Lower G.I Tract

Small intestine

• The small intestine follows the stomach in the digesting of food and is divided into 3 parts:  the duodenum (most superior), the jejunum and the ileum (most inferior). The small intestine stretches ~6m in length and is where most of the digestion of food occurs.  

• Circular and longitudinal smooth muscles of the small intestine are used to push food along via peristaltic waves. Small processes called microvilli increase the surface area of the intestine to help in the absorption of nutrients from the chyme. Proteins and carbohydrates are absorbed via active transport and facilitated diffusion. Of the chymes water volume, around 90% is absorbed also.

• The duodenum secretes bile received from the liver (see accessory organs). Bile emulsifies fats to increase surface area for absorption in the small intestine. The digestion of lipids is also greatly influenced by bile as well as the neutralisation of chyme. The duodenum also secretes pancreatic juice received from the pancreas (see accessory organs) which contains various digestive enzymes including amylase and trypsinogen.

• Other enzymes present in the duodenum include maltase, sucrose and lactase which aid in digestion of sugars. The Brunner’s glands present in the submucosal layer of the duodenum secrete bicarbonate to protect itself from the acidic chyme. Another benefit of alkaline secretion is that enzymes present in the duodenum can remain active to aid digestion processes (e.g. the breakdown of proteins)

• The jejunum contains larger villi and microvilli for optimum uptake of nutrients. Fewer Brunner’s glands are present and the main function of the jejunum is to transport chyme to the final section of the small intestine – the ileum using peristalsis, whilst absorbing nutrients from the chyme.

• The ileum contains Payers patches which contain lymphatic nodules to help provide protection against harmful bacteria. The ileum function is to absorb bile salts and vitamin B12 from chime as well as any other nutrients that have not been taken up by the jejunum.

Large intestine

• The large intestine is around 5ft in length and is situated around the small intestine (see diagram) starting just below the ileac region of the pelvis crossing the majority of the width of the abdominal cavity ending at the anus. The main function of the large intestine is water absorption from the remaining undigested food, followed by removal of the remaining substance from the body. The large intestine consists of the caecum (preceding section of large intestine; see diagram below) and the colon (succeeding section of large intestine) and is separated from the small intestine by the ileocecal valve which allows only unidirectional movement through the valve.

• The colon consists of the ascending, transverse, descending and sigmoid colons. The ascending colon extends from the caecum to the turning point of the colon, known as the hepatic flexure situated at the underside of the right lobe of the liver. The transverse colon is the longest section of the colon and stretches with slight decline from the hepatic flexure to the left colic flexure which is situated beneath the spleen and adjacent to the pancreas tail. The descending colon passes besides the lateral side of the left kidney to the sigmoid colon. The S-shaped sigmoid colon typically lies in the pelvic region. The main function of the colon is to remove water from the degested food matter

• Finally the rectum, which is a continuation of the sigmoid colon ends in the anal canal. Here, movement of faeces into the rectum causes a reflex that causes the internal anal sphincter to open. Defication from the anus occurs if both the internal anal sphincter and the voluntary external anal sphincter are open simultaneously.

^{Images courtesy of https://training.seer.cancer.gov/ss_module07_ugi/unit02_sec03_anatomy.html and Wikipedia under commons licence}