The Colon - Anatomy Flashcards
What is the caecum?
Most proximal part of the large intestine and is located between the ileum and the ascending colon.
Blind ending sac
Where can the caecum be found?
In the right iliac fossa of the abdomen. It lies inferiorly to the ileocaecal junction and can be palpated if enlarged due to faeces, inflammation or malignancy.
Superiorly, what is the caecum continuous with?
The ascending colon
Is the caecum intra or retroperitoneal?
Intraperitoneal
What is present between the caecum and the ileum?
The ileocaecal valve
What is the function of the ileocaecal valve?
Prevents reflux of large bowel contents into the ileum during peristalsis and is thought to function passively, as opposed to be a defined muscular sphincter
Where is the caecum derived from?
The embryological midgut
What is the caecum supplied by?
Via branches of the superior mesenteric vessels
Where is arterial supply of the caecum from?
The ileocolic artery - a branch of the SMA
The ileocolic artery divides into anterior and posterior caecal arteries which supply the caecum
What is venous drainage of the caecum provided by?
The corresponding ileocolic vein, and empties into the superior mesenteric vein
How is the caecum innervated?
By the ileocolic branch of the superior mesenteric plexus which follows the same course as the ileocolic artery and carries vagal and sympathetic nerve fibres.
Where is lymph from the caecum drained into?
The ileocolic lymph nodes (which surround the ileocolic artery).
What is the appendix?
A narrow blind-ended tube that is attached to the posteromedial end of the caecum
What does the appendix contain?
A large amount of lymphoid tissue but is not thought to have any vital functions in the human body
What is the appendix supported by?
The mesoappendix, a fold of mesentery which suspends the appendix from the terminal ileum
Where is the appendix derived from?
The embryologic midgut
Therefore, the vascular supply is via branches of the superior mesenteric vessels
Where is the arterial supply of the appendix from?
The appendicular artery
Where is the appendicular artery derived from?
The ileocolic artery, a branch of the superior mesenteric artery
Where is the venous drainage of the appendix via?
The corresponding appendicular vein
Where is the appendicular artery and vein contained?
Within the mesoappendix
What innervates the appendix?
The ileocolic branch of the superior mesenteric plexus. It accompanies the ileocolic artery to reach the appendix.
Where do the sympathetic afferent fibres of the appendix arise from?
T10 of the spinal cord - thus explaining why the visceral pain of early appendicitis is felt centrally within the abdomen.
Where does lymph fluid from the appendix drain into?
Lymph nodes within the mesoappendix and into the ileocolic lymph nodes (which surround the ileocolic artery).
Where is pain from appendicitis referred?
Initially, pain from the appendix and its visceral peritoneum is referred to the umbilical region. As the appendix becomes increasingly inflamed, it irritates the parietal peritoneum, causing the pain to localise to the right lower quadrant.
What can prolonged appendicitis lead to?
It can become necrotic and rupture, resulting in peritonitis (inflammation of the peritoneum).
What are the 4 parts of the colon?
Ascending, transverse, descending, sigmoid
Is the ascending colon intra or retroperitoneal?
Retroperitoneal
Describe the pathway of the ascending colon
- Ascends superiorly from caecum
- When it meets the right lobe of the liver, it turns 90 degrees to move horizontally. This turn is known as the right colic flexure (or hepatic flexure), and marks the start of the transverse colon.
Describe the pathway of the transverse colon
Extends from the right colic flexure to the spleen
Where does the transverse colon end?
Left colic flexure / splenic flexure (colon turns 90 degrees to point inferiorly)
How is the colon attached to the diaphragm at the left colic flexure?
By the phrenicocolic ligament
Is the transverse colon intra or retroperitoneal?
Intraperitoneal - enclosed by the transverse mesocolon
Describe the pathway of the descending colon
After the left colic flexure, the colon moves inferiorly towards the pelvis
Is the descending colon intra or retroperitoneal?
Retroperitoneal
Where does the sigmoid colon begin?
When the descending colon begins to turn medially
Describe the pathway of the sigmoid colon
Extending from the left iliac fossa to the level of the S3 vertebra. This journey gives the sigmoid colon its characteristic “S” shape.
Where is the sigmoid colon found?
In the left lower quadrant of the abdomen
How is the sigmoid colon attached to the posterior wall?
By the sigmoid mesocolon
What are the paracolic gutters?
Two spaces between the ascending/descending colon and the posterolateral abdominal wall
Why are the paracolic gutters clinically important?
They allow material that has been released from inflamed or infected abdominal organs to accumulate elsewhere in the abdomen.
How can the LI be distinguished from the SI?
- Omental appendices
- Teniae coli
- Haustra
- Diamter
What are omental appendices? Where are they found?
Attached to the surface of the large intestine. Small pouches of peritoneum, filled with fat
What are teniae coli? Where are they found?
Running longitudinally along the surface of the large bowel are three strips of muscle, known as the teniae coli. They are called the mesocolic, free and omental coli
What are haustra?
The teniae coli contract to shorten the wall of the bowel, producing sacculations known as haustra.
Describe the diameter of the LI compared to the SI?
The large intestine has a much wider diameter compared to the small intestine
Where do the features of the LI cease?
At the rectosigmoid junction, where the smooth muscle of the teniae coli broaden to form a complete layer within the rectum.
What parts of the colon are derived from the midgut?
Ascending colon and proximal 2/3 of the transverse colon
What parts of the colon are derived from the hindgut?
Distal 1/3 of the transverse colon, descending colon and sigmoid colon
As a general rule, what are midgut-derived structures and hindgut-derived structures supplied by?
Midgut-derived structures are supplied by the superior mesenteric artery, and hindgut-derived structures by the inferior mesenteric artery.
Where does the ascending colon receive arterial supply from?
Two branches of the SMA; the ileocolic and right colic arteries
The ileocolic artery gives rise to colic, anterior cecal and posterior cecal branches – all of which supply the ascending colon.
What is the transverse colon supplied by?
The transverse colon is derived from both the midgut and hindgut, and so it is supplied by branches of the SMA and IMA:
- Right colic artery (from the SMA)
- Middle colic artery (from the SMA)
- Left colic artery (from the IMA)
What is the descending colon supplied by?
A single branch of the IMA; the left colic artery.
What is the sigmoid colon supplied by?
Via the sigmoid arteries (branches of the IMA)
What is the marginal artery?
Provides collateral supply to the colon – thereby maintaining arterial supply in the case of occlusion or stenosis of one of the major vessels.
As the terminal vessels of the SMA and IMA approach the colon, what happens?
They split into many branches, which anastomose with each other
These anastomoses form a continuous arterial channel which extends the length of the colon – the marginal artery
What arises from the marginal artery to supply the colon?
Long, straight arterial branches (called vasa recta)
What is the venous drainage of the ascending colon?
Ileocolic and right colic veins, which empty into the superior mesenteric vein
What is the venous drainage of the transverse colon?
Middle colic vein, which empties into the superior mesenteric vein.
What is the venous drainage of the descending colon?
Left colic vein, which drains into the inferior mesenteric vein.
What is the venous drainage of the sigmoid colon?
Drained by the sigmoid veins into the inferior mesenteric vein.
What do the superior mesenteric and inferior mesenteric veins ultimately empty into?
The hepatic portal vein.
What is purpose of SMV and IMV emptying into hepatic portal vein?
This allows toxins absorbed from the colon to be processed by the liver for detoxification
What are the midgut-derived structures of the colon innervated by?
Receive their sympathetic, parasympathetic and sensory supply via nerves from the superior mesenteric plexus.
What are the hindgut-derived structures of the colon innervated by?
Receive their sympathetic, parasympathetic and sensory supply via nerves from the inferior mesenteric plexus:
- Parasympathetic innervation via the pelvic splanchnic nerves
- Sympathetic innervation via the lumbar splanchnic nerves.
Where is the lymphatic drainage of the ascending and transverse colon into?
The superior mesenteric nodes
Where is the lymphatic drainage of the descending and sigmoid colon into?
The inferior mesenteric nodes
Where does the lymph from the superior mesenteric and inferior mesenteric nodes ultimately empty into?
The thoracic duct
