The rectum is named the straight gut, though in humans it curves
Rectum is Latin for straight, a label inherited from the Greek physician Galen. The trouble is that Galen dissected animals, whose rectum really is straight. In people it bends along the curve of the sacrum, so one of anatomy's oldest names describes the wrong species.
The adult human rectum runs about 12 centimetres, from the end of the sigmoid colon around the third sacral vertebra down to the anal canal, which adds roughly another 4 centimetres. Near its lower end it widens into the ampulla, named after a Roman bottle, where stool waits before it leaves. Unlike the rest of the colon it lacks the three separate muscle ribbons called taeniae coli; they merge a few centimetres higher into one continuous sheet that wraps the whole rectum. Much of what we know of its detailed anatomy comes from Andreas Vesalius's descriptions in 1543.
Its lining is the standard gut pattern of a single layer of columnar cells, but with more mucus-producing goblet cells than anywhere else in the digestive tract. Where rectum meets anus the lining changes abruptly to several layers of flat cells. Blood arrives from two directions: the superior rectal artery serves the upper two thirds, and the middle and inferior rectal arteries the lower third. The upper rectum, called Sudeck's point, sits on the boundary between two circulations, which makes it vulnerable when blood flow falls.
Storage is regulated by pressure. As the walls stretch, receptors signal the urge to go once pressure passes about 18 mmHg, and reflex emptying follows around 55 mmHg. Continence depends on two sphincters and the resting tension of the puborectalis sling; relaxing that sling straightens the angle between rectum and anus. Curiously, stimulant laxatives such as senna trigger peristaltic waves in the colon but not in the rectum itself, producing a sense of fullness instead.
Its position makes it clinically useful. Rectal temperature typically reads about half a degree Celsius above oral readings, though ear and forehead thermometers have reduced the practice. Medicines can be absorbed there when swallowing is impossible, and an ultrasound probe placed in the rectum can image the nearby prostate.
Source: Rectum
