: rectums or recta) is the final straight portion of the large intestine in humans and some other mammals, and the gut in others.

[3] Its diameter is similar to that of the sigmoid colon at its commencement, but it is dilated near its termination, forming the rectal ampulla.

The rectum follows the shape of the sacrum and ends in an expanded section called an ampulla where feces is stored before its release via the anal canal.

The microanatomy of the wall of the rectum is similar to the rest of the gastrointestinal tract;[9] namely, that it possesses a mucosa with a lining of a single layer of column-shaped cells with mucus-secreting goblet cells interspersed, resting on a lamina propria, with a layer of smooth muscle called muscularis mucosa.

[10] There are a higher concentration of goblet cells in the rectal mucosa than other parts of the gastrointestinal tract.

The rectum receives fecal material from the descending colon, transmitted through regular muscle contractions called peristalsis.

[11] An internal and external anal sphincter, and resting contraction of the puborectalis, prevent leakage of feces (fecal incontinence).

Colonoscopy and sigmoidoscopy are forms of endoscopy that use a guided camera to directly view the rectum.

[17] When symptoms such as weight loss, bleeding through the rectum, or pain are present, additional investigations such as a CT scan may be ordered.

[17] If constipation persists despite simple treatments, testing may also include anal manometry to measure pressures in the anus and rectum, electrophysiological studies, and magnetic resonance proctography.

[17] In general however, constipation is treated by improving factors such as hydration, exercise, and dietary fibre.

They induce a sensation of rectal fullness and contraction that frequently leads to defecation, but without the distinct waves of activity characteristic of peristalsis.

[19] English rectum is derived from the Latin intestinum rectum[20] 'straight gut',[21][22] a calque[23][24] of Ancient Greek ἀπευθυσμένον ἔντερον, derived from ἀπευθύνειν, to make straight,[25] and ἔντερον, gut,[25] attested in the writings of Greek physician Galen.

[23][24] During his anatomic investigations on animal corpses, Galen observed the rectum to be straight instead of curved as in humans.