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143 Cards in this Set

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Anabolism

Uses raw material to synthesize essential compounds (building up)

Catabolism

Decomposes substances to provide energy cells need to function (breaking down)

The digestive system acquires nutrients from the

environment

Two essential ingredients required for catabolic reactions

1. Oxygen


2. Organic molecules broken down by intracellular enzymes (carb, protein, fat)

Other names for the digestive tract

Gastrointestinal tract or Alimentary canal

Digestive tract extends from

Oral cavity to anus

Digestive tract passes through

Pharynx, esophagus, stomach, small and large intestines

Oral cavity (mouth) functions

Ingestion, mechanical processing with accessory organs (teeth and tongue), moistening, mixing with salivary secretions

Pharynx function

Muscular propulsion of materials into esophagus

Esophagus function

Transport of materials into stomach

Stomach functions

Chemical breakdown of materials by acid and enzymes; mechanical processing through muscular contractions

Small intestine functions

Enzymatic digestion and absorption of water, organic substrates, vitamins, and ions

Large intestine functions

Dehydration and compaction of indigestible materials in preparation for elimination

Teeth function

Mechanical processing by chewing (mastication)

Another word for chewing

Mastication

Tongue functions

Assists mechanical processing with teeth, sensory analysis

Salivary glands function

Secretion of lubricating fluid containing enzymes that break down carbohydrates

Liver functions

Secretion of bile, storage of nutrients, many other vital functions

Gallbladder function

Storage and concentration of bile

Pancreas functions

Exocrine cells secrete buffers and digestive enzymes; Endocrine cells secrete hormones

6 functions of the digestive system


  1. Ingestion
  2. Mechanical processing
  3. Digestion
  4. Secretion
  5. Absorption
  6. Excretion

Ingestion

Occurs when materials enter digestive tract via the mouth (eating)

Mechanical processing


  • Crushing and shearing
  • Makes materials easier to propel along digestive tract

Digestion

The chemical breakdown of food into small organic fragments for absorption by digestive epithelium

Secretion

Is the release of water, acids, enzymes, buffers, and salts by epithelium of digestive tract and glandular organs

Absorption

Movement of organic substrates, electrolytes, vitamins, and water across digestive epithelium into interstitial fluid of digestive tract

Excretion

Removal of waste products from body fluids

Lining of digestive tract safeguards surrounding tissues against

  1. Corrosive effects of digestive acids and enzymes
  2. Mechanical stresses, such as abrasion
  3. Bacteria either ingested with food or that reside in digestive tract

Digestive organs and peritoneum are lined with serous membrane consisting of

  • Superficial mesothelium covering a layer of areolar tissue
  • Serosa (or visceral peritoneum)
  • Parietal peritoneum

Serosa (visceral peritoneum)

Covers organs with peritoneal cavity

Parietal peritoneum

Lines inner surfaces of body wall

Mesenteries

Double sheets of peritoneal membrane

Mesenteries function


  • Suspend portions of digestive tract within peritoneal cavity by sheets of serous membrane that connect parietal peritoneum with visceral peritoneum
  • Stabilize positions of attached organs
  • Prevent intestines from becoming entangled

Areolar tissue between mesothelial surfaces

Provides an access route to and from the digestive tract for passage of blood vessels, nerves, and lymphatic vessels

During embryonic development digestive tract and accessory organs are suspended in peritoneal cavity by

Dorsal mesentery


Ventral mesentery

What happens to the ventral mesentery

It disappears along most of the digestive tract

Where is the ventral mesentery remain in adults

lesser omentum and falciform ligament

The Lesser Omentum

  • Stabilizes position of stomach
  • Provides access route for blood vessels and other structures entering or leaving liver

The Falciform Ligament

Helps stabilize portion of liver relative to diaphragm and abdominal wall

The Dorsal Mesentery


  • Enlarges to form enormous pouch, called greater omentum
  • Extends infeiorly between body wall and anterior surface of small intestine
  • Hangs like an apron from lateral and infeior borders of stomach

Greater omentum


  • A large fold of the dorsal mesentery of the stomach
  • Hangs anterior to the intestines

Adipose tissue in greater omentum

  • Conforms to shapes of surrounding organs
  • Pads and protects surfaces of abdomen
  • Provides insulation to reduce heat loss
  • Stores lipid energy reserves

The Mesentery Proper

  • Thick mesenterial sheet
  • Provides stability
  • Permits some independent movement
  • Suspends all but first 25 cm (10 in) of small intestine
  • Associated with initial portion of small intestine (duodenum) and pancreas
  • Fuses with posterior abdominal wall, locking structures in position

The Mesocolon

A mesentery associated with a portion of the large intestine

Major organs of digestive tract


  • Oral cavity (mouth)
  • Pharynx
  • Esophagus
  • Stomach
  • Small intestine
  • Large intestine

Accessory organs of digestive tract


  • Teeth
  • Tongue
  • Salivary glands
  • Liver
  • Gallbladder
  • Pancreas

Peritoneal fluid is produced by

Serous membrane lining

Peritoneal fluid functions

  • Provides essential lubrication
  • Separates parietal and visceral surfaces
  • Allows sliding without friction or irritation

How much peritoneal fluid is absorbed daily

~ 7 liters

Ascites

Excess peritoneal fluid causing abdominal swelling

During development, mesocolon of ascending colon, descending colon, and the rectum:


  • Fuse to dorsal body wall
  • Lock regions in place

4 major layers of digestive tract

  1. Mucosa
  2. Submucosa
  3. Muscularis externa
  4. Serosa

The Mucosa

  • Inner lining of digestive tract
  • Mucous membrane consisting of epithelium moistened by glandular secretions

Mucosal epithelium

Simple or stratified depending on location, function, and stresses

Oral cavity, pharynx, and esophagus histology

  • Mechanical stresses

  • Lined by stratified squamous ephithelium

Stomach, small intestine, and most of large intestine histology

  • Absorption
  • Simple columnar epithelium with mucous cells

Where are enteroendocrine cells

Scattered among columnar cells of digestive epithelium

What do enteroendocrine cells do?

Secrete hormones that coordinate activities of the digestive tract and accessory glands

What does folding do for the digestive tract?

Increases surface area for absorption

What types of folds are in the digestive tract?


  • Longitudinal folds (disappear as digestive tract fills)
  • Permanent transverse folds

Lamina Propria

A layer of areolar tissue that contains:



  • Blood vessels
  • Sensory nerve endings
  • Lymphatic vessels
  • Smooth muscle cells
  • Scattered areas of lymphatic tissue

Muscularis mucosae

Narrow band of smooth muscle and elastic fibers in lamina propria

Smooth muscle cells of muscularis mucosae arranged in two concentric layers

  • Inner layer encircles lumen (circular muscle)

  • Outer layer contains muscle cells parallel to tract (longitudinal layer)

Submucosa

  • Layer of dense irregular connective tissue

  • Surrounds muscularis mucosae

  • Has large blood vessels and lymphatic vessels

  • May contain exocrine glands (secrete buffers and enzymes into digestive tract)

Submucosa surrounds

Muscularis mucosae

Submucosal Plexus also called

Meissner's plexus

Submucosal plexus

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

Muscularis externa

Dominated by smooth muscle cells arranged in inner circular layer and outer longitudinal layer

Muscularis externa is involved in


  • Mechanical processing
  • Movement of materials along digestive tract

Muscularis externa movements are coordinated by

Enteric nervous system (ENS)

Enteric nervous system (ENS)

Automatic - works on its own


Primarily innervated by parasympathetic division of ANS

Serosa

Serous membrane that covers the muscularis externa


(except in oral cavity, pharynx, esophagus, and rectum)

Adventitia

Fibrous sheath that firmly attaches the digestive tract to adjacent structures

The muscular layers of the digestive tract consist of

visceral smooth muscle tissue

Pacesetter cells function

Control rhythmic cycles along digestive tract

Smooth muscle cells in digestive tract undergo

spontaneous depolarization, triggering a wave of contraction through entire muscular sheet

How is a wave of contraction triggered through the muscular sheet

spontaneous depolarization by pacesetter cells along digestive tract


Where are pacesetter cells located

In muscularis mucosae and muscularis externa, surrounding the lumen of the digestive tract

Peristalsis


  • Waves of muscular contractions
  • Moves a bolus along the length of the digestive tract

Steps of peristaltic motion

  1. Contraction of circular muscles behind bolus
  2. Contraction of longitudinal muscles ahead of bolus
  3. Wave of contraction in circular muscles forces bolus forward

Segmentation


  • Cycles of contraction that churn and fragment the bolus, mixing the contents with intestinal secretions
  • Does not follow a set pattern, does not push materials in any one direction
  • * Mixing the food, pushing enzymes to outside

Control of digestive functions

  • Local factors
  • Neural mechanisms
  • Hormonal mechanisms

Local factors


  • Prostaglandins, histamine, and other chemicals released into interstitial fluid
  • May affect adjacent cells within small segment of digestive tract
  • Coordinate response to changing conditions (for example: variations in local pH, chemical, or physical stimuli)
  • Affect only a portion of tract (*local*)

Neural mechanisms

  • Control movement of materials along digestive tract and secretory functions
  • Motor neurons control smooth muscle contraction and glandular secretion located in myenteric plexus

Neural mechanisms - short reflexes

  • Responsible for local reflexes
  • Control small segments of digestive tract
  • Operate entirely outside of CNS control (sensory neurons, motor neurons, interneurons)

Neural mechanisms - long reflexes

  • Higher level control of digestive and glandular activities

  • Control large-scale peristaltic waves

  • Involve interneurons and motor neurons in CNS

  • May involve parasympathetic motor fibers that synapse in the myenteric plexus

Hormonal mechanisms


  • At least 18 peptide hormones that affect most aspects of digestive function and activities of other systems
  • Are produced by enteroendocrine cells in digestive tract
  • Reach target organs after distribution in bloodstream

Functions of the oral cavity

  1. Sensory analysis of material before swallowing
  2. Mechanical processing through actions of teeth, tongue, and palatal surfaces
  3. Lubrication (mixing with mucus and salivary gland secretions)
  4. Limited digestion of carbs and lipids

Oral mucosa epithelium

stratified squamous epithelium

Oral mucosa

  • Lining of oral cavity
  • Inferior to tongue is thin and vascular enough to rapidly absorb lipid-soluble drugs
  • Cheeks are supported by pads of fat and the buccinator muscles

Epithelial lining of cheeks, lips, and inferior surface of tongue

Is relatively thin, nonkeratinized, and delicate

Labia


  • Also called lips
  • Anteriorly, the mucosa of each cheek is continuous with that of the lips

Vestibule

Space between the cheeks (or lips) and the teeth

Gingivae

Gums



  • Ridges of oral mucosa
  • Surround base of each tooth on alveolar processes of maxillary bones and mandible

Functions of the tongue

  1. Mechanical processing by compression, abrasion, and distortion
  2. Manipulation to assist in chewing and to prepare material for swallowing
  3. Sensory analysis by touch, temperature, and taste receptors
  4. Secretion of mucins and the enzyme lingual lipase

3 pairs of salivary glands secrete into oral cavity

  1. Parotid salivary glands
  2. Sublingual salivary glands
  3. Submandibular salivary glands

Each pair of salivary glands has

distinctive cellular organization and produces saliva with different properties

Parotid salivary glands

  • Inferior to zygomatic arch
  • Produce serous secretion (enzyme salivary amylase - breaks down starches)
  • Drained by parotid duct which empties into vestibule at second molar

Sublingual salivary glands

"sub" = under
"lingual" = tongue


  • Covered by mucous membrane of floor of mouth
  • Produce mucous secretion
  • Acts as buffer and lubricant
  • Sublingual ducts on either side of lingual frenulum



Submandibular salivary glands

  • In floor of mouth
  • Within mandibular groove
  • Secretes buffers, glycoproteins, and salivary amylase
  • Submandibular ducts open immediately posterior to teeth, either side of lingual frenulum

Saliva

Glands produce 1.0-1.5 liters of saliva each day


99.4% water

Saliva functions


  • Lubricating the mouth
  • Moistening and lubricating materials in the mouth
  • Dissolving chemicals that stimulate taste buds and provide sensory information
  • Initiating digestion of complex carbohydrates by the enzyme salivary amylase

Control of salivary secretions

Autonomic nervous system


Parasympathetic - increases


Sympathetic - decreases

Dentin

A mineralized matrix similar to that of bone, does not contain cells

Pulp cavity

Receives blood vessels and nerves through the root canal

Root

  • Of each tooth sits in a bony socket
  • A layer of cementum covers dentin of root providing protection and anchoring periodontal ligament

Crown


  • Exposed portion of tooth
  • Projects beyond soft tissue of gingiva
  • Dentin covered by layer of enamel

Alveolar processes of the maxillae

Form maxillary arcade (upper dental arch)

Alveolar processes of the mandible

Form mandibular arcade (lower dental arch)

Primary dentition (deciduous teeth)


  • Also called primary teeth, milk teeth, or baby teeth
  • 20 temporary teeth

Secondary dentition


  • Also called permanent dentition
  • Replaces deciduous teeth
  • 32 permanent teeth

Pharynx (throat)

  • Common passageway for solid food, liquids, and air
  • Food passes through oropharynx and laryngopharynx to the esophagus

Regions of the pharynx


  • Nasopharynx
  • Oropharynx
  • Laryngopharynx

Esophagus

  • Hollow muscular tube
  • Conveys solid food and liquids to the stomach
  • Contracts with muscles to push food into stomach

Resting muscle tone

Esophagus prevents air from entering

3 layers of wall of esophagus

  1. Mucosal

  2. Submucosal

  3. Muscularis

Mucosa histology

Nonkeratinized and stratified squamous epithelium

Mucosa and submucosa form

Large folds that extend the length of the esophagus

Another word for swallowing

Deglutition

Swallowing

  • Can be initiated voluntarily
  • Proceeds automatically
  • Divided into 3 phases

3 phases of swallowing

  1. Buccal phase

  2. Pharyngeal phase

  3. Esophageal phase

Buccal phase of swallowing


  • Compression of bolus against hard palate
  • Retraction of tongue forces bolus into oropharynx

Pharyngeal phase of swallowing


  • Begins as bolus comes into contact with palatoglossal and palatopharyngeal arches and posterior pharyngeal wall

Esophageal phase of swallowing


  • Begins as the contraction of pharyngeal muscles forces bolus through entrance of esophagus
  • Bolus is pushed toward stomach by peristalsis

The approach of the bolus to the stomach triggers the

Opening of the lower esophageal sphincter

Major functions of the stomach

  1. Storage of ingested food

  2. Mechanical breakdown of ingested food

  3. Disruption of chemical bonds in food material by acid and enzymes

  4. Production of intrinsic factor

Intrinsic factor

Glycoprotein required for absorption of vitamin B12 in small intestine

Stomach shape

Like an expanded J

Regions of the stomach

  1. Cardia

  2. Fundus

  3. Body

  4. Pylorus

Histology of the stomach

Simple columnar epithelium lines all portions of stomach


Gastric pits

Shallow depressions in stomach that open onto gastric surface

Gastric glands


  • In fundus and body of stomach, extend deep into underlying lamina propria
  • Each gastric pit communicates with several gastric glands - Parietal cells & Chief cells

Parietal cells

Secrete intrinsic factor and hydrochloric acid (HCl)

Chief cells

Are most abundant near base of gastric gland that secrete pepsinogen

Pepsinogen

Is converted by HCl in the gastric lumen to pepsin

Pyloric glands


  • Located in pylorus
  • Produce mucous secretions scattered with enteroendocrine cells

Mucous secretions of pyloric glands


  • G cells produce gastrin
  • D cells release somatostatin

Somatostatin

  • Growth hormone

  • Hormone that inhibits release of gastrin

Regulation of gastric activity

Production of acid and enzymes by the gastric mucosa can be



  • Controlled by the CNS
  • Regulated by short reflexes of ENS
  • Regulated by hormones of digestive tract

3 phases of gastric control

  1. Cephalic phase
  2. Gastric phase
  3. Intestinal phase

Cephalic phase of gastric control


  • Takes place in brain (cephalic = head)
  • Triggered by sight, smell, taste, or thought of food
  • Perceived by CNS
  • Vagus nerve stimulates mucous cells to produce mucous, chief cells to produce pepsinogen, parietal cells to produce HCl, and G cells to produce gastrin, and movement of the stomach
  • Preparing for food

Gastric phase of gastric control

  • Bolus enters stomach
  • Stretches stomach
  • Increase in pH
  • Stretch receptors and chemoreceptors send signals to plexuses
  • Secretes more mucus, pepsinogen, HCl, and gastrin
  • Stomach moves more

Intestinal phase of gastric control

  • Chyme leaves stomach and enters small intestine (duodenum)
  • Stretches duodenum
  • Chemoreceptors sense pH changes and presence of lipids and carbs
  • Hormones secreted