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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
• Mouth Superior• Pharynx• Esophagus• Stomach• Small Intestine• Large Intestine• Rectum• Anus Inferior
Accessory Digestive Organs
• 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
• 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
Histology of the Alimentary Canal
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 GlandsSaliva – produced by salivary glands
– 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 1.5-
3.5
– 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 hepato-pancreatic 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