Facets of Digestion Flashcards
mechanical digestion
the physical breakdown of food into smaller particles
- cutting and grinding action of the teeth
- churning action of stomach and small intestines
- exposes more food surface to the action of digestive enzymes
Chemical digestion
a series of hydrolysis reactions that breaks dietary macromolecules into their monomers (residues)
-carried out by digestive enzymes produced by salivary glands, stomach, pancreas and small intestine
what are the chemical results of chemical digestion?
polysaccharides into monosaccharides
proteins into amino acids
fats into monoglycerides and fatty acids
nucleic acids into nucleotides
absorption
the uptake of substances into or across tissues
example: once food is digested (broken down to its simplest form) it is able to be absorbed by the small intestine.
what are the two anatomical subdivisions of the digestive system?
digestive tract (alimentary canal) and accessory organs
digestive tract (alimentary canal)
- 30 foot long muscular tube extending from mouth to anus
- mouth, pharynx, esophagus, stomach, small intestine, and large intestine
- gastrointestinal (GI) tract is the stomach and intestines
accessory organs
teeth, tongue, salivary glands, liver, gallbladder, and pancreas
key features of GI tract
- smooth muscle within the ‘muscularis externa’ that are responsible for perstalis
- enteric NS: note submucosal (Meissner) plexus and myenteric (Auerbach) plexus
peristalsis
involuntary rhythmic contractions that move food (pre-stomach) and chyme (post-stomach) through GI tract
enteric nervous system
a nervous network in the esophagus, stomach, and intestines that regulated digestive tract motility, secretion, and blood flow
- functions completely independently of the central nervous system
- ANS exerts a significant influence on its action
- nerves innervating the esophagus, stomach and intestine are known as the “Auerbach” plexus” (connected to the medulla oblongata)
motility and secretion of the digestive tract are control by?
neural, hormonal, and paracrine mechanisms
neural control: short and long reflexes
short (myentreic) reflexes: stretch or chemical stimulation acts through myentric plexus
-long (vagovagal) reflexes: parasympathetic stimulation of digestive motility and secretion
hormones
- chemical messengers secreted into bloodstream, and stimulate distant parts of the digestive tract
- gastrin and secretin
paracrine secretions (prostaglandins with local effect)
-chemical messengers that diffuse through the tissue fluids to stimulate nearby target cells
mastication
(chewing): breaks food into smaller pieces to be swallowed and exposes more surface to the action of digestive enzymes
Saliva is made up of?
salivary amylase, lingual lipase, mucus, lysozyme, immunoglobulin A, and electrolytes
salivary amylase
enzyme that begins starch digestion in the mouth
lingual lipase
enzyme that is activated by stomach acid and digests fat after the food is swallowed
mucus
binds and lubricates the mass of food and aids in swallowing
lysozyme
enzyme that kills bacteria
immunoglobulin A
an antibody that inhibits bacterial growth
what are the five electrolytes in saliva?
Na+, K+, CI-, phosphate, and bicarbonate
pharynx
a muscular funnel that connects oral cavity to esophagus and allows entrance of air from nasal cavity to larynx
esophagus
a straight muscular tube 25-30 cm long
- begins at level between C6 and the cricoid cartilage
- extends from pharynx to cardiac orifice of stomach passing through esophageal hiatus in diaphragm
- lower esophageal sphincter: food pauses at this point because of this
stomach
a muscular sac in upper left abdominal cavity immediately inferior to the diaphragm.
- mechanically breaks up food particles, liquefies the food, and begins chemical digestion of protein and fat
- most digestion occurs after the chyme passes on to the small intestine
chyme
soupy or pasty mixture of semi-digested food in the stomach
what are the four subregions on the stomach?
cardiac region (cardia), fundic region (fundus), body (corpus), and pyloric region
cardiac region (cardia)
small area within about 3 cm of the cardiac orifice
fundic region (fundus)
dome-shaped portion superior to esophageal attachment
body (corpus)
makes up the greatest part of the stomach
pyloric region
narrower pouch at the inferior end
-subdivided into the funnel-like antrum and narrower pyloric canal that terminates at pylorus (narrow passage to duodenum)
pyloric (gastroduodenal) sphincter
regulates the passage of chyme into the duodenum
What is the Vasovagal Reflex?
Explain the Hepatic Portal Circulation.
-stomach receives:
parasympathetic fibers from vagus, sympathetic fibers from celiac ganglia,
which are collectively called “vasovagal reflex”
-all blood drained from stomach and intestines enters hepatic portal circulation and is filtered through liver before returning to heart
what are the cells of gastric glands?
mucous cells, parietal cells, chief cells, and enteroendocrine
mucous cells
secrete mucus. predominate in cardiac and pyloric glands
parietal cells
found mostly in the upper half of the gland. secretes 3 things: hydrochloric acid (HCI), intrinsic factor, and a hunger hormone called ghrelin
chief cells
most numerous. secrete gastric lipase and pepsinogen. dominate lower half of gastric glands. absent in pyloric and cardiac glands.
enteroendocrine cells
concentrated in lower end of gland. secrete hormones and paracrine messengers that regulate digestion.
how is pepsinogen activated to pepsin?
hydrochloric acid (HCI) converts pepsinogen to pepsin which breaks down proteins to peptides. protein digestion begins in the stomach.
intrinsic factor
a glycoprotein secreted by parietal cells.
- essential to absorption of vitamin B12 by the small intestine: binds vitamin B12 and intestinal cells absorb this complex by receptor-mediated endocytosis
- vitamin B12 is needed to synthesize hemoglobin: prevents pernicious anemia
- secretion of intrinsic factor is the only indispensable function of the stomach
functions of hydrochloric acid
- activates pepsin and lingual lipase.
- breaks up connective tissues and plant cell walls
- converts ingested ferric ions (Fe3+) to ferrous ions (Fe2+)
- contributes to nonspecific disease resistance by destroying most ingested pathogens
gastric motility: how long does it take for food to leave the stomach? what about a fatty meal?
typical meal emptied from stomach in 4 hours. less time if the meal is more liquid. as long as 6 hours for a high fat meal.
gastritis
inflammation of the stomach can lead to a peptic ulcer as pepsin and hydrochloric acid erode the stomach wall. most ulcers are caused by acid-resistant bacteria, Helicobacter pylori that can be treated with antibiotics and Pepto-Bismol.
Gastroesophageal Reflux Disease (Gerd)
- reflux normally prevented by lower esophageal sphincter.
- complications include: strictures in esophagus, ulceration, and increased cancer risk.
functions of hepatocytes
- after a meal, the hepatocytes absorb from the blood (glucose, amino acids, iron, vitamins, and other nutrients for metabolism or storage
- removes and degrades (hormones, toxins, bile pigments, and drugs)
- secretes into the blood (albumin, lipoproteins, clotting factors, angiotensinogen, and other products)
- between meals, hepatocytes break down stored glycogen and releases glucose into the blood
gallbladder
a pear-shaped sac on underside of liver
-stores and concentrates bile by a factor of 20 by absorbing water and electrolytes
bile
yellow-green fluid containing minerals, cholesterol, neutral fats, phospholipids, bilt pigments, and bile acids
bile acids (bile salts)
steroids synthesized from cholesterol
-bile acids and lecithin, a phospholipid, aid in fat digestion and absorption
-80% of bile acids are reabsorbed in the ileum and returned to the liver
20% of the bile acids excreted in the feces
gallstones (biliary calculi)
hard masses in either the gallbladder or bile ducts. composed of cholesterol, calcium carbonate, and bilirubin
Pancreas
both an endocrine and exocrine gland
endocrine portion of pancreas
pancreatic islets that secrete insulin and glucagon
exocrine portion of pancreas
99% of pancreas that secretes 1200 to 1500 mL of pancreatic juice per day
-secretory acini release their secretion into small ducts that converge on the main pancreatic duct
Pancreatic Secretions
sodium bicarbonate -alkaline) raises pH of acidic chyme in duodenum
pancreatic amylase-digests starch
pancreatic lipase-digests fat
pancreatic protease-digests proteins (trypsin and chymotrypsin)
what three stimuli are chiefly responsible for the release of pancreatic juice and bile?
acetylcholine (A Ch), cholecystokinin (CCK), and secretin
acetycholine (A Ch)
from vagus and enteric nerves
-stimulates acini to secrete their enzymes during the cephalic phase of gastric control even before food is swallowed
cholecystokinin (CCK)
secreted by mucosa of duodenum in response to arrival of fats in small intestine
-strongly stimulates gall bladder to discharge bile into duodenum
secretin
released from duodenum in response to acidic chyme arriving from the stomach
- stimulates ducts of both liver and pancreas to secrete more sodium bicarbonate
- raising pH to level pancreatic and intestinal digestive enzymes require
- Alkalyzing Secretions!!!
what are the three regions of the small intestine?
duodenum, jejumun, and ileum
duodenum
the first 25 cm (10 inches) of the small intestine.
- receives stomach contents, pancreatic juice, and bile
- pancreatic enzymes take over the job of chemical digestion
- stomach acid is neutralized here
- fats are physically broken up (emulsified) byt the bile acids
jejunum
first 40% of small intestine beyond duodenum
-most digestion and nutrient absorption occurs here
ileum
forms the last 60% of the postduodenal small intestine
ilececal valve
sphincter formed by the thickened muscularis of the ileum
- protrudes into the cecum
- regulates passages of food residue into the large intestine
Small Intestine Villi
• Lipids (chylomicrons) are absorbed by specialized lymphatic capillaries called lacteals. • Intestinal crypts are analagous to gastric crypts
what is the purpose of segmentation of the small intestine?
to mix and churn not to move material along as in peristalsis
indigestible residue in large intestine
large intestine receives about 500 mL of indigestible residue per day
– reduces it to about 150 mL of feces by absorbing water and salts
– eliminates feces by defecation
anatomy of the large intestine
ascending colon, right colic (hepatic) flexure, transverse colon, left colic (splenic) flexure, and descending colon frame the small intestine
– sigmoid colon is S-shaped portion leading down into pelvis
-rectum - portion ending at anal canal
-anal canal – final 3 cm of the large intestine
rectum
portion ending at anal canal
• has 3 curves and 3 infoldings the transverse rectal folds(rectal valves)
anal canal
final 3 cm of the large intestine
-passes through levator ani muscle and pelvic floor
• terminates at the anus
• anal columns and sinuses – exude mucus and lubricant into anal canal during defecation
-large hemorrhoidal veins for superficial plexus in anal columns and around orifice
hemorrhoids
permanently distended veins that protrude into the anal canal or form bulges external to the anus
