Digestive bleeds can arise from mucosal ulcerations, growths that erode blood vessels, inflammation, ruptures of varicose veins within the oesophagus or rectum, or abdominal trauma. Although generally thought of as a distasteful subject, abnormal elimination, such as diarrhoea or vomiting, indicates that changes have occurred within the digestive system. Diarrhoea can be caused by overstimulation of bowel movements by the parasympathetic nervous system, which promotes digestive activity. Normally, bowel contents are propelled along by waves of muscle contraction at a speed that allows water to be absorbed, and therefore faeces to be well formed. Bowel wall movements push faecal matter into the rectum, stretching the rectal wall. This stretching triggers the defecation reflex. Assessing the risk of constipation requires keen observation of diet and fluid input, mobility, attempts at defecation, abdominal pain and mental alertness, and these are sometimes put together in risk assessment scales, which may be useful in some clinical areas.