Lesson 83 - Physiology of the Small intestine

What is lipase?

enzyme that breaks down fats into glycerol and free fatty acids

What is amylase?

enzyme that digests carbohydrates by breaking down starch into simpler sugars

What is trypsinogen?

inactive enzyme secreted by the pancreas

What is trypsin?

enzyme in small intestine to aid in protein digestion

What does trypsinogen get activated into?

trypsin

What does chyme contain?

polypeptides/oligopeptides, large emulsified fat globules, unchanged dietary carbohydrates

What breaks proteins down into polypeptides in the stomach?

HCl and pepsin

What breaks down carbohydrates into poly/oligosaccharides before the small intestine?

salivary amylase in omnivores

What must occur before the components leaving the stomach can be absorbed in the small intestine?

neutralization of gastric acid and hydrolysis of macromolecules into micromolecules

What do carbohydrates need to be absorbed?

monosaccharides

What do proteins need to be to be absorbed?

tri/dipeptides and amino acids

What do fats need to be to be absorbed?

free fatty acids/monoglycerides

What does the pancreas provide to the small intestine?

buffers and digestive enzymes

What does the bile provide to the small intestine?

buffers and fat emulsifier

What are the exocrine functions of the pancreas?

bicarbonate and digestive enzymes

Why does bicarbonate need to be excreted from the pancreas?

neutralize gastric acid and enable pancreatic enzymes to function

What pH do pancreatic enzymes need to function?

7.5-8

What are the major groups of digestive enzymes from the pancreas?

amylolytic, proteolytic, lipolytic

What is the amylolytic enzyme from the pancreas?

alpha-amylase

What does alpha-amylase digest?

most soluble carbohydrates, but not structural carbohydrates

What are soluble carbohydrates?

starch and glycogen

What does alpha-amylase do to the carbohydrates?

splits only 1-4 alpha glycosidic linkages

What is the end product from amylase?

polysaccharides into oligosaccharides and disaccharides

What are the proteolytic enzymes from the pancreas?

trypsinogen, chymotripsinogen, pro-elastase, procarboxypeptidases

What are proteolytic enzymes stored in?

vesicles in the pancreas

What is trypsinogen activated by?

enteropeptidase (enterokinase)

What activates the other proteolytic enzymes?

trypsin

What is the end result of proteolytic enzymes?

oligopeptides

What are the lipases from the pancreas?

pancreatic lipase, pro-phospholipase, cholesterolesterase

What is the end result of lipases?

triglycerides into fatty acids and monoglycerides

What do lipases depend on?

bile salts

What is colipase?

cofactor required to help pancreatic lipase

What is the way that the pancreas avoids autodigestion?

enzymes stored as zymogens, intracellular segregation or compartmentalization, pancreatic secretory trypsin inhibitor, plasma protease inhibitors

What are the forms of intracellular segregation that help prevent autodigestion?

zymogen granules

What are the plasma protein inhibitors?

alpha-macroglobulin, alpha1-proteinase inhibitor, anti-thrombin

What is the third largest group of plasma proteins?

plasma protease inhibitors

What happens with the pancreas during cephalic phase?

vagal stimulates the pancreas to start secreting enzymes and bicarbonate

What happens with the pancreas during gastric phase?

gastrin release stimulates the pancreas to start secreting enzymes and bicarbonate

What is the main phase of pancreatic stimulation?

intestinal phase

What are the hormones controlling the pancreatic stimulation during the intestinal phase?

cholecystokinin and secretin

What stimulates the release of secretin?

pH of chyme is <4.5

What is the result of secretin stimulation?

bicarbonate secretion from pancreas and gall bladder

What stimulates the release of cholecystokinin?

proteins and fats

What is the result of cholecystokinin stimulation?

pancreatic enzyme secretion and release of stored bile

What are the direct digestive functions of the liver?

facilitate digestion and absorption of fats and lipophilic vitamins and neutralize acidic chyme

How does the liver neutralize acidic chyme?

release of bicarbonate

How does the liver facilitate digestion of fats and lipophilic vitamins?

secretion of bile acids

What do hepatocytes secrete?

bile acids and lipophilic waste products

What are some lipophilic waste products?

bilirubin and cholesterol

What do duct cells of the liver secrete?

bicarbonate, water, electrolytes

What can happen if there is continuous absorption of electrolytes and water from bile stored in the gall bladder?

concentration of organic compounds

What are bile acids derived from?

cholesterol

What does cholesterol become?

cholic or chenodeoxycholic acids

What must happen for cholic or chenodeoxycholic acids to become bile salts?

conjugation with glycine or taurine

What are bile salts?

detergents

What are the functions of bile acids?

emulsification and transport

How do bile salts aid in emulsification?

attach to surfaces of fat globules and break it up into smaller pieces which increases surface area for lipases

How do bile acids aid in transport?

form aggregates with fatty acids and monoglycerides called micelles with hydrophilic ends pointing outwards and lipophilic pointing inwards which avoids re-condensation into triglycerides

What phases does bile juice get secreted?

some in cephalic and gastric but most in intestinal

What hormones control bile release during intestinal phase?

secretin and cholecystokinin

What sphincter controls the release of bile?

sphincter of Oddi

What is important about the sphincter of Oddi and horses?

not efficient barrier so bile is continuously released into duodenum

What is luminal digestion?

mixing and propelling of chyme along with pancreatic juices, bile, and buffers

What is membrane digestion?

digestive with mucosal enzymes before being absorbed

What are slow waves?

spontaneous rhythmical undulations of the resting membrane potential around a baseline

What cells have slow waves?

cells of cajal which are specialized smooth muscle cells

What helps conduct slow waves along a GI section?

gap junctions

What is the frequency of slow waves?

3-12/min

What is a spike potential?

when a slow wave peak reaches critical threshold potential and voltage gated calcium channels open which elicits a muscle contraction

What can modulate the amplitude of slow waves?

priming and de-priming factors

What are some priming factors?

gut wall stretching, parasympathetic stimulation, some GI hormones

How do priming factors affect GI motility?

increases motility becuase they depolarize the membrane allowing more slow waves to reach threshold

How do de-priming factors affect GI motility?

decreases motility because they hyperpolarize the membrane

What are some de-priming factors?

sympathetic stimulation and some GI hormones

What are the amplitude and frequency of action potentials directly related to?

force and duration of contraction

What are the patterns of motility in GI?

peristalsis, MMC, and segmentation

What is peristalsis?

circular contractions cranial to bolus and relaxation caudal to the bolus which occurs over long distances

What is segmentation?

localized contraction of mostly circular muscles over short distances to help mix and chop food

What helps prevent net forward movement of chyme with segmentation?

alternate segments contract

When do migrating motility complexes occur?

during fasting phases

What causes migrating motility complexes?

motilin

What is the function of the MMC?

prevent small intestinal bacterial overgrowth

What do stem cells differentiate into?

young enterocytes

What is the function of young enterocytes?

secretory cells

Where are young enterocytes?

along the crypts of lieberkuehn

What do young enterocytes differentiate into?

mature enterocytes, enteroendocrine cells, mucus cells, goblet cells, paneth cells

What is the function of mature enterocytes?

absorptive cells

What happens to mature enterocytes after about five days?

go into the gut lumen and are digested

What do paneth cells function like?

neutrophils

What do paneth cells contain?

alpha-defensins also known as cryptidins

What do young enterocytes secrete?

large volumes of intestinal fluid

What drives the secretion of intestinal fluid in young enterocytes?

active chloride secretion into the gut

What is actively secreting chloride into the gut?

cystic fibrosis transmembrane conductance regulator / cyclic-AMP dependent chloride channel

What is secreted due to chloride secretion?

sodium and water

What do mature enterocytes contain?

microvilli called brush border membrane

What does the brush border microvilli contain?

digestive enzymes and absorptive mechanisms

What does the brush border microvilli assist with?

membraneous digestion

What must occur to the end products of luminal digestion before absorption?

hydrolysis by BBM-bound enzymes (membrane digestion)