Week 1/2 - A(3) - Anatomy 5&6 - Large intestine (Colon, rectum, anal canal)

Question 1

What is the large intestine made up off from proximal to distal? What is the point where the large intestine passes from pevlis to perineum? What is the function of the large intestine?

Answer 1

Large intestine Colon - caecum, appendix, ascending / transverse / descending /sigmoid colon Rectum, anal canal, anus Distal half of anal canal & anus are in the perineum Functions - asborb water & electrolytes and store undigested material (faeces) until ready for expulsion

Question 2

Which parts of the large intestine are mobile and which are immobile?

Answer 2

Intraperitoneal structures are generally mobile, retroperitoneal are relatively fixed Caecum, appendix, transverse, sigmoid colon - intraperitoneal -> mobile Ascending / descending colon and upper 2/3rds of rectum - retroperitoneal-> immobile

Question 3

What are the paracolic gutters? Where are they found?

Answer 3

There are 2 paracolic gutters - left and right They are found between the lateral edge of the ascending and descending colon, and the abdominal wall

Question 4

Why are the paracolic gutters clinically important?

Answer 4

These gutters are clinically important because they allow a passage for infectious fluids from different compartments of the abdomen. For example; fluid from an infected appendix can track up the right paracolic gutter to the hepatorenal recess

Question 5

Does the splenic flexure or the hepatic flexure usually lie more superior? What is the longitduinal muscle layer that is collected into 3 bands known as? Where does it run from? What is produced by thes longitudinal muscle bands contract to produce?

Answer 5

Splenic flexure lies more superior than the hepatic 3 distinct lognitduinal bands of thickened smooth muscle running from caecum to distal end of sigmoid colon are known as the teniae coli. They contracts lengthwise to produce sacculations known as haustra- gives colon its appearance

Question 6

What are the little fatty tags of the large intestine known as? Do they exist on the small intestine? What is the normal appearance of faeces in the rectum on plain CXR known as?

Answer 6

These small fatty tags are known as appendices epiploicae - these are small fatty stores The appendices epiploicae do not exist on the small intestine This normal appearance is known as a mottled appearance

Question 7

Where do the caecum and the appendix lie? Where does the appendix usually sit in relation to the caecum? What is the arterial supply and venous drianage to both?

Answer 7

Caecum and appendix both lie in the right iliac fossa Position of appendix is variable but most often sits retrocaecally Blood supply to caecum -> ileocolic artery branch of SMA. Blood supply to appendix is -> appendicular artery branch of ileocolic Venous drainage to superior mesenteric vein

Question 8

In appendicitis where is pain initially felt? Where is maximum tenderness felt? Why is there this spread of pain?

Answer 8

In appendicitis pain is initially felt in the umbilical region, this is because the appendix is a midgut organ. As the appendix becomes more inflamed it begins to irritate the abdominal wall causing somatic pain which can be localised to the RIF. Specifically McBurney’s point - 1/3rd of the way between ASIS to umbilicus

Question 9

Let’s now discuss the arterial supply of the ascending, transverse, descending and sigmoid colon State the difference in arterial supply to these areas?

Answer 9

Ascending colon - Ileocolic artery - SMA Right colic artery - SMA Transverse colon - Proximial 2/3rds - Middle colic artery - SMA (midgut) Distal 1/3rd - Left colic artery - IMA (hindgut) Descending colon Left colic artery - IMA Sigmoidal arteries - IMA Sigmoid colon Sigmoidal arteries - IMA

Question 10

Let’s now discuss the venous drainage of the ascending, transverse,descending and sigmoid colon State the difference in venous drainage to these areas? Where in the colon does the parasympathetic supply change?

Answer 10

Ascending colon * Drains to the superior mesenteric vein Transverse colon * Proximal 2/3rds to the superior mesenteric vein * Distal 1/3rd to the inferior mesenteric vein Descending and sigmoid colon * Drains to the inferior mesenteric vein Pasaympathteic supply changes from CN X up to proximal 2/3rds of transverse colon to sacral splanchnic parasympathetic nerves (S2-4)

Question 11

There exists arterial anastmoses between the branches of the superior mesenteric artery and the inferior mesenteric artery What is the predominant one artery formed from the anastomoses known as? - what arteries contribute to this? Why is this important?

Answer 11

Marginal artery of Drummond is the main anastamosis - formed frommainly left colic, middle colic and right colic arteries Important as could help prevent intestinal ischaemia by providing an alternative (collateral) route by which blood can travel should there be eg an obstruction of a vessel

Question 12

What is the blood supply to the rectum?

Answer 12

Arterial supply - Superior rectal artery (terminal continuation of IMA) Middle rectal artery -(internal iliac artery) Inferior rectal artery (internal pudendal artery)

Question 13

What is the venous drainage of the rectum?

Answer 13

Superior rectum to portal system via inferior mesenteric vein but Middle and inferior rectum to internal iliac and internal pudendal vein This is the portal systemic anastamoses Portal part goes to the portal vein Systemic part goes to the IVC

Question 14

What is the line that divides the anal canal into upper and lower parts which differ in neurovascular supply? What is the line that marks the transition from the anal canal digestive mucosa to the skin?

Answer 14

This would be the pectinate line - separates the anal canal into upper 2/3rds and lower 1/3rd This is the white line (anocutaneous line) - marks the trnasition from the non keratinized squamous epithelium to the keratinized squamous epithelium (skin)

Question 15

What is the difference in neruovacular supply drainage to the rectum above and below the pectinate line?

Answer 15

Above pectinate line Nerve - autonomic - sacral splanchnic nerves (S2-4) Arterial - from terminal radicle branches of superior rectal from IMA Venous - to IMV Below pectinate line Nerve - somatic - pudendal Arterial- from internal ilaic artery Venous - to internal iliac vein

Question 16

What are the two major venous systems of the body? - where do they drain blood from? What are the 3 clinically important sites of venous anastamosis between these two sites? Is there valves in the portal systemic venous anastamoses?

Answer 16

Two major venous systems are the * hepatic portal venous system - drains venous blood from absorptive parts of GI tract/associated organs to liver for cleaning * systemic venous system - drains venous blood from all other organs/tissues to SVC or IVC into RA of heart Anastamoses - no valves in these veins * distal end of oeseophagus * skin around umbilicus * rectum/anal canal *

Question 17

What are the portal systemic anastamoses important? What happens to them clinically ie if there is portal hypertension? What are the signs?

Answer 17

At these sites, the presence of small collateral veins means blood can flow both ways Portal hypertension - > usually liver pathology The collateral veins will receive a much large volume of blood causing them to dilate, becoming varicose -> Oeseophageal and rectal varices, caput medusae

Question 18

What is the difference between rectal varices and haemorrhoids?

Answer 18

Rectal varices form in relation to portal hypertension - -> dilation of collateral veins between portal/systemic system Haemorrhoids are prolapses of the rectal venous plexuses - their development is not related to portal hypertension Related to raised pressure e.g. chronic constipation, straining, pregnancy

Question 19

Where does the sigmoid colon begin and where does it become the rectum? Where does the rectum nd and what does it become?

Answer 19

Sigmoid colon begins at the pelvic brim (anterior to left external iliac artery) and ends anterior to the 3rd sacral vertebra where it becomes the rectum The rectum begins at the 3rd sacral vertebra and ends 2cm in front of the tip of the coccyx where it pierces the pelvic diaphragm to become the anal canal

Question 20

Pelvic floor is formed from muscles and fascial layers The muscular part is known as the pelvic diaphragm Which muscles form the pevlic diaphragm and what is their nerve supply?

Answer 20

Pelvic diaphragm Levator ani - innervated by pudendal nerve (S2,3,4) and nerve to levator ani (S3,4) Coccygeus - innervated by branches direct from sacral plexus (S4,5)