Chapter 17 : The Digestive System
INTRODUCTION The breaking down of larger food molecules into smaller molecules is called digestion.
The passage of these smaller molecules into blood and lymph is termed absorption.
The organs that collectively perform digestion and absorption compose the digestive system and are usually divided into two main groups: those composing the gastrointestinal (GI) tract or alimentary canal and accessory structures. The GI tract is a continuous tube extending from the mouth to the anus. Functionally, anything contained within that tube can be thought of as being outside the body. The accessory structures include the teeth, tongue, salivary glands, liver, gallbladder, and pancreas.
OVERVIEW OF DIGESTIVE PROCESSES (Additional to book)
Digestion includes six basic processes:
ingestion is the taking of food into the mouth (eating).
secretion cells within the digestive system secrete about 9 L a day of water, acid, buffers, and enzymes.
mixing and propulsion contractions of smooth muscles in the walls of the GI tract help to mix food and secretions, and to propel the food down the tract (peristalsis).
mechanical and chemical digestion mechanical digestion is the physical breaking down of food into smaller particles to increase the surface area for chemical digestion and also mix food and secretions. Chemical digestion is the breaking down of large molecules which cannot be absorbed, into smaller pieces which can be taken into the body. This is usually accomplished by enzymes.
absorption once the nutrients are small enough, they can be taken into the body by active or passive processes. The materials are absorbed and then pass into the blood stream where they can be distributed throughout the body.
defecation Anything that cannot be absorbed into the body, as well as bacteria which live and die in the GI tract are eliminated in feces.
LAYERS OF THE GI TRACT
The wall of the alimentary canal is made up of four layers. The development of these layers varies with the region of the GI tract, since certain regions are specialized for certain functions. The layers from innermost (closest to the lumen) to outermost are:
1. Mucosa The mucosa is the lining of the GI tract, and as the name implies, is a mucous membrane. It consists of the epithelium, an underlying layer of areolar connective tissue called the lamina propria, and a thin layer of smooth muscle called the muscularis mucosae.
In the mouth, esophagus and anal canal the epithelial layer is mainly nonkeratinized stratified squamous epithelium . This type of epithelium guards against abrasion. In the stomach and intestines are lined by simple columnar epithelium which functions in secretion and absorption. Among these cells are enteroendocrine cells, which secrete hormones into the blood stream. There are also glands in this layer that secrete enzymes and mucus. In some regions it has folds and projections that increase the surface area for absorption.
The lamina propria contains many blood and lymphatic vessels and scattered lymph nodules. This layer binds the epithelium to the muscularis mucosae and provides the vessels to transport nutrients absorbed from the tract. Here we also find most of the mucosa-associated lymphoid tissue. (MALT). These patches of lymphatic tissue are found throughout the GI tract in the tonsils, small intestine, appendix, and large intestine. There are as many immune cells in the GI tract as there are in the rest of your body. (there are more bacteria in and on you than you have cells 2-10 X)
The muscularis mucosae contains two layers of smooth muscle cells, and inner circular layer, and an outer longitudinal layer, which cause the mucosa to fold. They also cause local movement of the folds, which increases surface area for absorption.
The submucosa is areolar connective tissue which binds the mucosa to the muscularis layer. It contains many blood vessels as well as the submucosal plexus which is a part of the autonomic nerve supply to the muscularis layer and to the blood vessels. The plexus also innervates secretory cells in the mucosal glands, and thus is important in controlling secretions by the GI tract. The submucosa may also contain glands and lymphatic tissue.
3. Muscularis or Muscular layer
The muscularis of the mouth, pharynx and superior part of the esophagus contains skeletal muscle which allows for voluntary swallowing. Skeletal muscle also forms the external anal sphincter, also under voluntary control. Throughout the remainder of the GI tract, there are two layers of smooth muscle: an inner circular layer and an outer longitudinal layer. Contractions of these muscles physically breakdown food, mix it with secretions, and move it along the tract in a motion called peristalsis.
The muscularis also contains the myenteric plexus which consists of fibers from the sympathetic and parasympathetic nervous systems. This plexus mostly controls the motility of the GI tract - the strength and frequency of the contractions of the muscularis. Together, the submucosal and myenteric plexuses have as many neurons as does the spinal cord.
The serosa is a serous membrane composed of connective tissue and simple squamous epithelium. Inferior to the diaphragm the serosa is also called the visceral peritoneum. It secretes a serous fluid which lubricates the tissues, so that the intestines can move freely within the abdominal cavity.
The mouth is formed by the cheeks, hard and soft palates, lips, and tongue, which aid mechanical digestion in the process of mastication (chewing food and mixing it with saliva).
The oral cavity is the space between the palate and tongue.
The vestibule is the space between the cheeks and lips and teeth and gums.
Cheeks - lateral walls of the mouth. Skin, pads of subcutaneous fat, and muscles associated with expression and chewing, and inner linings of nonkeratinized stratified squamous epithelium.
Lips- contain skeletal muscles and sensory receptors for determining the temperature and texture of food - remember primary somatosensory cortex
Tongue The tongue forms the floor of the oral cavity. It is composed of skeletal muscle covered with mucous membrane. The upper surface and sides of the tongue are covered with papillae which provide friction for food. Some papillae contain taste buds.
It is connected to the floor of the mouth by a membranous fold called the frenulum. It prevents the tongue from moving to far posteriorly, if too short = tongue-tied.
The posterior region of the tongue is called the root, and is attached to the hyoid bone. The root is covered with masses of lymphatic tissue called the lingual tonsils.
The palate forms the roof of the oral cavity and is made up of a hard anterior portion and a soft posterior portion. The hard palate is formed by the palatine process of the maxillary bone in front and the palatine bones in the back.
The soft palate is muscular arch, which extend posteriorly and downward as the uvula. Soft palate and uvula move up during swallowing and close off the nasopharynx.
In the back of the mouth are two muscular folds that run down the sides of the soft palate- anterior and posterior pillars. Between them are the palatine tonsils.
The last set of tonsils, are the pharyngeal tonsils or adenoids, and are located on the posterior wall of the pharynx, above the border of the soft palate. They may enlarge and block the passage between the pharynx and nasal cavity, and need to be surgically removed.
Teeth The teeth project into the mouth and are adapted for mechanical digestion. A typical tooth consists of two principal portions: crown, and a root
( and neck). Teeth are composed primarily of dentin - a bonelike substance, and are covered by enamel, the hardest substance in the body. Inside the tooth is the pulp cavity which contains blood vessels, nerves, and a connective tissue called pulp. There are two dentitions deciduous and permanent (32). Incisors cut, cuspids tear and shred, molars grind.
Saliva moistens food particles, and begins the chemical digestion of carbohydrates. Saliva acts as a solvent, dissolving food so that it can be tasted. Bicarbonate ions in saliva helps to buffer acids, so that the pH of saliva is near neutral, us. 6.5 to 7.5. This is a good pH for the action of salivary enzymes, and it also protects the teeth from dissolving in a highly acidic environment (bone in vinegar).
There are many small salivary glands scattered throughout the mucosa of the tongue, palate and cheeks. The major portion of saliva is secreted by the salivary glands, which lie outside the mouth and pour their contents into ducts that empty into the oral cavity.
There are three pairs of salivary glands: parotid, submandibular, and sublingual glands.
Salivary glands have two types of cells: serous cells and mucous cells. The serous cells produces a water fluid.
Saliva is 99.5 % water. It also contains ions, various organic substances, and lysozyme (kills some, but not all oral bacteria), and salivary amylase. Amylase splits starch and glycogen into dissacharides. There are also buffers in salvia to buffer acidic foods. About 1 -1.5 liters of saliva are produced each day. Salivation is controlled by the nervous system. The parasympathetic system promotes the secretion of large amounts of watery saliva, and the sympathetic system virtually turns it off - small quantities of a viscous saliva are produced. During dehydration saliva is not produced, resulting in a dry mouth which encourages the consumption of water. Salivation can begin with the touch or taste of food, or even the sight or smell of food . Salivation continues after meals - washes and buffers, and in response to irritating foods or nausea.
Parotid glands - anterior to and slightly inferior to each ear. Largest of the salivary glands. The parotid duct empties into the mouth through the cheek, at the level of the second molar. Mumps - parotid glands - 25-30% of males also have inflammation of testes - rare cases sterility.
Submandibular glands - on floor of the mouth, on the inside surface of the lower jaw. These glands secrete a more viscous fluid than the parotid glands. Ducts open onto the floor of the mouth, on either side of the frenulum.
The sublinqual glands are the smallest of the three sets of salivary glands. They are located on the floor of the mouth, inferior to the tongue. Their cells are primarily mucus secreting cells, and the fluid produced is thick and stringy. The gland empties onto the floor of the mouth through many ducts.
The Pharynx is the cavity posterior to the mouth, which connects the nasal and oral cavities. It can be divided into three parts:
1. nasopharynx - sup. to soft palate. Allows for passage of air. Eustachian tubes open into this region.
2. The oropharynx - posterior to the mouth, down to the upper boarder of the epiglottis. passageway for food and air.
3. The laryngopharynx - inferior to oroph. Upper epiglottis to the lower boarder of the cricoid cartilage. Passageway to the esophagus.
Physiology of Digestion in the Mouth Through mastication (chewing), food is mixed with saliva and shaped into a bolus. Salivary amylase converts polysaccharides (starches) to disaccharides (maltose) and lingual lipase acts on triglycerides. Not much actually happens in the mouth, but the enzymes may work for about an hour before stomach acid degrades them.
Physiology of Deglutition Deglutition, or swallowing, moves a bolus from the mouth to the stomach. It consists of three stages:
1. the first stage is the voluntary stage. Food is chewed, mixed with saliva, and the bolus is moved into oropharynx.
2. Receptors around the opening in the oropharynx trigger the pharyngeal stage . Involuntary - triggers the swallowing reflex -passage through the pharynx into the esophagus - breathing stops and things close off:
1. the soft palate rises and prevents food from entering nasal cavity.
2. The hyoid bone and larynx are elevated; the epiglottis closes off the top of the trachea so that food is less likely to enter.
3. The tongue is pressed against the soft palate, sealing off the oral cavity from the pharynx.
4. The longitudinal muscles in the pharyngeal wall contract, pulling the pharynx upward toward the food.
5. The lower portion of the inferior constrictor muscles relax, opening the esophagus.
6. The superior constrictor muscles (circular muscles) contract, stimulating a peristaltic wave to begin in other pharyngeal muscles. This wave forces food into the esophagus.
And finally : the esophageal stage (involuntary - through the esophagus into the stomach).
The esophagus is a collapsible, muscular tube that connects the pharynx to the stomach. It passes through the diaphragm at the esophageal hiatus (hiatal hernia ). It passes a bolus into the stomach by peristalsis. The upper third contains skeletal muscle, the middle third contains skeletal and smooth muscle, and the lower third is all smooth muscle. It contains an upper and lower esophageal sphincter. The upper esophageal sphincter is attached to the cricoid cartilage, and the elevation of the larynx during swallowing causes the sphincter to relax. Food is passed down the esophagus by peristalsis - contraction of the circular and longitudinal muscles of the muscularis layer. Passage of food is aided by mucus produced by glands in the esophagus. The lower esophageal sphincter is located just above the diaphragm. It also relaxes during swallowing. (Us. Closed to prevent reflux of stomach acid, etc. into esophagus.
Anatomy: The stomach connects the esophagus to the duodenum. The principal anatomic subdivisions of the stomach are the cardia, fundus, body, and pylorus. Pyloric sphincter. When empty - folds of mucosal and submucosal layers - called rugae. In addition to the layers of circular and longitudinal muscle, the muscularis layer of the stomach also has a third muscle layer of oblique fibers (innermost - most highly developed near the opening of the esophagus and in the body). Primary function of stomach is to receive food, mix it with gastric juice, begin digestion of proteins, and move food to the small intestine. Absorption occurs only to a limited extent.
Histology: The mucous membrane of the stomach is thick. The top layer is made of simple columnar epithelium. Epithelial cells also extend down into the lamina propria forming many narrow channels called gastric pits and columns of secretory cells called gastric glands. These glands contain three types of secretory cells (exocrine glands) :
The mucous surface cells and the mucous cells (goblet cells) produce mucus.
The chief cells secrete pepsinogen and gastric lipase.
The parietal cells secrete HCl, which helps convert pepsinogen to pepsin,(proteolytic enzyme, pepsin can also convert pepsinogen to pepsin)
and intrinsic factor, which is involved in the absorption of B12.
The enteroendocrine cells or G cells are located in the pyloric antrum and secrete the hormone gastrin.
Physiology of Digestion and Absorption in the Stomach
The substance produced by mixing food with gastric juice is called chyme. Mechanical digestion consists of mixing waves, which move the chyme forward and backward in the stomach, forcing small amounts (5-15 ml)out through the pyloric sphincter. As soon as gastric secretion begins, it inactivates the enzymes from the mouth. Chemical digestion : The parietal cells secrete H+ and Cl- ions which form HCl. HCl protects against microbes and starts to denature proteins, and stimulates the secretion of hormones which promote the flow of bile and pancreatic juice. Enzymatic digestion of proteins begins in the stomach by the action of pepsin. Pepsin breaks proteins down into smaller peptide fragments.( The stomach is protected by secretion of an inactive enzyme and a mucus layer.)
Gastric lipase breaks down the butterfat in milk to fatty acids and monoglycerides. It functions best at a pH of 5-6, so its role is limited.
Regulation of Gastric Secretion and Motility
Gastric secretion is regulated by neural and hormonal mechanisms. Stimulation of gastric secretion occurs in three phases: cephalic (reflex), gastric, and intestinal.
The cephalic phase consists of reflexes initiated by sensory receptors in the head. The thought, sight, smell, or taste of food causes the cerebral cortex and hypothalamus to send impulses to the medulla oblongata, which sends parasympathetic signals via the vagus nerve to the submucosal plexus. Postganglionic fibers then stimulate the glands of the stomach to secrete and the smooth muscle to contract.
In the gastric phase, food in the stomach stimulates stretch receptors in the stomach wall. Also, chemoreceptors watch for a rise in pH which indicates more food has reached the stomach. These receptors send impulses to the parasympathetic fibers in the submucosal plexus, and stimulate peristalsis and secretion of gastric juice. The release of acetylcholine by the parasympathetic fibers stimulates the secretion of gastrin by the G cells. Some chemicals in food (caffeine) may directly stimulate G cells. Gastrin stimulates secretion of gastric juice and gastric motility, closes the lower esophageal sphincter, and relaxes the pyloric sphincter , but only till the pH drops below 1.5. It is again stimulated as the pH rises.
The ACh and gastrin stimulate the release of HCl in the presence of histamine, which is produced by mast cells in the mucosa. The histamine receptors on the parietal cells are H2 receptors, which can be blocked by drugs such as Tagamet and Zantac.
The intestinal phase:
When food first contacts the intestines, it stimulates the cells of the duodenum to release a hormone called intestinal gastrin, believed to be identical to gastrin.
Then, as more food enters the intestine, the signals become inhibitory. When chyme containing fatty acids and glucose enters the small intestine, it triggers enteroendocrine cells of the intestine to secrete two hormones:
The presence of proteins and fats cause the release of cholecystokinin (CCK) which decreases gastric motility and inhibits stomach emptying.
Fats also cause the release intestinal somatostatin which inhibits the release of gastric juice.
Distension or stretching of the duodenum and the presence of fatty acids, amino acids and glucose in the chyme in the duodenum active nerve impulses which go to the medulla oblongata, which inhibit parasympathetic stimulation of the stomach and stimulate sympathetic activity.
Gastric Absorption - The stomach wall is impermeable to most substances, but it does absorb some water, ions, certain lipid -soluble drugs (esp. aspirin) and alcohol.
The rate at which the stomach empties depends on the fluidity of the chyme and the type of food. Liquids us. pass through quickly, but solids remain until they are well mixed with gastric juice. The stomach empties itself in 2-4 hours after a meal. Foods rich in carbohydrates spend the least time in the stomach, proteins stay longer, and fat-laden food delays emptying the most , about 3-6 hours, because of CCK production.
When enough chyme enters the duodenum to begin to stretch the walls of the small intestine it activates the enterogastric reflex. (Begins in the intestines and ends in the stomach) . This reflex decreases parasympathetic impulses to the stomach, inhibiting peristalsis, and therefore filling of the intestine. Also, if the chyme entering the intestine is fatty, CCK is released which also inhibits peristalsis.
Vomiting is a complex reflex that empties the stomach in the opposite direction. Irritation or distension of the stomach or intestines can trigger vomiting. Other stimuli include unpleasant sights, smells, sounds, tastes, emotions, dizziness, mechanical stimulation of the back of the throat and certain drugs. Sensory impulses travel to the vomiting center in the medulla oblongata, which initiates motor responses. Take a deep breath, raising the soft palate, closing the epiglottis, relaxing the circular muscles at the bottom of the esophagus, contracting the diaphragm so that it presses downward on the stomach, and contracting the abdominal wall muscles to increase the pressure in the abdominal cavity. The stomach is squeezed from all sides, forcing its contents upward and outward.
Anatomy The pancreas is retroperitoneal. It is divisible into a head, body, and tail and is connected to the duodenum via the pancreatic duct and accessory duct. In most people the pancreatic duct joins with the common bile duct to form the hepatopancreatic ampulla which enters the duodenum at the major duodenal papilla.
Histology Pancreatic islets (islets of Langerhans) secrete hormones (1%), and clusters called acini (AS- i-neye - 99% ) secrete pancreatic juice.
Pancreatic Juice Pancreatic juice contains water, some salts, sodium bicarbonate, and enzymes which digest all the food types. Pancreatic juice contains enzymes that digest starch (pancreatic amylase), proteins (trypsin, chymotrypsin, elastase, and carboxypeptidase), triglycerides (pancreatic lipase), and nucleic acids (ribonuclease and deoxyribonuclease). All the proteolytic enzymes are released in an inactive form. Trypsin is activated when it comes in contact with an enzyme in the brush boarder of the intestine, and trypsin activates the other proteolytic enzymes.
Regulation of Pancreatic Secretions Pancreatic secretion is regulated by neural and hormonal mechanisms. During the cephalic and gastric phases the parasympathetic N.S. also stimulates the pancreas to secrete pancreatic juice. When fatty acids, glucose and amino acids enter the small intestine, CCK and secretin are produced. Secretin stimulates the flow of bicarbonate rich pancreatic juice, and CCK stimulates the production of digestive enzymes.
Anatomy The liver is covered by peritoneum and a deeper layer of dense irregular connective tissue. The liver has left and right lobes separated by the falciform ligament. Associated with the right lobe are the caudate and quadrate lobes.
Histology The lobes of the liver are made up of lobules that contain specialized epithelial cells called hepatocytes (liver cells),arranged in plates around a central vein. The liver contains sinusoids which are partially lined with stellate reticuloendothelial (Kupffer's) cells, which are fixed macrophages. Hepatocytes produce bile that is carried by bile canaliculi, which empty into small bile ducts which merge to form the right and left hepatic ducts. These unite to leave the liver as the common hepatic duct. The common hepatic duct joins the cystic duct from the gallbladder to form the common bile duct. Bile enters the cystic duct and is temporarily stored in the gallbladder. After a meal, bile is released from the gall bladder into the small intestine.
Blood Supply The liver receives blood from two sources. From the hepatic artery it obtains oxygenated blood, and from the hepatic portal vein it receives deoxygenated blood containing newly absorbed nutrients from the gastrointestinal tract. Branches of both the hepatic portal vein and the hepatic artery supply blood to the sinusoids. Branches of the hepatic portal vein, hepatic artery, and bile duct typically accompany each other in their distribution through the liver, these structures are called a portal triad. Blood leaves the liver by the hepatic vein.
Bile Bile has a pH of about 7.6-8.6, and contains water, bile acids, bile salts, cholesterol, a phospholipid called lecithin, bile pigments, and several ions. Bile acids role in digestion is the emulsification of dietary lipids. Bilirubin, from the breakdown of heme, is the principal bile pigment.
Regulation of Bile Secretion Bile secretion is regulated by neural and hormonal mechanisms. Parasympathetic impulses can increase the production of bile by twofold. CCK causes the contraction of the wall of the gall bladder and also relaxes the sphincter of the hepatopancreatic ampulla. Secretin stimulates the secretion of bile that is rich in bicarbonate ions. Increased blood flow through the liver increases bile production, as does large amounts of bile salts in the blood.
Functions of the Liver:
Carbohydrate metabolism : liver can break glycogen into glucose, or convert glucose to glycogen(glycogenesis) or triglycerides(lipogenesis) .It can also convert lactic acid and certain amino acids into glucose through gluconeogenesis. It can also convert other sugars into glucose
Lipid metabolism: The liver stores some triglycerides, synthesizes cholesterol, and breaks down fatty acids.
Protein metabolism: hepatocytes synthesize most plasma proteins, such as albumin, prothrombin and fibrinogen. The liver can move or remove amino groups on amino acids, and converts the resulting ammonia into urea for excretion in the urine.
Removal of drugs and hormones: The liver can detoxify substances such as alcohol, or secrete drugs, such as antibiotics, into bile. It can alter or secrete thyroid hormones and steroid hormones.
Excretion of bilirubin and synthesis of bile salts.
Storage: in addition to glycogen, the liver stores fat soluble vitamins (A,D,E and K) and B12, and minerals such as iron and copper.
Phagocytosis: The Kupffer's cells phagocytize worn-out red and white blood cells, and also some bacteria.
Activation of vitamin D: along with the skin and kidneys, the liver activates vitamin D.
Anatomy The gallbladder is a sac located on the inferior and posterior surface of the liver.
Physiology The gallbladder stores and concentrates bile. Cholecystokinin (CCK) stimulates ejection of bile into the common bile duct.
SUMMARY: DIGESTIVE HORMONES (782) The four major hormones that regulate digestive processes are gastrin, gastric inhibitory peptide (GIP), secretin, and cholecystokinin (CCK). Gastrin and GIP exert their major effects on the stomach, whereas secretin and CCK affect the pancreas, liver, and gallbladder most strongly.
Anatomy The major events of digestion and absorption occur in the small intestine . The small intestine extends from the pyloric sphincter to the ileocecal sphincter. It is about 10 ft or 3M long, and its surface area is further increased by the presence of circular folds, villi and microvilli. It is divided into duodenum, jejunum, and ileum. The duodenum is the shortest segment (12 fingers wide) and is retroperitoneal. The ileum is the longest segment, and joins the large intestine at the ileocecal sphincter (valve). Circular folds are found from the proximal portion of the duodenum to the middle of the ileum. They enhance absorption by increasing the surface area and causing the chyme to spiral, rather than pass straight through the intestines.
Histology The mucosa of the small intestine contain villi, small projections which increase the surface area. In the center of each villus is a core of lamina propria which contains an arteriole, a venule, a capillary network and a lacteal. The epithelium is simple columnar epithelium that contains absorptive cells, goblet cells, enteroendocrine cells and Paneth cells. The free ends of the epithelial cells have microvilli, called the brush boarder, which further increase surface area. The brush boarder may also contain several digestive enzymes. The mucosa contains many cavities lined with glandular epithelium. These cells form the intestinal glands or the crypts of Lieberkuhn and secrete intestinal juice. Paneth cells are found in the deepest parts of the crypts, and secrete lysozyme and are capable of phagocytosis. Also here are the enteroendocrine cells which secrete CCK, secretin and GIP. The submucosa of the duodenum contains duodenal glands which secrete an alkaline mucus.
The lamina propria has an abundance of MALT. There are numerous aggregated lymphatic follicles or Peyer's patches in the ileum.
Intestinal Juice and Brush Border Enzymes Intestinal juice is slightly alkaline and contains water and mucus. The intestinal and pancreatic juices provide a vehicle for the absorption of nutrients at the microvilli. The absorptive cells synthesize several enzymes and insert them in the plasma membrane of the microvilli. These brush border enzymes digest carbohydrates, proteins, and nucleotides at the surface of mucosal epithelial cells.
Physiology of Digestion in the Small Intestine
There are two basic movements of the small intestine. Segmentation is the contraction of the circular muscles to create segments in the intestine. Alternating contracts mix the chyme, like squeezing alternate ends of a toothpaste tube. The second movement is peristalsis which moves chyme slowly along the intestine. Both movements are controlled by the autonomic N.S.
The chyme entering the small intestine consists of partially digested carbohydrates, proteins and lipids. The completion of digestion is a collective effort of the pancreatic juice, bile, and intestinal juice in the small intestine.
Carbohydrates are broken down into monosaccharides.
Proteins are broken down into amino acids, dipeptides and tripeptides.
Lipids are broken down into fatty acids and monoglycerides.
Nucleic acids are broken down into pentoses, phosphates and nitrogenous bases, which are absorbed by active transport.
Regulation of Intestinal Secretion and Motility
The most important mechanism is local reflexes that respond to the presence of chyme. The hormone known as vasoactive intestinal polypeptide (VIP) stimulates the production of intestinal juice. Segmentation movements depends mostly on intestinal distention. Peristalsis increases when most nutrients and water have been absorbed, and the walls of the intestine are stretched less. Parasympathetic impulses increase motility; sympathetic impulses decrease motility.
Physiology of Absorption in the Small Intestine Absorption occurs by diffusion, facilitated diffusion, osmosis, and active transport; 90% occurs in the small intestine.
Carbohydrates are absorbed by facilitated diffusion or active transport.
Proteins are absorbed by active transport, mostly in the duodenum and jejunum.
Lipids are absorbed by simple diffusion. Monosaccharides, amino acids, and short-chain fatty acids pass into the blood capillaries. Long chain fatty acids are transported to the epithelial cells inside of micelles, time spheres with bile salts on the outside. Within the epithelial cells monoglycerides are broken down into fatty acids and glycerol, and then recombined to form triglycerides. These aggregate into large globules with phospholipids, cholesterol and proteins, called chylomicrons. Chylomicrons leave the cells by exocytosis, and are taken up into the lacteals.
Water is reabsorbed by osmosis. The absorption of water depends on the absorption of electrolytes, monosaccharides and amino acids to establish a concentration gradient that promotes water absorption by osmosis.
Vitamins : fat soluble vitamins are included in micelles and absorbed by simple diffusion. Most water soluble vitamins are absorbed by simple diffusion. B12 is actively reabsorbed.
This is the largest serous membrane of the body. The parietal peritoneum lines the walls of the abdominal cavity, and the visceral peritoneum covers some of the organs in the abdomen. Others, such as the kidneys, pancreas and a section of the small intestine, lie behind the parietal peritoneum, and are said to be retroperitoneal. As in the pericardium and pleura, there is a potential space between the two layers of peritoneum called the peritoneal cavity. This potential space can become the site of the accumulation of several liters of fluid. This condition is called ascites. Unlike the pericardium and pleura, the peritoneum has several large folds which hold the organs of the abdomen together. One such fold is called the mesentery. This fold binds the small intestine to the posterior abdominal wall. The mesocolon binds the large intestine to the posterior abdominal wall. These folds carry blood and lymph vessels to the intestines, and hold them loosely to allow for a great amount of movement. The falciform ligament attaches the liver to the anterior abdominal wall. The lesser omentum is two folds which suspend the stomach and duodenum from the liver. The greater omentum drapes over the transverse colon and small intestines. It contains a lot of adipose tissue and is sometimes referred to as the fatty apron, or lace apron. It also contains many lymph nodes. These nodes may produce antibodies to combat infections of the peritoneum. Infection of the peritoneum is peritonitis. Like pleurisy, it can be very painful. In cases of acute infection, it can be life threatening.
The functions of the large intestine are the completion of absorption, the manufacture of certain vitamins, the formation of feces and the expulsion of feces from the body.
Anatomy The large intestine extends from the ileocecal sphincter to the anus. Its subdivisions include the cecum, colon, rectum, and anal canal. At the junction of the ileum and the large intestine is the ileocecal sphincter. The cecum is a blind pouch about 6 " long. The appendix is attached to the cecum. The open end of the cecum joins the colon. The colon is divided into several parts: the ascending colon, the right colic (hepatic) flexure, the transverse colon, the left colic (splenic) flexure, the descending colon, and sigmoid colon. The ascending and descending colon are retroperitoneal, but the transverse and sigmoid colon are not. The rectum is the last 8" of the GI tract, and the last inch is called the anal canal, and ends at the anus.
Histology The large intestine has no villi or permanent circular folds. The mucosa consists of simple columnar epithelium containing mostly absorptive and many goblet cells. The absorptive cells take in primarily water, and the goblet cells produce mucus. There are thickened bands of longitudinal muscles in the muscularis called taeniae coli (TEE -nee-ee Koli ), which gathers the colon into a series of pouches called haustra.
Physiology of Digestion in the Large Intestine
The passage of chyme into the large intestine is regulated by the ileocecal valve. Usually this sphincter is closed. Immediately after a meal, there is a gastroileal reflex in which peristalsis increases in the ileum, forcing the chyme into the cecum. Gastrin also relaxes the sphincter. As food passes into through the ileocecal valve, it accumulates in the cecum and ascending colon. Mechanical movements of the large intestine include haustral churning. In this process the haustra remain relaxed while they fill up. When distended to a certain point, the walls contract and squeeze the contents up to the next haustra. Peristalsis also occurs, though at a slower rate than in the small intestine. The final type of movement is called mass peristalsis. This begins at the middle of the transverse colon, and is a strong peristaltic wave which forces the colonic contents into the rectum. This is called the gastrocolic reflex, and is initiated by the presence of food in the stomach.
The last stages of chemical digestion occur in the large intestine through bacterial action; no enzymes are secreted by the colon. The bacteria ferment any remaining carbohydrates and release hydrogen, carbon dioxide and methane. Bacteria also convert the remaining proteins to amino acids and simpler substances. Some vitamins are synthesized by bacteria, including some B vitamins and vitamin K.
Absorption and Feces Formation in the Large Intestine The large intestine absorbs water, electrolytes, and vitamins. The chyle has become feces which consist of water, inorganic salts, epithelial cells, bacteria, and undigested foods.
Physiology of Defecation The elimination of feces from the rectum is called defecation. The distention of the rectal wall stimulates stretch receptors which send messages to the sacral spinal cord and initiate the defecation reflex. Motor impulses travel back through parasympathetic fibers to the descending colon, sigmoid colon, rectum and internal anal sphincter. The longitudinal muscles contract, increasing the pressure. Defecation is aided by voluntary contractions of the diaphragm and abdominal muscles and relaxation of the external anal sphincter.