The Digestive System

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The Digestive System
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Functions of the Digestive System

• Process food
• Extracts nutrients from the food
• Eliminates the waste

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Four Stages of Digestion

• Ingestionselective food intake
• DigestionMechanical Chemical breakdown of food
  into usable form
• AbsorptionUptake of nutrient molecules into the
  blood or lymph
• DefecationElimination of undigested wastes

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Two Subdivisions

• Digestive Tract
• Accessory Organs

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Digestive Tract

• Aka alimentary canal
• About 30 feet long (9 m)
• Oral cavity
• Pharynx
• Esophagus
• Stomach
• Small intestine
• Large intestine

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Accessory Organs

• Teeth
• Tongue
• Salivary glands
• Liver
• Gall bladder
• Pancreas

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Mouth

• Also known as oral or buccal cavity
• Ingestion of food
• Taste other sensory receptors for food
• Mastication (chewing)
• Deglutition (swallowing)
• Speech
• Respiration

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Tongue

• Manipulates food between the teeth
• Extract food particles from between teeth
• Contains the taste buds

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Teeth

• Collectively called dentition
• Adults have 16 teeth in the mandible 16 teeth in
  the maxilla
• Incisorschisel-like cutting teeth
• Caninespuncture shred
• Premolars Molarsbroad surfaces for crushing
  chewing
• Responsible for mastication or chewingbreaking
  food into smaller pieces so more surface area is
  exposed for enzymatic activity

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Saliva

• Moistens the Mouth
• Digests a little starch fat
• Cleanses the teeth
• Inhibits bacterial growth
• Dissolves molecules for the taste buds
• Moistens food and binds particles together to aid
  in swallowing

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Saliva contains

• 97.0 to 99.5 Water
• Salivary Amylaseenzyme that begins starch
  digestion
• Lingual Lipaseenzyme activated by stomach acid
  digests food after it is swallowed
• Mucusbinds to lubricates the food
• Lysozymeenzyme that kills bacteria
• IgAAntibody that inhibits bacterial growth
• Electrolytessodium, potassium, chloride,
  phosphate bicarbonate ions

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Salivary Glands

• Three pairs
• Parotid glandbeneath skin anterior to earlobe
  swells during mumps
• Submandibular glandhalfway along body of madible
• Sublinguial glandlocated in the floor of the
  mouth
• Secrete about 1 to 1.5 L of saliva a day

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Esophagus

• Tube leading from pharynx to stomach
• 20 to 30 cm long behind the trachea
• Sphincter muscle normally prevents
  gastroesophogeal reflux (backflow of food from
  stomach into esophagus) which causes heartburn

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Swallowing

• Involves 22 muscles in the mouth, pharynx, and
  esophagus
• Controlled by the swallowing center in the
  medulla oblongata pons via several of the
  cranial nerves

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Swallowing

• Buccal phasetongue presses against food to form
  bolus push it into the oropharynx
• The tongue keeps food from re-entering the mouth
  the soft palate prevents food from entering the
  nasal cavity and the epiglottis prevents food
  from entering the larynx

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Swallowing

• Pharyngeal-esophageal phaseThe pharyngeal
  muscles drive the bolus downward, finally into
  the esophagus
• The esophagus moves food by peristalsiswaves of
  muscular contractions that pushes the bolus ahead
• Liquid reaches the stomach in 1 to 2 seconds
• Food reaches the stomach in 4 to 8 seconds