The terminal part of the large intestine is called the rectum. It is located between the sigmoid colon and the anus. The whole Latin name of rectum is rectus intestinum, which means “straight intestine.” It is part of the digestive system whose main function is to store digestive wastes until the time a person is ready to defecate.
In humans, the rectum has a length of about 4 to 6 inches (10 to 15 cm). At the junction between the sigmoid colon and the rectum, it has a circumference of about 6 inches (15 cm). The circumference increases to about 14 inches (35 cm) at the rectal ampulla, which is its terminal portion. This ampulla looks like a pear or a balloon, and provides a large storage capacity.
The rectal walls expand as the rectum becomes filled with waste materials from the digestive system. This expansion stimulates the stretch receptors located on the walls, which in turn trigger an involuntary urge to defecate. During defecation, the increased pressure within the rectal walls forces the anus to open. Similar to the propelling forces that act on food in the rest of the gastrointestinal tract, peristaltic waves also act on feces. The peristaltic mechanism, coupled with rectal shortening, leads to successful defecation.
When a person does not follow the urge to defecate, the rectum responds by returning the waste material to the sigmoid colon, where additional water absorption occurs. If a person ignores the urge to defecate for an extended period, too much water is reabsorbed from the waste materials. The end results are hardened stools and difficulty of defecation, or constipation.
The rectoanal junction is called the anorectal line or pectinate line. Above this line are distinct parts of the rectal mucosa called columns of Morgagni, or simply rectal columns, which are able to accommodate contracting or dilating forces. The pectinate line is important in classifying hemorrhoids. If a hemorrhoid is located above the pectinate line, it is called an internal hemorrhoid, which is painless. A hemorrhoid located below the pectinate line is called an external hemorrhoid, which is often painful.
A condition called rectal prolapse or "mick" can occur that causes the rectal walls to collapse into the anus. Rectal prolapse occurs when the supporting ligaments and muscles weaken, and also when there is increased pressure inside the pelvis. Rectal prolapse occurs in association with old age, prolonged constipation, frequent diarrhea, pregnancy, and nerve problems. In severe cases, the whole rectum protrudes through the anus, which must be surgically corrected.