ANATOMY OF THE GI TRACT

Before we discuss the treatment of colorectal cancer, let’s review a few points of anatomy.  Some of the terms that you will encounter during your treatment, like “lymph nodes” or “anal sphincter” may be unfamiliar to you. Knowing what they mean will help you understand the concepts better.

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Digestive Tract
Our digestive tract is a tube that starts at the mouth, includes the esophagus, stomach, small intestine and large intestine (also called colon), and ends at the rectum and anus.

As food travels down this canal, it is processed into small particles by various chemicals called enzymes. The particles are gradually absorbed into the blood stream, to be distributed throughout the body and to serve as a source of energy.

The large intestine is the last portion of the alimentary canal and consists of the colon and the rectum.  By the time the digested mixture gets down to the last part the colon, it is mostly waste (also called stool or feces).

Colon 
The colon is about five to six feet long.  In is divided into several parts: the ascending, transverse, and descending colon, which lie folded in your abdomen, and the sigmoid colon, which is mostly in the pelvis.  The main function of the colon is to absorb excess water from the stool  to make it firm.  When not enough water is absorbed, you may experience diarrhea.  If too much water is absorbed, the result might be constipation. The human body has an ability to compensate for the loss of part, or even most, of the colon without major loss of efficiency.

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Rectum and Anus
The last eight or ten inches of the digestive tract, the rectum, lies deep inside the pelvis. The rectum is the reservoir where stool is held prior to defecation.

The rectum ends at the anus—the opening to the outside of the body. A circular band of muscles, called the anal sphincter surrounds the anus to keep it closed, so that stool does not leak out. If cancer invades the area near the anal sphincter, or if the muscles around the sphincter need to be damaged during surgery to remove the cancer, the surgeon will create an alternative opening at the end of the digestive track, called a colostomy.

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Large Intestine Wall
The wall of the large intestine is composed of several layers. The serosa is the outer layer.  The muscularis is the middle layer, where action of muscle fibers squeeze the intestine, pushing the digested food forward. The inner layer is the mucosa. It contains cells that help absorb the digested food or the excess water, and help keep bacteria out of the blood stream.

The cells of the mucosa reproduce and die very rapidly—renewing the lining of the colon approximately every six days. This rapid turnover is one of the factors that may lead to cancerous changes within the cells.  It is in this layer that most types of colorectal cancer arise.

Blood Vessels and Lymphatic System
Arteries and veins carry blood to and from the intestine, supplying it with oxygen, and collecting nutrients to be used by the body. This network of blood vessels fans out in an apron-shaped layer called the mesentery. Cancerous tumors can create their own blood vessels to ensure the growth of the tumor.

An important concept to understand is the lymphatic system. Lymph is the fluid that leaks out of the blood vessels and accumulates between cells. Lymph ducts collect this fluid and return it to the main circulation. Along the way, lymphatic fluid is filtered through small bean-shaped structures called lymph nodes, which trap debris such as bacteria, or escaped cancer cells. You may think of the lymphatic system as a network of sewer lines, although it also plays an important role in your immune defense system.

By examining the lymph nodes for presence of cancer cells, one can get a good indication of whether the tumor has begun to spread from the colon to the rest of the body.

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