Digestive System

Página 1

22.7

Bulk of each tooth consists of mineralized matrix similar to that of bone

Dentin: Differs from bone because it does not contain cells

Cytoplasmic processes extend into dentin from cells making up pulp in central pulp cavity

Components of a Tooth

Crown of Tooth: Projects into oral cavity form surface of gingiva

Neck of Tooth: Marks boundary between crown and root

Root of Tooth: Sits in bony cavity called tooth socket or tooth alveolous

Occlusal Surface: Portion of crown that is used for crushing, slicing, or chewing

Enamel: Covers dentin of crown

Hardest biologically manufactured substance

Contains calcium phosphate in crystalline form

Gingival Sulcus: Surrounds neck of each tooth

Epithelium is bound to tooth at base of sulcus

Epithelial attachment prevents bacteria from entering deeper tissues around root

Cement: Covers dentin of root

Less resistant to erosion than is dentin

Periodontal Ligament: Collagen fibers of this extend from dentin of root to compact bone of alveolus, creating a strong joint named gomphosis

Root Canal: A narrow tunnel within root of tooth

Pulp cavity receives blood vessels and nerves through this

Opening into root canal is called apical foramen

Types of Teeth

Incisors: Blade-shaped; located at front of mouth

Useful for clipping or cutting

Have a single root

Canines (cuspids): Conical; Sharp ridgeline and pointed tip

Used for tearing or slashing

Have a single root

Premolars (bicuspids): Flattened crowns with prominent ridges

Used to crush, mash, and grind

Have one or two roots

Molars: Large flatted crowns with prominent ridges

Used for crushing and grinding

Typically have three roots in upper jaw, two in lower

Teeth are collectively called dentition; two dentitions form during development

Deciduous Teeth (primary teeth): At 2 years of age, children normally have 20 deciduous teeth

2 incisors, one canine, one pair of deciduous molars

Permanent Teeth: Three additional molars appear on each side of upper + lower jaw as person ages

32 teeth at this point

22.8

Pharynx: An anatomical space and common passageway for food, liquids, and air

Food passes through oropharynx and laryngopharynx on its way to esophagus

Regions of Pharynx

Nasopharynx

Oropharynx

Laryngopharynx

Esophagus: Hollow muscular tube; Actively moves food and liquids into stomach

Inferior third has smooth muscle

Middle third contains mixture of skeletal and smooth muscle tissue

Circular + longitudinal layers of muscular layer contain skeletal muscle fibers in superior third of esophagus

Mucosa + Submucosa have large folds that extend length of esophagus

Folds allow for expansion during passage of large bolus

Muscle tone in walks keep lumen closed, except when you swallow

Mucosa of esophagus contains nonkeratinized, stratified squamous epithelium

Muscularis mucosae consists of an irregular layer of smooth muscle

Layers of esophageal wall are similar to other portions of digestive tract, other than shape of lumen and structure of muscular layer of esophagus

No serosa, but an adventitia exists (anchors esophagus to posterior body wall

Innervated by parasympathetic + sympathetic fibers from esophageal plexus

Deglutition (AKA swallowing): Complex process that starts voluntarily but proceeds automatically

Buccal Phase: Begins with compression of bolus against hard palate; strictly voluntary

Retraction of tongue forces bolus into oropharynx and assists in elevation of soft palate

Seals off nasopharynx

Pharyngeal Phase: Begins when tactile receptors on palatal arches and uvula are stimulated

Motor commands from swallowing center in medulla oblongata direct a coordinated pattern of muscle contraction in pharyngeal muscles

Esophageal Phase: begins as contraction pf pharyngeal muscles forces bolus through entrance to esophagus

Bolus is pushed towards stomach through process of peristalsis

Approach of bolus triggers opening of lower esophageal spicter

Trip takes around 9 seconds fo rnormal bolus

Dry or poorly lubricated bolus travels more slowly

Secondary peristaltic waves may be required to push bolus all the way to stomach

22.9

Peritonial Cavity: Serous membrane that lines the abdominopelvic cavity; encloses digestive tract and accessory organs

Potential space between parietal and visceral layers of peritoneum

Visceral Peritoneum: Covers organs

Parietal Peritoneum: Lines abdominal walls

Dorsal and Ventral Mesenteries: Suspend the digestive tract and accessory organs

Ventral Mesenteries disappear later along most of digestive tract except for

Lesser Omentum: Between stomach and liver

Stabilizes position of stomach

Provides an access route for blood vessels and other structures entering or leaving the liver

Greater Omentum: Between liver and anterior abdominal wall (AKA falciform ligament)

The dorsal mesentery of stomach

Forms enormous pouch that extends inferiorly between body wall and anterior surface of small intestine

Adipose tissue conforms to shapes of surrounding organs

Provide padding and protection across the anterior and lateral surfaces of abdomen

Mesentry Proper: Dorsal Mesentery of small intestine

Mesocolon: Dorsal mesentery of large intestine

Serous Membrane

Produces peritoneal fluid

Provides essential lubrication

7 liters are secreted and absorbed each day

Ascites: Accumulation of peritoneal fluid creates abdomindal swelling

22.10

Stomach is a muscular, expandable, J-shaped organ with three layers in muscular layer

Chyme: Material in stomach that consists of food combined with saliva & secretions of gastric glands

Viscous

Highly acidic

Soupy mixture

Regions of stomach

Fundus: Portion superior to junction between stomach and esophagus

Contacts inferior, posterior surface of diaphragm

Cardia: Superior medial portion of stomach within 3 cm of junction between stomach and esophagus

Contains abundant mucous glands whose secretions coat junction with esophagus

Help protect tube from acid and enzymes in stomach

Body: Area of stomach between fundus and curve of J

Largest region of stomach

Functions as mixing bowl for ingested food and secretions produced in stomach

Pyloric Part: Forms sharp curve of J

Changes shape as mixing movements occur during digestion

Pyloric Antrum: Region connected to the body of the stomach

Pyloric Canal: Empties into duodenum (proximal segment of small intestine)

Pyloric Orifice: Stomach outlet

Opens into duodenum

Pyloric Sphincter: Regulates release of chyme into duodenum

Rugae: Prominent but temporary mucosal folds

Allow gastric lumen to expand

As stomach fills, rugae flatten out until, at maximum distension, they almost disappear

Muscular Layer: Includes inner oblique later of smooth muscle

Strengthens stomach wall

Assists in mixing and churning to form chyme

22.11

Four major functions of Stomach

Temporary storage of ingested food

Mechanical digestion of ingested food

Chemical digestion of food through action of enzymes and acid

Production of intrinsic factor

Layers of Stomach Wall

Mucosa

Composed of simple columnar epithelium»

Produces layer of alkaline mucus

Protects epithelial cells against acid and enzymes in gastric lumen

Life span of a gastric

epithelial cell is only 3–7 days

Submucosa

Muscular Layer

Serosa

Gastric glands: Secrete most of acid and enzymes involved in gastric digestion

Dominated by parietal cells and chief cells

Gastric glands in pyloric part secrete mucus and hormones involved in coordination and control of digestive activity

Gastric Pits: Shallow depression that open onto gastric surface

Stem cells at base or pit divide to replace superficial cells that shed into chyme

Each pit communicates with several gastric glands that extend deep into lamina propria

Cells of Gastric Glands

Parietal Cells: Secrete intrinsic factor (glycoprotein that helps absorb vitamin B12 across intestinal lining)

Necessary for erythropoeisis

Also secrete hydrochloric acid

Keeps stomach contents at ph 1.5-2.0

Alkaline Tide: Influx of bicarbonate ions

Hydrogen ions are generated as carbonic anhydrase converts CO2 and H2O to carbonic acid. Dissociation occurs

An anion counter-transport mechanism ejects bicarbonate ions into interstitial fluid and imports chloride ions into cell

Chloride ions diffuse across cell and exit through open chloride channels into lumen of gastric gland

Hydrogen ions are transported into lumen of gastric gland

G Cells: Enteroendocrine cells that produce a variety of hormones

Chief cells: Secretes pepsinogen (inactive proenzyeme

HCL converts pepsinogen to pepsin (active proteolytic enzyme)

22.12

Circular Folds: Series of transverse folds in intestinal lining

Permanent features that do not disappear when small intestine fills

Intestinal Villi: Finger-like projections in mucosa of small intestine

Lacteal: Transport materials that cannot enter blood capillaries