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BIOL 204
~
Topic
Digestion
Ferrara
8
Objective 1
Functions:
• ingest food
• alter food
• absorption
• eliminate waste
Functions of the Digestive System
Objective 2
The Structural Plan
The digestive system consists of:
•a tube, alimentary canal or gastrointestinal (GI) tract
•accessory structures which secrete products into the GI tract
Note:
Salivary Glands:
secrete saliva into the
oral cavity
Pancreas:
secretes pancreatic juice
into the duodenum
Liver, Gall Bladder:
secrete bile into the
duodenum
Objective 3
Terms For Physiology
Ingestion:
taking food into the body
Propulsion:
moving food within the digestive tract; most of this
movement is accomplished by deglutition (swallowing) and
peristalsis
Peristalsis
Mechanical Digestion:
breaking down ingested material
using grinding or crushing forces,
such as mastication (chewing)
and segmentation
Segmentation
Chemical Digestion:
breaking down ingested material by
hydrolysis
specific enzymes are required
Sucrase
Sucrase + H2O
Glucose + Fructose
Lipase
Lipid + H2O
Glycerol + Fatty Acids
Absorption:
the transport of digested end products across
the wall of the GI tract into the blood or lymph
Defecation
elimination of solid, unabsorbed waste products
from the body
Study suggestion
For each portion of the
GI tract list if there is
any:
• mechanical digestion
• chemical digestion
(what is digested,
what enzymes)
• absorption
Something like this…..
Objective 4
How Digestive Processes Are Regulated
Various control mechanisms are used to regulate the type of chemical
and mechanical activities that occur in the GI tract and the rate at
which the processes proceed. The control mechanisms are sensitive
to the volume and composition of lumenal contents.
What is regulated:
motility and secretions
Receptors
found in the wall of the GI tract respond to
stimuli
Mechanoreceptors:
monitor the stretch of the wall of the GI
tract
Chemoreceptors:
monitor the chemistry of the components;
solute concentration (osmolarity), pH, presence
of complex nutrients and end products of
digestion
Reflexes:
may lead to stimulation or inhibition of the GI tract
and its associated glands
Short Reflexes:
No CNS involvement;
Controlled by intrinsic nerve
plexuses (enteric nervous
system) and hormones produced
by enteroendocrine cells
Long Reflexes: involves CNS
Objective 5
Falciform Ligament:
The Peritoneum
binds the liver to the anterior abdominal
wall and the diaphragm
Falciform
Ligament
Greater Omentum: attaches to the greater curvature of the
stomach
Lesser Omentum: lies between the lesser curvature of the stomach
and the liver
Greater Omentum
Lesser Omentum
Mesentery: attaches the small intestine to the posterior abdominal wall
Mesocolon: attaches the transverse colon to the posterior abdominal wall
Mesentery
Other terms:
Retroperitoneal structures: lie posterior to the peritoneum; includes
kidneys, pancreas, portions of the large intestine
Intraperitoneal structures: lie within the peritoneum
Infraperitonal structures: lie below the peritoneum; includes urinary
bladder and uterus
Retroperitoneal
Infraperitoneal
Objective 6
GI Tract Tunics
Tunica Mucosa
-innermost layer
Sublayers:
Epithelium
Lamina propria
Muscularis mucosae
Functions:
-lines the lumen
-secretes mucus,
enzymes
-lymph nodes protect
-muscle contracts and
creates folds that
increase surface area
in some places
Tunica Submucosa
external to the mucosa
Contains abundant blood vessels, collagenous and elastic fibers, lymph
nodes and nerves (submucosal plexus) and in some places, glandular
cells
Functions to nourish the wall of the GI tract; the submucosal plexus
regulates the muscular and glandular activity of the mucosa
Submucosal plexus (at the arrow) regulates the
muscular and glandular activity of the mucosa
Tunica Muscularis
external to the mucosa
Consists of smooth muscle, usually arranged in two layers (outer,
longitudinal; inner circular)
The myenteric plexus is a group of nerve fibers that is located between
the circular and the longitudinal layers
Functions include segmentation and peristalsis; the circular muscle
thickens in some places, forming valves called sphincters
The myenteric plexus controls the motility of the GI tract
Myenteric Plexus – between the circular and
the longitudinal smooth muscle layers
motility of the GI tract
Tunica Serosa:
outermost layer
Below the diaphragm is equivalent to the visceral peritoneum
(mesothelium covering loose areolar tissue)
Above the diaphragm is called the tunica adventita and is made of
dense, irregular connective tissue
Function is protection
Objective 7
The Oral (Buccal) Cavity
Boundaries of the Oral Cavity:
Anterior:
lips (labia)
Lateral:
cheeks
Inferior:
tongue
Superior:
hard palate
anteriorly and
soft palate
posteriorly
Posterior:
fauces (opening
into the
oropharynx)
Subdivisions of the Mouth:
Vestibule:
Oral Cavity :
Proper
area between the labia and the anterior surfaces of the
teeth
area between the posterior surfaces of the teeth and the
fauces
Structures Associated With
The Soft Palate:
Uvula:
fingerlike projections
extending down from
the free edge of the
palate
Palatoglossal Arches:
lateral arches that
anchor the soft palate to
the tongue
Palatopharyngeal Arches:
lateral arches that anchor
the soft palate to the wall
of the oropharynx;
represent the boundary
between the oral cavity
and the oropharynx;
contain the palatine
tonsils
Objective 8
Location: floor of the oral cavity
Functions:
• contains taste buds
• contains mucus and serous
glands
• during mastication, grips
food, mixes it with saliva
and compacts the food
into a bolus
• used to articulate speech
The Tongue
Structure:
interlacing bundles of skeletal muscle fibers
covered by a mucus membrane
it is anchored to the floor of the mouth by lingual
frenulum
Intrinsic Muscles of the Tongue:
Originate and insert in the tongue
Functions are to change the shape of the tongue
(speech, deglutition)
Extrinsic Muscles of the Tongue:
Originate outside of the tongue on the mandible, hyoid
bone, or temporal bone and insert on the tongue
The function to move the tongue (protraction, retraction,
elevation, depression, lateral movement)
Papillae are elevations on the tongues dorsal surface:
Filiform papillae: most numerous; are shaped like cones; scattered all
over the surface of the tongue; used to provide friction
Fungiform papillae are shaped like mushrooms and have a vascular core;
they are scattered all over contain taste buds
Circumvallate (vallate) papillae are the largest and most distinct; there
are 10-12 of them located in a “V” formation at the back of the tongue;
contain taste buds
Filiform papillae
Circumvallate papillae
Fungiform papillae
Taste Buds
Objective 9
Salivary and Buccal Glands
Saliva: secretion produced by extrinsic salivary glands and intrinsic salivary
glands (buccal glands)
Functions:
1. Cleanse the mouth
2. Solubilize chemicals in the food so that they can activate taste buds
3. Moistens food; makes it sticky so that it can form a bolus
4. Contains amylase an enzyme that digests carbohydrates
Intrinsic Salivary Glands are located within the oral mucosa and
distributed all over the mouth and the pharynx; continuously secrete
a small amount of watery saliva
Extrinsic Salivary Glands:
Parotid Glands, Submandibular Gland, Sublingual Gland,
Salivary Gland Tissue
Objective 10
Saliva
1.
Composition
•
97- 99.5% water
•
Solutes: electrolytes (Na+, K+, Cl-, PO42-, HCO3-)
amylase
mucin
lysozyme
IgA
wastes (urea, uric acid)
serum albumen
•
pH ranges from 6.75 – 7.00
•
Average daily volume 1000-1500 ml
2. Control of Salivation
Think about food
Note: parasympathetic increases
secretion; sympathetic decreases
Smell food
Activate chemoreceptors
or mechanoreceptors
in the mouth
See Food
Presence of Irritants
in the stomach and/or
small intestine
Salivatory Nuclei
(Lower Pons, Upper Medulla)
PNS Efferents in Facial (VII)
and
Glossopharyngeal (IX) nerves
Increased Saliva Output
Objective 11&12
Tooth Structure
crown
root
neck
enamel
dentin
cementum
periodontal ligament
root canal
apical foramen
Central incisors
Lateral incisors
Canines (Cuspids)
Premolars (Bicuspids)
Molars
Create a dental formula for adult dentition:
2I, 1C, 2PM, 3 M (upper jaw)
X2
2I, 1C, 2PM, 3 M (upper jaw)
Objective 13
The Pharynx
Location:
posterior to the oral cavity
Structure:
mucosa: stratified squamous epithelium
muscularis: inner longitudinal and outer circular
muscle; skeletal muscle
Objective 14
The Esophagus
Location:
in the mediastinum posterior to the trachea extends
from the pharynx to the stomach
Structure:
mucosa is stratified squamous epithelium
muscularis: upper third, skeletal muscle; mixture
of skeletal and smooth muscle and lower third,
smooth muscle
Functions:
move food to the stomach via peristalsis
Objective 15
A.
Digestive Processes Mouth/Pharynx/Esophagus
Chemical Digestion ; Salivary amylase begins the digestion of carbohydrate:
Salivary Amylase
Starch, glycogen
Dextrin
Mastication (Chewing)
• lips and cheeks help keep food between the teeth
• tongue mixes food with the saliva
• teeth grind and crush food into smaller pieces (helps prevent
damage to the GI tract and increases the surface area for
digestion)
Deglutition (Swallowing) – 3 phases
•
Buccal
•
Pharyngeal
•
Esophageal
1. Oral (Buccal) Phase: Voluntary phase

Tip of the tongue is raised against the hard palate

Tongue contracts to put pressure on the bolus

Bolus is forced into the pharynx

Coordinated by the trigeminal nerve (V)
2. Pharyngeal/Esophageal Phase

involuntary; coordinated by the swallowing centers in the upper
medulla and pons and activated by the vagus nerve (X)
Pharyngeal Phase
Esophageal Phase
bolus moves into the esophagus
bolus moves into the esophagus
the soft palate elevates, and larynx
rises (close off openings)
peristalsis: longitudinal/circular
muscle contraction
relaxation of the upper esophageal
sphincter
relaxation of the lower esophageal
sphincter and cardiac sphincter
pharyngeal muscles contract on bolus
How is the mouth blocked?
tongue
How is the opening to the nasopharynx blocked?
Soft palate
How is the glottis blocked?
epiglottis
Objective 16
Location:
The Stomach
left abdominopelvic cavity, largely in the left
hypochondriac region and epigastric region
Gross Structure:
volume ranges from 50 ml to 4L
it is about 25 cm long
Stomach Regions:
Cardiac, fundus, body, pyloris
Stomach Curvatures:
Greater (lateral), lesser
(medial)
Stomach Sphincters:
Cardiac (gastroesophageal),
pyloric
Rugae: macroscopic folds in an empty stomach
Objective 17
Stomach Histology
The stomach has four tunics:
A. Tunica Mucosa:
Gastric pits: invaginations which contain specialized secretory
cells (gastric glands)
Mucous Neck Cells: secrete a thin layer of protective mucus
Parietal Cells: secrete HCL (activates pepsinogen and helps prevent
bacterial infection) and intrinsic factor
Mechanism Used to Make HCl:
CO2
HCO3
Cl-
CO2 + H2O
carbonic anhydrase
H2CO3
HCO3-
H+
H+
K+
K+
Cl-
Chief Cells: secrete pepsinogen, an inactive precursor of pepsin;
pepsin initiates protein digestion:
HCl
Pepsinogen
Pepsin
Pepsin
Protein
Peptides
Enteroendocrine Cells: secrete gastrin, serotonin, histamine and somatostatin
Tunica Submucosa: typical
Tunica Muscularis: three layered – outer longitudinal,
middle circular, inner oblique
Tunica Serosa:visceral peritoneum
Objective 18
Secretory Activity of the Stomach
Gastric Juice:
•average daily volume = 2 to 3 liters
•includes water, HCl, mucus, intrinsic factor and enzymes
renin:
converts milk protein (casein) into a solid curd in infants
lipase:
digests lipid; not very active in adults
pepsin: initiates the digestion of proteins (note protein digestion
begins in stomach)
What is the role of HCl?
Regulation of Gastric Secretion:
Gastric secretion is controlled by hormones and neural pathways
using both short and long reflexes
Stimulus is either volume or composition of chyme
Response is either motility (peristalsis and segmentation) or
secretion of enzymes or juices
Autonomic innervation of the stomach is both sympathetic and
parasympathetic
Phases of gastric secretion:
1.
Cephalic Phase: occurs before food reaches the stomach
optic nerve (?)
olfactory receptors
taste buds
cerebral cortex
see food
smell food
taste food
think about food
hypothalamus
vagal nuclei of the medulla
vagus nerve (X)
increased secretion by gastric glands
Does this ring a bell?
2.
The Gastric Phase:
occurs after the bolus reaches the stomach
3.
The intestinal Phase:
occurs when chyme reaches the small
intestine
Objective 19
Stomach
Mechanical Activity of the
-
the three layered tunica muscularis causes directional movement
(peristalsis) and mixes chyme with gastric juice
1.
Stomach Filling:
-
as food is moved toward the stomach during deglutition, the
stomach smooth muscle relaxes (receptive relaxation); this is
triggered by the swallowing center of the medulla and is
accomplished by vagal efferents
swallowing center
vagus
(X)
stomach relaxes
-
when the bolus reaches the stomach, the stomach wall stretches; the
pressure of the food on the stomach wall activates a local reflex in
which the neurons use the neurotransmitter NO; this causes
relaxation of the stomach smooth muscle (receptive relaxation)
NO causes relaxation
bolus stretches the stomach
2.
Contractile Activity - Mixing
-
peristalsis is weak in the fundus and the cardiac region and stronger in
the pyloris
-
peristaltic waves are maintained by pacemaker cells in the longitudinal
smooth muscle layer of the greater curvature
-
each wave of peristalsis closes the pyloric sphincter
-
Contractile Activity - Emptying
-
peristaltic waves get stronger with time; when the pressure in the
stomach is very high, it can exceed the ability of the pyloric sphincter
to remain closed and chyme enters the duodenum
4.
Control of Gastric Emptying
-
the duodenum and the stomach work together to control stomach
emptying
-
liquids pass faster than solids and carbohydrates pass faster than
proteins which pass faster than lipids
-
gastric factors:
-
-
stretching of the stomach wall activates stretch receptors
which induce local and vagovagal reflexes; these reflexes
increase gastric activity
-
low acidity and the presence of peptides initiates the release
of gastrin by G cells; gastrin increases stomach motility
intestinal factors:
-
both neural mechanisms (the enterogastric reflex) and
hormonal factors (enterogastrones) are used by the small
intestine to control the rate of gastric emptying
What are the hormones and what do they do, i.e. stimulate or inhibit
gastric emptying?