The last section of the digestive voyage was focused on pancreatic secretions into the small intestine.  This section will deal with the different parts and structure of this tubular organ, extending from the stomach’s pyloric sphincter to the large intestine, and will include secretions, absorption, and movement.

As mentioned before, digestion in the small intestine involves hepatic and pancreatic secretions.  It completes chyme digestion, absorbs nutrients, and delivers what remains to the large intestine.

The small intestine is composed of three sections – the duodenum, the jejunum, and the ileum. Located in front of the upper three lumbar vertebrae and the right kidney, the C-shaped duodenum is the shortest section, and the most fixed, in relation to the jejunum and the ileum.

The jejunum and the ileum are more mobile, and lie fairly uninhibited in the peritoneal cavity.  The jejunum is the longest section of the small intestine and is followed by the ileum.  There is no clear division between these two sections, but the diameter of the jejunum is generally larger with the wall being more substantial.

Supporting tissue called mesentery suspends the two from the abdominal wall, providing the intestinal wall with blood and lymph vessels, as well as nerves.  The greater omentum blankets the entire abdominal organ area.

The interior surface of the small intestine exhibits a velvet-like quality due to intestinal villi.  These countless, miniscule protrusions are mucous membrane structures, and are most abundant in the duodenum and the beginning of the jejunum.  They protrude into the alimentary canal lumen, and come into contact with the contents of the intestine.  The villi enhance the surface area of the lining, increasing digestive absorption.  The villus themselves are covered with microvilli, again increasing surface area resulting in increased absorption.  In addition, the small intestine features circular folds of mucosa called the plicae circulares, these further increase the surface area of this highly effective absorbing organ.

Immediately prior to absorption, enzymes embedded in the mivrovilli break down remaining food molecules.  In addition to peptidases, these enzymes also include sucrase, maltase, lactase, and intestinal lipase.  These enzymatic secretions, along with those from the intestinal mucosa and the pancreas, allow the lymph and blood capillaries found within each individual villus to cart away absorbed nutrients.

Segmentation is the name given to the peristaltic process in the small intestine, dividing the chyme and moving it back and forth.  Generally segmentation is a slow process, with the chyme usually taking three to ten hours to travel the length of the small intestine.  However, if the distension of the intestinal wall becomes too great, or if it becomes irritated, this movement may increase to what is know as a peristaltic rush.  When this occurs, the movement of the chyme into the large intestine is greatly accelerated. Water, electrolytes, and nutrients rush into the large intestine unabsorbed, resulting in diarrhea.

The next, and final, chapter of the Digestive Voyage will concentrate on the large intestine.