The rectum is the final segment of the large intestine that connects the colon to the anus. It stores fecal matter produced in the colon until the body is ready to eliminate the waste through the process of defecation.
The rectum is a hollow muscular tube about 8 inches (20 cm) in length and 2.5 inches in diameter at its widest point. It extends from the inferior end of the sigmoid colon along the anterior surface of the sacrum and coccyx in the posterior of the pelvic cavity. At its inferior end, the rectum tapers slightly before ending at the anus.
- The mucosa forms the innermost layer of the rectum that is in contact with fecal matter. The mucosa is made of epithelial tissue that secretes mucus from specialized cells known as goblet cells. Mucus helps to protect the walls of the rectum and lubricate the feces as they pass through the rectum.
- Deep to the mucosa is the submucosa layer that supports the other layers of the rectum. Many blood vessels and nerves pass through the submucosa to provide nutrients, oxygen, and nerve signals to the mucosa and muscle tissue.
- Next is the muscularis layer, which contains layers of visceral (smooth) muscle. Contractions of the muscularis allow the rectum to expel feces during defecation.
- Finally, the serosa forms the outermost layer of the rectum and protects it from external damage. The serosa is made of a thin layer of simple squamous epithelium that secretes serous fluid to lubricate the exterior of the rectum and prevent damage caused by friction between the moving organs of the pelvic cavity.
Feces enter the rectum from the sigmoid colon, where they are stored until they can be eliminated through defecation. While feces are stored in the rectum, the walls of the rectum absorb some water and return it to the blood supply. Bacteria continue the fermentation of organic fecal matter that began in the colon and liberate some remaining nutrients that are absorbed by the rectal walls.
As feces accumulate and fill the rectum, they exert increasing pressure on the rectal walls. The distention of the rectum stimulates stretch receptors in the rectal walls to send nerve impulses to the brain. These impulses are integrated in the brain and result in feelings of discomfort and mounting pressure to empty the rectum through defecation. They also cause relaxation of the smooth muscle of the internal anal sphincter to allow defecation to proceed.