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PHYSIOLOGY:
Digestion, Absorption &
Defecation
Reference
 Guyton A., and Hall, J. Textbook of Medical
Physiology. 12th ed.
 Seeley, R., Stephens, T., and Tate, P.,
Anatomy & Physiology. 8th ed. McGraw Hill
Company Inc., (2008)
Functions of the Digestive
System
• Motility Functions
• Propulsion
• Mechanical Digestion
• Secretory Functions
• Chemical Digestion
• Regulation
• Absorptive Functions
Six basic processes are involved:
ingestion, secretion, propulsion,
digestion, absorption and
defecation.
Ingestion is taking food into the
mouth.
Secretion is the act of expelling a
liquid. The cells lining the GI tract
secrete about 9 liters (9.5 quarts) of
water, acid, buffers, and enzymes
each day to lubricate the canal and
Propulsion consists of alternating
contraction and relaxation of
smooth muscle in the walls of the GI
tract to squeeze food downwards.
Digestion has two parts, mechanical
and chemical.
Mechanical digestion is chewing up
the food and your stomach and
smooth intestine churning the food
Chemical digestion is the work the
enzymes do when breaking large
carbohydrate, lipid, protein and
nucleic acid molecules down into
their subcomponents -these and
others are the nutrients.
Absorption occurs in the digestive
system when the nutrients move
from the gastrointestinal tract to
the blood or lymph.
Defecation the act or process by which
solid or semisolid waste material
(feces) from the digestive tract are
eliminated via the anus.
Digestion in the Mouth
 Mechanical Digestion
 Mastication reflex
 Food enters mouth
 Relaxation of muscles of
mastication
 Stretch reflex causes contraction
Digestion in the Mouth
• Chemical Digestion by Saliva
• Serous secretion for digestion
of starches: amylase
• Mucous secretion for
lubrication
Swallowing
• Oral
• Pharyngeal
• Esophageal
DEGLUTITION (SWALLOWING)
accomplishes the propulsion of
ingested food the mouth to the
stomach.
Three stages:
Buccal
Pharyngeal
Esophageal
BUCCAL STAGE
tongue moves upward and backward
to push the food toward the pharynx.
PHARYNGEAL STAGE
coordinated involuntary actions
which direct food to the esophagus
closing the airway passages
ESOPHAGEAL STAGE
peristaltic waves propel food from
the esophagus to the stomach
NO Digestion in the
Esophagus
• No actual digestion
• Propulsion via peristalsis
• Mucous secretion for lubrication
Digestion in the Stomach
• Food storage in the fundus
• LES prevent reflux of stomach
contents
• Mechanical Digestion via
segmental contractions
• No absorption
Digestion in the Stomach
• Chemical digestion by Oxyntic cell
secretion
CELL
SECRETION
FUNCTION
Mucous
Neck Cells
Peptic
Chief cells
Mucus
Lubrication & protection
Pepsinogen
+ HCl to produce a
proteolytic enzyme Pepsin
for protein digestion
Parietal
Cells
HCl
Intrinsic
Factor
B12 Absorption
Chemical Digestion by
Pancreas
• Acini cells secrete pancreatic
digestive enzymes into the
hepatopancreatic duct to empty
into the small intestine
• Enzymes are not activated until
they are mixed with acidic chyme
Chemical Digestion by the
Pancreas
Trypsin
Chymotrypsin
Split proteins into
peptides
Carboxypolypeptidase Split proteins into AA
Pancreatic Amylase
Carbohydrate
breakdown
Fat digestion
Pancreatic Lipase
Cholesterolesterase
Phospholipase
Biliary Secretions by the
Liver & Gall Bladder
• For fat digestion & absorption
• Emulsify large particles for more
efficient lipase action
• Ease of absorption
• Means of excretion of waste
• bilirubin
Biliary Secretions by the
Liver & Gall Bladder
• Liver constantly produces bile
and is concentrated in the gall
bladder
• Biliary secretions include HCO3to neutralize the acidic chyme
Digestion & Absorption in the
Small Intestine
• Dominating chemical
digestion via
• Pancreatic secretions
• Biliary secretions
• Intestinal secretions
Digestion & Absorption in the
Small Intestine
Mucus
Protection & Lubrication
Peptidase
Maltase
Split peptides into individual amino
acids
Split maltose  glucose + glucose
Lactase
Split lactose  glucose + galactose
Sucrase
Split sucrose  fructose + glucose
Lipase
Split fats into  glycerol + free fatty
acids
Digestion & Absorption in the
Small Intestine
• Absorption via transport processes
• Water: diffusion through tight
junctions
• Carbohydrates: Na-Glucose
cotransport; fructose via facilitated
diffusion
Digestion & Absorption in the
Small Intestine
• Absorption via transport
processes
• Lipids: diffusion with bile salts
• Proteins: Na-Glucose cotransport or
endocytosis
Digestion & Absorption in the
Small Intestine
• Absorption via transport
processes
• Na: diffusion down a electrical
gradient
• Cl: diffusion via solvent drag
• HCO3-: indirectly by secretion of H+
•
*Other ions & vitamins are also absorbed
Absorption in the Large
Intestine
• Absorption of electrolytes occur but
backflow is prevented by tight
junctions
• Water follows as result of osmotic
gradient
• Secretion of mucus for lubrication
Mouth

Esophagus

Stomach
-chewing & lubrication (saliva)
-conduit
-digestion takes place
-secretes pepsin + HCl

Small intestine -duodenum
jejunum

ileum (functional reserve)
Large intestine -stores and concentrates
undigested material
colon: 3 limbs -ascending
transverse
descending
Rectum ampulla
- temporary storage for waste.
As the rectal walls expand, stretch
receptors from the nervous system
located in the rectal walls stimulate
the desire to defecate.
Colon innervation:
Intrinsic
1. Auerbach plexus- segmentation and
peristalsis
2. Plexus of Meissner- secretions and
sensation
Extrinsic
1. Parasympathetic increases and
relaxes sphincters
2. Sympathetic decreases motility and
contracts sphincters
The muscular wall of the cecum and
colon is innervated by:
Vagus nerve- cranial nerve X in PNS
Superior mesenteric plexus – SNS
cord segment T9-L2
The rectum and anal canal innervation:
Pelvic splanchnic nervesparasympathetic S2-4
Hypogastric nerve- sympathetic T11L2
Anal sphincters innervation:
Internal anal sphincter- pelvic
splanchnic nerves and hypogastric
nerves
External anal sphincter – pudendal
nerve
Defecation Reflex
• Integrated in the sacral region of
the spinal cord
• Stimulated by distention of the
rectal wall
• Weak contractions of rectal wall
• Relaxation of anal sphincter
Defecation Reflex
• May be inhibited by voluntary
constriction of external anal
sphincter
If the urge is not acted upon, the
material in the rectum is often
returned to the colon where more
water is absorbed.
If defecation is delayed for a
prolonged period the fecal matter
may harden, resulting in
constipation.
During defecation the chest muscles,
diaphragm, abdominal wall muscles,
and pelvic diaphragm all exert
pressure on the digestive tract and
ventilation temporarily ceases as
the lungs push the chest diaphragm
down in order to exert pressure.
Blood pressure rises.
Not good for high- risk cardiac
patients.
For defecation external anal sphincter
must relax.
Sphincter ani externus muscle for anal
and urethral are both closely linked
by the same nerves
Fecal incontinence is the inability to
control one's bowels.
When one feels the urge to have a
bowel movement, they may not be
able to hold it until they can get to a
toilet, or stool may leak from the
rectum unexpectedly.
Fecal incontinence
may be caused by physical injury
(such as damage to the anal
sphincter that may result from an
episiotomy)
intense fright, inflammatory bowel
disease, impaired water absorption
(diarrhea), and psychological or
neurological factors.
BEFORE DEFECATION
Fecal material passes to rectum by the
longitudinal muscular contraction.
When there is distension in the sigmoid
colon and rectum, the internal sphincter
relaxes at the same time as the
external sphincter contracts.
This allows some of the fecal contents to
enter the anal canal and be sampled by
its sensitive epithelium to determine if
its solid, liquid or gas.
If the individual decides to continue with
defecation, the intrarectal pressure
increases on straining. Added pressure
from abdominal straining or Valsalva
maneuver aids defecation. Straining
requires intact innervation of the lower
thoracic cord T6-T12.
The pressure should be sufficient to
overcome the external sphincter which
relaxes with the pelvic floor that
causing the rectum to straighten up
reducing the rectal angle.
This increase in intraabdominal pressure
forces stool into the rectum with
relaxation of the extenal anal
sphincter allowing defecation.
Fecal matter stimulate the anal canal
further relaxing the external anal
sphincter.
If the individual decides not to continue
with defecation, the rectum relaxes
further to allow accommodation of
the contents which further stimulates
the external sphincter to contract.
Upper motor neuron lesiondamage above the defecation
reflex center in the sacral cord
Results to:
1. Abnormal storage of feces
2. Inability to recognize urge and
distinguish contents
3. External sphincter dyssynergia
Lower motor neuron lesion –
damage within the reflex defecation
center
Results to:
1. Fecal retention
2. Oozing of stool through the flaccid
sphincter
3. Sensory and motor pathways are
disrupted so the patient is unaware of
urge to defecate and unable to
exercise voluntary external anal
sphincter
Assessment is through the
bulbocavernosus reflex
and the anal relfex
The bulbocavernosus reflex is a
palpable or visible contraction of the
anal sphincter when pressure is
applied to the glans penis or clitoris.
When contraction is present, (+) result.
This indicates that the reflex activity
of the sacral cord is intact and
therefore the SCI is an UMN lesion.
Should be tested soon after SCI, before
the spinal shock passes.
The anal reflex is a visible contraction
of the anal sphincter in response to a
pinprick. A positive response indicates
an UMN lesion.
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