br /><br />So constipation arises when feces remain in the colon too long because:<br /><br />  * the colon is naturally sluggish (has decreased ""motility,"" we say)<br />  * a low residue diet produces insufficient stimulation to the colon to trigger its normal propulsive activity and clear out the feces<br />  * a combination of the above two factors<br />  * there is obstruction of the lower GI tract<br />  o functional neurologic - the lower rectum and anus cannot properly relax to allow ev