Anatomical and physiological background

The small intestine is a long cylindrical tube that extends from the stomach to the caecum of the colon. The absolute length of the small bowel generally makes up to 80% to 90% of the entire gut length. In the abdomen most of the intestine is loosely suspended by the mesentery and it is looped upon itself. The diameter of the intestine is not constant but gradually decreases from the proximal to the distal part. For example, the diameter of the duodenum is 25–35 mm, that of the jejunum is ∼30 mm and that of the ileum is 20–25 mm.

The intestinal wall is a biological composite formed of four layers: mucosa, submucosa, muscular and serosa. The mucosa is the innermost layer and its primary function is to digest and absorb nutrients.

The submucosa consists mainly of connective tissue and serves a purely mechanical function. Septa of connective-tissue fibres carrying nerves, blood and lymphatic vessels penetrate into the muscle layer and form a fibrillary three-dimensional network. It maintains a stable organization of the wall and allows the intestine to undergo reversible changes in length and diameter, offering remarkable properties of stiffness and elasticity.

The muscle coat is made of two smooth muscle layers – a thick (inner) layer of circumferentially oriented smooth muscle cells and a thin (outer) layer of longitudinally oriented muscle elements. The two layers are distinct and separate, although there are intermediate bundles that pass from one layer to the other.