the digestive system part 1 Flashcards
Overview of the abdomen
The anatomical location for the
abdomen is from the diaphragm
(inferior thoracic aperture) to the pelvic
inlet / lower limbs
It freely communicates with the pelvis
inferior to the pelvis inlet
It is enclosed by the abdominal wall and
the inner large peritoneal cavity.
Organs either hang from mesenteries in
the peritoneal cavity or lie between the
abdominal wall and peritoneal cavity
Functions of the abdominal cavity
It contains and protects the major organs
Breathing
The abdominal wall relaxes to allow the
thoracic cavity to expand and contracts to reduce the thoracic cavity
(forcibly when coughing or sneezing)
Increasing abdominal pressure
Contraction of the abdominal wall assists in mictuation, defecation and childbirth
What is the regional anatomy
The abdomen can be divided as:
• 4 quadrants
• Median sagittal plane
• Axia plane at the level of the
umbilicus and L3/4
• 9 regions
• 2 sagittal planes at the mid clavicle
• Axial subcostal plane – level of lower
costal margin and L3
• Axial intertubercular plane – iliac
crests / L5
• Although there is some variation,
normally organs can be found in pre-
determined areas
What is the abdominal wall?
The anatomical landmarks for
the abdominal wall are the
• Superior - Xiphisternum and
lower costal margin
• Inferior – pelvis bones
• Posterior – spine
• It is made of 5 layers
• Skin- for protection
• Subcutaneous (adipose) tissue
and superficial fascia
• Muscles and fascia
• Extra-peritoneal fascia / fat
• Parietal peritoneum
What are the muscles of the abdomen?
There are 5 paired muscles to
meet the functions of the abdomen
• Each muscle has its own function
• It is separated in the midline by
the linea alba fascia
• Flat group (superficial to deep)
• External oblique
• Internal oblique
• Transversus abdominus
• Vertical group (enclosed in a rectus
sheath
• Rectus abdominus
• pyramidalis
What is Extraperitoneal fascia?
Deep to the muscles
Between the parietal peritoneum and the muscles of the abdominal wall
Contains different amounts of fat (adipose tissue) – there is more posteriorly than anteriorly
Anteriorly – called pre-peritoneal
Posteriorly – called retro-peritoneal
There are several organs / structures within the retroperitoneal fascia e.g. kidneys, descending and ascending colon
What is the Peritoneum?
• This is the innermost layer of the abdominal wall
• It is a closed sac for men, unclosed in women
(there is an opening for the uterine tubes)
• It is a
• Continuous serous membrane
• Layer of simple squamous epithelium
• Supported by connective tissues
• There are 2 layers names by its role / location
• Parietal – lines the abdominal wall / peritoneal cavity
• Visceral (serosal) – covering the organs
• Folds of the peritoneum (mesenteries) are
connected to the abdominal wall to suspend / hold the GI tract- to prevent friction
What is the peritoneal cavity?
• The cavity only contains minimal
serous fluid – 5-20ml
• Fluid has an important role in
peritoneal homeostasis
• It is divided into 2 parts
• Greater sac
• Most of the space
• From diaphragm to pelvis cavity
• Penetrating injuries go into this area
• Lesser sac (omental bursa)
• Smaller area
• Posterior to the liver and stomach
• Communicates with the greater sac
through the omental foramen
What are the peritoneal retroperitoneal organs?
Within the peritoneal cavity
• Stomach
• GI tract
• First 5cm of
duodenum
• Jejunum
• Ileum
• Caecum and
appendix
• Transverse colon
• Sigmoid colon
• Upper 1/3 of the
rectum
• Spleen
• Tail of pancreas
Outside the peritoneal
cavity
• Urinary
• Adrenal glands
• Kidneys
• ureters
• Vascular
• Aorta
• Inferior vena
cava
• Digestive
• Lower 2/3
rectum
What is the omenta?
2 layers which connect the stomach / first part of the duodenum to the other organs
• Greater omentum
• Largest peritoneal fold
• Highly vascularised and fatty
• Extends from the greater curvature of the
stomach over the transverse colon and loops of small bowel
• Passes posteriorly to combine with other folds and attach to the posterior abdominal wall
• Encloses the spleen
• Lesser omentum
• From the lesser curvature of the stomach,
connects to the inferior surface of the liver
through ligaments
• Anteriorly contains hepatic and gastric arteries, portal vein and bile duct
What is the mesenteries?
Surround and support loops of bowel,
connecting to the posterior abdominal wall
There are double layers connected
posteriorly to the abdominal wall which
encase the loops of bowel
It allows for some movement of bowel
Also contains the route for neurovascular andlymphatic structures
Main types are:
• Mesentery (loops of jejunum and ileum)
• Transverse mesocolon
• Sigmoid mesocolon
Ligaments
There are 2 layers of peritoneum connecting
organs to the abdominal wall
• They are usually names after the structures
they connect
• The falciform ligament attached the anterior
part of the liver to the anterior abdominal
wall
What is the digestive system?
It converts food into energy and
absorbs nutrients
• Consists of
• Gastrointestinal tract
• Accessory organs (liver, gallbladder
and pancreas)
• Assisted by hormones, enzymes and
bacteria
• Main processes
• Ingestion
• Propulsion
• Digestion (mechanical and chemical)
• Absorption
• Elimination
What is the GI tract?
Continuous hollow tube
• Approximately 30ft long in cadavers
• Food is always considered external to
the body, it only contacts the inner lining
• The tissues are similar along the length of the canal with slight modifications to aid
digestion
Parts
• Mouth
• Pharynx
• Oesophagus
• Stomach
• Small intestine
• Large intestine
• Rectum and anal
canal
Structure
There are 4 basic tissue layers
• Mucosa
• Sub-mucosa
• Loose connective tissue to bind
mucosa to the muscle layer
• Contains neurovascular and
lymphatic structures
• Muscularis
• Serosa
• Fibrous outer layer in the thorax
• Single serous layer membrane in
the peritoneum
Why is the muscoal layer?
Mucous membrane
• Columnar epithelium
• Functions for protection, secretion and
absorption
• Cell types vary according to its location
• Lamina propria
• Loose connective tissue
• Contains supporting blood vessels and
lymphoid tissues
• Muscularis mucosa
• Smooth muscle layer
• It is folded into layers to increase the
surface area for absorption
What are the muscles of the GI tract?
• 2 layers of mostly smooth muscle
• Inner circular
• Outer longitudinal
• They are separated by lymphatic
and neurovascular layers
• Waves of contraction cause
peristalsis to mix and push
contents along
• There are sphincters of thickened
circular muscle at points to act as
valves
Valves in respiratory tract are one way valves. Stops large bowel contents going back.
What are the nerve supply of the GI tract?
There is intrinsic and extrinsic innervation
• All of the digestive system is innervated by the
autonomic nervous system
• In the GI tract there are
• 100 million nerves
• Those in the myenteric plexus (between the
muscle layers) control motility
• Those in the submucosa control secretions
• Parasympathetic
• Increases secretions and motility
• Mostly the vagus nerve and sacral nerve
• Sympathetic
• Decreases secretions and motility
• This is your spinal nerves along the spine
Blood supply to the digestive
system – arterial
• Arterial
• Thoracic aorta
• Oesophageal arteries
• Abdominal aorta
• Coeliac arteries – liver,
spleen, stomach (T12 – L1)
• Superior mesenteric artery –
pancreas, small intestine
(L1/2)
• Inferior mesenteric artery –
colon and rectum (L3)
Blood supply to the
digestive system -
venous
Venous
• Hepatic postal system in the liver
Brief
overview of
digestion
What is the aim of digestion?
• To break down food in preparation for
absorption
• It is mechanical
• There is movement along the GI tract
• Breaking down the food increases the
surface area for absorption
• As the food passes through it mixes with
chemicals / secretions
• Chemical enzymes
• Amylase – carbohydrates (salivary
glands and pancreas)
• Proteases (pancreas) and hydrochloric
acid (stomach) – proteins
• Bile (pancreas) and lipases (pancreas0 -
fats
What is the mouth?
• Oral cavity
• Vestibule – the space
between your teeth,
gums, lips and cheeks
• The oral cavity is lined
with the mucous
membranes
• Contains mucus secreting
goblet cells
• Consists of squamous
epithelium
What is the tongue?
Main functions
• Mastication
• Swallowing
• speech
• The base is attached to the hyoid bone and to the floor by
the mucous membrane
• There are skeletal muscle fibres
• That within the tongue change the shape but not the position
• That originate from the skull and change the
position of the tongue
• The surface is stiffened squamous epithelium
• There are sensory receptors – taste buds
• Vascular supply
artery
• Arterial – lingual branch of the external carotid
• Vascular – lingual veins into the internal jugular vein
The role of the
tongue in
digestion
• Grips food and positions it
between the teeth
• Mixes food with saliva
• Forms a bolus of food
• Initiates swallowing
What are the Salivary glands?
• There are 3 bilateral pairs of glands which
release saliva into the mouth through ducts
• Parotid
• Submandibular
• Sub-lingual
Saliva- lubricate, helps with diesting food, has antibiprobagents. Helps wash food particles to prevent damage to teeth.
What is the oesophagus?
• This is a muscular tube
• It begins distal to the pharynx at around
the level of C6
• The procimal cricopharyngeal sphincter
prevents air entering the oesophagus at
respiration and aspiration of food
• Approximately 25cm long and 2cm in
diameter
• Lies in the midline posterior to the
trachea and anterior to the spine
• Passes through the diaphragm at the
level of T10 (oesophageal hiatus)
• At the inferior part
• It curves sharply (this helps
prevent regurgitation) and
superiorly into the stomach
at the level of T11
• The cardiac sphincter
prevents reflux of stomach
contents
Oesophagus -structure
• Outer layer is fibrous and attaches to the surrounding
structures
• There is a muscle layer for peristalsis
• There are no digestive enzymes in the mucosal layer – no
absorption
• The arterial supply
• Thorax – branches off the thoracic aorta – oesophageal
arteries
• Abdomen – inferior phrenic and left gastric artery
• The venous drainage
• Thorax – azygous vein
• Abdomen – left gastric vein
What is saliva?
Food is mixed with saliva which contains enzymes.
• The enzymes are only activated in liquid and help to dissolve chemicals in food to aid taste
• Saliva contains
• 99.5% water – dissolves food
• 0.5% solutes – sodium, potassium etc.
• Mucous lubricates food
• Immunoglobulin A –prevents attachments of microbes
• Enzymes – inhibit the grown of oral bacteria
• Salivation is controlled by the autonomic nervous system.
• Parasympathetic –
continuous secretion
• Sympathetic – works during stress or dehydration – dry mouth / thirst
• Chemicals in food stimulate the receptors in taste buds which result in secretion
• After eating there is a flow of saliva for cleaning and diluting any remaining
chemicals
What is Swallowing (deglutition)?
• 3 stages according to location of bolus; takes around 4-8 seconds (1 for fluid)
• Oral Stage
• Mouth closed, bolus forced posteriorly by voluntary muscles of tongue / cheeks
• Pharyngeal Stage
• When bolus reaches oropharynx, reflex action by swallowing centre in medulla
• Involuntary contraction of muscles to propel bolus into oesophagus
• Other routes closed:
• Soft palate elevates to close nasopharynx
• Tongue/pharyngeal folds close off mouth
• Larynx elevates and moves anteriorly to occlude trachea through epiglottis
• Oesophageal Stage
• Peristalsis only starts when stimulated when food in pharynx
• Propels food to stomach, lubrication by mucous
• Cardiac sphincter relaxes on peristalsis
How can we image the digestive
system?
X-rays
• AXR (abdominal x-rays)
MRI
• MR enterography
Fluroscopy
• Barium swallows
• Barium meals / follow throughs
• Defaecating proctograms
Ultrasound
• Abdominal ultrasound
• Small bowel
ultrasound for
inflammatory bowel
disease
CT
• Abdominal / pelvis CT
with / without
contrast
• CT colonography
Nuclear medicine
• Gastric emptying
scans
• Meckel scan
AXR’s – small and large bowel
How do we look at bowel on a AXR?
• Remember the 3,6,9 rule
• It can be really hard to differentiate between large and small bowel on an AXR
• Look for valvular conniventes and the teniae coli
• Small bowel is normally more central
• Large bowel often has a mottled appearance due to gas within faeces
• Marsh P Normal abdominal radiograph -
annotated x-ray.
Image characteristics: AXR – stomach
• The stomach may be
visible if it contains gas.
However, if it is
completely empty or
completely fluid filled you
may not see it.
• Generally, lies at around
T10 level
Image characteristics: AXR’s – liver and
pancreas
• Liver – the liver lies in the right
upper quadrant and is a generally
homogenous grey density
• The superior edge forms the contour
of the right hemidiaphragm
• Sometimes breast shadow can
overlie and make it difficult to see
• You will generally only see the
gallbladder if its abnormal or absent!
• The liver doesn’t normally extend
lower than the lower pole of the
kidney
Image characteristics: AXR – liver normal
variant – Riedel lobe
Common anatomical variant of
the liver
• Can be mistaken for a mass
• It is where the right lobe of the
liver is larger than normal and
extends caudally in the
abdomen
• Seen in up to 31% of patients,
with a female predominance
What is CT colonography?
• Screening test for colorectal cancer
• After failed or unsuitable endoscopy
or patient choice
• Shows the colon in much more
detail than a normal CT
• Patient drinks contrast – generally
gastrografin
• Gas is inserted to inflate the bowel
CT scan of the liver
• Generally done as a triple phase scan
• Looks for liver lesions and metastases
• 3 phase
• Late arterial
• Portal venous phase
• Delayed phase
• There can also be an additional non
contrast scan
• The liver gets approximately 25% of its
blood supply from the hepatic artery and
75% from the portal vein so needs many
phases for accurate assessment
• It can be hard to work out what a liver lesion
is, but this helps.
• Late arterial
• Will see the portal vein, not the hepatic
vein
• Good to look for hepatocellular cancer
• Portal venous
• Will see portal veins and the hepatic vein
• Will see if the liver is fatty
• Good to look for very vascular liver
tumours
• Delayed phase
• Some tumours may enhance compared
to the rest of the liver in this late stage
What is MRI Enterography?
• Non invasive technique for
assessing the small bowel
• Why is this good for patients
with inflammatory bowel
diseases?
• It can demonstrate acute
inflammation from disease
exacerbation and complications
• Patients have to drink mannitol
Fluoroscopy
• Barium swallow
• Dedicated test of the pharynx,
oesophagus and proximal stomach
• Patient drinks barium when
instructed
• Dynamic so good for functional
disorders
• Can be used to help patients
rehabilitating after stroke or with
neuro or muscular degenerative
disorders
• Small bowel follow through
• Evaluates the small bowel
dynamically
• Patient drinks barium and water
• Sometimes ant sickness
medication is given
• Is being less utilised in favour of
CT or MRI
• Defecating proctograms
• Evaluates the pelvic floor in
patients with difficulties in
defaecation or with constipation
Normal variants and different
appearances in imaging the
digestive system
• Reidel’s lobe
• Surgery- gallbladder removed.
• Cholecystectomy
• Weight loss procedures
Cholecystectomies
• Nearly all laparoscopic
• Gallbladder removed due to
stones or polyps
What is the stomach size shape and position?
Size and shape
• C/J shaped
• 15 – 25cm long
• Volume depends on the contents – up to
4L capacity
• When empty the rugae collapse and fold
in, shrinking the stomach
• Position
• Inferior to the diaphragm
• In the upper left quadrant / left epigastric
and hypochondriac region
What are the regions of the stomach?
• Cardia
• Surrounds the cardio-oesophageal
sphincter
• Fundus
• The section lateral to the cardia
• Body
• Mid portion
• Narrows inferiorly
• Pylorus
• Consists of the antrum and canal
• Contains the pyloric sphincter which is
continuous with the small intestine
What are the stomach layers?
• Serosa
• Serosa is continuous with
the visceral peritoneum
• 3 muscle layers which allow
churning and mixing
• Mucosa
• Made up of simple
columnar epithelium
• Contains mucus cells
• Has a protective alkaline
layer to protect from acids
and enzymes
• Gastric juices are secreted
from gastric glands – up to
2-3l a day
Mechanical digestion in the stomach
There are peristaltic waves every 15-20 seconds
Food is macerated
It is mixed with secretions to form chyme
The fundus section is mainly for storage-
less movement
The rugae allow for expansion and
increase the surface area
Chemical digestion in the stomach
The food may be in the fundus for more than an hour before churning begins but the salivary amylase continues to work.
Food is mixed with acidic gastric juices
• HCI acid
• Kills microbes
• Denatures proteins
• Promotes flow of bile
• Gastrin
• Increases mobility of stomach
• Relaxes the pyloric sphincter
• Pepsin
• Starts digestion of protein
• Breaks peptide bonds making smaller chains
Very small amounts of nutrients are absorbed.
The epithelial cells are impermeable to most materials.
Some water is absorbed.
Some drugs are absorbed
• Aspirin
• Alcohol
• The stomach empties 2-4 hours after eating
• Carbohydrate rich food is the quickest
• Fatty food is the slowest
• Each wave moves approximately 3mm of chyme through the sphincter into the duodenum
What is the small intestine?
• Food is prepared in the small intestine so it can
enter the cells of the body
• The small intestine is a muscular tube approx. 7m
long from the pyloric sphincter to the ileocaecal
sphincter
• Longest segment of digestive tract
• Parts
• Duodenum – 5cm peritoneal
• Rest of duodenum, jejunum, ileum are
retroperitoneal
• Suspended form the posterior abdominal wall by
the mesentery
• Surrounded by the large intestine
Small intestine - divisions
• Duodenum
• 25cm long (5%)
• Jejunum
• 2.5m long (40%)
• Ileum
• 3m long (55%)
What is the duodenum?
• C shaped
• Extends from the pyloric
sphincter to the jejunum
• Has 4 sections
• 1st – superior
• 2nd – descending
• 3rd – horizontal
• 4th - ascending
• Surrounds the head of the
pancreas
• Receives
• Gastric chyme from the
stomach
• Digestive juices from the
pancreas
• Bile from the liver
The mucosa has villi to
increase the surface
area.
Its function is to
breakdown food using
enzymes.
It uses hormones to
regulate the rate of
stomach emptying
Mechanical digestion in the small bowel
• There is localised mixing
contractions
• The chyme mixes with
digestive juices
• Food comes into contact with
the mucosa
• Contents are not moved along
•Contraction of circular muscle into segments
•Muscle fibres in middle of each segment contract –dividing segment further
•First fibres relax -> large segment again
•Repeats – chyme sloshed back and forth
•Most rapid in duodenum – 12 x per minute 8 x per minute in ileum
•After most of food absorbed segmentation stops and peristalsis begins
•Peristalsis migration reaches end of ileum in 90 – 120 minutes
•Chyme remains in small intestines 3 – 5 hours
What is the jejunum?
• Has villi
• Has large circular folds to
increase surface area of
mucosa
• Main function is to absorb
previously digested food from
the duodenum
What is ileum?
• There isn’t an obvious junction between the jejunum and ileum
• Has smaller and thinnerwalls than the jejunum
• Function
• Absorb B12 and bile salts
• Absorb remaining products not absorbed in the jejunum
What is the large bowel?
• Large because it has a larger diameter
• 1.5m long
• Differing diameters – 6-9cm
• Runs from the ileocaecal valve to the
anus
• No villi on the mucus membrane as
there is no chemical digestion
• There are no structural alterations to
increase surface area
• There are goblet cells with produce
mucus to ease the passage of faeces
• Teniae coli
• 2 bands of smooth muscle
• Runs from the caecum to the
rectosigmoid junction
• These contact lengthwise to
produce haustra
• This helps to move faeces
through the large bowel
• Do not have them in the
rectum and anus
What is the Caecum and appendix?
• Caecum – 6cm long
• Receives chyme from the
ileum
• Has the appendix hanging
from it
Colon structure
• Ascending
• 20cm long
• Has the hepatic flexure at distal
end
• Transverse
• Has hepatic and splenic flexure
at either end
• Descending
• Left side of pelvis
• Sigmoid
• S shaped. Projects medially
Mechanical digestion
in the large bowel
•Chyme enters cecum via ileo-caecal
sphincter (Usually slowly)
•Gastro-colic reflex:
• Food entering stomach stimulates
release of Gastrin into
blood
•Gastrin plays a part in ileo-caecal
sphincter relaxation
•Allows chyme to enter caecum from
ileum
•Faeces in caecum triggers mass
movement
Mass movement of faeces
When caecum becomes distended the contraction of the ileocaecal sphincter increases.
Chyme fills caecum and accumulates in ascending colon.
Haustral churning –distended by contents, walls contract moving contents to next haustrum.
Peristalsis also occurs but slower 2 – 3 contractions per minute
Mass peristalsis – strong peristaltic wave from mid transverse colon
Drives faeces into rectum
Occurs after meals – 3 – 4 times per day
Chemical digestion in the large
bowel
No enzymes secreted
Final stage of digestion – activity of bacteria in lumen
Bacteria:
Ferment any remaining carbohydrate
Releases hydrogen, CO2 and methane gas
Flatus – which becomes flatulence if excessive
Breakdown remaining proteins and amino acids
Decompose bilirubin to simpler pigments results in brown
colour
Absorption in the large bowel
• Water
• Most water is absorbed in the large bowel
• The large bowel is important in maintaining homeostasis
• Bacterial products
• Including vitamins – B and K
Defaecation
•Elimination of indigestible residue
•When faeces into rectum by mass movement
defecation reflex initiated
•Spinal reflex:
• Causes walls of sigmoid colon and rectum to
contract
• Anal sphincters relax
• Faeces in anal canal messages to brain
• Voluntary decision to open external sphincter
• If not contraction ends and walls relax
• Next mass movement initiates new reflex
What are the functions of the liver?
Digestive
Haematological
Metabolismof nutrients
Detoxification
Mineral and vitamin storage
Bileproduction
What is the liver ?
• Irregular wedge-shaped organ
• The liver is the largest gland in the
body
• 10-12cm craniocaudally
• 20-23cm transverse
• It is situated under the diaphragm
in the right upper quadrant
extending into the left upper
quadrant
• It is largely protected by the ribs
and overlies the stomach
• It is divided into 4 lobes
• Left lobe
• Right lobe
• Subdivided into the
caudate and quadrate
lobes
• The 2 main lobes are divided by
the falciform ligament
• The falciform ligament attached
the liver to the anterior abdominal
wall
• The liver is covered by a
fibroelastic capsule made
up of
• Visceral peritoneum
• A Glisson capsule
underneath which contains
blood and lymph vessels
and nerves
Blood supply to the liver
A large amount of blood is
needed for metabolic
functions
Hepatic artery – 400-500
ml/min
Hepatic portal vein 1000-
1200 ml/min
hepatic veins return blood
to the IVC
Hepatocytes
Liver cells are capable of
regeneration – damaged or
resected liver can regrow
What is the gallbladder?
• Sac like organ
• Situated on the inferior surface of the liver
• It stores and concentrates bile
(approximately 90mls)
• Bile passes from the liver to the
gallbladder via the right and left hepatic
ducts into the common hepatic duct
• There is resistance at the sphincter of
Oddi – this controls flow into the
duodenum and precents reflux
• The bile passes into the gallbladder via
the cystic duct
How does the
gallbladder work
Approx. 30 minutes after
eating the gallbladder
contracts
• This forces bile through the
cystic duct into the common
bile duct
• The sphincter of Oddi relaxes
• Bile passes into the duodenum
via the major duodenal papilla
The pancreas
• Approx. 20cm long
• It has a head, neck, body and
tail
• The head, neck and body are
retroperitoneal
• The head sits in the curve of
the duodenum, the tail touches
the spleen
• The body sits behind the
stomach
Functions of the pancreas
Exocrine function
• Has cells that
secrete enzymes
and alkaline
pancreatic juices
• These cause the
gallbladder to
contract and
release bile into the
duodenum
• Endocrine function
• There is secretion of
• Insulin –lowers blood glucose
• Glucagon –
raises blood
glucose
