Colorectal cancer affects the lining of the large intestine and rectum.

What is going on in the body?

The colon is also called the large intestine. The colon begins near the junction of the small intestine and extends to the rectum. The colon has four parts:

  • the cecum, which lies on the right side of the body
  • the ascending colon, which rises slightly as it crosses from the right to the left side
  • the transverse colon, which runs across the abdomen
  • the descending colon, which drops down from the left side of the body
  • the sigmoid colon, a U-shaped bend of bowel that leads toward the rectum

Colorectal cancer starts in the lining, or mucosa, of the bowel. It usually develops in one area of the bowel over a long period of time. It occurs on the left side in the descending colon 40% to 50% of the time. The cancer grows along the opening in the colon. It also can grow further into the lining and muscle tissue.

Like other tumors, colorectal cancer can spread to lymph nodes and other parts of the body.

What are the signs and symptoms of the disease?

The colorectal tumor can bleed into the inside of the bowel. Symptoms may include:

  • rectal bleeding
  • dark stools called melena, caused by blood in the stools
  • anemia, which is a low blood cell count, from blood or iron loss
  • changes in bowel habits, such as the frequency of bowel movements
  • smaller stools
  • mucus discharge from the rectum
  • vague abdominal distress
  • gas pain
  • hemorrhoids, which are dilated blood vessels in the rectal area.

A tumor can narrow or block the bowel. It can also perforate the bowel, causing infection or bleeding into the abdominal cavity. When colorectal cancer spreads to other sites in the body, it can cause:

  • liver cancer
  • jaundice
  • pain in the liver
  • loss of appetite and weight loss
  • lung cancer

Rarely, swollen lymph nodes are a sign of colorectal cancer. Unusual health problems sometimes associated with colorectal cancer include:

  • thrombophlebitis, an inflammation in the veins of the lower leg
  • unusual syndromes that change skin coloring
  • muscle problems