Understanding the Digestive System

Digestive System Overview

• System responsible for mechanical and chemical food breakdown
• Nutrients are absorbed as part of the digestive system
• Wastes are eliminated by the digestive system
• Digestion happens in several stages, not mutually exclusive

Stages of Digestion

• Ingestion of food is the first step
• Propulsion moves food along the GI tract
• Digestion can be mechanical or chemical
• Nutrients are absorbed
• Waste is eliminated through defecation

GI Tract Composition

• The GI tract is also known as the alimentary canal
• A convoluted cylindrical tube allows food passage and progressive digestion
• The tract's lumen is considered external to the body

Accessory Organs

• Accessory organs attach through ducts to the GI tract
• Substances are secreted into the lumen to assist with digestion

Clinical Conditions Examples

• Appendicitis, gallstones, and cystectomy are clinical conditions related to the digestive system

Embryological Development

• At week 4, the primordial gut is an undifferentiated tube with endoderm
• The primordial gut is anchored by the mesentery
• The gut tube divides into the foregut, midgut, and hindgut

Divisions of the Gut Tube

• The foregut develops into the esophagus, stomach, duodenum, liver, pancreas, and spleen
• The celiac trunk supplies many foregut portions
• The midgut develops into the jejunum, ileum, ascending, and transverse colon
• The superior mesenteric artery supplies the midgut
• The hindgut develops into the descending and sigmoid colon and rectum
• The inferior mesenteric artery supplies the hindgut

Liver Development

• The liver buds from the duodenum and projects ventrally in a double mesentery layer
• The falciform ligament connects it to the ventral abdominal cavity

Stomach Formation

• The stomach forms between the esophagus and duodenum within a double mesentery layer
• The lesser omentum is the peritoneum between the stomach and liver

Midgut Development

• The midgut projects from the posterior abdominal wall at approximately 6 weeks
• The connection to the posterior wall is through a double peritoneum layer
• The midgut is continuous with the yolk sac through the umbilicus

Midgut Elongation

• The midgut elongates and projects back, forming intestinal loops
• The yolk stalk degenerates, making the midgut unconnected to the abdominal wall

Length Increase in Hindgut

• The hindgut also grows longer, occupying the area between the foregut and midgut
• The foregut rotates 270° counterclockwise to achieve the adult abdominal organ arrangement

Colon Anchoring

• The transverse colon is anchored by the transverse mesocolon, a double mesentery layer

Omentum Development

• The double peritoneum layer between the stomach and transverse colon folds to form the greater omentum in a quadruple layer

Anchoring of Retroperitoneal Organs

• Retroperitoneal organs anchor to the abdominal wall, covered by parietal peritoneum

Secondary Retroperitoneal Organs

• Some peritoneal organs become secondarily retroperitoneal through absorption into the posterior abdominal wall

Peritoneum Defined

• The peritoneum is a serous membrane lining the abdominal cavity
• This is similar to the pleura in the pleural cavity

Parietal Peritoneum

• The parietal peritoneum lines the abdominopelvic wall's internal surface -It contacts the transversalis fascia and muscles of the inferior and posterior walls

Visceral Peritoneum

• The visceral peritoneum covers many abdominal organs in the cavity's front and middle sections

Classifying Organs

• Intraperitoneal organs are fully enclosed by visceral peritoneum
• Retroperitoneal organs lie against the posterior wall, lined only on their front by parietal peritoneum

Continuity of Peritoneum

• Parietal and visceral peritoneum is continuous
• Parietal peritoneum "jumps" off the posterior wall to reach organs, carrying arteries, veins, and nerves

Mesentery Function

• Mesentery is a double peritoneum layer extending from the posterior wall, mostly to the small intestine
• Envelopes the jejunum and ileum completely
• The superior mesenteric artery and vein and the vagus nerve run through it

Transverse Mesocolon

• Transverse mesocolon is a mesentery that projects to transverse
• Contains a neurovascular plane for the transverse colon

Coronary Ligament

• The coronary ligament is a peritoneal projection from the diaphragm to the liver's upper surface
• Peritoneum layers don't meet before extending from the liver, creating a bare area that directly contacts the diaphragmatic epimesium

Secondary Projections

• Visceral peritoneal double layers create secondary connections between organs

Lesser Omentum

• The lesser omentum connects the liver and GI tract
• Hepatogastric ligament connects liver to stomach
• Hepatoduodenal ligament connects liver to duodenum and contains the portal triad's vascular structures

Greater Omentum Connections

• The greater omentum connects stomach and transverse colon and folds into a quadruple layer
• The omentum can adhere to intestinal organs and help contain infection

Additional Ligaments

• The gastrosplenic ligament connects the stomach and spleen
• The splenorenal ligament connects the spleen and left kidney along the posterior abdominal wall

Potential Space

• The peritoneal cavity is a potential space between peritoneal layers
• It contains some fluid for organ movement during digestion and general movement
• Organs within are considered external to the peritoneal cavity

GI Tract Structure

• The GI tract is a convoluted tube allowing passage of ingested material
• It is divided into regions based on anatomical and histological differences

Histology of the GI Tract

• Most GI tract regions share a similar layering pattern
• Differences among regions reflect the modifications of the standard layers

Layers of the GI Tract

• The Mucosa is the innermost layer, surrounding the lumen, and has three further components
• The epithelium varies based on location and needs
• Below that, the lamina propria consists of areolar connective tissue
• The muscularis mucosae is a thin smooth muscle layer
• The submucosa consists of areolar and dense irregular connective tissue, with high vasculature, autonomic nerves (Meissner's plexus), and lymphatic tissue (MALT)
• The muscularis has thick smooth muscle layers
• This generates peristalsis
• It has inner circular and outer longitudinal layers, with the myenteric nerve plexus in between
• The adventitia is the outer areolar connective tissue layer, covered by visceral peritoneum (forming a serosa) in intraperitoneal organs

Esophagus details

• The esophagus is a 25 cm tube linking the oral cavity and stomach, the bottom 3 cm being abdominal
• It enters the abdomen via the diaphragm's esophageal hiatus with a sphincter at T10
• This constricted region prevents acid reflux/heartburn

Esophagus Tissue

• The esophagus is non-keratinized stratified squamous epithelium
• The muscle transitions from skeletal to smooth along its length
• The upper third is 100% skeletal for rapid contraction to quickly empty the pharynx to breathe
• The middle third is a skeletal/smooth blend
• the bottom third is 100% smooth (5-8 sec) for food to enter the stomach

Esophagus Movements

• Autonomic peristaltic contractions move food and fluids against gravity
• It terminates at the cardiac orifice, becoming the stomach, with an abrupt mucosal change at the Z-line

Swallowing Phases

• Peristaltic contractions project the bolus towards the stomach during swallowing
• When the bolus reaches the hiatus, the cardiac sphincter relaxes

Stomach Structure

• The stomach is J-shaped
• The superior concave surface is the lesser curvature (lesser omentum attachment)
• The inferior convex surface is the greater curvature ( greater omentum attachment)

Stomach Sections

• The stomach divides into the cardiac region continuous with the esophagus
• There is a cardiac notch on the superior surface indicating the start of the fundus region of stomach
• The fundus is the superior region that expands when full
• The body of the stomach is the largest portion, narrowing distally as the rugae increase
• The pylorus is the narrow terminal portion, sagging inferiorly as the pyloric antrum, ending in the pyloric canal and sphincter

Stomach Features

• The stomach bulges between the esophagus and duodenum, mixing food with enzymes
• Gastric folds (rugae) on the internal surface assist in mechanical digestion
• A mucosal layer containing columnar cells reduces contact with digestive juices
• Gastric pit cells secrete protective mucous, HCl, and pepsinogen
• The external stomach is surrounded by 3 smooth muscle layers
  • outer longitudinal layer
  • intermediate circular layer
  • innermost oblique layer

Duodenum Features.

• The duodenum is the first and shortest small intestine segment, receiving partially digested food
• Begins at the pylorus and ends at the duodenojejunal junction
• The majority is relatively immobile and lies against the posterior abdominal wall

Duodenum Sections

• The superior first segment has a mesentery branch, the only intraperitoneal part
• The attachment point for what is considered the hepatoduodenal ligament
• The descending region curves back to become retroperitoneal, receiving bile and pancreatic ducts
• The ducts merge to form the hepatopancreatic ampulla emptying bile/pancreatic juices into the duodenal papilla
• The horizontal region curves along the pancreas underside
• The ascending region is along the pancreas left side, becoming intraperitoneal at the jejunum
• The suspensory ligament (of Treitz) secures the ascending segment's terminal part

Jejunum and Ileum

• The jejunum starts at the duodenojejunal junction, located in the left upper infracolic cavity
• The ileum empties into the large intestine at the ileocecal valve in the right lower infracolic region
• Together, the jejunum and ileum Make up the Small Intestine are close to 6 m long
• The jejunal wall is thicker and more vascular than the ileal wall

Jejunum Details

• The jejunum contains large circular loops (plicae circulares) that decrease along the small intestine
• The mesentery is thinner, its blood supply forms large loops (arcades), projecting the vasa recta to the intestinal wall

Ileum Details

• The mesentery is thicker, with more fat; has smaller, branched arcades with shorter vasa recta

Large Intestine Structure

• Composed of a thin-walled column with a diameter larger than that of the small intestine
• Muscular layers are very thin relative to the diameter of the tube

Teniae Coli

• Three thin, longitudinal muscle bands throughout the colon's length
• These bands compress the large intestine into bulges (haustra) divided by partial invaginations

Appendices Projections

• Small fatty projections of the peritoneum primarily project from the teniae coli

Cecum

• The cecum is a pouch-like first region of the large intestine, receiving digested material from the ileum through the ileocecal valve

Appendix Description

• A worm-shaped vermiform appendix projects from the cecum's bottom
• The appendix is wrapped underneath the intestine but varies in position

Ascending Colon

• The ascending colon runs from the cecum to the right hepatic flexure
• Is retroperitoneal and reabsorbed during development

Transverse Colon

• The transverse colon runs from the hepatic flexure to the left splenic flexure
• Is intraperitoneal and surrounded by the transverse mesocolon

Descending Colon

• Runs inferiorly from the splenic flexure to the lower left quadrant
• In this region, the colon returns to being retroperitoneal.

Sigmoid Colon

• Named for its curved appearance, it curves medially and superiorly to form the rectum
• Occurs at the midline at level S3

Accessory Organ Outcroppings

• Outcroppings of the GI tract that secrete digestive substances into the GI tract to help assist digestion

Salivary Glands

• There are 3 sets of paired glands in the oral cavity
• Parotid glands are found superficially to the facial bones, anterior to the ear
• Submandibular glands are found inferior to the ramus of the mandible
• Sublingual glands are present in the anterior floor of the oral cavity

Saliva Content

• The saliva contains lysozyme to help with antimicrobial and antibacterial properties
• Salivary amylase helps initiates the breakdown of complex carbs

Liver Details

• Weighs 3-5 lbs and in the upper right quadrant of the abdomen, protected by ribs
• Divided by the falciform ligament, which anchors to the anterior wall
• Ends at the ligamentum teres, formed by the umbilical vein

Lobes of the Liver

• Functionally divided into right and left lobes
• The inferior of the left lobe contains the caudate and quadrate lobes

Hepatic Porta

• The Porta hepatis is within the hepatoduodenal ligament
• Hepatic arterioles supply oxygenated blood to the liver tissue
• Portal venuoles supply deoxygenated, nutrient rich blood
• Bile ductules send bile salts made by the liver to the duodenum

Liver Histology

• Cells arranged to form thousands of microscopic lobules
• Lobules arranged like a spoked wheel
• Numerous vascular bundles surround periphery of liver
• The central Single vein in central region of cells contains hepatocytes radiating from that region channels

Hepatocytes

• Hepatocytes arranged in cords, only cells thick that are surrounded by channel
• separated by channels to the central vein arranged like wheel spokes

Blood Drainage

• Each cord has a bile canaliculus for draining bile produced by hepatocytes

Liver Nutrient Processing

• The liver has "first pass" for nutrients from digestion, excluding fats through porta hepatis arrangement to allow the hepatic artery and portal vein to enter through the porta hepatis
• Branches travel through tissue along the periphery

Hepatic Vessels

• Hepatic artery supplies 25% of oxygenated blood
• Portal vein supplies 75% of nutrient rich blood to hepatocytes

Venous Action

• Both vessels drain in mixing of blood content in the sinusoids
• Central vein collecting all blood to hepatic veins to the inferior vena cava

Bile processing

• Bile ductules collect bile to flow towards hepatic duct in porta hepatis

Liver Functions

• The liver is considered to be the metabolic center
• Plasma glucose amounts are maintained by glucose uptake and release that releases or uptakes glucose by the liver

Insulin Action

• During digestion, the Pancreas releases Insulin
• As Passes the Liver, Insulin Signals the Cells to Uptake Glucose
• This uptake helps lower glucose coming by the Liver to Limits the Quantity returning by the Vena Cava

Glucagon Action

• During the fasted state, the Pancreas is triggered to Release the Glucagon
• This stored glucose releases back to help circulation and hypoglycemia
• The hepatic portal arrangement of this allows for nutrients absorbed by digestion

Bile production

• Bile continuously made and salts from absorbed fats, used in digestion, for emulsification
• Stored until the need for digestion

Gall Bladder Location

• Is connected through duct which produces common bile duct, and the gall bladder produces 2 way direction from gall bladder
• The cystic duct permits 2 way flow into/out of gall bladder

Gall Bladder Function

• Accumulates concetrated bile in order for absorbed state in sectretes

Gall Bladder details

• The contains, body neck terminates in the cystic cystic duct merges to liver in order to form bile from liver

Flow Regulation

• The sphincter regulates the overall flow out of digestive contents of the bladder and contraction of the bladder assists the contents for digestion

Pancreas Location

• Located posterior to the stomach to make up the posterior omental bursa

Pancreas Composition

• Has three: separte, middle, and tail regions
• The inferomedial head region curves under itself as the uncinate process
• the pancreas surrounds the centrally located pancreatic duct that fuses with the common bile duct at the hepatopancreatic ampulla to drain digestive substances to the duodenum

Functions of the Pancreas

• Histologically composed of a majority of acinar cells connected by connective tissue that have an exocrine function
• Some cell portions have ducts to drain to the duodenum
• There is accessory pancreatic duct that drains from the accessory pancreatic duct and exocrine and endocrine function

Endocrine Functions in Pancreas

• The endocrine regions which function in the hepatic system, involve numerous clusters of cell hormones that secrete insulin
  • Alpha cells - produce glucagon hormone
  • Beta cells – produce insulin hormone

Acinar Cells

• Most of the pancreas is composed by these cells
• These are to secrete enzymes as function

Other Cell Functions

• Produce hormones and enzyme production for carb metabolism for tissues and to the liver
  • Generates precursors for trypsin and chymotrypsin, digestive enzymes to break down protein

Functionality by location

• Insulin stimulates take of glucose in tissue, and stimulates the hepatic cells to release sugars for circulation
  • stimulates hepatic tissue to break down glycogen and release glucose into circulation

Size and Position of the Spleen

• It is lymphatic large organ near the splenic region
• It is flexible in abdomen towards digestion process

Anchoring

• Anchored to the abdominal wall posteriorly by the splenicorenal ligament, to the stomach anteriorly by the gastrosplenic ligament

Function

• Involved highly to the liver function, as it filters out material like RBC cells and heme groups in the immune system/RBC's