The Colon (Large Intestine): Function and Anatomy

The colon (also known as the large intestine) is a U-shaped, segmented tube made of muscle that is located below the stomach and connects to the rectum at the end of the colon. The colon and rectum are about 2 meters (6.5 feet) long.

The large intestine lays around the edges of the abdominal cavity and is surrounded by the spleen, liver, pancreas, bladder, and reproductive organs.

The colon works alongside the stomach and small intestine to remove stool and maintain fluid and electrolyte balance. Understanding how the colon works is the first step to keeping your colon healthy.

Anatomy

The colon makes up most of the large intestine, beginning with where it connects to the small intestine, called the cecum. The five other sections of the colon include the following:

• Ascending colon (up from the cecum, in the lower right of the abdomen)
• Transverse colon (across the abdomen)
• Descending colon (down toward the final colon portion)
• Sigmoid colon (final part of the colon)
• Rectum (final part of digestive tract before the anus)

The cecum is the widest part of your colon and is approximately 5 centimeters (cm) long (almost 2 inches). It's part of what's also called the proximal, or right-sided, colon, including the ascending and transverse portions.

• Ascending colon: The ascending colon is about 10 cm long and begins at the hepatic flexure at the cecum. It runs along the right side of the abdomen, upward to beneath the ribcage, and connects to the transverse colon.
• Transverse colon: The transverse colon is about 50 cm long and lies close to your stomach, liver, and gallbladder. It connects to the descending colon, where the distal, or left-sided, part of the large intestine begins.
• Descending colon: The descending colon is about 10 cm long.
• Sigmoid colon: This portion of the colon is relatively S-shaped and gets its name from the Greek letter sigma. It is about 50 cm long and its primary function is to absorb water, vitamins, and some minerals from undigested food particles.
• Rectum: The final section of the colon is the rectum which runs about 15 cm long.

A tortuous colon is one that is longer than normal. In this relatively rare condition, the colon has extra twists and turns to be able to fit in the abdomen.

Function

The majority of the nutrients you eat are absorbed in the small intestine well before the "food" reaches your colon. The colon's main function is to receive chyme, the mix of digested food from the small intestine, and further digest it into a firm stool that passes from the body.

The colon must reabsorb water and electrolytes to form a stool. This is why, when you are dehydrated, you can get constipated and your stool may become hard and difficult to pass. The colon needs to pull more fluid from the stool for your body to use.

It is possible to live without a colon. People have portions of their colon removed in surgery, called surgical bowel resection, to treat several conditions including colon cancer. However, all 6 feet of your colon serve a purpose.

Layers of the Colon

The colon is about 6 feet long and is composed of four layers, each with a specific function. Starting at the innermost layer of the colon, the layers include the following:

• Mucosa: Divided into three sub-tissues, the mucosa surface is called the epithelium, where most colon and rectal cancers originate. The mucosa provides a lubricant, which helps aid in the passage of stool through the colon.
• Submucosa: The next layer of your colon, which is rich in blood vessels and nerves. The submucosa is a connective tissue layer connecting the mucosa to the next muscle layer.
• Muscularis Propria: The third layer is composed of opposing layers of muscle fibers—a set that runs horizontally and a set that runs around the colon. Once cancers reach through this layer, there is an increased chance that they can metastasize to other parts of the body.
• Serosa: This is the outermost layer of your colon. When cancer spreads through the serosa it has left the colon and metastasized.

When a diagnosis of colon cancer is made, the pathologist will determine what layer the cancer has reached (such as the innermost or outermost layer) to help determine the staging of the cancer.

The majority of colorectal cancers will start in the innermost layer, called the mucosa, and spread toward the outermost layer, or the serosa, if untreated.

Signs of a Colon Problem

Symptoms that might indicate a problem with your colon include:

• A change in bowel habits, such as diarrhea, constipation, or narrowing of the stool, that lasts for more than a few days
• A feeling that you need to have a bowel movement that’s not relieved by having one
• Rectal bleeding with bright red blood
• Blood in the stool, which might make the stool look dark brown or black
• Abdominal cramping or belly pain
• Weakness and fatigue
• Unintended weight loss

How to Keep Your Colon Healthy

Your colon is a simple organ with simple needs. Keep it hydrated and clean (by keeping waste moving through), and get routine screening exams to check the inner lining for polyps—growths that can precede cancer.

Many lifestyle-related factors have been linked to colorectal cancer. More than half of all colorectal cancers are linked to risk factors that can be changed, such as:

• Diets high in fat or sugar
• Excessive red meat consumption (more than 12 to 18 ounces a week
• Smoking
• Obesity
• Type 2 diabetes
• Excessive alcohol consumption

Help your colon function at its best by:

• Eating a high-fiber diet rich in whole grains, fruits, and vegetables
• Drinking plenty of water daily
• Limiting red meat and processed meat (hot dogs, salami, bacon, sausage, ham, etc.) consumption
• Getting daily exercise
• Avoiding tobacco products, including cigarettes, e-cigarettes, and snuff
• Limiting your alcohol intake
• Talking to your healthcare provider about a daily multivitamin if your diet is poor
• Doing any necessary screening exams to monitor your colon’s health
  

Summary

The colon, or large intestine, is the part of your gastrointestinal system that moves digested food from the small intestine and out of the body. Different segments make up the colon, beginning with the cecum and ending with the rectum, which will release stool to pass from the anus.

Colon health depends on several lifestyle factors, including lifestyle decisions about diet, exercise, smoking, and excessive alcohol use. Following a healthy lifestyle and routine colonoscopy screenings can help to prevent and detect cancer. Early detection saves lives, so discuss any colon health concerns with your healthcare provider.