It is highly adapted for digestion and absorption

The walls of small intestine are composed of four layers of tissues and there are some modifications of the peritoneum and mucous membrane

The surface area of the mucosa is greatly increased by permanent circular folds, villi and microvilli

It consists of three parts

The duodenum

The jejunum

The ileum

Is a C-shaped tube surrounding the head of the pancreas and is the shortest (25 cm long) but widest part of the small intestine.

It begins with the duodenal bulb and ends at the ligament of Treitz

Secretions from the gall bladder and pancreas are released into the duodenum through the hepatopancreatic ampulla, and the opening into the duodenum is guarded by the sphincter of Oddi

Is divided into four parts

Superior (First) Part

Descending (Second) Part

Transverse (Third) Part

Ascending (Fourth) Part

Is the middle section of the small intestine

In adult humans, the small intestine is usually between 5.5-6m long, 2.5m of which is the jejunum

The inner surface of the jejunum, its mucous membrane, is covered in projections called villi, which increase the surface area of tissue available to absorb nutrients from the gut contents

The ileum is the final section of the small intestine

It is separated from the cecum by the ileocecal valve

The function of the ileum is mainly to absorb vitamin B12 and bile salts and whatever products of digestion which were not absorbed by the jejunum

Onward movement of its contents which is produced by peristalsis

Secretion of intestinal juice

Completion of chemical digestion of carbohydrates, protein and fats in the enterocytes of the villi

Protection against infection by microbes that have survived the antimicrobial action of the hydrochloric acid in the stomach, by the solitary lymph follicles and aggregated lymph follicles

Secretion of the hormones cholecystokinin (CCK) and secretin

Absorption of nutrients.

Its length is about 1.5 metres long, which is about one-fifth of the whole length of the intestinal canal

Its subdivisions include

The cecum

Colon

Rectum

Anal canal

Is a pouch, connecting the ileum with the ascending colon of the large intestine

It is separated from the ileum by the ileocecal valve is considered to be the beginning of the large intestine

It is also separated from the colon by the cecocolic junction

The vermiform appendix is a fine tube, closed at one end, which leads from the caecum.

It is usually about 13 cm long and has the same structure as the walls of the colon but contains more lymphoid tissue

Has four parts

Ascending colon

Transverse colon

Descending colon

Sigmoid colon

This is a slightly dilated section of the colon about 13 cm long.

It leads from the sigmoid colon and terminates in the anal canal.

This is a short passage about 3.8 cm long in the adult and leads from the rectum to the exterior.

Two sphincter muscles control the anus

The internal sphincter, consisting of smooth muscle fibres, is under the control of the autonomic nervous system

The external sphincter, formed by skeletal muscle, is under voluntary control

The four layers of tissue described in the basic structure of the gastrointestinal tract are present in the large intestine.

The arrangement of the longitudinal muscle fibres is modified in the colon.

Running longitudinally along the surface of the large bowel are three strips of muscle, known as the teniae coli. They are called the mesocolic, free and omental coli.

The teniae coli contract to shorten the wall of the bowel, producing sacculations known as haustra.

The anal sphincters are formed by thickening of the circular muscle layer

Absorption

Microbial activity

Temporary storage of feces

Lubrication of feces by mucus

Defecation