Chapter 7 Gastrointestinal Physiology
Pathology note: The portal vein normally carries nutrient-rich blood from the intestines to the liver, after which the blood is shunted to the inferior vena cava through the hepatic vein. In cirrhosis, a variety of pathophysiologic changes result in elevated portal vein pressures, termed portal hypertension. Because the portal vein has multiple anastomoses with systemic veins, pressures likewise increase in these vessels. These systemic veins may then become abnormally dilated and are at increased risk for rupture. In the anterior abdominal wall, venous dilatation can result in caput medusae, a rather harmless clinical examination finding that nonetheless indicates severe liver disease. In the esophagus, venous dilatation can result in esophageal varices. Rupture of esophageal varices can be rapidly fatal.
(B, From Koeppen BM, Stanton BA: Berne and Levy Physiology, 6th ed. Updated ed. Philadelphia, Mosby, 2010, Fig. 26-2.)
7-3 Arrangement of mucosa of the small intestine. Circular folds (plicae circularis), villi, and microvilli significantly increase the surface area of the mucosa. Surface areas are shown in square meters.
Pathology note: In gastroesophageal reflux disease (GERD), the mucosal epithelium of the esophagus takes on the appearance of the gastric mucosal epithelium—it differentiates from a stratified squamous epithelium into a columnar epithelium. This process, whereby one cell type transforms into another, is termed metaplasia. Columnar metaplasia in the lower esophagus is called Barrett esophagus, which can be detected by endoscopy and substantially increases the risk for development of esophageal adenocarcinoma and stricture from scarring. Patients who develop Barrett esophagus require periodic endoscopic surveillance.
Pathology note: In Hirschsprung disease (aganglionic megacolon), the neural crest cells that form the myenteric plexus fail to migrate to the colon. Newborns with this condition are likely to be severely constipated, and imaging studies may reveal a massively dilated colon proximal to the aganglionic segment.
7-4 Innervation of the gastrointestinal tract by the autonomic nervous system. The myenteric plexus synapses mainly on the inner circular and outer longitudinal muscles, whereas the submucosal plexus synapses mainly on the muscularis mucosae and epithelial cells of the mucosa.
(From Damjanov I: Pathophysiology. Philadelphia, Saunders, 2008, Fig. 7-5.)
Clinical note: The PNS stimulates intestinal motility by releasing acetylcholine onto neurons of the myenteric plexus. Therefore, cholinergic drugs should never be given to a patient if an intestinal obstruction is suspected. The resulting increase in pressure could rupture a viscus, resulting in potentially lethal peritonitis.
Pharmacology note: Metoclopramide is a D2 receptor antagonist and 5-HT3 antagonist that is a prokinetic agent useful in the treatment of nausea and vomiting, particularly in patients who have delayed gastric emptying (gastroparesis).
Clinical note: Patients with a long history of poorly controlled diabetes mellitus can sometimes develop severe gastric motility dysfunction, termed gastroparesis. In diabetes, gastroparesis can occur as a result of damage to the autonomic nerves supplying the stomach. These patients may suffer from intractable nausea and vomiting because of the failure of the stomach to empty after a meal. In such patients, promotility agents such as metoclopramide can provide substantial symptomatic relief. A more aggressive option is to surgically implant a gastric pacemaker, although this is rarely done.
|Potassium bicarbonate||Neutralizes bacterial acid, preventing digestion of tooth enamel and dentine (prevents cavities)|
|Lingual lipase||Initiates lipid digestion|
|Salivary amylase||Initiates carbohydrate digestion|
|Mucins||Lubricate food bolus, are primary determinant of viscosity|
|Lysozyme||Initiates bacterial lysis|
|Immunoglobulins||Offer immune protection|
Clinical note: Sjögren syndrome is an autoimmune disorder characterized by lymphocytic infiltration of exocrine glands, mainly affecting the salivary and lacrimal glands. It is relatively common in elderly people (3% to 5% of those >60 years of age) and is characterized by dry mouth (xerostomia) and dry eyes (keratoconjunctivitis sicca). Low levels of saliva may cause dysphagia (difficulty swallowing) and increased dental caries; a deficiency in tear production may cause corneal ulceration and scarring. Pilocarpine, a muscarinic receptor agonist, is effective in increasing salivary production, and artificial tears can be used for treating dry eyes.
(From Koeppen BM, Stanton BA: Berne and Levy Physiology, 6th ed. Updated ed. Philadelphia, Mosby, 2010, Fig. 27-1.)
Pharmacology note: The muscarinic acetylcholine receptor mediates the effects of the PNS on the salivary glands. Blockade of this receptor can substantially decrease salivary secretions. This effect is associated with several classes of drugs, most notably antimuscarinic drugs (e.g., atropine, ipratropium), but also with drugs that have anticholinergic side effects, especially the antipsychotics and tricyclic antidepressants.