10.13.20 Lesson #7 Large Intestine

Here we are, almost at the end of our journey through the north to south process of digestion! Today we are going to talk about the large intestine which is about 5 feet long and about 2.5 inches in diameter. It starts at the ileocecal sphincter followed by the cecum, colon, rectum, and anal canal. It is all attached to the abdominal wall by mesentery, which is an organ with its own story!

The ileocecal sphincter connects the small intestine to the large intestine just to the inside of your right hip bone. The little room where this valve opens into is called cecum. The coiled tube at the lower end of the cecum is your appendix! Despite the rate at which these are removed, the appendix has the important role of storing a high density of lymph nodes that control the bacteria entering the large intestine. These can trigger an immune response.

From the cecum, chyme heads up the right side of the abdomen as the ascending colon, reaches the underside of the liver, turns left and goes through your middle near your lower ribs (transverse colon) until it reaches the underside of your spleen, then heads down the left abdomen to the top of the left hip bone (descending colon) where it curves toward the middle of your body (sigmoid colon), and then goes directly south to the rectum. The last 1 inch of rectum is made of the anal chanal and anus.

• Blausen.com staff (2014). "Medical gallery of Blausen Medical 2014". WikiJournal of Medicine 1 (2). DOI:10.15347/wjm/2014.010. ISSN 2002-4436.

You will notice the colon has a lumpy, puckered look. This is due to muscles on the outer wall and inner wall that run the length of the colon. This makes a series of pouches called haustra. While there are absorptive cells with microvilli present, there are no other structural features to increase surface area like there is in the small intestine.

The main goal of the large intestines is to convert chyme to feces. Feces are made of indigestible fibers, unabsorbed digested materials, bacteria and products of bacterial decomposition, mucosa, old cells from the GI tract, inorganic salts, and water. This mass is moved along by slow waves of peristalsis. While it is moving along, bacteria are fermenting any remaining carbohydrates and proteins. The amino acids and bilirubin (from bile) are broken down giving feces their brown color! Some B vitamins and Vitamin K are bacterial products that are absorbed in the colon along with some water and ions.

While the large intestine seems quite straight forward, it has a large impact on our health and wellbeing. Come back next week for a quick look at one of the fastest developing new frontiers in health, our microbiome! Email me if you have any questions or other areas you’d like to delve into!

References:

Tortora and Derrickson, Introduction to the Human Body, 10th Edition

NTA slides 2019, Digestion and Elimination