BHS 116.2 – Physiology
Notetaker: Vivien Yip
Date: 1/23/2013, 1st hour
Page1
Clicker Q
True or false, aceylcholine inhibits gastrin secretion
 False
 Acetylcholine is parasympathetic stimulation and stimulates digestion
Question dropped from test
Adrenal Cushing syndrome ocular symptoms
- Least likely to occur:
o Glaucoma
o Diplopia
o Cataracts
o Retinopathy
 Inhibit glucose uptake, hyperglycemia is major symptom
 Hypertension in addition to hyperglycemia
 Will get retinopathy and cataracts
 Could possibly get diplopia with fat distribution in the eye
 Trigger neuronal defects resulting in EOM palsies and diplopia (reason why it is
dropped)
Lecture 15 GI Secretory Functions continued and absorption in the GI tract
Describe the various secretions of the GI tract as well as their functions and regulations – pancreas, liver, small
intestine, large intestine
Pancreatic secretion
- CHO digestion w/ salivary amylase
- Begin some protein digestion with pepsin in the stomach
- But minimal digestion of nutrients up to this point
- Pancreas play major role in changing that
o Secrete vast majority of enzymes that will break down the nutrient components
- Endocrine function
o Islets: insulin and glucagon
- Exocrine function
o Acinar cells
 Glandular cells
 Release a number of digestive enzymes to break down CHO (carbohydrates), PRO
(protein), fats
o Ductal cells
 Leave to the ducts and release contents
 Release sodium bicarbonate and neutralize acidic chyme
- Each cell type produce very important things in digestive process
Pancreatic Secretion
- List of major enzymes released from exocrine portion of pancreas
- Released in their inactive form (-ogen)
o Trypsinogen
 Digests proteins into peptides
o Chymotrypsinogen
 Digests proteins into peptides
o Procarboxypeptidase
 Digests peptides into amino acids
o Pancreatic amylase
 Digests carbohydrates into disaccharides
o Pancreatic lipase
 Primary lipid enzyme
 Digests fat (TG) into fatty acids and monoglycerides
o
BHS 116.2 – Physiology
Notetaker: Vivien Yip
Date: 1/23/2013, 1st hour
Page2
o
Cholesterol esterase
 Hydrolyzes cholesterol esters
o Phospholipase
 Digests phospholipids into fatty acids
- Proteolytic enzymes are secreted in their inactive forms
- Also trypsin inhibitor released with them to help prevent activation of trypsinogen until it gets to GI tract
- Small enzyme on duodenal epithelial cells called enterokinase
o Once these proteases enter the duodenum, the enterokinase will convert trypsinogen into active
trypsin
o Trypsin is now formed, it will convert procarboxypeptidase to carboxypeptidase and
chymotrypsinogen to chymotrypsin
o Now all proteases are active
- Lipase is active already
- Pancreatic amylase is already active when secreted
- Ductal epithelial cells will release bicarbonate to increase the pH and reduce acidity of chyme
o Enzymes work much better when pH is neutralized
Regulation of pancreatic secretion
- Neuronal (from vagus)
o Acetylcholine (parasympathetic nerves)
- Hormonal
o CCK
 When food enters
the duodenum,
this is secreted
 Cause acinar cells
to release
enzymes
o Secretin
 Response to acid
in duodenum
 Activate ductal
epithelial cells to
release sodium
bicarbonate to
reduce acidity of
chime
-
Acid in duodenum causes secretin release
Fat and protein cause CCK release
CCK will cause acinar cells to release enzymes to break down those components
Secretion of bile by the liver
- Produced by the liver
- 2 important functions
o Primary aids in fat digestion, without bile present, we would have very inefficient digestion of
lipids
 Emulsify fat particles into smaller particles
o Role in absorption
 aiding the transfer of fats into the intestinal epithelium once they are broken down
- Also functions to get rid of waste products and cholesterol
Bile is continuously made in the liver
- Continuously secreted
- If no digestion occurring, no chyme in small intestine, bile will go into gallbladder and stored until it is
needed
- No active digestion, bile is still continuously secreted by the liver
BHS 116.2 – Physiology
Notetaker: Vivien Yip
-
Date: 1/23/2013, 1st hour
Page3
(hormonal regulation) CCK stimulates the gallbladder to contract and release bile into the small intestine
(neuronal regulation) Acetylcholine causes gall bladder contraction
Secretions of Small intestines
- Primarily mucous, water
- First part of duodenum: Brunner’s gland
o Where chyme is most acidic, just came from stomach
o Some sodium bicarb from pancreas to neutralize that
o Secrete alkaline mucus
 protects epithelium, both from acid and the proteases
 help neutralize pH
o Mucus is released just in response to tactile stimuli
 As soon as chyme is released
 Physical pressure on the epithelial cells
 Aided by vagus innervation
 CCK and secretin also help with stimulating alkaline mucus secretion
o Sympathetic stimulation inhibits this process
- Further down intestine: Crypts of Leiberkuhn
o Small valleys throughout entire length of small intestines
o Goblet cells secrete mucus
o More of a protective mechanism to help movement of chyme through small intestines
o Enterocytes, main epithelial cell
 Secrete and absorb water and electrolytes
 about 2L/day
- Regulated by neuronal and hormonal influences (CCK and secretin)
- Non secretory function
o Small digestive enzymes physically attached to surface of enterocytes
o Not secreted, membrane bound
 Break down anything that is left over from the other enzymes
 Peptidase (peptides  amino acids)
 Sucrose, maltase, isomaltase, lactase (disaccharides  monosaccharides)
 Intestinal lipase (fats  glycerol + FA)
Secretion of the Large Intestine
- Only really secretes mucus to protect epithelial lining and help movement of chyme
Describe the digestion process for CHO, PRO, fats and their absorption by the GI epithelium – enzymes, bile,
transport into and out of epithelium
Digestion of CHO
- Digestion starts in mouth w/ salivary amylase
- Only 3 major sources of CHO in the normal human diet
o Starches (large polysaccharides)
 Salivary amylase will break it down into maltose
 Any of the larger polysaccharides will be broken down by pancreatic amylase
o Lactose (disaccharide in milk)
o Sucrose (disaccharide, cane sugar)
- CHO are broken down into monosaccharides after disaccharides
- 3 monosaccharides
o Glucose
o Galactose
o Fructose
 These are the only ones absorbed in GI tract
- Digestion begins in the mouth w/ salivary amylase (enzyme ptyalin)
- Continues in esophagus and for a brief time in stomach until the acid inhibits function of the amylase
- CHO digestion finishes in the duodenum of the small intestines
BHS 116.2 – Physiology
Notetaker: Vivien Yip
Date: 1/23/2013, 1st hour
Page4
o
-
Disaccharides then are broken down into monosaccharides by the enzymes that are attached to the
surface of the enterocytes
Maltase, lactase, sucrose (on surface of enterocytes), complete the last digestive reaction
-
-
-
o
o
o
Starches, big
polysaccharides
broken down by
pancreatic and salivary
amylase into
disaccharides
Further broken down
by lactase, sucrase and
maltase found on the
surface of the
enterocytes into
monosaccharides
Glucose is found in
one of the components
in all of the
disaccharides
Maltose = gluc + gluc
Lactose = galactose + gluc
Fructose = sucrose + glucose
Protein digestion does not start until it gets to the stomach
- Starts w/ pepsin, requires HCl
- Pepsinogen is what is secreted and needs the conversion to pepsin to begin that process
- Only enzyme to break down collagen (component of muscle in meat)
- Vast majority of protein digestion occurs in the pancreas
o Pancreatic enzymes
 Break down into tripeptides, dipeptides, amino acids
o Peptidase enzymes on surface of the enterocytes will carry out last process (lining the intestinal
villi)
 Break down any peptides left over into amino acids
 Some dipeptides are taken up by the enterocytes, there is a peptidase inside the
enterocytes that will break it down
 Vast majority of the peptides is broken down already at the surface of enterocytes
-
-
-
Most of the break down occurs via
pepsin and pancreatic proteolytic
enzymes
Smaller peptides are further broken
down to aminopeptidase on surface
of enterocytes
If some dipeptides get through, will
have intracellular peptidase that
will break down into individual
amino acids
BHS 116.2 – Physiology
Notetaker: Vivien Yip
Date: 1/23/2013, 1st hour
Page5
Digestion of fats
- More complicated process
- Most abundant dietary fat is triglyceride
- Do get a little digestion in the stomach – but is mechanical, not enzymatic
- Just by force of stomach, can break larger fat particles into smaller fat particles
- Essentially, all of fat digestion occurs in the small intestine
- First step is emulsification of fat by breaking down the larger fat particles into smaller fat particles and
getting them to be water soluble so lipase can physically break down and digest the fat particles
- Mixing movements that occur with the small intestines, help physically break down
- Bile also plays key role in emulsification process
- Fat is not water soluble
- Bile salt will bind to these water insoluble fat molecules and incorporate into the large fat particles into
smaller fat particles
- Bile salts are both lipid soluble and insoluble components, water soluble portion are exposed on outside,
which makes the fat droplets water soluble
- Now broken down to smaller parts, lipase can act on these surfaces
Pancreatic lipase is the most important enzyme in fat digestion
- Left over is fatty acid and glycerol
-
-
Large fat particle, primarily triglycerides
Bile salts help emulsify into smaller lipid particles so
pancreatic lipase can act on it
Lipase can break it down to glycerol and free fatty
acids
Bile salts will now bind to individual fatty acids and
help solubilize those fats and allow them to be brought
to surface of enterocytes so they can be broken down
there
o Aid in absorption
Role in digestion
Absorption in the stomach
- Only a few highly lipid soluble substances are
absorbed in the stomach (except for alcohol and
aspirin)
o No nutrient absorption in the stomach
Describe the various folds and absorptive surfaces of the small intestines
Vast majority of absorption occurs in the small intestines
- Absorptive surface area of the small intestine is increased almost 1000 fold due to folding
o Tremendous amount of absorptive area for nutrients released from enzymatic processes
- Gross circular fold, can physically see
- Next we have forming of villi, blood vessels and lymph vessels projecting
- Enterocytes are all lining the villi
- On the surface of each enterocytes are microvilli (brush border)
o Finger like projections on the surface
Describe the process of absorption of ions and in the GI tract
Absorption of ions
- Water is absorbed by osmosis into pericellular spaces (follows Na+)
- Na+ is absorbed into the blood by diffusion into and between the enterocytes followed by active transport
out of the enterocytes
BHS 116.2 – Physiology
Notetaker: Vivien Yip
Date: 1/23/2013, 1st hour
Page6
CHO absorption
- All CHO are absorbed as monosaccharides
o Glucose and galactose via sodium co-transport
mechanism
o Fructose is transported via facilitated diffusion
mechanism and is converted into glucose in the
epithelium (or in the liver)
- Sodium is prevalent in the lumen, monosaccharides, glucose
taken up through active transport
- Fructose via GLUT 5 is (main transporter) for enterocytes– does
not require insulin
- Once those monosaccharides are inside the enterocytes, they
need to be pumped out
- Both fructose and glucose leave enterocytes via GLUT 2
- Taken up into the blood, carried to cells or liver to be stored
Protein absorption
- Taken up as individual amino acids or as di or tripeptides
- This is a process of co transport with sodium
- Sodium amino acid transporter in membrane of enterocytes
- Same process as CHO
- Small peptides taken up, intracellular peptidase will break it
down into individual amino acids
Only amino acids leave enterocytes to be taken up by the blood through
facilitated diffusion
High concentration inside the cell, facilitated diffusion out into the
blood
Fat absorption
- Primarily taken up as monoglycerides and free fatty acids
- Dissolved in bile micelles which makes them water soluble and can get down to villi and microvilli to
diffuse into the cell
- Diffuse directly through enterocyte membrane, lipid soluble
- When they get inside the enterocyte, they are reconverted to triglycerides
- Triglycerides packaged into chylomicrons
o Chylomicrons contain triglycerides, cholesterol and protein
- Chylomicrons have to undergo exocytosis to leave the cell
- Do not enter blood
o Capillaries cannot uptake chylomicrons (too large) and no receptors
o Instead, they enter the lymph system
Absorption in large intestine
- Mostly water and electrolytes in the chyme are absorbed in the proximal half of the large intestines
- Bacteria digests small amounts of cellulose
- Bacteria can produce vitamins:
o Vitamins K, B12, thiamine, Riboflavin
Clicker Q
Where does digestion of proteins begin?
- Stomach