luminal stimuli to initiate GI reflexes?
Wall distension, chyme osmolarity, chyme acidity, and chyme concentration of specific digestion products: monosac, Fas, peptides, aas, etc.
What is the enteric NS?
GI’s own nervous system: submucosal plexus and myenteric plexus
How does activity in one half of the enteric NS affect the other?
Interrelated because the axons of one synapse with the neurons of the other
In which direction can impulses conduct along plexuses of the enteric NS?
Both directions: stimulation at one plexus point can affect other areas, like SI to stomach and lower intestinal tract muscle
What is the difference in influence of myenteric vs submucosal plexus?
Submucosal is mostly for secretory activity, and myenteric is mostly for smooth muscle activity
What kinds of neurons are in the enteric NS?
Adrenergic and cholinergic neurons; and neurotransmitter-releasing neurons
What are the main effectors of enteric NS neurons?
Muscle cells and exocrine glands
What kinds of neurons, sympathetic or parasympathetic, innervate the plexuses of the enteric NS?
Both! Allow CNs to influence motility and secretory activity of GI tract
What are the two types of neural0reflex arcs?
Short reflexes and long reflexes
What is a short reflex?
From receptors through nerve plexuses to effector cells
What is a long reflex?
From receptors to CNS by afferent nerves, then back to nerve plexus and effector cells through autonomic neurons
Where do hormones for regulating GI activity come from?
Cells scattered in the stomach and SI
What stimulates endocrine cells to secrete regulatory hormones to the GI tract?
Chyme: if it touches the endocrine cell on the luminal side, the endocrine cell will secrete hormones to the basolateral side into the blood to reach target cells by circulation
What are the four GI hormones?
Secretin, cholecystokinin (CCK), gastrin, and glucose-dependent insulinotropic peptide (GIP)
What are the two generalizations of GI hormones?
They participate in feedback control to regulate one aspect of the GI environment, and they affect multiple kinds of target cells
What triggers CCK secretion into the blood from the SI?
Fatty acids and amino acids in the SI
What happens when CCK is released into the blood after being triggered by fatty acids and amino acids?
Stimulates the pancreas to increase digestive enzyme secretion; causes gallbladder to contract and deliver bile salts
What is potentiation?
How secretin (stimulates pancreatic bicarb secretion) and CCK (weak stimulus for pancreatic bicarb secretion) TOGETHER cause major secretion of bicarb: whole is greater than sum of parts
Why does potentiation for secretin and CCK exist?
Because even though CCK is a weak stimulus for bicarb secretion from the pancreas, it amplifies the stimulation of secretin
What is the point of potentiation?
Small changes in plasma concentration of one GI hormone can have large effects on the actions of other GI hormones
What is leptin?
Peptide hormone: released from adipose cells, influences good intake and metabolic rate; induces satiety
What is ghrelin?
Peptide hormone: released from stomach during fasting; induces hunger
What are incretins?
GI hormones that alter insulin secretion from pancreatic islet cells
Which of the four main GI hormones is an incretin?
GIP: glucose-dependent insulinotropic peptide
What are the three phases of neural and hormonal control of the GI system?
Cephalic, gastric, and intestinal
Gastrin: what kind of hormone, where produced, stimuli for release?
Peptide; stomach; AAs, peptides in stomach, parasympathetic nervous