Bowel Loops

It is necessary to recognize the axis of the root of the mesentery and the undulating, changeable nature of the small bowel loops to understand the basis for the radio-logic identification of their mesenteric and antimesen-teric borders.1 Figure 14-9 illustrates certain constant geometric properties of the mesentery. The base of the mesentery, from which the jejunum and ileum are suspended, is essentially a series of alternating convexities and concavities connected by intervening limbs. Any single limb is thus shared by a convexity and a concavity on either side. A limb or straight segment of small bowel, therefore, connects one convex loop with the next con cave loop. Any curved line presents basically two surfaces—one convex, the other concave—determined by the point of reference. In the small bowel, the point of reference used is the root of the mesentery. By definition, therefore, the concept of convexity or concavity of bowel loops is determined by the axis of the root of the small bowel mesentery. Figure 14-9, top illustrates that limbs 1 and 2 constitute the convex loop A, limbs 2 and 3 form the concave loop B, and limbs 3 and 4 produce the convex loop C.

Another phenomenon of the ruffled nature of the mesentery and small bowel loops results, depending on positional relationships to the mesenteric root. If rotation occurs toward the left (Fig. 14-9, right), there is no essential change in the convex-concave relationships of bowel loops. However, if rotation on the mesenteric root occurs toward the right (Fig. 14-9, left), there may be a complete reversal of these relationships. Figure 149, left shows that now loop A is concave, B, convex; C, concave; etc. It must also be recognized that in vivo the small bowel loops do not act synchronously in rotating in position on the mesenteric root. One coil may be

Small Ruminant Elbow Radiographs

Fig. 14—8. Changeable position of small bowel loops.

(a and b) Radiographs of the intubated small intestine 1 day apart, demonstrating striking variations in the positions of the bowel loops. (X marks the duodenojejunal junction).

(c and d) Tracings illustrate that changes in position and arcing convexities of some bowel segments are more marked than in others. (Reproduced from Meyers. )

Fig. 14—8. Changeable position of small bowel loops.

(a and b) Radiographs of the intubated small intestine 1 day apart, demonstrating striking variations in the positions of the bowel loops. (X marks the duodenojejunal junction).

(c and d) Tracings illustrate that changes in position and arcing convexities of some bowel segments are more marked than in others. (Reproduced from Meyers. )

Fig. 14-9. Diagram of the ruffled edge of the mesentery.

This immediately suspends the small bowel loops. (Top) Vertical (sagittal). (Left) Rotation to the right. (Right) Rotation to the left. The dashed line indicates the root of the small bowel mesentery.

(Reproduced from Meyers.1)

Small Bowel Mesentery

Fig. 14-9. Diagram of the ruffled edge of the mesentery.

This immediately suspends the small bowel loops. (Top) Vertical (sagittal). (Left) Rotation to the right. (Right) Rotation to the left. The dashed line indicates the root of the small bowel mesentery.

(Reproduced from Meyers.1)

horizontal (transverse), the next vertical (sagittal), the next in the coronal plane, and the next obliquely oriented between any of these three planes. The variability is further increased by the fact that these mesenteric projections may extend toward the left or toward the right and may alternate. Position of the patient, peristaltic dynamics, and factors of intraabdominal pressure are likely determinants.

Thus, several conclusions can be derived from these observations that can be accurately applied in the radiologic-anatomic identification of small bowel loops.1 Figure 14-10 illustrates these:

1. The concave margin of a small bowel loop, facing toward the axis of the root of the mesentery is the mesenteric border. This is true whether the loop is positioned to the left or to the right of the mesenteric root. This is shown in segments 1, 2, 5, and 6 in Figure 14-10.

2. The convex margin of a small bowel loop, facing away from the axis of the root of the mesentery, constitutes the anti-mesenteric border. This is also true, of course, whether the loop projects to the left or to the right of the mesenteric root. *

3. Even if loops are suspended in the midline, their mesenteric and antimesenteric borders can be identified similarly if they project in the coronal plane. This is shown in segment 3 in Figure 14-10.

4. The precise distinction between mesenteric and antimesenteric borders can be routinely applied in a detailed small bowel series (Fig. 14-11). When viewed in a frontal x-ray, a loop projecting in the sagittal or horizontal plane may not present its mesenteric and anti-mesenteric borders for definition. This occurs most often near the midline overlying the mesenteric root, as shown in segment 4 of Figure 14-10. Pressure films or radiographs with the patient positioned obliquely may render such a loop convex and allow delineation of its mesenteric and antimesenteric borders. Viewed in the axial or sagittal plane, these intestinal borders are easily appreciable on computed tomography or magnetic resonance imaging (Figs. 14-12 and 14-13).

* It can further be appreciated that consequent to the fixation of the mesenteric ruffles, the convex margin of a bowel loop facing toward the axis of the root of the mesentery may at times also constitute the mesenteric border of that particular loop (Fig. 14-1 and loops A and C in Fig. 14-9b). However, while this may be anatomically true, its radiologic application would be untrustworthy.

Fig. 14—10. Variable orientations of small bowel loops to mesenteric root.

The mesenteric border (arrows) constitutes the concave margin of a loop facing the root. The antimesenteric border is the convex margin of a loop facing away from the root. (Reproduced from Meyers.1)

Antimesenteric Border BowelAntimesenteric Border Bowel

Fig. 14—11. Biphasic enteroclysis study of the small intestine.

Suspended by the fanlike ruffles of the mesentery, the 2022 ft of small bowel course in an undulating pattern, resulting in the variously termed coils of bowel or intestinal loops. Each coil or loop presents a concave mesenteric and convex antimesenteric contour. The normal folds are symmetric.

Mesenteric Margin Colon

Fig. 14—12. Intestinal borders.

Computed tomography identifies fat-containing leaf of mesentery with vascular branches extending to the concave mesenteric border of small bowel loop (SB).

Fig. 14—12. Intestinal borders.

Computed tomography identifies fat-containing leaf of mesentery with vascular branches extending to the concave mesenteric border of small bowel loop (SB).

Wiley Barnard

Fig. 14—13. Intestinal borders.

Two different examples of tuberculous peritonitis illustrate the radiation of thickened vessels coursing through the mesentery to the mesenteric borders of multiple small bowel loops.

(Courtesy of Emil Balthazar, M.D., Bellevue Hospital, New York University School of Medicine, New York, NY.)

(b) Sagittal Tl-weighted fat suppressed MR image. The terminal intestinal branches (arrows) fan out to the bowel loops. Ascites is present.

(Reproduced from Semelka RC, Ascher SM, Reinhold C: MRI of the Abdomen and Pelvis. Wiley-Liss, New York, 1997.)

Fig. 14—13. Intestinal borders.

Two different examples of tuberculous peritonitis illustrate the radiation of thickened vessels coursing through the mesentery to the mesenteric borders of multiple small bowel loops.

(Courtesy of Emil Balthazar, M.D., Bellevue Hospital, New York University School of Medicine, New York, NY.)

(b) Sagittal Tl-weighted fat suppressed MR image. The terminal intestinal branches (arrows) fan out to the bowel loops. Ascites is present.

(Reproduced from Semelka RC, Ascher SM, Reinhold C: MRI of the Abdomen and Pelvis. Wiley-Liss, New York, 1997.)

Essentials of Human Physiology

Essentials of Human Physiology

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  • J Lowe
    What is bowel loop in human body?
    1 year ago

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