Digestive System Dr Anderson Rowan University Digestive System

Digestive System Dr. Anderson Rowan University

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 Pharynx Esophagus Stomach Small Intestine Large Intestine Rectum Anus Superior 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 – p. H 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

Splanchnic 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 • Lamina Muscularis mucosae – muscle layer that produces muscle contractions for peristalsis

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

Tongue Musculature Strongest muscle in the body - Right?

Salivary Glands Saliva – 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=umnn. A 50 IDIY

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 alimentary 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 p. H between 1. 53. 5 – Chief Cells • Produces pepsin in the acid of the stomach – Enteroendocrine cells • Hormones that regulate gut function – E. g. Gastrin

Stomach Epithelium

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 B 12 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 p. H 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 – https: //www. youtube. com/watch? v=o 18 Uyc. WRsa. A

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 p. H 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)