514 Chapter 7. Pathophysiology of the gastrointestinal tract ( I. Hul´ın, I. Duriˇˇ s et al.) quent bulks of chyme from the stomach into the dis- the bowel thus determines its typical shape. The tended small intestine is inhibited. The intestinoin- circular musculature is separated into sections which testinal reflex procures the hindrance of intestinal are referred to as haustra. Muscular contractions motility in cases when some part of the intestine is reduce and enlarge the haustra. extremely distended. The gastroileal reflex arises in The large-intestinal mucosa contains cylindric coincidence with increased gastric motility and secre- cells which absorb water and electrolytes. Mucus- tion. It causes an increase in the motility of ileum. producing cells create a protective layer on the mu- In this way,a more rapid evacuation of the ileum cosal surface. is procured,in order to continue the acceptance of The motor and secretory acitivities of the large more chyme. The gastroileal reflex is probably reg- intestine are regulated from the myenteric plexus ulated by neurotensin. During long-term fasting,or which is to a great extent independent from other sleep,a slow contraction proceeds from the stomach systems. The parasympathetic innervation of the co- to the terminal ileum. In this way this reflex procures ecum,ascendent colon and the first part of the trans- the evacuation of gastric and intestinal contents into verse colon originates from the vagus nerve which the large intestine. The gastrocolic reflex (urgency in this area of the large intestine stimulates rhyth- to defecate after meal) is pronounced especially in mic contractions. The distal colon is parasympa- childhood. thetically innervated from the pelvic nerves. The The ileocoecal junction is procured by a valve internal anal sphincter is innervated by the sym- (sphincter) which is enclosed at rest. Its opening pathetic nervous system (the sympathetic system is stimulated by peristaltic waves. After the transi- enhances contractions) and parasympathetic system tion of a small part of chyme it encloses again. This (the parasympathetic system supports relaxation). mechanism inhibits both regurgitation of chyme and It is permanently in state of contraction. It relaxes extreme distension of the large intestine. only for a short time in coincidence with the disten- sion of the rectum and at defecation. The external sphincter is innervated by sacral branches of spinal nerves. The sympathetic innervation originates in the coeliac and superior mesenteric ganglia. Destruc- 7.9 The large intestine tion of the lower part of medulla spinalis paralyses the external sphincter whereas the internal sphinc- ter does not respond. The sympathetic activity in the area of the large intestine modulates intestinal The large intestine is approximately 1.5 m in reflexes,somatic sensations and pain. length. It is constituted by the coecum,appendix, Basic movements of the large intestine are repre- colon (ascendent,transverse,descendent,sigmoid), sented by segmental contractions which always in- rectum and the anal canal. The coecum accepts volve an entire haustrum. Peristaltic movements in- the chyme from the ileum. The vermiform ap- volve several haustra. They procure the transition pendix might have its significance in elimination of of the faecal mass aborally to the rectum. The gas- unusual components of food present in the bowel. trocolic reflex enhances peristaltic movements of the The sphincter in the distal part of the sigmoid colon large intestine after the stomach filling,which stim- controlling the transport of chyme into the rectum ulates defecation. It is probable that gastrin partic- is in comparison with the ileocoecal sphincter less ipates in this reflex. demarcated. It is an intestinal part which fulfils the Fluid is absorbed in the large intestine by diffu- task of a sphincter of a longer section. The anal canal sion or by active transport. Under the influence of encloses the internal sphincter constituted by smooth aldosteron,the diffusion of sodium into the cells and musculature and the external sphincter which is con- its active transport through the basolateral mem- stituted by striated muscles. brane into the interstitial fluid increase. Together The longitudinal musculature of the large intestine with sodium,also chlorides – as complementary an- constitutes three stripes referred to as colic taeniae ions – are absorbed. They enter the cells on the basis (taeniae coli). The fact that they are shorter than of exchange for bicarbonates. The active transport 7.10. Malabsorption 515 of sodium currently increases the gradient for the dif- sequence of defective digestion. The accepted food fusion of potassium from cells into the lumen. The must be initially processed down to basic compo- substances being absorbed in the large intestine in- nents. These acitivites enable further steps to be clude also the chains of fatty acids which are pro- carried out with subsequent resorption. The pro- duced during fermentation. Neither saccharides,nor cess takes place within the lumen,on the mucosal amino acids are absorbed in the large intestine. surface (contact digestion). A significant role in Under physiological circumstances,the gastric the process of digestion is carried out by the pan- contents,owing to the presence of HCl,is almost creas. It produces lipase,colipase and proteases, sterile. The bacterial reproduction in the duode- especially trypsin. Therefore malabsorption is the num is inhibited by bile and antibodies. Duode- cardinal symptom of chronic pancreatitis. In order num contains only a minimal amount of aerobic,and to procure the optimal digestion of fat,the chyme no anaerobic microbes. Anaerobic microbes harbour must contain a sufficient amount of bile acids which the more remote part of the ileum – the ileocoecal are necessary for normal lipolysis and formation of area. The intestinal flora mostly includes Escherichia micelles. A decreased production of bile salts can coli,Clostridium Welchi and streptococci. Bacte- develop in consequence of its decreased synthesis in ria significantly participate in the metabolism of bile the liver,however more often it is caused by cholesta- acids,namely in the reabsorption of bile components, sis in coincidence with hepatic cirrhosis. A further elimination of toxic metabolites from bile and the cause resides in an increased deconjugation of bile breakdown of fibrous material. The intestinal bac- acids caused by bacterial overgrowth. terial flora participates in the metabolism of oestro- Bile acids deficiency may be caused also by dis- gens,androgens,lipids,various nitrogen substances eases of the ileum or ileal bypass by leading to insuf- and drugs. ficient recirculation of bile acids. The liver in these cases is not able to synthesize bile acids de novo in a sufficient amount. This condition results in mal- absorption of fat and deficiency of fat-soluble vita- mins. These global disturbances can be simultane- ously present in specific defects of digestion. Thus, 7.10 Malabsorption e.g. lactase deficiency on the surface of enterocytes deteriorates the absorption of saccharides. This con- dition can result in intolerance to milk and clinical symptoms as flatulence,intestinal distension and di- The main function of the gastrointestinal tract is arrhea. It occurs more frequently in black people. to digest and absorb the nutrients serving as the It can occur as a consequence of a diffuse defect of source of energy. In addition to the latter,an or- mucosa in coincidence with other diseases. ganism needs small amounts of other significant sub- stances for the optimal procurement of specific func- tions. 7.10.2 Inadequate absorption (malab- In a precise pathophysiological sense,the term sorption) malabsorption refers to a defective absorption of nu- trients by the intestinal mucosa,whereas an impaired The inadequate absorption can appear in normal in- hydrolysis of nutrients is referred to as maldigestion. tact digestion. Most frequently it is caused by a However,as the processes of digestion and absorption reduced surface for absorption of substances. It de- are very closely associated,all aspects of impaired di- velops after intestinal resection due to mesenterial gestion and absorption are in clinical practice com- infarction,bypass due to morbid obesity,or due to monly referred to as malabsorption syndrome. Crohn’s disease. In other cases,the surface necessary for absorp- 7.10.1 Inadequate digestion tion may be sufficiently large,but insufficiently in- (maldigestion) volving the processes of absorption per se. These impairments are caused by the defects in mucosal The malabsorption syndrome often develops in con- cells (inborn or acquired). Examples of this condi-.