Food Physiology
http://www.greathampers.co.uk
Digestion
Food In
Digestion takes place in the gastrointestinal tact
which is a long tube about 30ft in length.
Digestive Challenges:
1.
2.
3.
4.
Waste Out
It has to deal with a huge variety of food types
It has to prevent self digestion
It has to avoid penetration by infectious agents
It must maintain mass balance because the system
secretes roughly 7 liters of fluid a day, most of
which must be reabsorbed.
Food Types
Protein
Carbohydrates
Lipids
Vitamins
Minerals
?
Four main Processes:
1. Ingestion
2. Digestion (Chemical, Mechanical)
3. Absorption
4. Egestion
30 to 120 hours to go from ingestion
to egestion.
Figure 21-2a
Major subdivisions include:
1. Oral Cavity
2. Esophagus
3. Stomach
4. Small + Large Intestine
5. Cloaca – meaning sewer (Latin)
Figure 21-2a
Accessory Organs:
1. Tongue
2. Teeth
3. Oral Glands
4. Pancreas
5. Liver
6. Gall bladder
7. Host of nerve and glandular systems
Figure 21-2a
Oral Cavity
Adults have 32 teeth, 16 in
each Jaw.
On one half of each Jaw:
1. Two incisors
2. One canine
3. Two premolars
4. Three molars (incl Wisdom)
Oral Cavity
In general the arrangement of
teeth depends on feeding habits.
Humans being omnivores tend
not to a greatly specialized teeth.
Weasel - Carnivore
Sabertooth tiger
Oral Cavity
Children only have 20 teeth,
so-called milk teeth. These
are replaced at about 6
years.
At 18 years or more,
additional molars appear in
the form of “Wisdom” teeth.
Tooth Structure
Three layers:
1. Pulp
2. Dentine
3. Enamel
Pulp contains blood vessels and nerves.
Dentine is largely inorganic (70%) but
it is alive in the sense that it can be
replaced.
Enamel is a crystalline calcium phosphate
and fluoride material. It is brittle and the
hardest material in the human body. It is
essentially dead. Not replaceable.
Salivary Glands and Saliva
We all secrete roughly 1.5 L saliva
per day.
At least four major glands surrounding
the mouth.
Functions of Salvia:
Moisten food, allow easy swallowing
Contains:
1. R-Protein (Binds B12)
2. Mucus
3. Enzyme Amylase
1. Digests starch to maltose + dextrin
4. Lysozyme
1. Digests bacterial cell walls
5. Lipase
1. Digests triglycerides to fatty acids
and glycerol.
Swallowing
Reflex action once food touches the back of the
mouth.
Epiglottis closes the
larynx diverting food
(we hope) to the
esophagus.
Muscle Diversion
Cardiac muscle tissue forms the bulk
of the wall of the heart. Like skeletal
muscle tissue, it is striated. High
fatigue tolerance, force not
as strong as skeletal. Contraction
graded.
Skeletal muscle, prone to fatigue, but
can operate very quickly and with
great force. Contraction all or none.
Smooth muscle, high fatigue
tolerance, slow movements.
Contraction graded and tends to
be involuntary.
Esophagus (Oesphagus)
Long muscular tube that moves food
to the stomach by peristalsis.
The muscle of the esophagus
is a smooth muscle under
involuntary control.
The esophagus ends at the
cardia (or cardiac) sphincter
which is a one way valve.
Figure 21-4a
Volume roughly up to 2 L but
can go to 4/6L in extreme
conditions.
The stomach connects the
esophagus to the duodenum.
It is located on the left side.
Immediately beneath the stomach
is the pancreas.
At the lower end of the stomach is
the pyloric sphincter value.
Figure 21-2b
The stomach is quite muscular
containing at least three major
layers of muscle. Each layer either
runs lengthwise, circular or
obliquely.
The inner surface is highly
folded and secretes gastric juice.
The stomach is well supplied
with blood vessels and the vagus
nerve.
Figure 21-2b
Figure 21-2c
Figure 21-25
Parietal Cells – HCI Secretion
CA = Carbonic Anhydrase
Figure 21-6
Stomach Digestion
The liquid content of the stomach is called Chyme.
Food stays in the stomach for about 1 to 3 hours (Rule of thumb)
Some small molecules are absorbed by the stomach:
Glucose
Alcohol
Aspirin
Caffeine`
Drugs in general
Figure 21-26
Stomach Digestion
Enzyme and proteins secreted by the stomach:
Inactive Pepsinogen is secreted by the Chief Cells and is converted
to Pepsin by the low pH of the stomach.
Intrinsic factor – Used later in the small intestine to absorb
vitamin B12, secreted by Parietal cells.
Pepsin is an endopeptidase. Pepsin is effective against collagen (meat!)
Leu, Phe, Trp
Figure 21-26
Stomach Digestion
Enzyme and proteins secreted by the stomach:
Rennin (Chymosin) found in cow stomachs - aspartic acid protease
The gene is present in humans but is inactive. Causes cleavage
of phenylalanine and methionine in casein (milk protein). Also
used in cheese making but is now made recombinantly in
organisms such as E. coli.
Some gastic lipase is also produced in the stomach.
Amylase in the saliva is slowly inactivated by the low pH
Figure 21-26
Stomach Hormones: all small peptides
Hormones secreted by the stomach:
Gastin: The hormone gastrin causes an increase in the secretion
of HCl, pepsinogen and intrinsic factor from parietal cells
in the stomach. Produced the G-Cells. Inhibited by low pH
but stimulated by neuronal, mechanical stretching and
protein fragments in the stomach.
Histamine: Stimulates HCI production via Gastin
Somotostatin: Inhibits Gastin, Parietal cells and Chief Cells.
Figure 21-26
Post Stomach Anatomy
1. Duodenum
2. Jejunum
3. ilium
4. ileocecal value
5. Cecum
6. Colon
7. Anus
Duodenum
Duodenum: 25-30 cm long
The word Duodenum means “12” because
this part of the intestine is 12 fingers in breadth.
The bile duct and pancreatic duct enter
the Duodenum at a common point.
The bile duct originates from the liver and
the pancreatic duct from the pancreas
www.gopetsamerica.com
Duodenum
Duodenum: Brunner’s glands (also called duodenal glands) are found in
the Duodenum. These glands secrete bicarbonate ions (so does the pancreas),
mucus and a growth factor that stimulates epithelial proliferation. In addition
throughout the small and large we find specialized goblet cells which secrete
only mucus.
www.humanillnesses.com
Bile
Bile is a yellow/green alkaline liquid secreted by hepatocytes in the liver.
Bile is stored in the gallbladder between meals and is discharged into the
Duodenum after a meal.
About 1 L a day is made and is what gives
feces it’s characteristic color.
http://www.billcasselman.com
Bile - Composition
Bile contains the following components:
1. Water
2. Cholesterol
3. Bile pigments: bilirubin, biliverdin
These are excretion products from the
breakdown of hemoglobin.
4. Lecithin (phophatidylcholine)
5. Bile salts: Essential for fat digestion
http://www.billcasselman.com
Bile Pigments: biliverdin and bilirubin
Heme
Biliverdin
Bilirubin – Yellow pigment,
also responsible for the color
of bruises.
Bile Salts
Bile salts (Na) are derived from Cholesterol
via conjugation with glycine or taurine.
a) Glycocholate
b) Taurocholate
Gall stones result from precipitation of
bile salts.
Glycocholate
Taurocholate
www.nlm.nih.gov
Bile Salts
The production of bile salts is also
a mechanism for controlling the
level of Cholesterol. However
about 10 to 25% of the bile salts are
reabsorbed in the large intestine,
presumably to recover
some of the Cholesterol.
About 50% of all Cholesterol
made or consumed by the body
is made into Bile salts.
Small Intestine Hormones: all small peptides
CCK = Cholecystokinin is a hormone that is produced throughout
the small intestine. The release of CCK is stimulated by fatty acids,
amino acids and polypeptides. Causes gallbladder to contract and
inhibits emptying of stomach, stimulates pancreatic enzymes and
bicarbonate.
Secretin: This is a hormone mainly produced in the Duodenum in
response to acidic chyme that enters the Duodenum. It inhibits
acid secretion and stimulates bicarbonate production and hepatic
bile flow.
Small Intestine Hormones: all small peptides
Motilin: This hormone regulates digestive motility and is released by
neurons in the small intestine.
GIP = Glucose dependent insulinotropic peptide: Inhibits acid
secretion and stimulates insulin release - probably most
important function (Used to be called gastric inhibitory polypeptide).
The effect that GIP has on insulin release means that, paradoxically,
glucose is taken up quicker by cells when given orally than when
administered by injection.
Gallbladder
The gallbladder is an active organ where
water and NaCl is actively removed
to concentrate the bile. The walls of
the gallbladder contain smooth
muscle that contract the bladder, this
action is under hormonal (CCK) and
nervous control. The hormonal response
is stimulated by fatty acids entering
the Duodenum.
Ach = acetylcholine
The Pancreas
The pancreas is both an endocrine and
and enzyme secreting (exocrine) organ and
as such its cellular morphology reflects this
dual function.
Acinar cells or an acinus,
these secrete pancreatic
enzymes + bicarbonate
islets of Langerhans
containing beta cells. These
cells are hormonal and secrete
into the blood stream hormones
such as Insulin, Glucagon and
somatostatin.
http://www.dkimages.com/
Pancreatic Juice
The pancreatic juice contains a number of components, including
proenzymes, water and bicarbonate (required to neutralize the HCl
from the stomach).
A proenzyme is an inactive enzyme that must be modified before becoming active.
1. Trypsinogen
2. Chymotrypsinogen
3. Procarboxypeptidase
4. Pancreatic lipase
5. Pancreatic amylase
- Inactive
- Inactive
- Inactive
- secreted in active form but requires colipase for activity
- secreted in active form
There are also some RNAses and DNAse, etc.
A disease called pancreatic necrosis occurs when the pancreas produces active forms
of the proteases, this results in the pancreas digesting itself.
Activation of Proenzymes
The small intestine
secretes an enzyme called
enteropeptidase. This
enzyme is responsible for
activating trypsinogen
into its active form
Trypsin.
Trypsin in turn not only
activates the other
proenzymes but also
activates more
trypsinogen. This creates
a positive feedback.
Activation of Proenzymes
CCK = Cholecystokinin is a hormone that is produced throughout
the small intestine. The release of CCK is stimulated by fatty acids,
amino acids and polypepides.
Hormonal Control of the Pancreas
Pancreatic Juice - Amylase
Starch or Glycogen (Glycogen is more highly branched)
http://www.vivo.colostate.edu
Small Intestine
The small intestine in general includes goblet cells, enzyme
and hormone releasing cells, Payer’s patches, large nervous
network, rich blood and lymph system and transport
proteins. Final digestion occurs on the intestinal borders.
1. Jejunum: Less goblet cells (mucus) and generally lack’s
payers patches (lymph nodes), 2.5 m long. The work
Jejunum means “empty” because at death the
Jejunum is found to be often empty of contents.
2. ileum: Payer’s patches, absorbs B12, large surface area.
2-4 m long. The word ileum means “twisted”.
Figure 21-2d
Figure 21-2e
Figure 21-13
Protein Structure: Terminology
Figure 21-16a
Endopeptidases
Examples: Stomach Pepsin, Pancreatic Trypsin
Figure 21-16b
Exopeptidases
Figure 21-16c
Figure 21-17
Protein Digestion: Summary
Carbohydrate Digestion: Summary
Starch
Glycogen
Maltotriose (breakdown product of starch, glycogen)
Sucrose:
Lactose:
Maltose:
Trehalose
Glucose, Fructose
Sucrase
Glucose, Galactose
Lactase
Glucose, Glucose
Maltase
Glucose, Glucose (insect sugar): Trehalase
We DO NOT digest cellulose (or lignin – wood - for that matter)
Carbohydrate Digestion: Summary
Saccharose = Sucrose!
Carbohydrate and Proteins
All free amino acids and sugars go directly into the
blood stream and on to the liver.
Diversion:
Basic Circulatory System
Carotid
Jugular
Pulmonary vein
Pulmonary artery
Aorta
Vena Cava
Hepatic vein
Hepatic artery
Hepatic portal vein
Renal
Coronary
iliac
Mesenteric Artery
Basic Circulatory Systems
Figure 21-30
Fat Digestion
Figure 21-30
Hormone Action Summary Sheet
Figure 21-31 – Overview (1 of 4)