The Digestive System

The Digestive System

• Acquires nutrients from the environment
• Consists of the digestive tract and accessory organs
• Primary functions include breaking down and absorbing nutrients, and eliminating waste

Anabolism and Catabolism

• Anabolism synthesizes essential compounds
• Catabolism breaks down compounds to provide cells with energy

Digestive Tract

• Also known as the gastrointestinal (GI) tract or alimentary canal
• Extends from the oral cavity to the anus, forming a muscular tube

Accessory Organs

• Include teeth, tongue, and various glandular organs

Integrated Processes

• Ingestion
• Mechanical digestion and propulsion
• Chemical digestion
• Secretion
• Absorption
• Defecation

Digestion Defined

• Ingestion occurs when food enters the oral cavity
• Mechanical digestion and propulsion involves crushing, shearing, and propelling food
• Chemical digestion breaks down food into small fragments via digestive epithelium
• Secretion releases water, acids, enzymes, buffers, and salts, using the epithelium of the digestive tract, glandular organs, and gallbladder.
• Absorption involves movement of organic molecules, electrolytes, vitamins, minerals, and water across digestive epithelium into interstitial fluid
• Defecation eliminates wastes from the body. Feces consist of compacted, dehydrated wastes.

Lining of the Digestive Tract

• Must be safeguarded against corrosive acids/enzymes, mechanical stress, and bacteria

Peritoneum

• A serous membrane lining the peritoneal cavity
• Superficial mesothelium covers a layer of areolar tissue
• Visceral peritoneum (serosa) covers organs within the peritoneal cavity
• Parietal peritoneum lines inner surfaces of the body wall
• Peritoneal fluid, produced by the serous membrane lining, allows frictionless sliding of surfaces

Peritoneal Fluid

• About 7 liters are produced and absorbed daily
• Very little peritoneal fluid is present in the peritoneal cavity at one time
• Ascites involves abdominal swelling from buildup of peritoneal fluid

Mesenteries

• Double sheets of peritoneal membrane
• Suspend portions of the digestive tract
• Connect parietal peritoneum with visceral peritoneum
• Route for blood vessels, nerves, and lymphatic vessels
• Stabilize organ positions and prevent entanglement

Embryonic Development

• The digestive tract and accessory organs are suspended in the peritoneal cavity by dorsal and ventral mesenteries
• Ventral mesentery usually disappears, but persists in adults as the lesser omentum and falciform ligament

Lesser Omentum and Falciform Ligament

• Lesser omentum stabilizes the stomach's position
• Provides an access route for vessels entering/leaving the liver
• Falciform ligament stabilizes the liver

Dorsal Mesentery

• Enlarges to form the immense greater omentum, which extends inferiorly between the body wall and the small intestine's anterior surface
• It hangs like an apron from the stomach

Greater Omentum

• Adipose tissue conforms to shapes, pads, and protects abdominal surfaces
• Insulates, stores lipid energy reserves, and contributes to a "beer belly"

Mesentery Proper

• Provides stability and permits independent movement
• Suspends all but the first 25 cm of the small intestine
• Fuses with the abdominal wall, locking organs in place when associated with the duodenum and pancreas
• Retroperitoneal organs are posterior to the peritoneal cavity

Mesocolon

• Associated with part of the large intestine
• During development, the mesocolon of the ascending colon, descending colon, and rectum fuse to the posterior body wall.

Histology of the Digestive Tract

• Four major layers include the mucosa, submucosa, muscular layer, and serosa

Mucosa

• Innermost lining of the digestive tract
• Consists of mucous membrane
• The lining varies depending on the region
• Longitudinal folds in the empty stomach
• Small intestine contains permanent transverse folds

Epithelium

• Moistened by glandular secretions
• Mucosal epithelium is either simple or stratified, depending on stresses
• Stratified squamous epithelium is located in the oral cavity, pharynx, esophagus, and anal canal
• Simple columnar epithelium is located in the stomach, small intestine, and most of the large intestine

Enteroendocrine Cells

• Secrete hormones coordinating digestive tract and accessory gland activities

Lamina Propria

• Areolar tissue containing blood vessels, sensory nerve endings, lymphatic vessels, smooth muscle cells, and scattered lymphatic tissue

Muscularis Mucosae

• Deep to the lamina propria in most digestive tract areas
• Is made of smooth muscle and elastic fibers
• Has smooth muscle cells arranged in two concentric layers, inner being circular and outer being longitudinal

Submucosa

• Dense, irregular connective tissue that binds mucosa to muscular layer
• Contains numerous blood vessels and lymphatic vessels
• May contain exocrine glands that secrete buffers and enzymes

Submucosal Neural Plexus

• Innervates the mucosa and submucosa
• Consists of sensory neurons, parasympathetic ganglionic neurons, and sympathetic postganglionic fibers

Muscular Layer

• Dominated by smooth muscle cells in inner circular and outer longitudinal layers
• Involved in mechanical digestion and moving materials
• Movements coordinated by the enteric nervous system (ENS)

Myenteric Plexus

• A network of parasympathetic ganglia, sensory neurons, interneurons, and sympathetic postganglionic fibers
• Located between the circular and longitudinal muscle layers

Serosa

• Serous membrane covering the muscular layer
• Encloses most of the digestive tract within the peritoneal cavity
• Adventitia, a dense network of collagen fibers, attaches the digestive tract to other structures in areas where serosa is absent

Motility

• Visceral smooth muscle tissue has rhythmic cycles that are controlled by pacesetter cells with spontaneous depolarization
• Waves of contractions spread throughout entire muscular sheet

Peristalsis

• Muscular contractions that move a bolus along the digestive tract
• Circular muscles contract behind the bolus while those ahead relax
• Longitudinal muscles contract, shortening adjacent segments
• Contractions force the bolus forward

Segmentation

• Cycles of contraction that churn and fragment the bolus, by mixing intestinal secretions
• Does not follow a set pattern or push materials in any one direction

Regulation of Digestive Functions

• Local, neural, and hormonal mechanisms contribute

Local Factors

• pH, volume, or chemical composition can have direct effects
• Stretching the intestinal wall stimulates contractions
• Local factors may stimulate chemical release like prostaglandins or histamine

Neural Mechanisms

• Visceral motor neurons control smooth muscle contraction and glandular secretion from the myenteric plexus
• Short reflexes control small segments and operate outside of CNS control
• Long reflexes involve interneurons and motor neurons in the CNS to control large-scale peristalsis

Hormonal Mechanisms

• Enteroendocrine cells produce peptide hormones, affecting almost every aspect of digestion
• These hormones affect separate systems via blood

Oral Cavity

• Contains the tongue, teeth, and salivary glands
• Is used for ingestion and mechanical digestion

Oral Cavity Functions

• Sensory analysis of food before swallowing
• Mechanical digestion via teeth, tongue, and palatal surfaces
• Lubrication with mucus and saliva
• Limited chemical digestion of carbohydrates and lipids

Oral Mucosa

• The oral cavity's lining consists of stratified squamous epithelium
• Cheeks, lips, and the tongue's inferior surface are thin and nonkeratinized
• Thin, vascular mucosa under the tongue allows rapid absorption of lipid-soluble drugs
• Cheeks' mucosae are supported by fat pads and buccinator muscles

Oral Vestibule

• The space between the cheeks (or lips) and teeth

Gingivae (Gums)

• Ridges of oral mucosa that surround the base of each tooth on alveolar processes

Uvula

• Dangling process at the soft palate's posterior margin, preventing premature pharynx entry

Palatoglossal and Palatopharyngeal Arches

• Palatoglossal arch: between soft palate and tongue base
• Fauces: space between oral cavity and pharynx, bounded by the soft palate and tongue base
• Palatopharyngeal Arch: soft palate to pharyngeal wall

Tongue

• Compression, abrasion, and distortion while digesting mechanically
• Chewing assistance and food preparation for swallowing
• Sensory analysis by touch, temperature, and taste receptors
• Mucin and lingual lipase secretion

Gross Anatomy of the Tongue

• Tongue's anterior body and posterior root
• Inferior midline frenulum
• Extrinsic muscles perform all gross movements
• Intrinsic muscles are smaller and perform precise movements

Teeth

• Assisted by the tongue during chewing

Dentin

• A mineralized matrix in teeth, and does not contain cells

Pulp Cavity

• Interior chamber of tooth
• Receives blood vessels and nerves through the root canal via the apical foramen

Root

• Sits in a tooth alveolus

Cement

• Covers and protects the dentin of the root
• Anchors periodontal ligament, which extends from root dentin to alveolar bone, creating a gomphosis articulation

Crown

• Projects beyond gingiva and is separated from the root by the neck; gingival sulcus surrounds neck

Enamel

• Covers dentin
• Forms the occlusal (biting) surface

Cusps

• Elevations/projections of occlusal surface
• Maxillae and mandible alveolar processes form dental arcades

Types of Teeth

• Incisor
• Canine
• Premolar
• Molar

Incisor Teeth

• Blade shaped, located at the front of the mouth
• Used for clipping/cutting
• Have a single root

Canine Teeth

• Conical with a single, pointed cusp (cuspids)
• Used for tearing/slashing
• Have a single root

Premolar Teeth

• Flattened crowns, bicuspids with two prominent, rounded cusps
• Used to crush, mash, and grind
• Have one or two roots

Molar Teeth

• Very large, flattened crowns with four to five prominent, rounded cusps
• Used for crushing and grinding
• Have two to three roots

Sets of Teeth

• Deciduous (primary, milk, baby) and permanent (adult) teeth

Deciduous Teeth

• 20 temporary teeth, with five on each side of the upper and lower jaws
• Includes 2 incisors, 1 canine, and 2 deciduous molars

Permanent Teeth

• Replace deciduous teeth upon eruption
• 32 permanent teeth with 8 on each side of the upper and lower jaws
• Includes 2 incisors, 1 canine, 2 premolars, and 3 molars

Salivary Glands

• Three major pairs secrete into the oral cavity: parotid, sublingual, and submandibular
• Each pair has a unique cellular arrangement, producing distinctive saliva

Parotid Glands

• Inferior to zygomatic arch
• Produce serous secretion, containing salivary amylase to break down starches
• Drained by the parotid duct

Sublingual Glands

• Covered by the mucous membrane
• Numerous sublingual ducts open along both sides
• Produce mucus, which function as a buffer

Submandibular Glands

• Located in the mandibular groove
• Secrete buffers, glycoproteins (mucins), and salivary amylase
• The submandibular ducts open on each side posterior to teeth

Saliva

• 1.0–1.5 liters produced daily from submandibular (70%), parotid (25%), and sublingual (5%) glands
• 99.4% water, the remaining 0.6% is electrolytes, buffers, glycoproteins, antibodies, enzymes, and wastes

Functions of Saliva

• Cleaning oral surfaces
• Moistening/lubricating food
• Maintaining a pH of about 7.0
• Controlling bacterial populations and limiting acid production
• Dissolving taste-bud chemicals
• Initiating carbohydrate digestion with amylase

Regulation of Salivary Secretions

• Salivary glands have parasympathetic and sympathetic innervation
• Parasympathetic efferents originate in the medulla oblongata

Saliva Secretion

• Stimulated by any object in mouth, brainstem nuclei, and higher centers
• Parasympathetic stimulation accelerates secretion

Mastication

• Chewing
• Forced from oral cavity to vestibule, back across teeth

Muscles of Mastication

• Closes jaws and slides the lower jaw side to side
• Tongue compacts chewed food into a moist, rounded bolus that is fairly easy to swallow

Pharynx and Esophagus

• Transports the food bolus from the oral cavity to the stomach

Pharynx (Throat)

• Common passageway for food, liquid, and air

Regions of the Pharynx

• Nasopharynx
• Oropharynx
• Laryngopharynx
• Food passes through on its way to the esophagus

Esophagus

• Hollow muscular tube that conveys food and liquids to the stomach
• Approximately 25 cm long and 2 cm wide
• Begins posterior to the cricoid cartilage
• Enters abdominopelvic cavity through the esophageal hiatus
• Innervated by sympathetic/parasympathetic fibers
• Prevents air entry/backflow

Histology of the Esophagus

• Three layers: mucosa, submucosa, and muscular layer

Esophagus Mucosa

• Contains nonkeratinized stratified squamous epithelium
• Forms large folds with the submucosa
• Consists of smooth muscle

Esophagus Submucosa

• Contains esophageal glands that produce mucus

Esophagus Muscular Layer

• Contains inner circular and outer longitudinal layers
• Adventitia anchors esophagus

Deglutition

• Swallowing is initiated voluntarily, but proceeds automatically after the swallowing reflex begins

The Swallowing Reflex

• Tactile receptors stimulate with bolus
• Information is relayed to the swallowing center of medulla oblongata
• Consists of buccal, pharyngeal, and esophageal phases

Stomach

• J-shaped organ that receives food bolus for chemical and mechanical digestion

Major Functions of Stomach

• Temporary storage, mechanical digestion with contractions, and chemical digestion with enzymes

Chyme

• Partially digested food mixed with acidic secretions

Gross Anatomy of Stomach

• Shaped like a J, located between T7 and L3 vertebrae
• Short lesser curvature forms the medial surface
• Long greater curvature forms the lateral surface
• Anterior and posterior surfaces are rounded
• Shape and size vary

Regions of Stomach

• Cardia, fundus, body, and pyloric part

Cardia

• Superior, medial portion
• Abundant mucous glands

Fundus

• Superior to stomach-esophagus junction
• Contacts the diaphragm

Body

• Between the fundus and the curve of the J
• Largest region of the stomach
• A mixing tank for ingested food

Pyloric Part

• Between the body and duodenum
• Shape changes during digestion
• Pyloric antrum connects to the body
• Pyloric canal empties into the duodenum
• Pylorus is muscular tissue surrounding pyloric orifice

Pyloric Sphincter

• Thick circular muscle layer within the pylorus

Rugae

• Prominent folds in mucosa, allowing for stomach expansion

Muscularis Mucosae and Muscular Layer

• Contain extra smooth muscle layers
• Oblique layer in addition to circular and longitudinal layers

Histology of Stomach

• Simple columnar epithelium lines all portions, forming a protective mucus that covers the interior surface
• The surface consists of shallow depressions called gastric pits

Gastric Pits

• Shallow depressions on the gastric surface
• Contain mucous cells at the base (neck). These cell actively divide and replace superficial cells.

Gastric Glands

• Found in the fundus and body of the stomach
• These extend deep into the lamina propria
• Communicate with gastric pits and contain parietal and chief cells
• Secretes approximately 1500 mL of gastric juices daily

Parietal Cells

• Common along the proximal gastric glands portions
• Secrete intrinsic factor, a glycoprotein
• Helps absorb vitamin B12
• Indirectly secrete hydrochloric acid (HCl)

Chief Cells

• Most abundant near base of gastric glands
• Secrete pepsinogen, an inactive proenzyme converted to pepsin

Pepsin

• Active proteolytic enzyme converted from Pepsinogen with the help of HCl

Stomachs of Newborn Infants

• Makes enzymes important for digestion of milk
• Rennin (chymosin)
• Gastric lipase

Pyloric Glands

• Located in pyloric part of stomach
• Produce mucous secretions

Enteroendocrine Cells

• Produce at least severn different hormones

G-Cells

• Produce gastrin
• Stimulate secretion by parietal and chief cells
• Stimulate contractions of gastric wall

D-Cells

• Produce somatostatin
• Inhibit release of gastrin

Chemical Digestion

• Some carbohydrates are digested by salivary amylase, and lipids by lingual lipase
• As stomach becomes more fluid, the pH approaches 2.0
• Digestion occurs as preliminary protein
• Note, no nutrients are NOT absorbed in the stomach

Regulation of Gastric Activity

• Acid and enzyme production by the gastric mucosa are controlled by the CNS, reflexes of ENS, and hormones

Three Overlapping Phases of Gastric Control

• Cephalic phase
• Gastric phase
• Intestinal phase

Pancreas

• Accessory digestive organ, similar to the pancreas and liver
• Produces secretions that aid in chemical digestion
• Lies posterior to stomach, in the retroperitoneal position
• Wrapped in connective tissue capsule

Gross Anatomy of Pancreas

• Head, body and tail

The Head

• Bread shaped, in the loop formed during duodenums

Body

• Slender, extends towards the spleen

Tail

• Short and rounded

Pancreatic Duct

• Delivers pancreas secretions to the duodenum

Histology

• Lobules are separated by connective tissue partitions
• Ducts in lobules branch repeatedly and end in pancreatic acini
• Contain simple cuboidal epithelium

Pancreatic Islets

• Endocrine tissues of pancreas
• Scattered among pancreatic acini
• Account for about 1% of pancreatic cells
• Make insulin and glucagon

Exocrine Cells

• Acinar cells and epithelial cells of duct system
• Secrete alakaline pancreatic juice into the small intestine
• About 1000 mL per day
• Contains the digestive enzymes, water, and ions
• Is controlled by hormones made in in the duodenum

Pancreatic Enzymes

• Pancreatic alpha-amylase
• Pancreatic lipase
• Nucleases
• Proteolytic enzymes Pancreatic Alpha Amylase a carbohydrase that breaks down certain starches
• Almost identical to salivary amylase

Pancreatic Lipase

• Breaks down certain complex lipids
• Releases products that are easily absorbed, like fatty acids

Nucleases

• Breaks down RNA or DNA

Proteolytic Enzymes

• Break apart proteins
• Proteases, break complex structures
• Peptidases small pep tide chains into smaller chains

Proenzymes

• Make up 70%
• Secreted as inacitve, and activated only after small intestine

Pancreatic Proenzymes

• Trypsinogen, is converted with trypsin
• Chymotrpsinogen- converted to active with trypsin
• Procarboxypeptidase, converted to active trypsin
• Proelastase, converted to elastase

Liver

• Largest visceral organ
• Essential metabolic and synthetic functions
• Lies in right hypochondriac and epigastric regions
• Wrapped with a tough, fibrous capsule
• Liver is covered with a visceral peritonium
• Faliciform, marks division of tissue

4 Liver Lobes

• Left and right lobes
• Caudate lobe
• Quadrate lobe

Liver Blood Vessels

• Converge into the hepatic portal
• One third of blood is arterial
• Two thirds is venous

Heptatocytes

• (liver cells) adjust levels of ciruclating nutrients through absorbtiona nd secretion

Histology

Each lobe of the liver is divided by tissue, into aprxxiametely 100,000 lobules

Lobules

-The liver's basic functinoal unit

• Rougly 1 mm idna, form with irregular plates, contains sinusoids Many Kupper cells

Hepatic System

Liver lobulesare hexagonal

• 6 corners, containg a portal vein
  • contain interlobular -interlobulara artery, interlobular bile duct

Bila Duct

• Liver secrets it, in networks via canilculi
• Right and left ducts are formed

Bile Ducts

• Collecting form all, forming a duct
• Bile duct: empties into duodenal ampulla

Bile Duct

• Union Formed by the cystic duct, passing towards stomach
• penetrates the auadum

Blood Levels of Liver

• Liver extracvuts

Physiology

• Liver has over 200 functinos

Regulatory functions

• Affect: Carbohydrate mebstosim,- Lpid and Amino acid m, -Waste removal, Mineral storage,-d=drug inaxtication 25% Digestive System Lecture Note
• Largest store is fluid in body Hemagtolucal Regulatuiion by liver
• Antogen presentation
• Synthesis of plasma -Removal of circulating hormones
• Storage of toxisn

Productions

Bile -Emulicficatuiion: Bile breaks down

Gall bladder

Hollow

CCK

• Hepatoprancatic Relaxes, gal ladder contacts

CCK

Physiology

• Full gal bladder, 40=-70
• water and is absorved, become concentrated

Small Intestines

3 Segments

Dueudnrum Jephum Ileusm

Small Intestine

Lactial vessel in all walls

Villi

finger like projections

intestinal Glands

Contain endodroneg

Intestine Functions

Gland, juic , reflex

Reflexes

Gasteronetic

Nuerial

intestes

G cells

Hormones

Secreting

Liver

• Inhibits acid Duadumen has 3 parts Colystolic

Large intestine

divided in water Feces

Large intestine Frames, ends, liver

Colon

• Tenia coli Appenducides colon Transverse Lacks vilil Mucus lubricate Lacks Flatus Gastron Positive mass movenments Refelxion 2-13 Brush border Absorption Absorption Liplds, complex proteins Not by action transport Increased absorbtionn Increases B Vitamic Digestive Water Liver cancer Devisi0n rates
• Dehydrations Nerversous ,,, endoccrinet