Digestive System

The basics

Functions of the Digestive System

Acquires nutrients from environment

Catabolism: Decomposes substances to provide energy cells need to function

Anabolism: Uses raw materials to synthesize essential compounds

Catabolic Reactions

Require two essential ingredients

1. Oxygen

2. Organic molecules broken down by intracellular enzymes

  • For example, carbohydrates, fats, and proteins

The Digestive Tract

Also called gastrointestinal (GI) tract or alimentary canal

Is a muscular tube that extends from oral cavity to anus

Passes through pharynx, esophagus, stomach, and small and large intestines

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Six Functions of the Digestive Tract

1. Ingestion- Occurs when materials enter digestive tract via the mouth

2. Mechanical processing

Crushing and shearing

Makes materials easier to propel along digestive tract

3. Digestion (chemical digestion) The chemical breakdown of food into small organic fragments for absorption by digestive epithelium

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

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

6. Excretion Removal of waste products from body fluids, the process called defecation removes feces

The Lining of the 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

Histological Organization of the Digestive Tract

Four major layers of the digestive tract

1. Mucosa

  • Epithelium
  • Lamina Propria
  • Muscularis mucosae

2. Submucosa

3. Muscularis externa

4. Serosa

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1. The Digestive Epithelium

•Is the inner lining of digestive tract

•Is a mucous membrane consisting of:

•Epithelium, moistened by glandular secretions

Mucosal epithelium is simple or stratified depending on location, function, and stresses


Oral cavity, pharynx, and esophagus

  • Mechanical stresses

  • Lined by stratified squamous epithelium


Stomach, small intestine, and most of large intestine

  • Absorption
  • Simple columnar epithelium with mucous cells


Enteroendocrine cells

  • Are scattered among columnar cells of digestive epithelium
  • Secrete hormones that coordinate activities of the digestive tract and accessory glands

The Lamina Propria

Consists of 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 arranged in two concentric layers

Inner layer encircles lumen (circular muscle)

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

2. Submucosa

Is a layer of dense irregular connective tissue that surrounds muscularis mucosae

  • Has large blood vessels and lymphatic vessels
  • May contain exocrine glands
  • Secrete buffers and enzymes into digestive tract


Submucosal Plexus /Also called Meissner’s plexus innervates the mucosa and submucosa

Contains:

  • Sensory neurons
  • Parasympathetic ganglionic neurons
  • Sympathetic postganglionic fibers

3. The Muscularis Externa

Is dominated by smooth muscle cells arranged in:

Inner circular layer

Outer longitudinal layer

Involved in:

  • Mechanical processing
  • Movement of materials along digestive tract

Movements coordinated by enteric nervous system (ENS)

•Sensory neurons

•Interneurons

•Motor neurons



ENS

Innervated primarily by parasympathetic division of ANS sympathetic postganglionic fibers

The myenteric plexus (Auerbach’s plexus)

4. The Serosa

Serous membrane covering muscularis externa

Except in oral cavity, pharynx, esophagus, and rectum where adventitia, a dense sheath of collagen fibers, firmly attaches the digestive tract to adjacent structures.

Movement of Digestive Materials

Facilitated by muscular layers of digestive tract composed of visceral smooth muscle tissue

•Has rhythmic cycles of activity controlled by pacesetter cells in muscularis mucosae and muscularis externa

•Surrounding lumen of digestive tract

•Cells undergo spontaneous depolarization triggering wave of contraction through entire muscular sheet

Peristalsis

Peristaltic Motion

1. Circular muscles contract behind bolus while circular muscles ahead of bolus relax

2. Longitudinal muscles ahead of bolus contract shortening adjacent segments

3. Wave of contraction in circular muscles forces bolus forward

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Segmentation

Cycles of contraction

  • Churn and fragment the bolus
  • Mix contents with intestinal secretions

Does not follow a set pattern, does not push materials in any one direction

Control of Digestive Functions

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

-variations in local pH, chemical, or physical stimuli

-Affect only a portion of tract


Neural mechanisms - Short reflexes

•Are 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

•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

•Glossopharyngeal, vagus, or pelvic nerves


Hormonal mechanisms

At least 18 peptide hormones that:

  • Affects most aspects of digestive function
  • Are produced by enteroendocrine cells in digestive tract
  • Reach target organs after distribution in bloodstream
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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 by mixing with mucus and salivary gland secretions

4. Limited digestion of carbohydrates and lipids

Oral Mucosa

Lining of oral cavity

  • has stratified squamous epithelium of cheeks, lips, and inferior surface of tongue
  • Is relatively thin, nonkeratinized, and delicate
  • 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


Labia

Also called lips

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

Vestibule is the 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

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Tongue

Manipulates materials inside mouth

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

Salivary glands

Three pairs secrete into oral cavity

1. 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


2. Sublingual salivary glands

•Covered by mucous membrane of floor of mouth

•Produce mucous secretion

•Acts as a buffer and lubricant

•Sublingual ducts on either side of lingual frenulum


3. Submandibular salivary glands

•Each pair has distinctive cellular organization

•And produces saliva with different properties

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Saliva facts

•Glands produce 1.0–1.5 liters of saliva each day

•70 percent by submandibular glands

•25 percent by parotids

•5 percent by sublingual glands


% 99.4 water

% 0.6 includes:

•Electrolytes (such as Na+, Cl−)

•Buffers (such as HCO3−)

•Glycoproteins (mucins)

•Antibodies

•Enzymes

•Waste products

Functions of saliva

  • 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 (ptyalin or alpha-amylase)

Control of Salivary Secretions

By autonomic nervous system

via parasympathetic and sympathetic innervation

Parasympathetic accelerates secretion by all salivary glands

•Salivatory nuclei of medulla oblongata influenced by:

•Other brain stem nuclei

•Activities of higher centers

The teeth

•Tongue movements pass food across occlusal surfaces of teeth

•Chew (masticate) food

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Types of Teeth

1.Incisors

•Blade-shaped teeth

•Located at front of mouth

•Used for clipping or cutting

•Have a single root


2. Cuspids (canines)

•Conical

•Sharp ridgeline

•Pointed tip

•Used for tearing or slashing

•Have a single root


3. Bicuspids (premolars)

•Flattened crowns

•Prominent ridges

•Used to crush, mash, and grind

•Have one or two roots


4. Molars

•Very large, flat crowns

•With prominent ridges

•Used for crushing and grinding

•Have three or more roots

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Dental Succession

During embryonic development, two sets of teeth form

  • Primary dentition, or deciduous teeth

    20 temporary teeth of primary dentition

    Five on each side of upper and lower jaws

    2 incisors

    1 cuspid

    2 deciduous molars

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  • Secondary dentition, or permanent dentition

    •Replaces deciduous teeth

    •32 permanent teeth

    •Eight on each side, upper and lower

    •2 incisors

    •1 cuspid

    •5 molars

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Mastication (chewing)

•Close the jaws

•Slide or rock lower jaw from side to side

•Chewing involves mandibular:

•Elevation and depression

•Protraction and retraction

•Medial and lateral movement

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Lining of the digestive tract

Folding increases surface area for absorption

  • Longitudinal folds, disappear as digestive tract fills

  • Permanent transverse folds (plicae circulares)

The Digestive Organs and the Peritoneum

Lined with serous membrane consisting of:

Superficial mesothelium covering a layer of areolar tissue:
Serosa, or visceral peritoneum covers organs within peritoneal cavity

The parietal peritoneum lines inner surfaces of body wall

Serous membrane contains peritoneal fluid

Is produced by serous membrane lining

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


About 7 liters produced and absorbed daily, but very little in peritoneal cavity at one time

Ascites – excess peritoneal fluid causing abdominal swelling

Mesenteries

Areolar tissue between mesothelial surfaces

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

Stabilize positions of attached organs and prevents intestines from becoming entangled

Mesentery Development

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

  • Dorsal mesentery
  • Ventral mesentery


Later disappears along most of digestive tract except at the lesser omentum and at the 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 position of liver relative to diaphragm and abdominal wall

Dorsal Mesentery

Enlarges to form an enormous pouch, called the greater omentum

Extends inferiorly between the body wall and the anterior surface of small intestine

Hangs like an apron from lateral and inferior borders of stomach

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

Mesentery proper

Is a thick mesenterial sheet

Provides stability

Permits some independent movement

Suspends all but first 25 cm (10 in.) of small intestine

Is associated with initial portion of small intestine (duodenum) and pancreas

Fuses with posterior abdominal wall, locking structures in position

Mesocolon

A mesentery associated with a portion of the large intestine

Transverse mesocolon supports transverse colon

Sigmoid mesocolon supports sigmoid colon

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

  • Fuse to dorsal body wall

  • Lock regions in place