3. GI structural anatomy

Question 1

name the 5 main layers of the anterolateral abdominal wall

Answer 1

1. skin
2. superficial fascia (1 layer above umbilicus, 2 layers below: fatty superficial and membranous deep)
3. muscles
4. transversalis fascia
5. parietal peritoneum

Question 2

describe the muscular arrangement of the abdominal wall

Answer 2

1. external obliques - fibres run superolateral to medioinferior
2. internal obliques - fibres run superomedial to inferolateral
3. rectus abdominis - run vertically near midline
4. transversus abdominis - run horizontally

Question 3

describe the surface markings associated with rectus abdominis

Answer 3

• LINEA ALBA: fibrous groove separating RA muscles in midline, runs from xiphoid process to pubic symphisis
• LINEA SEMILUNARIS: curved tendinous intersections found on either side of RA muscles
• TENDINOUS INTERSECTIONS: horizontal fibrous strips separating RA muscles

Question 4

What is the rectus sheath?

Answer 4

• aponeurosis of external and internal obliques and transverse abdominis
• envelops the midline rectus abdominis muscles

Question 5

what does the arcuate line mark and where is it found

Answer 5

• lower limit of posterior layer of rectus sheath (distal to this line, RAs lie directly on transversalis fascia)
• point at which epigastric vessels pierce rectus abdominis
• 1/2 way between umbilicus and pubic crest

Question 6

what are the 2 main functions of sphincters in the GI tract

Answer 6

1. control movement along tube

2. prevent reflux of material

Question 7

name the 7 main sphincters of the GI tract

Answer 7

i) upper oesophageal
ii) lower oesophageal
iii) pyloric sphincter
iv) sphincter of Oddi
v) ileo-caecal valve
vi) internal anal
vii) external anal

Question 8

describe the arterial blood supply to the GI tract

Answer 8

branches of aorta:

• coeliac trunk - foregut
• superior mesenteric artery - midgut
• inferior mesenteric artery - hindgut

Question 9

describe the venous drainage of the GI tract

Answer 9

blood goes to liver portal system via portal vein

Question 10

name 4 differences between small and large intestiens

Answer 10

1. large is much shorter
2. large is much wider
3. large has crypts not villi
4. external longitudinal muscle is incomplete: forms 3 distinct bands - TAENIAE COLI - that contract to form sacculations - HAUSTRA

Question 11

describe the arterial and nervous supply of the large intestine

Answer 11

Determined by embryological origin:

• ascending colon + proximal 2/3 transverse (midgut derivatives): superior mesenteric artery and superior mesenteric plexus
• distal 1/3 transverse + descending and sigmoid colon (hindgut derivatives): inferior mesenteric artery and inferior mesenteric plexus

Question 12

describe 3 functions of the large intestine

Answer 12

1. water absorption (ascending and early transverse)
2. numerous bacteria: immune defence, digest small amounts of cellulose, produce vitamins (K, B12, thiamine…)
3. storage (late transverse and descending)

Question 13

how is the epithelium in ascending and early transverse colon specialise for water absorption

Answer 13

• ENaC (induced by aldosterone): Na+ reabsorption generated gradient for water mov.
• tight junctions between epithelial cells much tighter than in small intestine, preventing sig. amounts of ion back-diffusion

Question 14

describe the reflex pathways involved in defecation

Answer 14

1. GASTROCOLIC REFLEX (mediated by enteric NS): stimulates mass movements - intense and prolonged peristaltic contraction that clears an area of large intestine… push faeces into rectum
2. DEFECATION REFLEX (spinal reflex mediated by pelvic nn.): stimulated by distension of rectum… relex relaxation of internal anal sphincter… voluntary relaxation of external anal sphincter… defecation