Medical brain notes 30

  THE GASTROINTESTINAL TRACT AND ITS ACCESSORY ORGANS The GIT may be described as a tubular system with distinctive regions that begin with the mouth leading to the pharynx, esophagus, stomach, the small and large intestines and terminating at the anus ( Figure 11.1  ( A )). It is associated with several accessory digestive organs, such as the pancreas, liver and gall bladder ( Figure 11.1  ( B )). The walls of the GIT have a fairly common substructure ( Figure 11.2  ( A ) and ( B )) and its interior is called the lumen. The GIT and accessory organs are bound to each other and to the inner wall of the abdomen by the  peritoneum . This is a strong, colorless membrane with a smooth surface that consists of two parts; the parietal peritoneum, which lines the abdominal cavity, and the visceral peritoneum that covers most of the organs in the abdomen ( Figure 11.3 ). The thin space between the two parts, called the peritoneal cavity, is filled with serous fluid. In m...

  DIGESTION Digestion is the hydrolytic breakdown of nutrient macromolecules and compound lipids to smaller products that can be absorbed. The hydrolytic reactions are catalyzed by a variety of enzymes: proteases that digest pro-teins; carbohydrases that digest carbohydrates; lipases that catalyze the hydrolysis of lipids and nucleases that degrade DNA and RNA. Digestion occurs in the mouth, to a small extent, stomach and small intestine, and most absorption of nutrients occurs in the small intestine and that of water in the large intestine. MOUTH In the mouth, teeth break the food into smaller portions increasing the surface area upon which digestive enzymes can act. Three pairs of salivary glands, the submandibular located under the jaws, the sublingual located under the tongue and the parotid situated in front of the ears ( Figure 11.6 ), secrete saliva into the mouth. The saliva contains amylase ( Figure 11.7 ), water and mucus. The water helps to dissolve nutrients, while the ...

  ABSORPTION OF THE PRODUCTS OF DIGESTION The large surface area of the small intestine allows the rapid absorption of the products of digestion. The enzymes concerned with the final stages of diges-tion of a number of nutrients are located in the brush border of the entero-cytes as described or even within their cytoplasm. This ensures that the final products of digestion are produced near or within the absorp-tive surface of the GIT. Enterocytes are joined together by tight junctions that ensure material cannot leak from the lumen. Absorption by enterocytes is largely active and selective and they have a high metabolic rate because the transport of materials across their membranes requires considerable amounts of metabolic energy. A membrane-bound Na + /K + -ATPase uses a major pro-portion of this energy to catalyze the hydrolysis of ATP in the presence of Na +  and K + . The free energy from the hydrolysis is used to expel three Na +  from the cell and to pump two K + ...

  ACTIVITIES OF THE LARGE INTESTINE The large intestine is so named because its diameter is greater than that of the small intestine though it is, in fact, much the shorter of the two. Fluid, con-taining the unabsorbed products of digestion, directly enters from the small intestine at a junction that is also the site of the vestigial cecum and appen-dix ( Figure 11.22 ). Absorption of Na +  and water occurs over the surface of the large intestine, which also acts as a reservoir for material resistant to diges-tion by GIT enzymes. However, bacterial action on this material releases some nutrients from food, for example certain vitamins as well as about 200–2000 cm 3  of gas in 10–14 episodes per day. The final waste together with bacteria forms the feces, which passes to the last section of the GIT, the rectum, and is eliminated through the anus. Two sphincter muscles control elimination: the first of smooth muscle opens involuntarily in response to pressure within the rec...

  THE SMALL INTESTINE AND HOMEOSTASIS Within enterocytes a portion of the monosaccharides absorbed are converted to lactate by glycolysis. Excess nonessential amino acids, especially glutamine, are used to synthesize alanine and ammonia ( Figure 11.23 ). These products are then delivered to the liver in the hepatic portal vein. Converting some of the absorbed nutrients to lactate and alanine reduces the metabolic load on the liver because it can easily regenerate pyruvate from them. Pyruvate is a versa-tile liver metabolite; it is a substrate for the TCA cycle, allowing the formation of ATP during oxidative phosphorylation but it can be used for the biosynthe-sis of glucose and glycogen, ketone bodies, fatty acids and all but two of the nonessential fatty acids and cholesterol. The GIT is a significant contributor to nutrient homeostasis both during and after nutrient absorption because the formation of lactate and alanine continues even when absorption ceases.

  HORMONAL CONTROL OF GIT SECRETIONS The GIT produces a large number of hormones many of whose functions are not well understood, although some of them, together with neuronal activi-ties, are concerned with coordinating the secretions of various digestive juices. Endocrine cells are scattered throughout the entire GIT in clusters forming a diffuse portion of the endocrine system . Over 25 peptides have been extracted and characterized from the GIT. No deficiency states are known for any of these peptides although hormone-secreting tumors have been described. The G cells in the antral and pyloric regions of the stomach produce gastrin. Gastrin occurs in a number of molecular forms, for example gastrin 17  and gas-trin 34  are composed of 17 and 34 amino acid residues respectively. Gastrin 17  is the most active and has a half-life of about 8 min. Its precursor, gastrin 34 , has a half-life of approximately 40 min. The release of gastrin is stimulated by food entering...

  DISORDERS OF THE GIT AND ACCESSORY ORGANS Disorders of the GIT and its accessory organs can affect the mouth, esopha-gus, stomach, pancreas, liver, bile duct, small and large intestines. Some of the disorders affect the exocrine pancreas, liver, stomach, small and large intes-tines. DISORDERS OF THE EXOCRINE PANCREAS Acute pancreatitis is a severe, rapid inflammation of the pancreas with varying degrees of edema , hemorrhage and tissue necrosis. It arises because of an inappropriate activation of pancreatic enzymes which then autodigest pancreatic tissue. Normally, these enzymes are inactive until they reach the duodenum. The cause of acute pancreatitis is unclear, although excessive alcohol intake is believed to have a major role, but viral infections, drug reactions and pancreatic cancer have also been implicated. The clinical features of acute pancreatitis include attacks of severe abdominal pain that may extend to the back, vomiting, fever and shock. The leakage of pancreatic...

  DISORDERS OF THE LIVER, GALL BLADDER AND BILE DUCT Jaundice is  the yellow discoloration of tissues due to an accumulation of bilirubin ( Figure 11.5 ). Many disorders of the liver give rise to jaundice, although clinical jaundice may not be seen until the concentration of bilirubin in the serum is greater than 50 µmol dm –3 . The causes of jaundice can be pre-hepatic, hepatic or posthepatic.   The causes of prehepatic jaundice include hemolysis, where there is an increased breakdown of hemoglobin producing large amounts of bilirubin that overloads the conjugating mechanism. Such bilirubin is mostly uncon- jugated and commonly occurs in newborn babies. If the concentration of serum bilirubin approaches 200 µmol dm –3 , then phototherapy  is used to degrade it, otherwise its high concentra-tion may cause damage to the brain called  kernicterus . Other causes of pre-hepatic hyperbilirubinemia include hemolytic disease of the newborn due to Rhesus incompatibility...