CHAPTER 23 - THE DIGESTIVE SYSTEM
â Digestive system is responsible for taking in food, break it down, absorb nutrients into
blood and eliminate indigestible wastes
â 2 main organ groups of the digestive system
o Alimentary canal ï GI tract
ï§ Muscular tube that extends from mouth to anus ï (Food in alimentary
canal considered to be outside the body because the canal is open to the
external environment at both ends)
â¢ Mouth, pharynx, esophagus, stomach, small intestine, large
o Accessory digestive organs ï lie external to the GI tract and connected by ducts
ï§ Secrete saliva, bile and digestive enzymes
ï§ Teeth, tongue, salivary glands, liver, gallbladder and pancreas
The digestive process
â Ingestion ï food enters digestive process via the mouth
â Propulsion ï the movement of food through peristalsis
â Mechanical digestion ï breakdown of food into smaller pieces
o Chewing, food churning in stomach, segmentation in small intestine
â Chemical digestion ï breakdown of food molecules (e.g., carbs) into building blocks
(e.g., simple sugars)
o Occurs in mouth, stomach, small intestine
â Absorptionï nutrients transported from alimentary canal into blood and lymphatic
â Defecation ï elimination of indigestible substances as feces
â Scheme 1: 4 lines divide abdominal wall into 9 regions (tic-tac-toe grid)
o Midclavicular lines ï vertical lines of grid
o Subcostal plane ï superior horizontal line
ï§ Connects inferior points of costal margin
o Transtubercular plane ï inferior horizontal line
ï§ Connects tubercles of iliac crests
o Superior 3 regions ï right hypochondriac, epigastric, left hypochondriac
o Middlde 3 regions ï right lumbar, umbilical, left lumbar
o Inferior 3 regions ï right iliac (inguinal), hypogastric (pubic), left iliac (inguinal)
â Scheme 2: A simple method of sectioning the anterior abdominal wall into 4 regions
o Right and left upper quadrants
o Right and left lower quadrants
Peritoneal cavity and peritoneum
â Peritoneum ï most extensive serous membrane
o Visceral peritoneum ï surrounds digestive organs/covers the external surfaces
o Parietal peritoneum ï lines the body wall
â Peritoneal cavity ï a slit-like potential space between the visceral and parietal
peritoneum. o Contains lubricating serous fluid to allow the organs to glide easily along one
â Double layer of peritoneum that extends to the digestive organs from the body wall.
o Holds organs in place
o Are sites of fat storage
o Provides a route for circulatory vessel and nerves
â Most mesenteries are dorsal, extending from the alimentary canal to the posterior
o Greater omentum ï connects the greater curvature of the stomach to the
posterior abdominal wall.
ï§ Contains a great deal of fat
â Ventral mesentery in the superior abdomen extends from the stomach and liver to the
anterior abdominal wall (mesenteries can be called ligaments)
o Falciform ligament ï binds anterior aspect of liver to anterior abdominal wall and
o Lesser omentum ï runs from liver to the lesser curvature of the stomach
ANATOMY OF THE ALIMENTARY CANAL
â Same 4 layers from esophagus to anus
o Mucosa/mucous membrane ï innermost layer
ï§ Contains 3 sub-layers: epithelium, lamina propia, muscularis mucosae
o Submucosa ï external to the mucosa; layer of CT
ï§ Contains blood and lymphatic vessels, nerve fibers
o Muscularis externa ï external to the submucosa
ï§ 2 layers of smooth muscle:
â¢ Circular muscularis ï inner layer whose fibers orient the
circumference of the canal (squeezes the gut)
â¢ Longitudinal muscularis ï outer layer whose fibers orient along the
length of the canal (shortens the gut)
o Serosa ï outermost layer
ï§ Is the visceral peritoneum
ï§ Parts of the alimentary canal that are not associated with the peritoneal
cavity lack a serosa and have an adventitia (ordinary fibrous CT) as their
outer layer e.g., esophagus.
â Primarily found in the walls of visceral organs e.g., urinary bladder, uterus, intestines.
â 6 major locations: the iris of the eye, and in the walls of the circulatory vessels,
respiratory tubes, digestive tubes, urinary organs, and reproductive organs.
â Fibers elongated and one central nucleus
â Grouped into sheets (right angles to each other)
o Longitudinal layer ï parallel to long axis of organ; externally located
o Circular layer ï deeper layer, fibers run around circumference of organ; constricts
a hollow organ. Mouth and associated organs
â Mucosal lined cavity
â Anterior opening is the oral orifice.
â Divided into 2:
o Vestibule ï slit between the teeth and the cheeks (or lips)
o Oral cavity proper ï region of mouth that lies internal to the teeth.
Lips and cheeks
â Composed of a core of skeletal muscles:
o Lips ï orbicularis oris
o Cheeks ï buccinators muscles
â Labial frenulum ï median fold that connects the internal aspect of each lip to the gum
â Forms roof of the mouth
â Has 2 distinct parts:
o Hard palate ï anteriorly; forms a rigid surface against which the tongue forces
food during chewing.
o Soft palate ï posteriorly; rises to close off the nasopharynx when swallowing
â Interlacing fascicles of skeletal muscle
â Houses most of the taste buds
â Mix the food with saliva and form it into a compact mass called a bolus.
o Intrinsic muscles ï within the tongue and not attached to bone; change shape of
tongue e.g., rolling, but they do not change its position.
o Extrinsic muscles ï external to the tongue; extend to the tongue from bones of
the skull and the hyoid bone; alter the position of the tongue e.g., protrude it,
retract it, and move it laterally.
o Lingual frenulum ï fold of mucosa on the undersurface of the tongue.
ï§ Secures tongue to floor of mouth and limits its posterior movements.
â Found in sockets called alveoli in the gums of the maxilla and mandible
â Humans have 2 sets of teeth/dentitions
o By 21 years old, primary dentition (deciduous teeth) have been replaced by the
â Teeth are classified according to their shape and function:
o Incisors ï nipping off pieces of food
o Canines ï tear and pierce
o Premolars (bicuspids) and molars ï have broad crowns with rounded cusps for
â Tooth structure
o 2 main regions:
ï§ Crown ï exposed region covered by a layer of enamel (hardest substance
in the body)
â¢ Dentine underlies enamel cap and forms bulk of the tooth. â¢ Pulp cavity in the center of the tooth is filled with dental pulp, a
loose CT containing the toothâs vessels and nerves.
ï§ Root ï in the socket
â¢ The part of the pulp cavity in the root is the root canal.
â¢ External surface of the tooth root is covered by a calcified CT
o Attaches the tooth to the periodontal ligament or
ï Anchors tooth in bony socket of the jaw.
o Continuous with the gum/gingiva.
ï§ These two regions meet at the neck near the gum line.
â Produce saliva
â All salivary glands are compound tubuloalveolar glands.
â Small intrinsic salivary glands ï within tongue, palate, lips, cheeks
o Saliva from these glands keeps the mouth moist at all times.
â Large extrinsic salivary glands ï lie external to the mouth but connect to it through their
o Secrete saliva only during eating or anticipation of a meal causing the mouth to
o Parotid glands ï largest extrinsic gland; lies anterior to the ear
ï§ Parotid duct runs parallel to zygomatic arch
ï§ Surgery on this gland can lead to paralysis because the branches of the
facial nerve run through the parotid gland on their way to the muscles of
ï§ Contain only serous cells.
o Submandibular glands
ï§ Lies along medial surface of mandible
ï§ Contain both serous and mucous cells.
o Sublingual glands
ï§ Lies in floor of oral cavity, inferio