Small Bowel

3-5m in life, 6-7m in death. 
Jejunum 40%, ileum 60%. 

 

Jejunum

Ileum

Diameter (cm)

3-4

2-5-3.5

Mucosal folds

Prominent
Thicker

Absent distally
Thinner

Length of vasa recta

Long

Short

Vascular arcades

1-2

3-5

Colour

~Pink (more vascular)

~White

Fat in mesentery

Not to wall

(windows)

To bowel wall

-           prominent mucosal folds mean that mucous membrane can be felt through the jejunal wall (a shirt sleeve through a coat sleeve): feels double the ileum.

-           Peyer’s patches are prominent in ileum also, giving the whitish mucous membrane appearance.

Small bowel mesentery (257)

Begins L of L1-2 ® oblique downward path to R S-I joint, crossing D3/D4, aorta, IVC, R ureter.
SMA and SMV enter mesentery where it crosses the duodenum, and run in base at first; then pass into mesentery towards the left.
Much longer mesentery than its root causes convolutions of the bowel.

Blood Supply of Small Bowel (286)

Entirely by SMA (branches: inferior p-d, middle colic, Rcolic, ileocolic &  15 intestinal branches.
- at termination, main branch carries on into mesentery, while ileocolic continues on in root of mesentery toward caecum; important for R hemi
- can help distinguish by noting branches coming off only on L of SMA.
Intestinal branches form vascular arcades ® vasa recta (end arteries; no intramural plexus cf stomach). 
If > 2-4cm of small bowel excluded from blood supply, will cause a colour change. 

Veins correspond.  -> SMV (291); don't always lie close to arteries

Lymph mural --> intermediate --> SMA.

Nerves: parasympathetic --> (motor and secretory).  Afferents (uncertain function).

Sympathetic (vasoconstrictor; T9-10).  Pain follows sympathetics, felt in umbilicus (T10)

Ileal Pouch
1. If necessary, make a relaxing incision in the peritoneum of the mesentery to allow length.
- if the apex can be brought to 6cm below the inferior symphasis pubis then should be fine.
2. Blood supply important
- need a preserved ileocolic artery, which may not be possible in cancer.
3. J pouch fashioned by side to side stapling
- then anvil in the enterotomy.

Meckel’s Div & other abnormalities

Meckel’s: rule of 2’s: 2% of people, 2 feet from caecum, 2in long (though in practice length variable and site may be more proximal).  Is intestinal end of vitello-intestinal duct, apex may adhere to umb.

-           heterotopic gastric mucosa in 10%, pancreatic mucosa less common.

-           has rudimentary mesentery with blood supply via ileal branch of SMA.