This section describes the large intestine and its structures.

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Anatomy

The Large Intestine, so named for it's diameter, extends from the ileocecal valve to the anus and is attached to the posterior abdominal wall by the mesocolon.  The mesocolon is divided into ascending, transverse, descending, and sigmoid or pelvic portions, according to the segment of the colon to which it gives attachment.  The large intestine is approximately 1.5 to 1.8 meters (5-6 feet) in length and about 6 centimeters (2.5 inches) in diameter.

There are several principal regions of the colon:

The Cecum is a blind pouch measuring about 6 centimeters (2.5 inches) in length.  It is a sac-like portion of the large bowel which contains the entry of the appendix and the end of the small intestine.  It begins at the ileocecal valve and functions to guard the opening from the ileum to the large intestine and allows undigested materials to pass from the small to the large intestine.  It also prevents reflux of undigested material back into the ileum.

The Appendix or Vermiform Appendix is a small, finger-sized structure, approximately 8 centimeters (3 inches) in length and is attached to the inferior portion of the ileum and adjacent portion of posterior abdominal wall by the mesoappendix.   The adjective "vermiform" literally means "worm-like" and reflects the narrow, elongated shape of this intestinal appendage.  The appendix is longest in childhood and gradually shrinks throughout adult life. The wall of the appendix is composed of all layers typical of the intestine, but it is thickened and contains a concentration of lymphoid tissue.  The internal diameter of the appendix, when open, has been compared to the size of a matchstick. The small opening to the appendix eventually closes in most people by middle age.

The Colon begins at the open end of the cecum and is the longest part of the large intestine.  It is divided into three portions.  The Ascending Colon ascends on the right side of the abdomen and extends to the inferior surface of the liver.  This portion of the colon is located retroperitoneally.  The right colic (hepatic) flexure is an abrupt left turn in the colon at inferior surface of the liver and signifies the beginning of the Transverse Colon.  The Transverse Colon extends from the right colic (hepatic) flexure to the left colic (splenic) flexure and extends across the abdomen from right to left.  It is not located retroperitoneally and ends at the left colic (splenic) flexure, which is an abrupt right turn of the colon curving beneath the inferior end of the spleen.  The next area--the Descending Colon, extends downward from the left colic (splenic) flexure to the level of the iliac crest.  Like the Ascending Colon, it is located retroperitoneally.  The next area, the Sigmoid Colon, begins near the left iliac crest, projects medially and terminates at the Rectum which is at approximately the level of the third sacral vertebra.

 

The Rectum is the last 20 centimeters (8 inches) of the gastrointestinal tract and is located anterior to the sacrum and coccyx.  The terminal 2-3 centimeters (1 inch) of the rectum is called the Anal Canal.  The Anal Canal is comprised of mucous membranes arranged in longitudinal folds called anal columns.  It contains a network of arteries and veins.  The opening of the anal canal to the exterior is the Anus which is guarded by an internal sphincter of smooth (involuntary) muscle and external sphincter of skeletal (voluntary) muscle and is normally held closed.

 

Blood Supply

The arterial blood supply to the large intestine originates in the Superior and Inferior Mesenteric Arteries.  The superior mesenteric artery feeds branches supplying the right and transverse colons of the large intestine.  The Right Colic Artery supplies the ascending colon, the Middle Colic Artery supplies the transverse colon, and the Ileocolic Artery supplies the Ileum and ascending colon

The inferior mesenteric artery feeds branches supplying the left large intestinal structures.  The  Left Colic Artery supplies the transverse and descending colons.  The Sigmoid Arteries supply the descending and sigmoid colons.  The Superior Rectal Artery supplies the rectum.

 

All blood returning to the heart from the digestive tract returns via the Hepatic Portal Vein. Venous blood is drained from the colon from branches that form venous arches similar to those of the arteries. These eventually drain into the superior and inferior mesenteric veins, which ultimately join with the splenic vein to form the portal vein.   The Inferior Mesenteric Vein combines with the Splenic Vein to dump into the Hepatic Portal Vein.  The Left Colic Vein, the Sigmoidal Veins, and the Superior Rectal Veins are tributaries to the inferior mesenteric vein. The Superior Mesenteric Vein also dumps directly into the hepatic portal vein.  Tributaries of this vein include the Right and Middle Colic Veins and the Ileocolic Vein.

 

Innervation

The Large Intestine has its own intrinsic nervous system called the Enteric Nervous System (ENS) that functions independently and is influenced by the autonomic nervous system.  Parasympathetic stimulation tends to increase motility and secretion and to open valves.  Conversely, Sympathetic stimulation decreases motility and secretion and closes valves.

The entire structure of the ENS is arranged into two ganglionated plexuses. The larger, myenteric (Auerbach's) plexus, situated between the muscle layers of the muscularis externa, contains the neurons responsible for motility and for mediating the enzyme output of adjacent organs. The smaller, submucosal (Meissner's) plexus contains sensory cells that "talk" to the neurons of the myenteric plexus, as well as motor fibers that stimulate secretion from epithelial crypt cells into the gut lumen. It is responsible for reception of sensory stimulation (ie distention by food) The submucosal plexus contains fewer neurons and thinner interganglionic connectives than does the myenteric plexus, and has fewer neurons per ganglion. Electrical coupling between smooth muscle cells enables signals to rapidly alter the membrane potential of even those cells that have no direct contact with neurons and ensures that large regions of bowel--rather than small groups of muscle cells--will respond to nerve stimulation.

 

Histology

 

 

The gastrointestinal tract is composed of three microscopic layers. Each layer is important for either maintaining peristalsis or the digestive functions of the gut. The innermost layer is the Mucosa. The mucosa consists of specialized cells known as epithelial cells. These cells can be arranged in a single layer as seen in the esophagus, or in multiple layers as seen in the stomach and intestine. The epithelium serves to reduce friction and provide a protective barrier from the concentrated enzymes that are released into the inside of the intestine known as the lumen. This layer consists of simple columnar epithelium, lamina propria (areolar connective tissue), and muscularis mucosa (smooth muscle.)  Here, absorptive cells function primarily in absorption of water and goblet cells secrete mucous to lubricate contents as they pass.  There are no villi on goblet cells of the large intestine.  It is the first layer that nutrients must pass through to reach the blood stream. Without this barrier, other layers of the gut would be autodigested by enzymes and toxic substances would have free passage into the blood stream. The mucosal layer also consists of a thin layer of muscle tissue. This is referred to as the Muscularis mucosa. The main function of this portion of the mucosa is to aid in propelling nutrients in a uniform direction from the lumen to the submucosa. Finally, there is some connective tissue in the mucosa that serves to keep all the structures together and in somewhat fixed positions. The next layer encountered--moving from inside the lumen to the outside of the gut--is the submucosa. This is not considered a separate layer from the mucosa, but it does have some distinct properties. This layer is made up of connective tissue that contains blood vessels, nerves and lymphatics. Absorption into the blood stream takes place in this layer. Once the nutrients have successfully passed through the mucosal layer, they will come in contact with the blood vessels here. Through passive and active diffusion, the nutrients will be absorbed into the blood stream and carried through the circulatory system to the rest of the body. The nerves in this layer help coordinate peristalsis and absorption. Meanwhile, the lymphatic vessels carry immune system cells that help fight infection. When bacteria enters the gut and gets past the mucosal barrier into the submucosa, the cells in the lymphatic tissue will identify the foreign substance and attack and destroy the bacteria, thereby preventing the spread of infection. The second true layer encountered is the muscular layer of the gut (muscularis externa). This is composed of two discrete layers: an inner layer of muscle that runs in an up and down fashion (longitudinal muscle); and an outer layer of muscle that runs in a circular fashion (circular muscle).  Portions of the longitudinal muscle are thickened forming three longitudinal bands called taeniae coli. The longitudinal bands alternate with wall sections with little or no longitudinal muscle.  Bands run the length of most of the large intestine and tonic contractions of the bands gather the colon into haustra (series of pouches) – giving the colon its puckered appearance. The main function of these two layers is to provide peristalsis to the gut. The peristaltic movement is like that of an inchworm, creating a slow wave of contractions that starts in the esophagus and continues through the rectum. The two muscular layers work in a complimentary fashion to squeeze the food and products of digestion through the digestive tract. The slow waves of peristalsis are known as the migrating motor complex (MMC). These waves are constant, continue between meals, and are only interrupted by mass movements. The mass movements propel the food rapidly through the gut when challenged with food and products of digestion. The third and final layer of the gut is the serosa. This layer is mostly composed of connective tissue and gives strength to the long digestive tract. It helps suspend the gut in the thoracic and abdominal cavities by attaching itself to surrounding structures. Although not rigidly fixed, the organs and compartments of the gastrointestinal tract will remain in constant relationships to one another and surrounding organs thanks to their serosal attachments.

Physiology

The large intestine actively absorbs sodium from the ascending and transverse colon. This is then followed by the passive absorption of chloride and water. About 350ml is absorbed from the 500ml of chyme (thick liquid made of partially digested food and stomach juices; made in the stomach and moves into the small intestine for further digestion) entering the colon.  The hormone gastrin produced in the stomach relaxes the ileocecal sphincter allowing increased passage of chyme into large intestine following a meal. Peristalsis is a slow process in the large intestine at approximately one centimeter per hour and is stimulated by food and exercise. During these times, large segments of the ascending and transverse colon contract simultaneously which drives the feces about 1/3 of the way down the colon. These massive contractions are called mass movements and they push the feces into the descending colon, sigmoid colon, and rectum where feces is stored.  150 grams of fecal material then has to be eliminated. This includes 100 grams of water and 50 grams of solids. The length of time the food residue remains in the large intestine will determine the amount of water absorbed.  The large intestine also secretes an alkaline mucus. This lubricates the feces and facilitates their passage through the intestine. The mucus also contains bicarbonate which maintains colonic pH. The mucosa is also protected by the bicarbonate, which neutralizes acids produced by bacterial fermentation.  Stretch receptors of the rectal walls are stimulated as a result of mass movements. This initiates the defecation reflex. Defecation occurs when relaxation of the smooth muscle of the internal anal sphincter, and relaxation of the skeletal muscle of the external anal sphincter occurs. Voluntary control of the skeletal muscle of the external sphincter allows an individual to prevent defecation.

The last stage of digestion occurs via the activity of the bacterial flora of the large intestine.  Bacteria ferment remaining carbohydrates and release hydrogen, carbon dioxide, and methane gas.  Bacteria then convert some of remaining proteins to amino acids--some of which contribute to the odor of feces.  Bacteria also decompose bilirubin to more simple pigments giving feces its brown color.  They also produce several necessary vitamins as byproducts of their metabolism which are absorbed in the colon – including some B vitamins and Vitamin K.

 

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Basic Study Questions

  1.  Name the regions of the large intestine as digestion progresses.

  2.  Which cells absorb water and which lubricate the intestine?

  3.  What nervous system is responsible for gut innervation?

  4.  From what two major arteries does blood supply to the large intestine originate?

  5.  Blood supply from the large intestine returns via what major vein?

 

Additional Study Questions