Digestive System: Anatomy and Processes

• Gastroenterology is the study of the digestive tract

General Anatomy and Digestive Processes

• The digestive system processes food, extracts nutrients, and eliminates the indigestible portion
• This process happens in five steps: ingestion, digestion, absorption, compaction, and defecation

Digestion Steps

• Ingestion is the intake of food
• Digestion is the breakdown of food into a usable form via mechanical and chemical digestion
• Mechanical digestion is the physical breakdown of food into smaller particles, for example: cutting or grinding of food by the teeth, or churning contractions of the stomach and small intestine
• Mechanical digestion creates more surface area for digestive enzymes to act on
• Chemical digestion uses chemical reactions to break macromolecules down into their monomers
• Absorption is the uptake of nutrients into the epithelial cells of the digestive tract, then into blood or lymph
• Compaction is the absorption of H2O, and consolidation of leftover food residue into feces
• Defecation is the elimination of feces from the body

The Digestive Tract

• The digestive system is divided into two divisions, the digestive tract and the accessory organs
• The digestive tract is also known as the alimentary canal
• This is a long, muscular tube which is open to the environment at both ends (mouth and anus), and is about 30 ft (9 m) long in the average adult
• Material within the digestive tract is NOT considered to have entered the body until it is absorbed via digestive tract epithelial cells
• The wall of the digestive tract has four layers, mucosa, submucosa, muscularis externa, and serosa

Layers of the digestive tract

• Mucosa has three layers: epithelium, lamina propria, and muscularis mucosae
• The epithelium is simple columnar in most of the digestive tract for absorption and secretion
• The epithelium is stratified squamous from the oral cavity through the esophagus, and in the lower anal canal and anus for areas which endure greater abrasive forces
• The lamina propria is a thin layer of connective tissue
• The muscularis mucosae is a thin layer of smooth muscle that contracts to cause grooves and ridges, increasing surface area and nutrient absorption
• The submucosa is a thick layer of CT containing blood vessels, lymphatic vessels, nerves, and glands
• The muscularis externa contains two layers of smooth muscle whose fibers are perpendicular to each other
• The inner circular layer encircles the digestive tract, forms sphincters in certain locations, and thickened layers of circular muscle which can open and close to regulate the passage of materials through the digestive tract
• Outer longitudinal layer muscle fibers whose muscle fibers run down the length of the digestive tract
• Serosa is a thin layer of areolar tissue, covered by simple squamous mesothelium

Enteric Nervous System

• The enteric nervous system, or "brain in the gut", is NOT found in all portions of the digestive tract, but begins in lower esophagus and ends just before the rectum
• The esophagus, stomach, and intestines have their own nervous network which regulates movement, secretion, and blood flow in the digestive tract
• The enteric nervous system can operate independent of the Central Nervous System (CNS) and is considered to be part of the Autonomic Nervous System (ANS)
• The Vagus Nerve (cranial nerve X) innervates the ENS
• It contains two types of neurons, Meissner's (Submucosal) Plexus and Auerbach's (Myenteric) Plexus
• Meissner's Plexus is located in the submucosa and controls movement of the muscularis mucosa and glandular secretions
• Auerbach's Plexus controls peristalsis of the muscularis externa, moving food byproducts down the digestive tract

Accessory organs

• Accessory organs consist of the teeth, tongue, salivary glands, liver, gall bladder and pancreas

Peritoneum

• Peritoneum is a double layered, serous membrane which forms the lining of the abdominal cavity
• Parietal peritoneum lines the abdominal wall
• Visceral peritoneum covers suspended organs in the abdominal cavity
• mesenteries and omenta are folds in the peritoneum, which cover some organs of the GI tract and carry vessels and nerves to the organs
• Mesentery covers and suspends the small intestines and stomach
• Greater Omentum hangs down from greater curvature of stomach to loosely cover the small intestines
• Lesser Omentum extends from the Liver and gallbladder to the lesser curvature of the stomach

Relationship of organs to the peritoneum

• Intraperitoneal organs are any organ which is enclosed within the peritoneum
• Retroperitoneal organs are organs in the posterior abdomen, which are NOT enclosed by the peritoneum
• Retroperitoneal organs: Kidneys, duodenum, ascending colon, descending colon, transverse colon, and pancreas

Regulation of the Digestive Tract

• Motility and secretion of the digestive system are controlled via neural control and hormonal control
• The Neural Control is composed of the Myenteric (short) Reflexes and Vasovagal (long) Reflexes
• Stretching and chemical stimulation of digestive tract acts through the Auerbach's Plexus to produce contractions in nearby regions of the muscularis externae, peristalsis
• Act through ANS nerve fibers, Sensory fibers carry information about digestive tract to hypothalamus, which sends motor response back to digestive tract, Regulates digestive motility and secretion, Parasympathetic (PSNS) branches of the Vagus Nerve especially important in this
• Hormonal Control produces hormones such as gastrin and secretin which stimulate digestive functions
• Gastrin stimulates secretion of HCI from parietal cells
• Secretin reduces pH in the duodenum, by stimulating synthesis of bicarbonate and water from the pancreas, and drains into the duodenum

Mouth Through the Esophagus

• The mouth is also known as the oral cavity or buccal cavity
• Buccal relates to the mouth or the cheeks

Functions of the mouth

• Ingestion of food, taste and mastication (chewing) and chemical digestion
• Enzymes in saliva break down starch
• The mouth is enclosed by cheeks, lips, palates, and gum, the anterior opening of the mouth is the oral fissure
• The lining of the mouth cavity is an epithelium
• Keratinized, stratified squamous occur where greater abrasive forces occur which is located in the gums and hard palate
• Nonkeratinized, stratified squamous occurs everywhere else in mouth

Cheeks and more

• Cheeks, lips, and gums retain and position food within the mouth, assist in speech, allow sucking and blowing actions
• Median fold of tissue which anchors lips to gums
• Vestibule is the space between the teeth and the lips/cheeks, anterior to the teeth

Functions and features of the mouth

• The tongue is a large, muscular organ on the floor of the oral cavity
• Tongue assists in speech, and positions food in mouth during mastication
• It is composed of nonkeratinized stratified squamous epithelium
• the Surface of tongue is covered in raised bumps called papillae (there are several different types), taste buds are located in the papillae
• Body of the tongue is the anterior 2/3 of tongue and occupies the oral cavity Root of the tongue is the posterior 1/3 of tongue. Root of the tongue occupies the oropharynx, and contains the lingual tonsil The Lingual Frenulum is a medial fold which attaches tongue to floor of mouth, contains lingual glands which form a portion of saliva
• The palate separates the oral cavity from the nasal cavity to make it possible to breathe while chewing food, and is composed of two structures, the hard and soft palate

Palate features

• Hard Palate is made up of the maxillae and the palatine bones at the roof of the oral cavity
• Soft Palate is located posterior to the hard palate, and is made up of skeletal muscle and glandular tissue which contains the uvula
• The uvula is a cone shaped tissue which hangs medially from the soft palate, and prevents food from entering the nasal cavity on swallowing or digestive tract while chewing

The teeth

• The teeth are collectively called the dentition
• Teeth break food down into smaller portions so it makes food easier to swallow
• Teeth increase surface area for digestive enzymes to act on
• Adults normally have 32 teeth which is 16 in the maxilla (upper jaw) and 16 in the mandible (lower jaw)
• Dentition Formula indicates that from the midline outward to the rear of each jaw there are 2 incisors,1 canine, 2 premolars and 3 molars

Features and details of teeth

• 3rd molar is also known as wisdom teeth and may not be present in all people
• Each tooth is embedded in a socket called an alveolus
• The tooth is anchored to alveolus via periodontal ligaments which hold teeth firmly in place, but allow slight movement of teeth during chewing
• The gingiva (gum) is a soft tissue which covers alveolar bone
• The portion of tooth above the gingiva
• The space within the crown which is occupied by pulp, a mass of loose CT, vessels, and nerves
• The portion of tooth embedded in alveolar bone contains a narrow passage in the root filled with pulp
• Dentin is mostly composed of hard, yellowish tissue so in the crown, the dentin is covered with a hard, white, mineralized substance called enamel and in the root, the dentin is covered by a CT called cementum
• The apical foramen contains small passages in the mandible and maxilla which allow nerves and vessels to enter the teeth

Features of the teeth in development

• They are developed beneath the gingiva and erupt (emerge) at certain times
• Deciduous Teeth (Baby Teeth) have 20 deciduous teeth, which erupt between 6-30 months
• Permanent Teeth have 32 teeth which Erupt between 6-25 years of age

Saliva

• Aqueous fluid produced by salivary glands
• Saliva contains electrolytes, lysozymes, IgA, mucous, salivary amylase (breaks down starches) and lipases (break down fats), and its' functions include to:
• Moisten and clean mouth, inhibit bacterial growth, dissolves food particles to stimulate taste buds, to bind food particles into a soft mass (bolus) and begins digestion of starch and fats

Salivary glands

• Parotid Glands are located at the angle of the mandible, and empties into the mouth near 2nd premolar tooth
• Submandibular Glands are located along the body of the mandible, and empties under the tongue, lateral to the lingual frenulum
• Sublingual Glands are located in floor of mouth, and has multiple ducts which empty throughout floor

salivation stimulation

• Humans produce 1.0-1.5 L of saliva daily, and is stimulated in response to food in mouth (PSNS innervation) or stress (SNS innervation)
• Presence of food stimulates the salivatory nuclei of the brain stem.
• Signals glands via ANS branches of the Facial (CN VII) and Glossopharyngeal (CN IX) nerves
• Produces abundant, watery saliva: PSNS innervation
• Produces thicker saliva with more mucous: SNS innervation
• Conserves H2O, This is why mouth can feel sticky and dry during stressful situations

Pharynx

• Pharynx is the point where respiratory and digestive tracts intersect
• It contains circular muscle, which propels food downward on swallowing
• The inferior most muscle of the pharynx is known as the Upper Esophageal Sphincter (UES), and when NOT swallowing, UES remains constricted to keep air out of the esophagus
• The esophagus is a straight muscular tube about 25-30 cm (about 10 in) long located posterior to the trachea
• The esophagus passes through the diaphragm via an opening called the esophageal hiatus before ending at the entrance to the stomach
• The esophagus contains a circular muscle, which propels food downward on swallowing, and the inferior most muscle of the esophagus is known as the Lower Esophageal Sphincter (LES)
• This prevents gastric acid, in the stomach, from ascending into the esophagus because stomach acid in the esophagus is corrosive to the tissues and produces a burning sensation known as "heartburn"
• Anatomy: the mucosa contains nonkeratinized stratified squamous epithelium, the submucosa contains esophageal glands which secrete a lubricating mucous into the lumen of the digestive tract, and the muscularis externa has upper 1/3 - skeletal muscle, middle 1/3 - mix of smooth and skeletal muscle and lower 1/3 - smooth muscle
• Deglutition (aka Swallowing) is a complex mechanism involving more than 22 muscles

Swallowing

• This is coordinated by the swallowing center of the medulla oblongata, with the Trigeminal Nerve, Facial Nerve, Glossopharyngeal Nerve, and Hypoglossal Nerve (CN V, VII, IX, XII)
• Deglutition occurs in 3 phases: The Oral Phase, The Pharyngeal Phase and The Esophageal Phase
• The Oral Phase includes voluntary control, manipulation of chewed food into a bolus, and the bolus pushes epiglottis posteriorly and descends into laryngopharynx
• The Pharyngeal Phase is involuntary control in which the palate and tongue prevent food from entering nasal cavity, the epiglottis prevents food from entering respiratory tract and esophagus widens to receive food
• The Esophageal Phase occurs through the wave of involuntary contractions, known as peristalsis, which propels food down the digestive tract
• Circular muscle above bolus constricts and circular muscle below bolus relax
• The stomach is a large, muscular sac which mechanically & chemically breaks down food into a pasty mixture known as chyme

Stomach

• The gross Anatomy of the stomach includes four regions
• The regions are: cardia, fundus, corpus and pyloric region
• Cardia is the small region where esophagus meets the stomach
• Fundus is the portion superior to the esophagus
• Corpus (Body) is the largest region distal to cardia
• Pyloric region is the narrow point on inferior portion of stomach which is made up of three components: antrum (funnel-like opening), pyloric canal (narrow portion with the antrum), and pylorus, which is the narrow passage into the duodenum of the small intestine, and is surrounded by thick ring of smooth muscle called the pyloric sphincter to control passage of chyme into the duodenum The inferior/lateral border of the stomach
• The greater Curvature is found on the inferior border
• The upper Curvature is located on the superior/medial border of the stomach

Stomach nerves

• Stomach receives PSNS innervation from Vagus Nerve (CN X)
• Stomach receives SNS innervation from Celiac ganglia
• Oxygenated blood is delivered via the Celiac Trunk of the Abdominal aorta
• Deoxygenated blood is drained the Hepatic Portal Circulation
• The microscopic anatomy of the stomach reveals that the stomach wall is similar to rest of digestive system with some variations and the mucosa contains gastric pits lined with simple columnar epithelium and holds gastric glands

Gastric glands features

• Gastric glands produce 2-3 L of a fluid called gastric juice per day and contain mucous cells along with parietal cells and chief cells to make secretions
• Apical surface of cell secretes mucin which when combined with H₂O forms mucous through: Secrete Hydrochloric (HCI) acid Produces low pH within the stomach: Parietal cells Secretions help liquefy food to form chyme, and Converts Ferric Iron (Fe3+) to Ferric Iron (Fe2+) Intrinsic factor: Binds to Vitamin B12 and allows B12 to be absorbed by the small intestines which is required for Hb synthesis and Deficiency in intrinsic factor produces a disease called pernicious anemia and Typically, due to autoimmune destruction of parietal cells
• Ghrelin: Appetite regulating hormone
• Serete gastric lipase and an inactive Pepsinogen • Breaks down dietary fat in stomach through Cheif cells The chief cells release an inactivated enzyme, Pepsinogen.
• Also contain Enteroendocrine cells Secretes various hormones which regulate digestion
• when the stomach is full the submucosa is flat, when stomach is empty, the submucosa forms wrinkles called rugae
• Lamina Propria contains tubular glands

Muscularis Externae

• Muscularis Externae has 3 layers, outer longitudinal, middle circular and inner oblique
• It spirals around the stomach, and mixes chyme
• Gastric Motility:
• As you swallow, stretching of the pharynx is detected by mechanoreceptors, which relay the information to the medulla oblongata then the stomach via the Vagus Nerve (CN X)
• This initiates peristaltic contractions which mix chyme and force it towards the pyloric region
• The pylorus only allows very small molecules (< 7mm) and can only eject 3 mL of chyme into the duodenum at a time
• This is to allow the duodenum enough time to adequately neutralize the acidity of the chyme
• All of the rest of the chyme is forced back up into the body of the stomach, where it continues to undergo peristaltic contractions
• This repetitive downward propulsion breaks the food down into smaller and smaller particles
• If duodenum becomes overfilled (detected by stretching of duodenal wall), it inhibits gastric motility, recieving less chyme from stomach

typical meal emptying

• typical The typical meal is emptied from stomach in 4-6 hours
• The time varies based on foods being digested
• Vomiting is a forceful ejection of contents, from the stomach, out the mouth
• Produced by thoracic expansion and abdominal contractions
• Creates a pressure gradient which opens up the esophagus Vomiting occurs when rising thoracic pressure and abdominal contractions force the UES open, forcing stomach contents up the digestive tract and out the mouth and It is Controlled via the emetic center of the medulla oblongata

features that protect the stomach

• The stomach has protections to keep from gastric acid from damaging the tissue's
• Those consist of a Mucous Coat which is highly alkaline, resists action of stomach and coats the apical surface of epithelial cells and the formation of Tight Cell Junctions

Regulation of Gastric functions

• The nervous and endocrine system regulate gastric activity, which occurs in 3 overlapping phases; The Cephalic phase, The Gastric Phase and The Intestinal Phase
• Cephalic Phase is a phase is in which stomach responds to sight, smell, taste, or thought of food
• This information sent to the hypothalamus, then relayed to the enteric nervous system via the Vagus Nerve (CN X)
• Stimulates secretion of gastrin to increase HCI secretion by parietal cells
• The Gastric Phase is the stage where swallowed food activates gastric activity and stimulates secretion of : Gastrin and HCI from parietal cells Pepsinogen from chief cells
• Intestinal Phase: Stage where duodenum responds to influx of chyme
• Unlike other 2 phases which stimulate increased gastric activity, this stage moderates gastric activity
• Stretching of duodenal walls initiates the enterogastric reflex, and Duodenum sends inhibitory nerve signals to enteric nervous system which decreases gastric activity
• • sends the signal via the Vagal stimulation
• Duodenum produces and releases secretin which lowers pH of the duodenum
• The liver, gallbladder, and pancreas assist in the digestion process where in addition to chyme, the small intestines also receive digestive secretions from the liver and pancreas and the ducts from these organs enter the small intestines in the duodenum