Structure of the tissue

A fundamental goal in constitutive modelling is to predict the mechanical behaviour of a material under various loading states. A biological tissue is a collection of cells, and extracellular matrices, that perform various specialized functions. There are four basic primary tissues types: muscular, nervous, epithelial and connective. Muscle tissue produces mechanical work through contraction–relaxation. For example, skeletal muscles are responsible for locomotion primarily through voluntary muscle contraction, cardiac muscles provide the active pumping of blood from the heart, and smooth muscles, which are part of the organs of the digestive tract, facilitate peristalsis, propulsion, microcirculation, etc.

Nervous tissue provides communication among organs and systems predominantly by electrical signals. Neurons are responsible for the production and propagation of the waves of depolarization in the myelinated and unmyelinated nerve fibres, smooth muscle syncytia and other cell aggregates. Neuroglial cells are a diverse group of morphoelements that play a supportive, mainly trophic, role.

Epithelial tissue covers the outer and inner surfaces of most of the organs. Various types of cells line the digestive, reproductive and urinary tracts, blood vessels, ducts, etc. They act as a protective barrier and are instrumental in selective regulation of the transport of specific agents and substrates.

Connective tissue includes a diverse set of cells surrounded by a large amount of extracellular matrix. Its main function is to provide a level of mechanical support to the organ.

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.