THE DIGESTIVE SYSTEM
PART II
BY
MEDICAL PHYSIOLOGY
DEPARTMENT – FACULTY OF
MEDICINE – MINIA UNIVERSITY
DEFINITIONS
❑ Steatorrhea: means passing of greasy bulky pale
stools due to deficient bile (i.e. required for fat
digestion).
❑ Choleretic: it is a substance that increases the bile
secretion from the liver (e.g. bile salts, vagal
stimulation and secretin hormone).
❑ Cholagugue: it is a substance that helps the
evacuation of the gall bladder (e.g. Cholecystokinin
hormone (CKK).
JAUNDICE
It is a yellowish discoloration of the skin, sclera and mucous
membranes due to increased serum bilirubin level ≥ 2 mg/dl.
DEFINITION
JAUNDICE
TYPES AND CAUSES OF JAUNDICE
Points Haemolytic Jaundice Obstructive Jaundice Hepatocellular Jaundice
Cause
• Incompatible blood
transfusion.
• All causes of
haemolytic anaemia
(e.g. hereditary
spherocytosis, sickle cell
anaemia, poisons…
• Stones in the common
bile duct (CBD).
• Cancer head of
pancreas.
• Liver diseases (e.g.
hepatitis, cancer, liver
cirrhosis, bilharzias…
Blood
Anaemia
Bilirubin
Present
Haemobilirubin
Absent
Cholebilirubin
Absent
both
Urine Colour Normal Dark brown brown
Stool Colour Darker than normal Very pale pale
Gastrointestinal Motility
I. MASTICATION (CHEWING)
DEFINITION:
The chewing movements break the food into small pieces and
stimulate the secretion of saliva to make a suitable food bolus
ready for swallowing.
The act of chewing is partly voluntary and partly reflex in
nature.
Functions of Mastication:
1. Break down of food into small particles to increase the
surface area of food exposed to the action of digestive
enzymes.
2. It stimulates salivary, gastric pancreatic and bile secretions.
3. Reduce mechanical damage of GIT.
4. Development of satiety through stimulation of smell and
taste receptors.
I. MASTICATION (CHEWING)
Definition: It is the passage of food from mouth to stomach.
Stages (Phases) of Deglutition:
1. Buccal Stage:
In which food passes from mouth to pharynx. It is the voluntary
phase.
2. Pharyngeal Stage:
In which food passes from the pharynx to the esophagus. It is
involuntary.
This phase is sometimes called the Deglutition Reflex:
• Stimulus: touching the pharynx (swallowing receptors) by
food.
• Afferent: trigeminal nerve.
II. DEGLUTITION (SWALLOWING)
• Center: swallowing center in medulla oblongata.
• Efferent: glossopharyngeal and vagus nerve.
• Responses: as follows;
a. Protective Reflexes:
➢ Elevation of soft palate to close posterior nasal opening to
prevent regurgitation of food to nose.
➢ Elevation of the larynx to be covered by epiglottis to
prevent entry of food to the trachea.
➢Approximation of the vocal cords.
➢ Temporary apnea as swallowing center inhibits respiratory
center few seconds.
b. Pharyngeal Peristalsis:
Contraction of pharyngeal muscles and relaxation of
upper part of esophagus (upper esophageal sphincter)
to push the food bolus to the esophagus.
N.B.
In anesthesia, cough and swallowing center are
depressed and the protective reflexes don't occur so
secretion or vomitus may accumulate in pharynx and
enter the trachea → choking (i.e. inability to
breathe).
3. Esophageal stage:
• In which food passes from esophagus to stomach.
• It is involuntary.
Stages of Deglutition (i.e. Swallowing).
III. GASTRIC MOTILITY
Three types;
1. Receptive Relaxation:
• A reflex relaxation of stomach wall initiated by the movements of the
pharynx and esophagus and increases when food enters the stomach
especially in the proximal unit of the stomach (i.e. fundus and body).
• Function; it allows the stomach to accommodate up to 1.5 liters of
fluids without marked increase in intragastric pressure (i.e. plasticity).
2. Gastric Peristalsis:
• Peristaltic waves occur in the distal motor unit of the stomach (i.e.
antrum and pylorus) shortly after food intake.
• Function; it helps in mixing the food with gastric juice and pushing it to
the duodenum.
3. Hunger Contractions:
• Strong peristaltic contractions that occur in the body of the
stomach when it is empty for long time (e.g. prolonged
fasting).
Mechanism:
• Prolonged fasting ….. Causes hypoglycemia ….. Activates
the feeding center which in turn ….. Activates vagal nucleus
in medulla oblongata ….. Hunger contractions.
• That is why insulin stimulates hunger contractions while
glucose inhibits it
VOMITING
• Definition: It is a reflex evacuation of upper GIT contents
(mainly stomach) through the mouth.
• It is preceded by salivation, sweating, rapid heart rate and
sensation of nausea.
Causes of Vomiting:
1.Reflex vomiting; as a result of smell, taste, vision, severe
pain, motion sickness …
2.Central vomiting; as a result of hypoxia, head trauma,
drugs, ketosis …
Effects of Persistent Vomiting:
1.Dehydration (i.e. water loss).
2.Alkalosis (due to loss of gastric HCl).
3.Potassium depletion.
4.Tetany.
Treatment of Vomiting:
1.Treat the cause.
2.Anti-emetic drugs.
3.Correct the complications by giving I.V. fluids, electrolytes
(K+) and acidifying salts.
IV. INTESTINAL MOTILITY
Two types;
Points Peristaltic movementSegmenting
movement
Definition
A wave of deep
circular contraction
followed by a wave
of relaxation.
Ring like contractions;
appear at regular
interval along the
intestine.
Mechanism Neurogenic. Myogenic.
Function Abolished by cocaine.Not abolished by
cocaine.
Effect of local
anaesthetic
(e.g. cocaine)
Push intestinal contents.Help digestion and
absorption.
DEFECATION
Definition:
It is evacuation of the colon through the anus.
Mechanism:
• It is pure automatic reflex in animals and infants of
human.
• It is under voluntary control in adult human.
Defecation Reflex:
• Stimulus: distension of rectum by stools.
• Receptors: stretch receptors in rectal wall.
• Afferent: pelvic nerve.
• Center: defecation center in spinal cord (S 2, 3 and 4).
• Efferent: pelvic nerve.
• Response; either:
➢ If conditions are favorable → contraction of the rectal wall
and relaxation of internal anal sphincter → Defection.
➢ If the conditions are not favorable → the cerebral cortex
sends inhibitory impulses to the defecation center in the
spinal cord → Temporary inhibition of defecation.
DISORDERS OF DEFECATION
Definition: In normal persons, the frequency of defecation
ranges from several times per day to once every 2-3
days.
Constipation is an abnormal condition characterized by
prolonged interval between defecation associated with
difficulty in performing the act.
Causes of Constipation:
1. Repeated voluntary inhibition of the act. This leads to
overstay of stool in the colon, resulting in more water
reabsorption from it.
1. CONSTIPATION
2. Decreased motility of the colon as occurs in
prolonged ingestion of food with little residue, thus
the bulk of stool becomes ineffective to initiate
defecation reflex, e.g. old age, lack of muscular
exercise, debilitating disease and emotional stress
(i.e. spastic colon).
Effects of Constipation:
1. Abdominal discomfort and distension.
2. Nausea and anorexia.
3. Headache.
2. DIARRHEA
• DEFINITION: Abnormal condition characterized by
increased frequency of defecation with excretion of
watery stool.
Causes of Diarrhea:
• Hypermotility of the small intestine due to Intestinal
infection either viral or bacterial
Effects of Diarrhea:
Diarrhea is much more dangerous than constipation
especially in infants, because it may lead to loss of water
(dehydration), electrolytes and bicarbonate (i.e. metabolic
acidosis).
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