Digestive Disorders

Disorders and diseases in the digestive system are among the most common problems in the body. Some or these, such as vomiting, are normal responses to ingesting toxins or excess food, while others, such as ulcers, may have complicated causes including stress.

Vomiting

Vomiting is a useful physiological defence response and aids in the rejection of undesirable food from the stomach by expelling it out through the oesophagus and mouth. The vomiting reflex is initiated by activation of sensory receptors in the stomach wall and both chemoreceptors and stretch receptors may be involved. Among the stimuli normally activating these receptors is the presence of too much food, which causes excessive stretching of the stomach. Poisons and microbial toxins initiate vomiting by acting on the chemoreceptors of gastric mucosa. These sensory signals are communicated by sensory fibres in the vagus to a vomiting centre in the brain medulla. This centre also responds to certain toxic substances in the blood. Activation of the vomiting centre results in a complex of reflex responses: the glottis closes to keep the vomit from entering the respiratory passages: the cardiac sphincter in the lower oesophagus opens, massive contractions of the abdominal and respiratory muscles occur to exert external pressure on the stomach the vagus nerve to stimulates the stomach vigorously. Finally, a strong wave of reverse peristalsis moves from the pylorus to the cardia. As a result the stomach contents are expelled eliminating the source of toxicity and discomfort.

Gastrointestinal ulcers

Gastrointestinal ulcers are wounds occurring in the inner lining of the stomach and small intestine, particularly in the duodenum. Only 10% of ulcers occur in the stomach. The rest are associated with the duodenum. Stomach ulcers are, however, the more dangerous. There is little doubt that ulcers are caused by the corrosive and noxious effects of acid on the gut wall. At first these ulcers are superficial, but if exposure to acid continues unchecked, the wound deepens reaching the vascular layers deep in the wall and bleeding occurs. The bleeding is worsened by digestion of food which increases both acid secretion and stomach motility. This bleeding, which makes ulcers painful and dangerous, can be detected by the presence of fresh blood clots in the faeces. Several factors and conditions may contribute to ulcers. One is the excessive production of acid which may result from increased activity of the vagus nerve or gastrin secretion. Indeed, in many serious ulcer cases cutting of the vagus nerve to the stomach (vagatomy) markedly ameliorates the condition. The cause of the increased vagal activity is not known. Gastrin producing tumours of the pancreas also cause ulcers. Another cause may have to do with too little resistance to acid by the gut wall. It is widely believed (though not fully established) that stress causes ulcers at least in certain individuals. In rats, a few hours or exposure to stressful situations such as immobilisation can cause widespread gastric and duodenal ulcers. These effects may be due to the catabolic affects of high levels of corticosteroids secreted by the adrenal cortex during stress, which weaken the gut wall's resistance to acid. Other stress hormones, such as adrenaline, are less likely to cause ulcers.

Diarrhoea

Diarrhoea is characterised by excessive and frequent discharge or watery faeces. The condition is sometimes caused by an increase in intestinal motility delivering great quantities of watery chyme to the large intestine. The colon has the inability to absorb the excess water and this causes watery and frequent faecal discharges. Different factors may be responsible for the increased motility. Certain fruits, such as prunes, contain substances that naturally increase intestinal motility. Diarrhoea can also be caused by the actions of certain toxins on the epithelial cells of intestinal glands. For example, cholera toxin causes the intestinal glands to secrete large quantities of electrolytes (sodium chloride and sodium bicarbonate) and water into the lumen. This is followed by osmosis. A cholera victim can lose about 10L (2 gallons) of water per day: a lethal condition if not treated.

Certain diarrhoeas are caused by enzyme deficiency in the small intestine. For example certain individuals lack the enzyme lactase, produced by mucosal cells. Therefore, they cannot digest lactose, the sugar in milk and dairy products. The undigested lactose increases the lumen osmolarity, resulting in decreased water absorption in the small intestine and increased chyme delivery to the colon causing diarrhoea. Diarrhoea may also be of nervous (psychogenic) origin. For example, anxiety increases parasympathetic activity to the lower bowels, increasing intestinal motility which in turn decreases absorption time, leading to diarrhoea.

Constipation

Reduced intestinal motility, particularly of the large intestine, is responsible for constipation, a common digestive disorder. The reduced motility increases the storage time which in turn increases the amount of water absorbed from the faeces. Dried faeces are less bulky and therefore less likely to initiate movements. Causes of constipation are not well understood, but learning to inhibit the defecation reflex during childhood may be one cause. Dietary habits may be another. Increased fibre content (raw vegetables, fruits) in the diet improves faecal bulk which in turn stimulates colon motility and defecation. Although the average adult defecates once daily many healthy people have less frequent bowel movements. Indeed, mild and occasional constipation does not pose any physiological problems but prolonged constipation is accompanied by abdominal discomfort, headaches, loss of appetite, and even depression. Sudden prolonged constipation however, may be due to diseases of the colon.