AHE II 3.2 - DIGESTIVE PART 1

• The course objective is to explore how the body obtains nutrients and eliminates waste.
• It involves tracing anatomical structures of the digestive system and recalling their roles.

Lecture Learning Objectives

• Describe the basic organization and functions of the digestive system.
• Define terminology related to digestion.
• Recall the gross anatomy (including epithelia) and digestive functions of the oral cavity and pharynx; the esophagus; the stomach; the small intestine; the large intestine; and the rectum, anal canal, and anal sphincter.
• Describe the process of swallowing through its three phases, linking each phase to its innervation.
• Compare and contrast the four tunics of the GI tract wall.
• Identify and describe the peritoneum, recognizing its anatomical terminology, and recalling intraperitoneal and retroperitoneal organs.
• Describe the greater and lesser omenta, specifically its anatomy and attachments.
• Describe the three abdominal mesenteries, specifically its function and attachments.
• Recall the roll and structures of the pelvic diaphragm in the GI tract.

Basic Digestive Functions

• Ingestion: Introduction of materials into the oral cavity.
  • A bolus is a mixture of chewed food and saliva.
• Digestion: Breakdown of large food items into smaller structures and molecules.
  • Chyme is a paste-like processed bolus found in the stomach.
  • Mechanical digestion is physical and includes mastication.
  • Chemical digestion involves enzymes.
• Propulsion: Movement of materials through the GI tract.
  • Peristalsis involves ripples of muscular contraction, moving materials through the GI tract.
  • Segmentation refers to churning and mixing movements in the small intestine.
• Secretion: Production and release of fluids like acid, bile, and digestive enzymes.
  • The function is to digest and protect/lubricate.
• Absorption: Passive or active transport of electrolytes, digestion products, vitamins, and water across the GI tract epithelium into blood or lymph vessels.
• Elimination: Removal of undigestible materials and waste products secreted by accessory organs, compacted into fecal material, and eliminated via defecation.

GI Segments

• Oral cavity and pharynx
• Esophagus
• Stomach
• Small intestines (duodenum, jejunum, and ileum)
• Large intestines (cecum, ascending colon, transverse colon, descending colon, sigmoid colon, and rectum)

Oral Cavity

• The oral cavity functions as the entrance to the GI tract and opens posteriorly into the pharynx.
• Mechanical digestion (mastication) occurs in the oral cavity with teeth, tongue, cheeks, lips, and palate.
• Chemical digestion, involving enzymes in saliva produced by salivary glands, begins in the oral cavity.
• The mouth is lined with nonkeratinized stratified squamous epithelium, which protects against abrasive activities associated with digestion.
• The oral cavity's anterior boundary includes the teeth, lips, and oral fissure.
  • The oral vestibule is the space between the cheeks/lips and gums.
• The posterior boundary is the oropharynx isthmus.
• The superior boundary includes the hard and soft palates.
• The inferior boundary includes the mylohyoid muscle, mucous membrane, and tongue.
• The oral cavity proper is enclosed by dental arches.
• Gingivae (gums) cover the alveolar processes and neck of teeth.
• The palate forms the roof of the oral cavity.
  • The hard palate makes up the anterior 2/3 and includes the palatine process of the maxilla and palatine bone.
  • The soft palate makes up the posterior 1/3 and consists of skeletal muscle.
  • During swallowing, the uvula and soft palate elevate to close off the nasopharynx and prevent food from entering the nasal cavity; this process is innervated by cranial nerve X (CN X).
• Palatine and lingual tonsils are located at the oropharynx isthmus.
  • It monitors ingested food and drinks for antigens and initiate immune responses.

Muscles of Mastication

• Masseter: Elevates the mandible
• Buccinator: Forms the cheeks and compresses cheeks to hold solid foods in place against teeth while chewing
• Temporalis: Elevates the mandible
• Orbicularis oris: Forms the lips, creating the oral fissure
• Medial pterygoid: Elevates the mandible
• Lateral pterygoid: Protracts the mandible

Tongue

• Mechanical digestion mixes and manipulates ingested materials with the tongue, which helps compress materials against the palate to form a bolus.
• The tongue assists in swallowing.
• The root of the tongue is attached to the mandible and hyoid bone.
• The oral part of the tongue makes up the anterior 2/3.
  • Includes Somatic sensory innervation and motor innervation.
• The pharyngeal part of the tongue makes up the posterior ½.
  • Includes Small nodules of lymphatic tissue- lingual tonsils, somatic sensory innervation, and motor innervation.
• All Intrinsic tongue muscles have cranial nerve XII (CN XII) innervation

Extrinsic Tongue Muscles

• Genioglossus- CN XII
• Hyoglossus- CN XII
• Styloglossus- CN XII
• Palatoglossus- CN X

Pharynx

• The pharynx serves as a common space for respiration and digestion.
• The pharynx is divided into three regions: the nasopharynx, oropharynx, and laryngopharynx.
• The oro- and laryngopharynx are lined with nonkeratinized stratified epithelium, which protects against the abrasiveness of ingested materials.
• During swallowing, the larynx elevates to meet the epiglottis and prevents food from entering the trachea.

Pharyngeal Muscles

• Tensor veli palatini: Innervated by CN V and tenses the soft palate and opens the pharyngotympanic tube.
• Levator veli palatini: Innervated by CN X and elevates the soft palate.
• Walls are formed by superior (SC), middle (MC), and inferior (IC) constrictor muscles, all innervated by CN X.
• Stylopharyngeus- CN XII.
• Styloglossus - CN XII.
• Hyoglossus – CN XII.

Swallowing Process (Deglutition)

• The swallowing process moves ingested materials from the oral cavity to the stomach.
• It consists of three phases: voluntary, pharyngeal, and esophageal.
• Voluntary phase: In the oral cavity, chewing forms a bolus, which is directed towards the oropharynx by the tongue and hard palate.
• Pharyngeal phase: Begins when the bolus enters the oropharynx; tactile receptors trigger the swallowing reflex controlled by the medulla oblongata. During this phase, the soft palate and uvula elevate to block the passageway into the nasal cavity. The larynx and laryngeal opening elevate towards the epiglottis, which covers and seals the glottis and prevents food from entering the trachea.
• Esophageal phase: A fully involuntary phase where food passes through the esophagus into the stomach.
• Voluntary phase: Tongue- CN X (palatoglossus) and XII (all others), Mastication- CN V, Saliva secretion- CN VII and IX
• Pharyngeal phase: Soft palate- CN X (levator veli palatini), Pharynx- CN X, Larynx- CN X
• Esophageal phase: CN X

Peritoneum and Associated Structures

• The abdominal cavity is lined with a moist serous membrane called the peritoneum.
  • Parietal peritoneum: Lines the surface of the body wall.
  • Visceral peritoneum: Lines the surface of internal organs; the mesentery links the visceral and parietal peritoneum.
  • Peritoneal cavity: A potential space created by the parietal and visceral peritoneum; serous fluid is secreted into this space to prevent friction, allowing organs to move freely.
• Intraperitoneal organs: Organs surrounded by the visceral peritoneum, including the stomach, most of the duodenum, jejunum, ileum, cecum, appendix, transverse colon, and sigmoid colon.
• Retroperitoneal organs: Lie against the posterior abdominal wall, with only the anterolateral surfaces covered by the peritoneum; these include the ascending colon, descending colon, and rectum.
• Greater sac: Larger space beginning at the diaphragm and continues into the pelvic cavity.
• Omental bursa: Space posterior to stomach and liver and continuous with greater sac through the omental foramen.

Clinical Connections: Peritoneum

• Its large surface area facilitate in rapid spread of disease through the peritoneal cavity.
• Allows for treatment and medical procedures.
• Ventriculoperitoneal shunt- Excessive cerebrospinal fluid in individuals with obstructive hydrocephalus can be drained via a catheter into the peritoneal cavity which is then absorbed.
• Peritoneal dialysis- The peritoneum can be used as a location for fluid and electrolyte exchange due to its large surface area.

Omenta

• Omenta - peritoneum that passes from the stomach and proximal duodenum to other viscera
• Greater omentum: A peritoneal fold attached to the inferior border of the stomach (greater curvature) and proximal duodenum that forms the inferior portion of the omental bursa; it folds posteriorly.
• Lesser omentum: Extends from the lesser curvature of the stomach (superior/medial portion) and proximal duodenum to the inferior surface of the liver and contains right and left gastric vessels.
  • It divides into the hepatogastric and hepatoduodenal ligaments, the latter containing vessels entering the liver, including the hepatic artery proper, bile duct, and hepatic portal vein.

Mesenteries

• Mesenteries- Peritoneal that attach viscera to posterior abdominal wall.
• Allows some movement, location of vessels and nerves

1. Mesentery- Connects jejunum and ileum
2. Transverse mesocolon- Connects transverse colon
3. Sigmoid mesocolon- Connects sigmoid colon

• Peritoneal ligament- Attaches organs to one another or organ to lateral abdominal wall.

Layers of Abdominal GI Tract

• The tube from the esophagus through the large intestine is composed of four concentric layers or tunics.
  • Mucosa: Innermost layer that includes Inner epithelial lining – simple columnar (exception- esophagus with nonkeratinized stratified squamous), lamina propria- areolar CT, and Muscularis mucosae - thin layer of smooth muscle at the boundary of the mucosa and submucosa
  • Submucosa: It is Areolar or dense irregular CT and the location of lymphatic tissue, mucin secreting glands, vessels, and nerves
  • Muscularis: It is a two-layer of smooth muscle with exceptions- esophagus and stomach - Includes Inner circular layer and outer longitudinal layer and the Location of enteric nerve plexus ganglia
  • Adventitia or Serosa: It Is the Outer most layer comprised of areolar CT and Includes visceral peritoneum

Esophagus

• The esophagus is a tubular passageway for swallowed materials from the pharynx to the stomach.
• When not passing a bolus of food, the esophagus is completely flattened.
• The esophagus is lined with nonkeratinized stratified squamous epithelium.
• The thoracic esophagus passes the diaphragm via the esophageal hiatus.
• Abdominal esophagus is near the abdomen.

Stomach

• The stomach is a muscular J-shaped organ located in the upper left quadrant of the abdomen.
• It processes food bolus into chyme through Contractions of thick muscularis layer and Gastric secretions of acid and enzymes
• It has the following four regions: Cardia,Fundus,Body, and Pyloris.
• It may have pyloric antrum and Pyloric canal
• It has Greater and Lesser curvature
• Gastric folds (rugae) can be found in the Stomach.

Small Intestine

• It finishes chemical digestion process in ~12 hours
• Absorbs up to 90% of nutrients and water
• ~ 6 meters (20 ft) in length uncontracted
• Extends from pyloris of stomach to cecum of large intestines
• It is comprised of three segments which include the Duodenum, Jejunum, and Ileum
• It has Circular folds- comprised of mucosal and submucosal layers for Slow down movement of chyme; microvilli further increase surface area for nutrient absorption

Duodenum

• The duodenum is a short C-shaped segment beginning at pyloric sphincter that wraps around the head of pancreas
• connects to jejunum at duodenojejunal flexure
• It is Mostly retroperitoneal
• The major duodenal papilla serves as a valve opening for bile and pancreatic juices to enter in the small intestine.
• Circular folds are within the Duodenum

Jejunum and Ileum

• The jejunum is a middle region of small intestines, this where occurs the Primary location for chemicaldigestion and nutrient absorption; Intraperitoneal can be found and it is suspended by a mesentery proper.
• The Ileum - which the last portion of small intestines, Also, intraperitoneal as well a suspended by mesentery proper- can be found. Terminates at the ileocecal valve which is the valve-sphincter that controls the entry of materials into the large intestines.

Large intestine

• It extends ~1.5 m from distal ileum to anus
• Absorbs fluids and salts, forms fecal material
• It has the following five components - Cecum, Vermiform appendix,Colon, Rectum, Anal canal

Cecum & Appendix

• The cecum is Sac-like structure that extends inferiorly from level of ileocecal valve and Located in the right groin and sits within the right iliac fossa.
• Open to the ileum via the ileocecal valve
• TheVermiform appendix is Thin, hallow, fingerlike sac lined by lymphocyte-filled lymphatic nodules, Most commonly attached to posteromedial wall of cecum, and Partially suspended from ileum by mesoappendix

Clinical Connections: Appendicitis

• Abdominal emergency
• Occurs when the appendix is obstructed by fecalith or enlargement of the lymphoid nodules and it will result in.
• The bacteria proliferate and invade surrounding tissue, which becomes damaged due to pressure necrosis; perforation may ensue
• Pain initially beings in central/periumbilical region; progresses to lower right groin
• Visceral sensory enters spinal cord at the level of T10, causing diffuse pain related to peristaltic movement and Later, appendix contracts and irritates the parietal peritoneum in the right iliac fossa causing focal pain

Colon

• Ascending Colon originates at level ileocecal valve and Bends 90° left forming right colic flexure (hepatic flexure) inferior to liver
• The transverse colon Forms left colic flexure just inferior to spleen and Connected to posterior abdominal wall by transverse mesocolon (splenic flexure)
• The descending origin Begins as left colic flexure and Travels inferiorly through left flank into left groin
• The sigmoid colon Terminates at rectum
• Features:
  • Omental appendices- peritoneal-covered accumulations of fat
  • Taenia coli- narrow bands of longitudinal muscle in the colon walls
  • Haustra- sacculations of the colon
  • Paracolic gutters- depressions on lateral margins of colon where materials can pass through one regions of peritoneal cavity to another

Rectum and Anal Canal

• Rectum- the Expanded to store accumulated fecal material prior to defecation Contains transverse folds, rectal valves, to maintain retention of fecal material
• Anal canal which is the Last portion of large intestines and Passes in a posterior direction through opening in levator ani muscles. It contains the anal sinuses and valves which secrete mucin to aid in defecation and it is Surrounded entire anal canal and Relax during defecation,

Pelvic Diaphragm

• It is the muscular part of pelvic floor and Includes muscles that contribute to Gl function -Levator ani is a Support pelvic viscera while Maintaining closure of rectum and vagina. Maintain an angle between rectum and anal canal and Reinforce external anal sphincter and is vaginal sphincter in women.
• Coccygeus Supports pelvic viscera and Pulls coccyx forward after defecation