Ganong's Review of Medical Physiology, 23rd Edition

CHAPTER 28

Gastrointestinal Motility 477

defecation occurs. Defecation is therefore a spinal reflex that
can be voluntarily inhibited by keeping the external sphincter
contracted or facilitated by relaxing the sphincter and con-
tracting the abdominal muscles.

Distention of the stomach by food initiates contractions of

the rectum and, frequently, a desire to defecate. The

response is called the

gastrocolic reflex,

and may be

amplified by an action of gastrin on the colon. Because of the

CLINICAL BOX 28–5

Constipation

Constipation refers to a pathological decrease in bowel

movements. It was previously considered to reflect changes

in motility, but the recent success of a drug designed to en-

hance chloride secretion for the treatment of chronic consti-

pation suggests alterations in the balance between secretion

and absorption in the colon could also contribute to symp-

tom generation. Patients with persistent constipation, and

particularly those with a recent change in bowel habits,

should be examined carefully to rule out underlying organic

disease. However, many normal humans defecate only once

every 2–3 d, even though others defecate once a day and

some as often as three times a day. Furthermore, the only

symptoms caused by constipation are slight anorexia and

mild abdominal discomfort and distention. These symptoms

are not due to absorption of “toxic substances,” because

they are promptly relieved by evacuating the rectum and

can be reproduced by distending the rectum with inert ma-

terial. In western societies, the amount of misinformation

and undue apprehension about constipation probably ex-

ceeds that about any other health topic. Symptoms other

than those described above that are attributed by the lay

public to constipation are due to anxiety or other causes.

FIGURE 28–8

Responses to distention of the rectum by

pressures less than 55 mm Hg.

Distention produces passive tension

due to stretching of the wall of the rectum, and additional active ten-

sion when the smooth muscle in the wall contracts.

(Reproduced with

permission from Davenport HW:

A Digest of Digestion,

2nd ed. Year Book, 1978.)

Pressures

Distention of rectum

Active

Passive

Some More Still more

Rectum

Internal

anal

sphincter

External

anal

sphincter

FIGURE 28–9
Sagittal view of the anorectal area at rest (above) and during straining (below).
Note the reduction of the anorectal an-
gle and lowering of the pelvic floor during straining.
(Modified and reproduced with permission from Lembo A, Camilleri, M: Chronic constipation. N Engl J Med
2003;349:1360.)

A AT REST

Pubis

Puborectalis

External anal

sphincter

Internal anal

sphincter

Anorectal

angle

Coccyx

B DURING STRAINING

Anorectal

angle

Descent of the pelvic floor