Digestion Lecture 1 Flashcards
(97 cards)
1
Q
Digestive System Consists of (2)
A
- Gastrointestinal (GI) Tract
- Accessory organs
2
Q
Gastrointestinal (GI) Tract
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- muscular tube also called digestive tract or
alimentary canal - Food passes along length from mouth to
anus
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Q
Accessory organs
Produce or store
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- produce or store secretions that flow into GI tract through ducts
4
Q
Functions of the Digestive Tract
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- Ingestion
- Mechanical digestion and propulsion
- Chemical digestion
- Secretion
- Absorption
- Defecation
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Q
Ingestion
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- Occurs when solid food and liquid enter the oral cavity
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Q
Absorption
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- Movement of nutrients across the digestive epithelium and into the bloodstream
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Q
Secretion
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- The release of water, acids, enzymes, buffers, and salts by the digestive tract epithelium and by accessor digestive organs
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Q
Defecation
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- Indigestible food is compacted into material waste called feces, which are eliminated by defecation
9
Q
Mechanical digestion and propulsion
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- Involves crushing and shredding of food in the oral cavity and mixing and churning in the stomach
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Q
Mechanical Digestion
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- Includes all movements that help digestion:
- Mastication (chewing)
- Swallowing
- Mixing
- Peristalsis
- Muscular contractions in the wall of the GI tract physically break down food by churning it and propelling it along the tract and also help dissolve foods by mixing them with fluids secreted into the GI tract
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Q
Chemical Digestion
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- Enzymes secreted by accessory organs and cells that line the tract break down the food chemically
- Mainly accomplished by using water to break down chemical bonds (hydrolysis)
- Fats are broken down into fatty acids and glycerol
- Carbohydrates are broken down into monosaccharides
- Proteins are broken down into amino acids
12
Q
Digestive tract
A
- Muscular tube about 10 m (33 ft) long
- begins with the mouth and ends with the anus
13
Q
Major organs
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- Oral cavity (mouth)
- Pharynx
- Esophagus
14
Q
Oral Cavity
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- Mechanical processing (with the teeth and tongue), moistening, mixing with salivary secretions
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Pharynx
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- Muscular propulsion of food into the esophagus
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Esophagus
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- Transport of materials to the stomach
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Stomach
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- Chemical breakdown and mechanical processing
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Small intestine
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- Enzymatic digestion and absorption
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Large intestine
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- Dehydration and compaction of indigestible
materials
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Q
A
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Q
Accessory digestive organs
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- Salivary glands
- Gallbladder
- Pancreas
- Liver
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Q
Salivary glands
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- Produce saliva containing mucins and enzymes
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Gallbladder
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- Stores and concentrates bile secreted by the liver
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Pancreas
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- Exocrine cells secrete buffers and digestive
enzymes - Endocrine cells secrete several hormones
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Liver
* Almost 200 known functions
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Layers of the GI Tract
* Four major layers of the digestive tract
* Same basic layers from esophagus to anal canal
* The layers are:
1. Mucosa
2. Submucosa
3. Muscular Layer
4. Serosa or adventitia
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Mucosa (inner lining), 3 components
1. Epithelium
2. Lamina propria
3. Muscularis mucosae
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Epithelium
| Mucosa (inner lining), 3 components
* Epithelium in the mouth, pharynx, esophagus, and anal canal is nonkeratinized, stratified squamous epithelium (for PROTECTION)
* Epithelium in stomach and intestines is simple columnar epithelium (for SECRETION and ABSORPTION)
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Lamina Propria
| Mucosa (inner lining), 3 components
* areolar connective tissue with blood and lymphatic vessels and mucosa-associated lymphatic tissue (MALT)
* MALT contains immune cells that protect against disease
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Muscularis mucosae
| Mucosa (inner lining), 3 components
* thin layer of smooth muscle making folds to increase surface area specifically in the stomach and the small intestine
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Submucosa
* Layer of dense irregular connective tissue
* Contains blood vessels and lymphatic vessels
* Also contains exocrine glands in some regions
* Secrete buffers and enzymes into the digestive tract
* Has a network of neurons called **submucosal
(Meissner’s) plexus**
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GI tract Muscular layer
* Smooth muscle in two layers
* inner circular layer
* outer longitudinal layer
* Involved in mechanical processing and
movement along tract
* Another network of neurons exists here called
the **myenteric (Auerbach’s) plexus**; between
muscle layers
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Serosa
* Outermost covering of organs along the
digestive tract in the abdominal cavity
* Also called the **visceral peritoneum** along the
digestive tract in the abdominal cavity
* No serosa in oral cavity, pharynx, esophagus,
and rectum
* Covered instead by **adventitia** (sheath formed
from a dense network of collagen fibers)
* Firmly attaches tract to adjacent structures
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Peritoneum
* Largest serous membrane of the body
* Divided into
* Parietal peritoneum
* Lines the inner surface of the peritoneal cavity
* Visceral peritoneum
* covers organs enclosed by the peritoneal cavity
* also called serosa
* Space between is peritoneal cavity and contains
lubricating serous fluid
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Peritoneum Serous membrane
* Continuously secretes peritoneal fluid into
peritoneal cavity
* ~7 liters/day secreted and reabsorbed
* Volume at any one time is about 50 mL
* Thin layer separates parietal and visceral surfaces
* Allows sliding movements without friction or irritation
* Rate of fluid moving into the cavity is accelerated
by liver disease, kidney disease, and heart failure
* Accumulation of peritoneal fluid can create abdominal
swelling (ascites)
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3 major peritoneal folds in the peritoneum
1. mesentery
2. omenta
3. falciform ligaments
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Peritoneal Folds: Mesentery
* Double sheets of peritoneal membrane
* Areolar tissue lies between mesothelial layers
* Provides access route for blood vessels, nerves, and
lymphatics
* Stabilizes attached organs
* Prevents entanglement of intestines
* Includes the mesocolon
* mesentery of the colon
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Peritoneal Folds: Omenta
* There are two omenta in the body, they are
continuations of the mesentery:
1. Greater omentum
2. Lesser omentum
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Greater omentum
* Attached to stomach and transverse colon
* Forms a large pouch extending inferiorly between
anterior body wall and anterior surface of the small
intestine
* Contains adipose tissue that provides padding and
protection
* Also contains blood vessels and lymphatic tissue
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Lesser omentum
* Suspends the stomach and duodenum from
the liver
* Provides an access route for blood vessels
and other structures entering/leaving the
liver
* hepatic portal vein
* hepatic artery
* common bile duct
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Falciform ligament
* Connects the liver to the anterior body wall
* The liver is the only digestive organ that is
attached to the **ANTERIOR** abdominal wall
* Remnant of part of the umbilical vein
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GI Tract Nerve plexuses
* Involved in local control of digestive activities
* Parasympathetic stimulation increases digestive muscle tone and activity
* Sympathetic stimulation decreases muscle tone and activity
* 2 major nerve plexuses:
1. Submucosal neural plexus (Meissner’s)
2. Myenteric plexus (Auerbach’s)
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Submucosal (Meissner’s) neural plexus
* Located in the submucosal layer
* Innervates the mucosa and submucosa
* Contains sensory neurons, autonomic nerve fibers
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Myenteric (Auerbach’s) plexus (mys, muscle & enteron, intestine)
* Network of sensory neurons and autonomic
nerve fibers
* Located in the muscularis externa between the
circular and longitudinal layers
* Works with the submucosal plexus to coordinate
local control of digestive activity
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Enteric Nervous System (ENS)
* The submucosal and myenteric plexuses make up the ENS
* Intrinsic set of nerves: the “brain of the gut”
* Neurons extending from esophagus to anus
* Quasi autonomous, meaning it can act on it’s own with some input from
the autonomic nervous system
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Autonomic Nervous System (ANS)
Extrinsic set of nerves
* Vagus nerve (CN X) supplies parasympathetic fibres to most of the GI tract
* EXCEPT the last 1/2 of the large intestine which is supplied from the sacral spinal cord
* Parasympathetic stimulation will increase secretion and motility by increasing activity of ENS
neurons
* Sympathetic nerve supply arises from thoracic and lumbar region of the spinal cord
* stimulation decreases secretions and activity by inhibiting ENS
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The Oral Cavity
* space that contains the tongue, teeth, and gums
* Lined by oral mucosa (stratified squamous epithelium)
* Keratinized in areas that are exposed to severe abrasion (superior tongue surface, hard palate)
* Thin, nonkeratinized lining on cheeks, lips, and inferior tongue surface
* Thin mucosa inferior to the tongue allows for rapid absorption of lipid-soluble drugs (example: nitroglycerin)
* Little to no nutrients are absorbed here
* Digestion of carbohydrates and lipids begins here
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Oral cavity boundaries
Superior Boundary
* **Hard palate**
* Formed by the palatine processes of the maxillary bones and horizontal
plates of the palatine bones
* **Soft palate**
* Muscular region posterior to the hard palate
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Oral Cavity Boundaries
Anterior and Lateral Boundaries
* **Cheeks**
* Form the lateral walls of the oral cavity
* Supported by pads of fat and the
buccinator muscles
* Anteriorly, cheek mucosa is continuous with
the labia
* **Labia (lips)**
* Form anterior boundary
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Oral Cavity Boundaries
Inferior Boundary
**Body of the tongue**
Anterior, mobile portion
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Oral cavity boundaries (continued)
Posterior boundary
* Uvula
* Root of toungue
* Lingual Tonsils
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Uvula
* Dangling process extending from the soft palate
* Helps prevent food from entering pharynx prematurely
* Swings upward during swallowing to prevent food from entering the nasopharynx
* Palatine tonsils (one located on either side of the oropharynx)
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Root of tongue
* Fixed portion projecting into the oropharynx
* Marked by a V-shaped line of vallate papillae
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Oral vestibule
* The space between the cheeks (or lips) and teeth
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Frenulum of the upper lip
* Attaches gums to upper lip
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Frenulum of the lower lip
Attaches gums to lower lip
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Gingivae
* Ridges of oral mucosa surrounding the base of each tooth
* Firmly attached to the periostea of the underlying bone
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Palatal arches
* Located on either side of the uvula
* **Palatoglossal arch**
* Extends between soft palate and base of tongue
* **Palatopharyngeal arch**
* Extends from soft palate to pharyngeal wall
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Tongue
* Accessory digestive organ
* Skeletal muscle covered by mucous
membrane
* Has extrinsic muscles that move the tongue side to side and in and out to maneuver food for chewing and forcing food to the back of the mouth for swallowing
* Intrinsic muscles originate in and insert into connective tissue within the tongue
* Dorsum and lateral surfaces are covered with papillae which contain taste buds and some receptors for touch
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frenulum of the tongue
Attaches tongue to floor of mouth
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Salivary Glands
* Salivary glands release saliva into the oral cavity
* Ordinarily, just enough is secreted to keep mouth and pharynx moist and clean
* When food enters mouth, secretion increases to lubricate, dissolve and begins the chemical digestion of food
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3 pairs of major salivary glands secrete most of the
saliva
1. Parotid glands
2. Submandibular glands
3. Sublingual glands
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Parotid Glands
Lie inferior to the zygomatic arch, deep to the skin, covering the mandible
* secretes saliva via parotid duct that pierces buccinator muscle
* Produce serous secretion containing large amounts of salivary amylase
* Provide ~25 percent of total saliva secretion
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Sublingual Glands
* Located beneath the tongue
* sublingual ducts open into the floor of the mouth
* Produce a mucous secretion that acts as a buffer and lubricant
* Provide ~5 percent of total saliva secretion
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Submandibular Glands
* Located medial and inferior to the body of the mandible and the submandibular ducts enter the
oral cavity lateral to the lingual frenulum
* Secrete a mixture of buffers, mucins, salivary amylase
* Amylase is an enzyme that breaks down starches
* Cells transport IgA antibodies into the saliva
* Provides protection against pathogens
* Provide ~70 percent of total saliva secretion
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Saliva
* Mostly water 99.5%
* 0.5% solutes – ions (sodium, potassium, chloride, bicarbonate and phosphate), dissolved gases, urea, uric acid, mucous, immunoglobulin A, lysozyme, and salivary amylase (acts on starch)
* Salivary glands produce 1.0–1.5 L of saliva each day
* Small glands called Ebner’s glands secrete lingual lipase
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Saliva Functions
* Constantly flushes oral surfaces
* Buffers keep pH of mouth near 7.0 and prevent buildup of acids
produced by bacteria
* Contains antibodies (IgA) and lysozyme to help control oral
bacteria populations
* Mixes with food to form a bolus to be easily swallowed
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Salivation controlled by
* Controlled by autonomic nervous system
* Parasympathetic stimulation promotes secretion of moderate amount of saliva
* Sympathetic stimulation decreases salivation
* The taste and smell of food are potent stimulators of salivary gland secretions (“cephalic phase” of digestion)
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Teeth or dentes
* Accessory digestive organ
* Dentin forms the majority of the tooth and is harder than bone
* Covered by enamel, which is even harder
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2 dentitions
* deciduous or primary teeth
* “baby teeth”
* permanent or secondary teeth
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Components of a tooth
* in different regions of the jaws vary in size, shape, and function
* The bulk of each tooth is composed of **dentin**
* Mineralized matrix similar to bone but contains no cells
* **Pulp cavity**
* The interior chamber of the tooth
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Occlusal surface
* Portion of the crown used for crushing, slicing, or
chewing
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Enamel
* Covers the dentin of the crown
* Hardest biologically manufactured substance
* Composed of calcium phosphate
* Requires calcium, phosphate, and vitamin D for formation and resistance to decay
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Periodontal ligament
* Creates gomphosis articulation between root dentin and alveolar bone
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Root canal
* Narrow tunnel within the root of the tooth
* Passageway for blood vessels and nerves to the pulp cavity
* Opening into the root canal is the apical foramen
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Crown
| Regions of a tooth
* Portion projecting into the oral cavity from the
surface of the gums
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Neck
| Regions of a tooth
* The boundary between the crown and root
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Root
| Regions of a tooth
* Portion below the gum line
* Sits in a bony tooth socket called an alveolus
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Digestion in the Mouth
* The mouth is the first place where digestion begins, both mechanical and chemical
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Mechanical digestion in the mouth
* Chewing or mastication
* Food is manipulated by tongue, ground by teeth, and mixed with saliva
* Forms a soft, flexible mass called bolus
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Chemical digestion in the mouth
* Salivary amylase secreted by salivary glands and acts on starches
* Lingual lipase secreted by lingual glands of tongue acts on triglycerides
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Salivary amylase
* secreted by salivary glands and acts on starches
* Only monosaccharides can be absorbed into bloodstream
* Continues to act until inactivated by stomach acid
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Lingual lipase
* secreted by lingual glands of tongue acts on triglycerides
* Active in the mouth, but becomes even more activated in acidic environment of stomach
* Acts to breakdown dietary triglycerides into fatty acids
and diglycerides
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Pharynx
* Membrane-lined cavity posterior to the nose and mouth (throat)
* Skeletal muscle walls of the pharynx play a key role in swallowing (along with esophagus)
* Continuous with the esophagus
* Common passageway for solid food,
liquids, and air
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Pharynx 3 regions
1. Nasopharynx (respiration only)
2. Oropharynx
3. Laryngopharynx
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The Esophagus Function
* Actively moves food and liquids to the stomach
* No enzymes produced here, no absorption
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The Esophagus Structure
* Hollow, muscular tube ~25 cm (10 in.) long and 2 cm (0.8 in.) wide
* Narrowest point at the beginning (posterior to cricoid cartilage)
* Descends posterior to the trachea
* Enters the abdominopelvic cavity through the esophageal hiatus (opening in the diaphragm)
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Hiatal hernia
* occurs when part of the stomach protrudes above the diaphragm through the esophageal hiatus
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Upper esophageal sphincter
* Band of smooth muscle that functions as sphincter
* Prevents air from entering the
esophagus
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Lower esophageal sphincter
| Cardiac sphincter
* At the inferior end of the esophagus
* Normally contracted (prevents
backflow of stomach contents)
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Swallowing or Deglutition
* Initiated voluntarily but proceeds automatically
* Facilitated by secretions of saliva and mucus
* Involves mouth, pharynx, and esophagus
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* Three phases of swallowing
1. Buccal Phase
2. Pharyngeal Phase
3. Esophageal Phase
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Buccal phase (or voluntary phase)
* Begins with compression of bolus against hard palate
* Tongue forces bolus into the oropharynx
* Also elevates soft palate (sealing off nasopharynx)
* Entry into the oropharynx triggers reflex response
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Pharyngeal phase
* Begins with stimulation of tactile receptors in uvula and palatine arches
* Motor commands from the swallowing center (in medulla oblongata) coordinate muscle contraction in pharyngeal muscles
* Larynx is elevated; epiglottis is folded; uvula and soft palate are elevated
* Bolus is moved through the pharynx into the esophagus
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Esophageal phase
Begins as bolus is forced through the entrance to the esophagus
* Bolus is pushed toward the stomach by peristalsis
* Approach of bolus triggers the opening of the lower esophageal sphincter
* Bolus enters the stomach
* Typical travel time is 9 seconds
* Liquids may travel faster
* A dry (poorly lubricated) bolus may require secondary peristaltic waves
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Peristalsis
* Wave of muscle contraction
* Food enters the digestive tract as a bolus
* Moist, compact mass of material
* Bolus is propelled along the tract by contractions of the muscularis externa (peristalsis)
* Circular muscles contract behind bolus
* Longitudinal muscles ahead of bolus contract
* Process repeats
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Periodontal Disease
* Most common cause for loss of teeth
* Occurs when dental plaque forms between gums and teeth
* Resulting bacterial activity may cause:
* **Gingivitis** (inflammation of the gums)
* Tooth decay
* Eventual breakdown of periodontal ligaments and
surrounding bone
* Link to cardiovascular disease?
* Inflammation and atherosclerosis
