Large Intestine – Tutorial
Please read Unit 10 – Introduction to Digestive System Tissues and study the image slider of the four major layers of the digestive tract wall prior to completing the activities in this chapter.
Introduction to the Large Intestine
The large intestine (large bowel) is a large diameter tube-like organ measuring approximately 3 inches (7.5 cm) in diameter and approximately 5 feet (1.5 m) in length that forms the last segment of the human digestive tract. The primary functions of the large intestine are to reabsorb water from digestive tract waste and temporarily store digestive waste until it is removed from the body by the process of defecation. The large intestine is shaped like an upside-down U, and it outlines much of the abdominal cavity inferior to the liver and stomach, essentially surrounding the coils of the small intestine. The large intestine is divided into three parts known as the cecum, colon, and rectum.
The first portion of the large intestine is the cecum, which is essentially a blind-ending pouch in the right iliac region of the abdominal cavity where contents of the ileum of the small intestine pass through the ileocecal valve (sphincter) and enter the large intestine. The appendix, which is a narrow, worm-like tube, is attached to the posterior-medial border of the cecum.
The colon is the longest portion of the large intestine, beginning at the cecum in the right iliac region of the abdominal cavity and ending at the rectum. The colon is divided into four segments: the ascending colon, transverse colon, descending colon, and sigmoid colon. The four segments of the colon essentially outline the entire abdominal cavity inferior to the liver and stomach and surround the coils of the small intestine. The wall of the colon is relatively thin compared to its diameter due in part to a relatively thin muscularis externa. The longitudinal layer of smooth muscle in the muscularis externa does not surround the entire circumference of the wall and is reduced to three longitudinal bands known as teniae coli that run along the entire length of the colon.
The rectum is the last portion of the large intestine extending from the inferior border of the sigmoid colon through the pelvic outlet and terminating at the anus. The rectum has an expandable wall that stretches to store feces prior to defecation. Two sphincter muscles control the opening and closing of the anus. The internal anal sphincter is composed of smooth muscle and under involuntary control. The external anal sphincter guards the most distal portion of the anal canal and must voluntarily relax to allow feces to leave the body during defecation.
The lumen of the large intestine is lined with simple columnar epithelium with large numbers of giant goblet cells that line the surface of intestinal glands that penetrate deep into the mucosa. The mucus produced by intestinal glands serves to lubricate digestive waste as it progresses along the lumen and becomes more compacted through the gradual process of water reabsorption. The intestinal glands of the rectum are similar in composition to those of the colon, but the intestinal glands of the rectum tend to be shorter and lined with even more goblet cells.
Tutorial: Use the image slider below to learn more about the structure and characteristics of the large intestine.
Microscopy: Use the image slider below to learn how to use a microscope to study the large intestine on a microscope slide.
Tutorial: Use the hotspot image below to learn more about the structure of the wall of the large intestine on a low power image.
Tutorial: Use the hotspot image below to learn more about the structure and function of the large intestine.
