Medical brain notes 27

  DIVISIONS OF THE DIGESTIVE SYSTEM   The two divisions of the digestive system are the ali-mentary tube and the accessory organs (Fig. 16–1). The  alimentary tube  extends from the mouth to the anus. It consists of the oral cavity, pharynx, esophagus, stomach, small intestine, and large intestine. Digestion takes place within the oral cavity, stomach, and small intestine; most absorption of nutrients takes place in the small intestine. Undigestible material, pri-marily cellulose, is eliminated by the large intestine (also called the colon).   The  accessory organs  of digestion are the teeth, tongue, salivary glands, liver, gallbladder, and pancreas. Digestion does not take place  within  these organs, but each contributes something  to  the digestive process.

  TYPES OF DIGESTION The food we eat is broken down in two complementary processes: mechanical digestion and chemical digestion.  Mechanical digestion  is the physical break-ing up of food into smaller pieces. Chewing is an example of this. As food is broken up, more of its sur-face area is exposed for the action of digestive enzymes. The work of the digestive enzymes is the  chemical digestion  of bro-ken-up food particles, in which complex chemical mol-ecules are changed into much simpler chemicals that the body can utilize. Such enzymes are specific with respect to the fat, protein, or carbohydrate food mole-cules each can digest. For example, protein-digesting enzymes work only on proteins, not on carbohydrates or fats. Each enzyme is produced by a particular diges-tive organ and functions at a specific site. However, the enzyme’s site of action may or may not be its site of production. These digestive enzymes and their func-tions are discussed in later sect...

  ORAL CAVITY   Food enters the  oral cavity  (or  buccal cavity ) by way of the mouth. The boundaries of the oral cavity are the hard and soft palates superiorly; the cheeks later-ally; and the floor of the mouth inferiorly. Within the oral cavity are the teeth and tongue and the openings of the ducts of the salivary glands. TEETH The function of the  teeth  is, of course, chewing. This is the process that mechanically breaks food into smaller pieces and mixes it with saliva. An individual  develops two sets of teeth: deciduous and permanent.   The  deciduous teeth  begin to erupt through the gums at about 6 months of age, and the set of 20 teeth is usually complete by the age of 2 years. These teeth are gradually lost throughout childhood and replaced by the  permanent teeth , the first of which are molars that emerge around the age of 6 years. A complete set of permanent teeth consists of 32 teeth; the types of teeth are inc...

  PHARYNX   As described in the preceding, the oropharynx and laryngopharynx are food passageways connecting the oral cavity to the esophagus. No digestion takes place in the pharynx. Its only related function is swal-lowing, the mechanical movement of food. When the bolus of food is pushed backward by the tongue, the constrictor muscles of the pharynx contract as part of the swallowing reflex. The reflex center for swallow-ing is in the medulla, which coordinates the many actions that take place: constriction of the pharynx, cessation of breathing, elevation of the soft palate to block the nasopharynx, elevation of the larynx and clo sure of the epiglottis, and peristalsis of the esophagus. As you can see, swallowing is rather complicated, but because it is a reflex we don’t have to think about mak-ing it happen correctly. Talking or laughing while eat-ing, however, may interfere with the reflex and cause food to go into the “wrong pipe,” the larynx. When that happens, the co...

  ESOPHAGUS   The  esophagus  is a muscular tube that takes food from the pharynx to the stomach; no digestion takes place here. Peristalsis of the esophagus propels food in one direction and ensures that food gets to the stom-ach even if the body is horizontal or upside down. At the junction with the stomach, the lumen (cavity) of the esophagus is surrounded by the  lower esophageal   sphincter  (LES or cardiac sphincter), a circular   smooth muscle. The LES relaxes to permit food to enter the stomach, then contracts to prevent the backup of stomach contents. If the LES does not close completely, gastric juice may splash up into the esoph-agus; this is a painful condition we call heartburn, or gastroesophageal reflux disease (GERD). Most people experience heartburn once in a while, and it is merely uncomfortable, but chronic GERD is more serious. The lining of the esophagus cannot withstand the cor-rosive action of gastric acid and will be damage...

  STRUCTURAL LAYERS OF THE ALIMENTARY TUBE   Before we continue with our discussion of the organs of digestion, we will first examine the typical structure of the alimentary tube. When viewed in cross-section, the alimentary tube has four layers (Fig. 16–4): the mucosa, submucosa, external muscle layer, and serosa. Each layer has a specific structure, and its functions contribute to the functioning of the organs of which it is a part.   Figure 16–4.  ( A ) The four layers of the wall of the   alimentary tube. A small part of the wall of the small intestine has been magnified to show the four layers typical of the alimentary tube. ( B ) Sagittal section through the abdomen showing the relationship of the peritoneum and mesentery to the abdominal organs.  QUESTION:  What is the function of the external mus-cle layer? MUCOSA The  mucosa , or lining, of the alimentary tube is made of epithelial tissue, areolar connective tissue, and two  thin lay...

  STOMACH   The  stomach  is located in the upper left quadrant of the abdominal cavity, to the left of the liver and in front of the spleen. Although part of the alimentary tube, the stomach is not a tube, but rather a sac that extends from the esophagus to the small intestine. Because it is a sac, the stomach is a reservoir for food, so that digestion proceeds gradually and we do not have to eat constantly. Both mechanical and chemical digestion take place in the stomach.   The parts of the stomach are shown in Fig. 16–5. The cardiac orifice is the opening of the esophagus, and the fundus is the portion above the level of this opening. The body of the stomach is the large central portion, bounded laterally by the greater curvature and medially by the lesser curvature. The pylorus is adja-cent to the duodenum of the small intestine, and the  pyloric sphincter  surrounds the junction of the two   organs. The fundus and body are mainly storage area...

  SMALL INTESTINE   The  small intestine  is about 1 inch (2.5 cm) in diam-eter and approximately 20 feet (6 m) long and extends from the stomach to the cecum of the large intestine Within the abdominal cavity, the large intestine encir-cles the coils of the small intestine (see Fig. 16–1).   The  duodenum  is the first 10 inches (25 cm) of the small intestine. The common bile duct enters the duo-denum at the ampulla of Vater (or hepatopancreatic ampulla). The  jejunum  is about 8 feet long, and the  ileum  is about 11 feet in length. In a living person, however, the small intestine is always contracted and is therefore somewhat shorter.   Digestion is completed in the small intestine, and the end products of digestion are absorbed into the blood and lymph. The mucosa (see Fig. 16–4) has simple columnar epithelium that includes cells with microvilli and goblet cells that secrete mucus. Enteroendocrine cells secrete the hormones...

  LIVER   The  liver  (Fig. 16–6) consists of two large lobes, right and left, and fills the upper right and center of the abdominal cavity, just below the diaphragm. The structural unit of the liver is the  liver lobule , a roughly hexagonal column of liver cells (hepatocytes). Between adjacent lobules are branches of the hepatic artery and portal vein. The capillaries of a lobule are sinusoids, large and very permeable vessels between the rows of liver cells. The sinusoids receive blood from both the hepatic artery and portal vein, and it is with this mixture of blood that the liver cells carry out their functions. The hepatic artery brings oxygenated blood, and the portal vein brings blood from the digestive organs and spleen (see Fig. 13–7). Each lob-ule has a central vein. The central veins of all the lob-ules unite to form the hepatic veins, which take blood out of the liver to the inferior vena cava.   The cells of the liver have many functions (whic...

  GALLBLADDER   The  gallbladder  is a sac about 3 to 4 inches (7.5 to 10 cm) long located on the undersurface of the right lobe of the liver. Bile in the hepatic duct of the liver flows through the  cystic duct  into the gallbladder (see Fig. 16–6), which stores bile until it is needed in the small intestine. The gallbladder also concentrates bile by absorbing water.   When fatty foods enter the duodenum, the enteroendocrine cells of the duodenal mucosa secrete the hormone  cholecystokinin . This hormone stimu-lates contraction of the smooth muscle in the wall of the gallbladder, which forces bile into the cystic duct, then into the common bile duct, and on into the duo-denum.

  PANCREAS   The  pancreas  is located in the upper left abdominal quadrant between the curve of the duodenum and the spleen and is about 6 inches (15 cm) in length. The endocrine functions of the pancreas were discussed, so only the exocrine functions will be considered here. The exocrine glands of the pancreas are called acini (singular: acinus) (Fig. 16–7). They produce enzymes that are involved in the digestion of all three types of complex food molecules.   Figure 16–7.  ( A ) The pancreas, sectioned to show the pancreatic ducts. The main pan-creatic duct joins the common bile duct. ( B ) Microscopic section showing acini with their ducts and several islets of Langerhans. QUESTION:  In part B, what do the acini secrete? The pancreatic enzyme  amylase  digests starch to maltose. You may recall that this is the “backup” enzyme for salivary amylase, though pancreatic amy-lase is responsible for most digestion of starch.  Lipase  c...

  COMPLETION OF DIGESTION AND ABSORPTION   SMALL INTESTINE   The secretion of the epithelium of the intestinal glands (or crypts of Lieberkühn) is stimulated by the  presence of food in the duodenum.   The intestinal enzymes are the peptidases and sucrase, maltase, and lactase.  Peptidases  complete the digestion of protein by breaking down short polypeptide chains to amino acids.  Sucrase ,  maltase , and  lactase , respectively, digest the disaccharides sucrose, maltose, and lactose to monosaccharides.   The enteroendocrine cells of the intestinal glands secrete the hormones of the small intestine. Secretion is stimulated by food entering the duodenum. A summary of the digestive secretions and their functions is found in Table 16–1. Regulation of these secretions is shown in Table 16–2.   ABSORPTION   Most absorption of the end products of digestion takes place in the small intestine (although the stomach does absorb wat...