Digestive System

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Digestive System

• Chapter 16

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The Digestive system includes

• The muscular digestive tract
• Various accessory organs

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Fig. 16.28
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Fig. 16.1
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Functions of the digestive system

• Ingestion
• Mechanical processing
• Digestion
• Secretion
• Absorption
• Egestion

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Fig. 16.2
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The digestive system organs and the peritoneum

• Some parts are retroperitonial
• Mesenteries
• Sheets of serous membranes that support portions
  of the digestive tract
• Provides padding, protection, insulation, and
  energy reserves

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Fig. 16.3
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Fig. 16.5
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Fig. 16.6
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Teeth

• Function in mastication of bolus
• Periodontal ligaments hold teeth in alveoli (bony
  sockets)

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Fig. 16.7
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Salivary glands

• Serous mainly contain salivary amylase and
  dissolve food in liquid o taste
• Mucous lubrication
• lysozyme

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The oral or buccal cavity

• Its functions include
• Analysis of material before swallowing
• Mechanical processing by the teeth, tongue, and
  palatal surfaces
• Lubrication
• Limited digestion

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The tongue

• Main functions include
• Mechanical processing
• Assistance in chewing and swallowing
• Sensation by touch, temperature, and taste
  receptors

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Salivary glands (three pairs)

• Parotid (serous), sublingual(serous mucous),
  and submandibular (mucous)
• Saliva
• watery solution
• electrolytes, buffers, glycoproteins, antibodies,
  enzymes
• Functions include
• Lubricating, moistening, and dissolving
• Initiating digestion of complex carbohydrates
  salivary amylase

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The pharynx

• Common passageway for food, liquids, along with
  air
• Lined with stratified squamous epithelium
• Pharyngeal muscles assist in swallowing
• tonsils

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The esophagus

• Carries solids and liquids from the pharynx to
  the stomach
• Materials pass through the esophageal hiatus (in
  diaphragm) and cardiac sphincter (also known as
  lower esophageal sphincter)

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Functions of the stomach

• Bulk storage of undigested food
• Mechanical breakdown of food
• Disruption of chemical bonds via acid and enzyme
  action
• Production of intrinsic factor (required for B12
  absorption)

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Stomach anatomy

• Rugae
• Ridges and folds in relaxed stomach flatten as
  the stomach fills
• Muscles
• Circular
• Longitudinal
• Oblique
• Pyloric Sphincter
• Regulates release of the chyme from the stomach

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Histology of the stomach

• Mucus cells mucous production
• Gastric glands
• 1. Parietal cells
• Secrete intrinsic factor and HCl pH of 1.5-2.0
  needed to kill most ingested microorganisms, to
  break down connective tissues in meat, for
  activation and function of pepsin

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• 2. Chief cells
• Secrete pepsinogen which is converted to pepsin,
  a protein-digestion enzyme

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• Pyloric glands
• Produce a mucous secretion
• Enteroendocrine cells scattered among the
  mucous-secreting (epithelial) cells produce
• Gastrin stimulates the secretion of chief and
  parietal cells
• Somatostatin inhibits release of gastrin

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Digestion and absorption in the stomach

• Preliminary digestion of proteins
• Pepsin
• Permits digestion of carbohydrates
• Very little absorption of nutrients
• However, some drugs are absorbed

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Fig. 16.10a
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Fig. 16.10b
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Small intestine

• Important digestive and absorptive functions
• Secretions and buffers provided by pancreas,
  liver, gall bladder (accessory structures)
• Three subdivisions
• Duodenum
• Jejunum
• Ileum
• Ileocecal sphincter
• Transition between small intestine (ileum) and
  large intestine (cecum)

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Histology of the small intestine

• Plicae
• Permanent transverse folds of the intestinal
  lining
• Villi
• Fingerlike projections of the mucosa (inner
  lining) increase SA (brush borders)
• Lacteals
• Lymphatic capillary in villus transports
  materials unable to enter blood capillaries
  (e.g., fatty acids)

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• Intestinal glands (located at the bases of the
  villi)
• Lined by enteroendocrine, goblet and stem cells
• Enteroendocrine cells produce several intestinal
  hormones (e.g., secretin)

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Fig. 16.13
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Fig. 16.14
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Fig. 16.16a
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Fig. 16.16bc
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Functions of the large intestine

• Reabsorb water and compact material into feces
• Absorb vitamins (e.g., K) produced by bacteria
• Store fecal matter prior to defecation

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The four areas of the colon

• Ascending
• Transverse
• Descending
• Sigmoid

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Physiology of the large intestine

• Reabsorption in the large intestine includes
• Water
• Vitamins K, biotin, and B5
• Organic wastes urobilinogens and
  stercobilinogens
• Bile salts
• Toxins

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The rectum

• Last portion of the digestive tract
• Terminates at the anal canal
• Internal and external anal sphincters internal
  sphincter consists of smooth muscle while the
  external consists of skeletal.

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Fig. 16.21
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Movement of digestive materials

• Visceral smooth muscle shows rhythmic cycles of
  activity
• Pacemaker cells
• Peristalsis
• Waves that move a bolus
• Segmentation
• Churn and fragment a bolus

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Peristalsis
Figure 24.4
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Coordination secretion and absorption

• Neural and hormonal mechanisms coordinate glands
• GI activity stimulated by parasympathetic
  innervation
• Inhibited by sympathetic innervation
• Enterogastric, gastroenteric and gastroileal
  reflexes coordinate stomach and intestines

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Carbohydrate digestion and absorption

• Begins in the mouth
• Salivary and pancreatic enzymes
• Disaccharides and trisaccharides
• Brush border enzymes
• Monosaccharides
• Absorption of monosaccharides occurs across the
  intestinal epithelia

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Lipid digestion and absorption

• Lipid digestion utilizes lingual and pancreatic
  lipases
• Bile salts improve chemical digestion by
  emulsification
• Lipid-bile salt complexes called micelles are
  formed micelles diffuse into intestinal
  epithelia and lipids are released into the blood
  via the lymphatic system as chylomicrons

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Protein digestion and absorption

• Low pH destroys tertiary and quaternary structure
  in stomach
• Pancreatic peptidases in small intestine small
  peptides
• Brush border enzyme
• amino acids
• Absorption

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• Vitamins
• Water soluble vitamins are absorbed by diffusion
• Fat soluble vitamins are absorbed as part of
  micelles
• Vitamin B12 requires the intrinsic factor

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Absorption

• Water
• Nearly all that is ingested is reabsorbed via
  osmosis
• Ions
• Absorbed via diffusion, primary active transport
  and cotransport

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The Components of the Digestive System
Figure 24.1