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