The lower GI tract

The lower end of the gastrointestinal (GI) tract is not one that people generally want to think about too much… until it causes problems.

In the lower GI tract both water and nutrients are absorbed and waste products are excreted from the body.

As chyme – food that has been physically and chemically broken down during its journey through the upper GI tract – exits the duodenum, it continues into the next two sections of the small intestine: the jejenum and the ileum. These are very similar in structure to each other, with the walls of the intestinal lining being folded and covered in finger-like projections called villi, which increase the surface area for nutrient absorption. Villi are covered in a mixture of goblet cells, which secrete mucus, and absorptive cells, which facilitate the movement of water and nutrients into the bloodstream. 

The small intestine ends with the ileocolic sphincter muscle, which ensures the chyme moves only one way into, rather than backwards from, the large intestine.

Over the next one and a half metres of tubing, chyme continues to be broken down, this time by bacteria. The first section of the large intestine is the caecum – a small pouch that acts as a reservoir for chyme – to which the appendix is attached. The colon, which is the largest part of the large intestines, follows, and this is divided into four regions. The first two are the ascending colon and transverse colon and it is here that water, sodium and some medicines are absorbed. What is left of the chyme is now referred to as faeces and is stored in the descending colon and sigmoid colon.

The structure of the colon is different to that of the rest of the GI tract, with layers of muscles running lengthwise with small breaks in between; this gives rise to the segmented appearance of the large intestine. There are three types of muscle movement that take place here: peristalsis, which pushes the contents along; segmentation, which causes churning and hence absorption of nutrients; and mass movement, which is a strong pulsation that occurs after eating and causes a significant shift in faeces along the tract.

At the end of the colon lies the rectum – a muscular cavity that acts as a short-term storage facility for faeces. It is sealed from the outside world by the anus, which is comprised of two sphincter muscles. The inner one which relaxes involuntarily as a result of mass movement of the colon and gives rise to the urge to defaecate. The outer sphincter opens under conscious control when the individual goes to the toilet to open their bowels.