A Short Guide to How the Colon and Rectum Work

When a person eats or drinks, food moves through the mouth, down the esophagus, and into the stomach. The food is digested in the stomach and nutrients from it are absorbed in the small intestine. Fluids from digestion are absorbed continuously as the waste, or fecal matter, moves through the length of the colon made up of the large and small intestine. The large intestine is a muscular tube 5 to 6 feet long with a diameter as wide as 5 inches. It is divided into two major areas: the colon and the rectum. The colon makes up approximately the first 5 feet of the large intestine. The last 10 inches to one foot is the rectum.

The colon is subdivided into five sections named for both their location and the direction that the waste moves through it.

• Liquid waste, called chime, enters the colon from the small intestine. The waste passes through an opening, called the ileocecal valve, into a pouch-like segment named the cecum.

• Circular muscle contractions, called peristalsis, move waste upward through the ascending colon. This section of the colon is located on the right side of the body so it is also called the right colon.

• The transverse colon carries waste across the abdomen to the left side of the body.

• Waste then travels downward through the descending, or left colon.

• The sigmoid colon is an S-shaped section that wraps back toward the center of the abdomen to the rectum.

The rectum comprises the last approximately 10 inches of the large intestine.

Solid waste remains in the sigmoid colon and the rectum until it is expelled through the anal canal.

The anus is the lowest opening of the digestive tract, where solid waste passes out of the body. The anus has two rings of sphincter muscles, one that is voluntary and another that is involuntary. The voluntary sphincter remains closed until a person has a bowel movement.

The structures of both the colon and rectum are comprised of four concentric rings of tissue. The inner lining of the colon and rectum is called the mucosa. It is the layer in direct contact with waste material and is constantly shedding and growing new cells.

The next layer, called the sub mucosa, supports the mucosa. The muscularis propria is a layer of strong muscle that makes the circular contractions, or peristalsis, to push waste through the entire length of the colon. The outermost layer of the colon is called the serosa.