Anatomy of the Small intestine and Large Intestine

Question 1

What are the mucosal layers of the small intestine?

Answer 1

Mucosa: Simple columnar epithelia which has villi to increase SA for absorption, basement membrane, lamina propria. It contains enterocytes, goblet cells to produce mucous and Paneth cells for release of anti-microbial proteins.

Submucosa: Loose Connective tissue with large blood vessels and glands

Muscularis Externa: Oblique muscle, circular muscle and longitudinal muscle.

Serosa: Mesothelium of simple squamous epithelia and loose connective tissue. It also has visceral peritoneum.

Question 2

What are enterocytes?

Answer 2

Enterocytes are absorptive cells involved in the uptake of Na+, Cl- and glucose into the small intestine cells. Enterocytes also produce mucin and antimicrobial proteins

Question 3

What are the lacteal ducts?

Answer 3

Lacteal ducts in the small intestine absorb fat and lipids to transport this to the lymphatic circulation for distribution to the peripheral tissues.

Question 4

What are the divisions of the small intestine?

Answer 4

Duodenum, jejunum and ileum. These are all covered by the greater omentum anteriorly.
Duodenum is partially intraperitoneal and retroperitoneal.
Jejunum and ileum are fully intraperitoneal.

Question 5

What is the function of the small intestine?

Answer 5

Duodenum makes up 1/6 of small intestine for digestion. Jejunum makes up 1/3- of small intestine and is the start of absorption. Ileum makes up 1/6 of the small intestine and is the main site of absorption of amino acids, lipids, monosaccharides, drugs and water.

Question 6

What are the divisons of the duodenum?

Answer 6

Superior duodenum-Intraperitoneal. Posteriorly, common bile duct and artery are present

Descending duodenum- retroperitoneal. It has the hepatopancreatic papilla to receive secretions from the common bile duct and the pancreatic duct.

Horizontal duodenum- retroperitoneal. Anteriorly, The superior mesenteric artery and posteriorly, the inferior vena cava and aorta.

Ascending duodenum-> duodenojejunal flexure to connect to the jejunum of the small intestine.

Question 7

What is the ligament of Treitz?

Answer 7

Ligament of Treitz is the double fold of peritoneum which supports and anchors the duodenojejunal flexure.

Question 8

What is the jejunum-ileum continuum?

Answer 8

The region of the small intestine which contains both the jejunum and ileum. Proximal 2/5 is formed by the jejunum. Distal 2/5 is formed by the ileoum.

Question 9

What are the plicae circularis?

Answer 9

Prominent folds in the jejunum which increases the SA for absorption.

Question 10

What are the arterial arcades?

Answer 10

Branches of anastomosing arches of the superior mesenteric artery which supplies the jejunum in the mesentery. The vasa recta are straight capillaries of the superior mesenteric arteries projecting from the arterial arcades.

Question 11

Difference between jejunum and ileoum?

Answer 11

Ileum has more fat and more arterial arcades with more numerous vasa recta that are shorter than the jejunum. Ileum has villi to increase SA instead of plicae circularis. It has a smaller calibre diameter less than 3cm and has a lower rate of absorption. Ileum absorbs more water, B12 and salts than the jejunum.

Question 12

What is the vascularisation to the small intestine?

Answer 12

Proximal duodenum: Gastroduodenal artery, a branch of the coeliac trunk.
Distal duodenum, jejunum and ileum are all supplied by the superior mesenteric artery.

Question 13

What are the divisions of the large intestine?

Answer 13

5cm long and is the site of water, salt and drug absorption. It consists of the intraperitoneal caecum, appendix and sigmoid colon. It also contains retroperitoneal ascending colon, transverse colon and descending colon.

Question 14

What are the mucosal layers of the large intestine?

Answer 14

Mucosa: Simple columnar epithelia which has villi to increase SA for absorption, basement membrane, lamina propria. It contains enterocytes and goblet cells.

Submucosa: Loose Connective tissue with large blood vessels and glands

Muscularis Propria: Outer longitudinal muscle, intramuscular space and inner circular muscle

Serosa: Mesothelium of simple squamous epithelia and loose connective tissue. It also has visceral peritoneum.

Question 15

Difference between large and small intestine?

Answer 15

Large intestines contain no villi, no plicae circularis and no Paneth stem cells.

Question 16

What are the features of the large intestine?

Answer 16

Longitudinal external muscle on its surface called the taeniae coli which contract to propel food/faces in the haustra. The haustra give the large intestine its segmented appearance and contribute to food propulsion. The appendices epiploic are the fat deposits which aid in colonic absorption and the propulsion of faeces.

Question 17

What is the ileocecal valve?

Answer 17

Ileocecal valve is 6-9cm which demarcates transition from small to large intestine. It varies anatomically between being in the retrocaecal or retroileal region. The ileocecal fold is a layer of peritoneum which over the valve. When chyme passes through the valve, it moves 2cm inferiorly in the large intestine.

Question 18

What is the rectum?

Answer 18

Rectum holds faeces and has transverse rectal folds to aid this. The rectum has no appendices epiploae and no haustra.

Question 19

What is the anorectal junction?

Answer 19

The anorectal junction is a bend in the rectum 5cm from the anus, which is supported by the puborectal sling.

Question 20

What is the puborectal sling?

Answer 20

A muscle with attachments to the right and left pubic symphysis and forms a supporting sling around the anorectal junction.

Question 21

What is the journey of faeces in the body?

Answer 21

Sigmoid colon solidifies faeces; Rectum holds faeces; Rectal ampulla stores faeces

Question 22

What are the divisions of the anal canal?

Answer 22

Anal canal is 5cm long and divided into a pre-pectinate line and post-pectinate line.

Question 23

What are the features of the anal canal above the pre-pectinate line?

Answer 23

Upper 2/3 lined with simple columnar epithelia, lymphatic drainage from the internal iliac node, vascularisation from the superficial rectal artery and vein and innervation from the inferior hypogastric nerves.
Internal haemorrhoids can occur (swelling of the veins) which is not painful.

Question 24

What are the features below the pectinate line?

Answer 24

it is drained by the superficial inguinal nodes. It is lined with the stratified squamous keratinising. Blood supply is via the middle and inferior gastric artery and vein with the innervation via the inferior rectal nerve. External haemorrhoids can occur here which is very painful.

Question 25

What are the sphincter muscles of the anus?

Answer 25

Internal sphincter is circular smooth muscle. It has sensory innervation from the parasympathetic fibres. Sympathetic motor supply is the inferior hypogastric plexus from T11-L2. Parasympathetic motor supply is the pelvic splenic nerve from S2-S4.

Question 26

What are the features of the external anal sphincter?

Answer 26

External anal sphincter is voluntary skeletal muscle which has deep, superficial and subcutaneous layer. It is innervated by the pudendal nerve supply for both sensory and motor to cause relaxation and contraction.

Question 27

What is the rectal ampulla?

Answer 27

The storage site in the rectum for faeces.

Question 28

What are the anal columns?

Answer 28

Vertical mucosal folds in the anus where the bottom is the anal sinuses which opens into the anal glands.