Download Digestive System

Digestive System
Dr. Anderson
GCIT
Digestive System
• Function – to process and sequester energy,
and essential nutrients from what we eat
• Two main divisions:
– Alimentary Canal – organs that food (or waste)
pass through
– Accessory Organs – organs that secrete
substances that aid the digestive process
Alimentary Canal
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Mouth
Pharynx
Esophagus
Stomach
Small Intestine
Large Intestine
Rectum
Anus
Superior
Inferior
Accessory Digestive Organs
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Teeth, tongue
Salivary Glands
Gall Bladder
Liver
Pancreas
Superior
Inferior
Digestive Processes
• Ingestion – eating
• Propulsion - peristalsis
• Mechanical Digestion – physically breaking up
food
• Chemical Digestion – chemically breaking
down food
• Absorption – getting digested nutrients into
the blood
• Defecation – removing waste products from
the body
Upregulation of Digestive Processes
• Sensors (both mechanoreceptive and
chemoreceptive) are present in the walls of the
alimentary canal
– Stretch receptors
– pH receptors
• Stimulation of these receptors causes changes in
the function of the digestive system
– Hormone release
– Peristalsis (movement of food or waste through the
system)
Intrinsic and Extrinsic Gut Influences
• Intrinsic - The “gut brain” = a plexus of nerves
in the wall in the GI tract allow organs to
communicate
– Coordinates physical digestion
• Extrinsic – Hormones can be secreted by
stomach and small intestine that will affect
other digestive organs
Digestive System - Anatomy
• Peritoneum – membrane
of connective tissue
– Visceral – surrounds
organs
– Parietal – lines inside of
abdominal cavity
• Peritoneal cavity
– Houses organs
– Produces peritoneal fluid,
allowing organs to move
easily during digestion
Mesentery
• Double layer of
peritoneum that
extends from the
digestive organs to the
parietal peritoneum
• Provides a pathway for
blood vessels and
nerves to reach
digestive organs
Appendicitis - Peritonitis
• Bacteria from a burst appendix leak into the
abdominal cavity causing infection and
inflammation
• Can be life threatening!
• Treatment – Lavage and antibiotics
Splanchic Circulation – Blood Supply
• Arterial blood from
aorta that serves the
digestive organs
• Travels through
mesentery
Histology of the Alimentary Canal
• Mucosa – Innermost layer of epithelium
– Secretes mucus, digestive enzymes, provides
barrier to pathogens and enzymes
• Epithelium – Produces mucus which lubricates food and
protects cells, also produces enzymes and hormones
• Lamina propia – Loose connective tissue – provides
barrier to bacteria
• Muscularis mucosae – muscle layer that produces
muscle contractions
Histology of the Alimentary Canal
• Submucosa – elastic, innervated, highly
vascular connective tissue
• Muscularis Externa – Smooth muscle layer
responsible for segmentation and peristalsis
• Serosa (Visceral Peritoneum) – Outer covering
of connective tissue
Gross Anatomy - Mouth
• Hard Palate – palatine bones in skull
• Soft palate – mostly skeletal muscle, rises to
close nasopharynx during swallowing
• Tongue – Interlacing bundles of muscles
– Grips and tastes food, assists in mechanical
digestion
– Papillae add roughness to the tongue, and house
taste buds
Salivary Glands
Saliva – produced by salivary glands
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–
–
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Moistens food
Begins digestion (complex carbs)
Dissolves chemicals
Cleanses the mouth
• Saliva contains enzymes and electrolytes
– Also immune-related chemicals (lysozyme, defensin,
mucus, antibodies)
Salivary Glands
• Located under tongue
(sublingual gland),
posterior to masseter
muscle (parotid gland)
and under lower jaw
(submandibular gland)
• Also secreted by buccal
glands, scattered
throughout the oral cavity
Control of Salivation
• Food ingestion
• Mechano and chemoreceptors in the brain
send messages to the salivatory nuclei in the
brain which increase salivation rates
• Neural input (smells, descriptions of food,
etc.)
– Pavlov’s dogs
The Esophagus
• Muscular tube that is superior to the stomach
– Serves to propel food into the stomach via peristalsis
– Passage is eased by lubricating mucus secreted from
the submucosa
• Joins the stomach at the gastroesophageal
sphincter
– Keeps stomach contents in the stomach
• “Heartburn” is the result of gastric juice moving past this
sphincter, and into the esophagus
• http://www.youtube.com/watch?v=umnnA50IDIY
The Stomach
• Very elastic – can vary from 50
ml to over 4 liters in volume!
– Folded when not holding
food (folds = rugae)
• Held in place by the omentum
– Attaches curvatures of
stomach to parietal
peritoneum
• Has oblique muscles that (in
addition to the other organs of
the alimetary canal, allow for
more movement, i.e. churning
of food)
Stomach Epithelium
• Gastric Glands – produce gastric juice
– Mucus neck cells
• Produces acidic mucus (function not yet known)
– Parietal Cells
• Secrete HCl - gives the stomach contents a pH between 1.53.5
– Chief Cells
• Produces pepsin in the acid of the stomach
– Enteroendocrine cells
• Hormones that regulate gut function
– E.g. Gastrin
Digestion in the Stomach
• HCl denatures proteins into simpler shapes,
making it easier for enzymes to catabolize
them
– Pepsin – enzyme that degrades proteins
– Rennin – degrades milk proteins in infants
• Intrinsic Factor – needed to absorb vitamin
B12 for erythrocyte production
Regulation of Gastric Secretion
• Cephalic reflex – sensory input from nose, eyes,
thoughts, etc. increase gastric secretion rates
• Gastric – distension of stomach (stretch), partially
digested proteins, and increases in pH stimulate
secretion
– Feedback loop?
• Intestinal – when intestines receive process food
from the stomach (chyme), the pyloric sphincter
to close and decreases gastric secretion rates
Gastric Motility and Emptying
• Motility serves to mix food and create chyme
– Stomach will relax when anticipating food
– Peristalsis – stomach moves food inferiorly (from
gastroesophageal sphincter to the pyloric sphincter)
– Only small amounts of chyme are allowed into the
small intestine by the sphincter
• The rest of the chyme in the wave is sent back into the
stomach for further mixing
– Pacemaker cells set the rhythm of peristalsis in
combination with neural and hormonal factors
– More stretch (more food) = more mixing
Vomiting
• Caused by gastric irritants (bacteria, alcohol,
alkaloids, etc.) but can also be from
psychological stimuli
• Coordinated reflex action of muscles
(diaphragm and abdominal wall), soft palate
(closes off nasopharynx most of the time) and
gastroesophageal sphincter (opens to
esophagus)
Small Intestine (SI)
• Digestion is completed here after receiving the
chyme from the stomach
• Extends from the pyloric sphincter of the
stomach to the ileocecal valve, where the SI joins
the LI
• Roughly 20 ft long (dead) but only 7-13 feet long
in living people
– Why so long?
SI Divisions
• Duodenum
– Receives chyme from
stomach, bile from the liver
and pancreatic juice
– Bile and pancreatic juice are
controlled by the hepatopancreatic sphincter
• Jejunum – extends from the
duodenum to the ileum
• Ileum – extends from
Jejunum to the large
intestine
Microscopic Anatomy of the SI
• Circular folds – deep folds of the mucosa that
slow food down for efficient absorption of
nutrients
• Villi – Fingerlike projections of mucosa
– Further increase of surface area for absorption
– Contain microvilli for further absorption
• Also release enzymes for digestion (carbs and proteins
Villi
Thin tissue and large
surface area allows
nutrients to dissolve into
blood and lymph for
distribution
SI Histology
• Besides villi, SI wall includes
– Intestinal crypts – cells that secrete intestinal juice
which facilitates absorbing nutrients from chyme
– Enteroendocrine cells – release hormones for
communication with other organs
– Immune cells
• T cells – Occur in Peyer’s patches
• Paneth cells that secrete lysozyme and defensin (AMPS)
Liver
• Largest gland in the
body
• Housed under ribcage,
superior and anterior
to stomach
• Comprised of four
lobes, separated by
ligaments
Liver Functions
• Produces bile which is exported to the
duodenum to aid digestion
– Bile serves to emulsify fat into solution for easier
digestion by enzymes (lipases)
• To filter and process blood
– Removal of dead cells, toxins, and metabolites
Blood Supply to Liver
• Hepatic Portal Artery -
• Hepatic Portal Vein -
Travel through
the lesser
omentum (specialized
peritoneum
attached to the
stomach)
Bile
• Very alkaline emulsifier produced by the liver and
exported via the common hepatic duct
• The common hepatic duct fuses with the cystic
duct (from the gall bladder) to form the bile duct
• Bile is then transported to the duodenum
• Bile salts are recycled in the gut, and reused in
digestion
Liver Structure
• The liver is made of hexagonal-shaped units
called hepatic (liver) lobules made of hepatocytes
• This tissue forms plates that radiate out from the
central vein
• In each lobule there are triads of tissue (portal
triad) consisting of
– Hepatic artery
– Bile duct
– Hepatic portal vein
Portal Triad
Liver Circulation
• Blood Enters liver
– 80% from digestive system (hepatic portal vein)
– 20% from heart (hepatic artery)
• Leaves these vessels and empties into liver
sinusoids
– Also have immune cells to filter out pathogens and
dead hemocytes
• Blood pools and is collected and returned to vena
cava via hepatic veins
Gall Bladder
• Located under the ventral surface of the liver
• Stores bile not immediately needed for digestion
– Also concentrates bile by removing water
• Gall Stones!
– Salts can precipitate out of the bile solution and form
“stones” that are very painful and may need to be
removed
Pancreas
• Lies deep to the greater curvature
of the stomach
• Produces pancreatic juice
– The main pancreatic duct delivers
pancreatic juice to the duodenum
by fusing with the bile duct
– High pH helps to neutralize
stomach acid
– Contains enzymes to break down
all organic macromolecules
(proteases, lipases, amylase)
Large Intestine
• Frames the small intestine on three sides
• Removes water from the food to reduce water
loss
• Stores feces prior to defecation
Large Intestine
Large Intestine Histology
• No folds, villi, etc. – Only water is absorbed,
which readily diffuses through mucosa
• Mucus produced by crypt cells ease the
passage of food as it is being dehydrated into
feces
• HUGE population of bacteria reside here
– Why?
Motility of the Large Intestine
• Powerful muscle contraction move the bulky,
relatively dry waste towards the rectum for
elimination
• Very slow, segmented movements – intiated by
stretch receptors
– As the LI fills, it contracts to move the waste forward
• Bulk help facilitate this process
– Eat your fiber!
Rectum
• Holds feces until defecation is convenient (or
unavoidable)
• Stretch receptors initiate muscle contraction
• Muscle in rectal walls flex, internal anal
sphincter is relaxed
• External (voluntary) anal sphincter is allowed
to open, allowing the excretion of feces
Rectum Anatomy
Physiology of Digestion
• Chemical Digestion
– Many organic macromolecules are polymers
• Proteins
• Carbohydrates
• Fats
• Accomplished via enzymatic action
Enzymes
• Reduce the energy needed for chemical
reactions to take place
– Both anabolism and catabolism
• In digestion, enzymes work to create monomers
from polymers, which are small enough to be
absorbed by cells
• Accomplished by hydrolysis - the addition of
water to break the chemical bonds in a molecule
Hydrolysis
Important Enzymes
• Carbohydrates – amylase, lactase, sucrase
• Proteins – pepsin, aminopeptidase
• Fats – lipases
• DNA/RNA – pancreatic nucleases,
nucleosidases
Absorption
• Depends on the molecule
– Polar – cannot dissolve directly into cell
membranes
– Non-polar – can dissolve directly into cell
membranes
• Monomers from digested molecules are
absorbed by cell in the alimentary canal and
diffuse into the blood
Absorption (pp. 898-901)
• Carbohydrate
– Transport into the cell facilitated by protein chaperones
and are pumped into capillaries via facilitated diffusion
• Protein
– Short peptides are absorbed by the epithelial cells in the
alimentary canal, further broken down by enzymes in the
cell and amino acids transported to capillaries
• Fats (Lipids)
– fat+bile salts form globules called micelles which are
absorbed by microvilli, they are then processed back into
fats again by the smooth endoplasmic reticulum (ER)
Table Assignments
• 1. Define the specific function of the organ as it relates
to digestion
• 2. How does your organ integrate with the organs that
are superior and inferior to it?
• 3. How are the specific actions of your organs
regulated?
• 4. Are there any pathologies that are specific to your
organ?
Liver