Physiology of the Gastrointestinal Tract

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These enteric neurons are grouped into two plexuses. The myenteric plexus plexus of Auerbach lies in the muscularis layer of the alimentary canal and is responsible for motility , especially the rhythm and force of the contractions of the muscularis. The submucosal plexus plexus of Meissner lies in the submucosal layer and is responsible for regulating digestive secretions and reacting to the presence of food.

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Extrinsic innervations of the alimentary canal are provided by the autonomic nervous system, which includes both sympathetic and parasympathetic nerves. In general, sympathetic activation the fight-or-flight response restricts the activity of enteric neurons, thereby decreasing GI secretion and motility. In contrast, parasympathetic activation the rest-and-digest response increases GI secretion and motility by stimulating neurons of the enteric nervous system. The blood vessels serving the digestive system have two functions.

They transport the protein and carbohydrate nutrients absorbed by mucosal cells after food is digested in the lumen. Lipids are absorbed via lacteals, tiny structures of the lymphatic system. Specifically, the more anterior parts of the alimentary canal are supplied with blood by arteries branching off the aortic arch and thoracic aorta. Below this point, the alimentary canal is supplied with blood by arteries branching from the abdominal aorta.

The celiac trunk services the liver, stomach, and duodenum, whereas the superior and inferior mesenteric arteries supply blood to the remaining small and large intestines.

The veins that collect nutrient-rich blood from the small intestine where most absorption occurs empty into the hepatic portal system. This venous network takes the blood into the liver where the nutrients are either processed or stored for later use. Only then does the blood drained from the alimentary canal viscera circulate back to the heart. The digestive organs within the abdominal cavity are held in place by the peritoneum, a broad serous membranous sac made up of squamous epithelial tissue surrounded by connective tissue.

It is composed of two different regions: the parietal peritoneum, which lines the abdominal wall, and the visceral peritoneum, which envelopes the abdominal organs. The peritoneal cavity is the space bounded by the visceral and parietal peritoneal surfaces.

Overview of the Digestive System

A few milliliters of watery fluid act as a lubricant to minimize friction between the serosal surfaces of the peritoneum. Figure 3. A cross-section of the abdomen shows the relationship between abdominal organs and the peritoneum darker lines. Inflammation of the peritoneum is called peritonitis. Chemical peritonitis can develop any time the wall of the alimentary canal is breached, allowing the contents of the lumen entry into the peritoneal cavity. For example, when an ulcer perforates the stomach wall, gastric juices spill into the peritoneal cavity.

Hemorrhagic peritonitis occurs after a ruptured tubal pregnancy or traumatic injury to the liver or spleen fills the peritoneal cavity with blood. Even more severe peritonitis is associated with bacterial infections seen with appendicitis, colonic diverticulitis, and pelvic inflammatory disease infection of uterine tubes, usually by sexually transmitted bacteria. Peritonitis is life threatening and often results in emergency surgery to correct the underlying problem and intensive antibiotic therapy.

When your great grandparents and even your parents were young, the mortality from peritonitis was high. Aggressive surgery, improvements in anesthesia safety, the advance of critical care expertise, and antibiotics have greatly improved the mortality rate from this condition. Even so, the mortality rate still ranges from 30 to 40 percent. The visceral peritoneum includes multiple large folds that envelope various abdominal organs, holding them to the dorsal surface of the body wall.


Within these folds are blood vessels, lymphatic vessels, and nerves that innervate the organs with which they are in contact, supplying their adjacent organs. The five major peritoneal folds are described in Table 2. Note that during fetal development, certain digestive structures, including the first portion of the small intestine called the duodenum , the pancreas, and portions of the large intestine the ascending and descending colon, and the rectum remain completely or partially posterior to the peritoneum.

Thus, the location of these organs is described as retroperitoneal. The digestive system includes the organs of the alimentary canal and accessory structures. The alimentary canal forms a continuous tube that is open to the outside environment at both ends. The organs of the alimentary canal are the mouth, pharynx, esophagus, stomach, small intestine, and large intestine.

The accessory digestive structures include the teeth, tongue, salivary glands, liver, pancreas, and gallbladder. The wall of the alimentary canal is composed of four basic tissue layers: mucosa, submucosa, muscularis, and serosa. The enteric nervous system provides intrinsic innervation, and the autonomic nervous system provides extrinsic innervation.

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Gastrointestinal system anatomy

Module 7: The Digestive System. Search for:. Overview of the Digestive System Learning Objectives By the end of this section, you will be able to: Identify the organs of the alimentary canal from proximal to distal, and briefly state their function Identify the accessory digestive organs and briefly state their function Describe the four fundamental tissue layers of the alimentary canal Contrast the contributions of the enteric and autonomic nervous systems to digestive system functioning Explain how the peritoneum anchors the digestive organs.

Disorders of the Digestive System: Peritonitis Inflammation of the peritoneum is called peritonitis. Critical Thinking Questions Explain how the enteric nervous system supports the digestive system. What might occur that could result in the autonomic nervous system having a negative impact on digestion?

What layer of the alimentary canal tissue is capable of helping to protect the body against disease, and through what mechanism? Show Answers The enteric nervous system helps regulate alimentary canal motility and the secretion of digestive juices, thus facilitating digestion. If a person becomes overly anxious, sympathetic innervation of the alimentary canal is stimulated, which can result in a slowing of digestive activity.

The lamina propria of the mucosa contains lymphoid tissue that makes up the MALT and responds to pathogens encountered in the alimentary canal. Licenses and Attributions. CC licensed content, Shared previously. Mucosa-associated lymphoid tissue and other lymphatic tissue defend against entry of pathogens; lacteals absorb lipids; and lymphatic vessels transport lipids to bloodstream. There, the food is chewed and mixed with saliva, which contains enzymes that begin breaking down the carbohydrates in the food plus some lipid digestion via lingual lipase.

Chewing increases the surface area of the food and allows an appropriately sized bolus to be produced. Food leaves the mouth when the tongue and pharyngeal muscles propel it into the esophagus. This act of swallowing, the last voluntary act until defecation, is an example of propulsion , which refers to the movement of food through the digestive tract.

It includes both the voluntary process of swallowing and the involuntary process of peristalsis. Peristalsis consists of sequential, alternating waves of contraction and relaxation of alimentary wall smooth muscles, which act to propel food along Figure 1. These waves also play a role in mixing food with digestive juices. Peristalsis is so powerful that foods and liquids you swallow enter your stomach even if you are standing on your head.

Digestion includes both mechanical and chemical processes. Mechanical digestion is a purely physical process that does not change the chemical nature of the food. Instead, it makes the food smaller to increase both surface area and mobility. It includes mastication , or chewing, as well as tongue movements that help break food into smaller bits and mix food with saliva. Although there may be a tendency to think that mechanical digestion is limited to the first steps of the digestive process, it occurs after the food leaves the mouth, as well.

Segmentation , which occurs mainly in the small intestine, consists of localized contractions of circular muscle of the muscularis layer of the alimentary canal. These contractions isolate small sections of the intestine, moving their contents back and forth while continuously subdividing, breaking up, and mixing the contents.

By moving food back and forth in the intestinal lumen, segmentation mixes food with digestive juices and facilitates absorption. In chemical digestion , starting in the mouth, digestive secretions break down complex food molecules into their chemical building blocks for example, proteins into separate amino acids. These secretions vary in composition, but typically contain water, various enzymes, acids, and salts.

The process is completed in the small intestine. Food that has been broken down is of no value to the body unless it enters the bloodstream and its nutrients are put to work. This occurs through the process of absorption , which takes place primarily within the small intestine. There, most nutrients are absorbed from the lumen of the alimentary canal into the bloodstream through the epithelial cells that make up the mucosa. Lipids are absorbed into lacteals and are transported via the lymphatic vessels to the bloodstream the subclavian veins near the heart. The details of these processes will be discussed later.

In defecation , the final step in digestion, undigested materials are removed from the body as feces. Digestive System: From Appetite Suppression to Constipation Age-related changes in the digestive system begin in the mouth and can affect virtually every aspect of the digestive system. Swallowing can be difficult, and ingested food moves slowly through the alimentary canal because of reduced strength and tone of muscular tissue.

Physiology of the Gastrointestinal Tract - 6th Edition

Neurosensory feedback is also dampened, slowing the transmission of messages that stimulate the release of enzymes and hormones. Pathologies that affect the digestive organs—such as hiatal hernia, gastritis, and peptic ulcer disease—can occur at greater frequencies as you age.

Problems in the small intestine may include duodenal ulcers, maldigestion, and malabsorption. Problems in the large intestine include hemorrhoids, diverticular disease, and constipation. Conditions that affect the function of accessory organs—and their abilities to deliver pancreatic enzymes and bile to the small intestine—include jaundice, acute pancreatitis, cirrhosis, and gallstones. In some cases, a single organ is in charge of a digestive process. For example, ingestion occurs only in the mouth and defecation only in the anus.

However, most digestive processes involve the interaction of several organs and occur gradually as food moves through the alimentary canal Figure 2. Some chemical digestion occurs in the mouth. Some absorption can occur in the mouth and stomach, for example, alcohol and aspirin.

Neural and endocrine regulatory mechanisms work to maintain the optimal conditions in the lumen needed for digestion and absorption. These regulatory mechanisms, which stimulate digestive activity through mechanical and chemical activity, are controlled both extrinsically and intrinsically. The walls of the alimentary canal contain a variety of sensors that help regulate digestive functions. These include mechanoreceptors, chemoreceptors, and osmoreceptors, which are capable of detecting mechanical, chemical, and osmotic stimuli, respectively. Stimulation of these receptors provokes an appropriate reflex that furthers the process of digestion.

This may entail sending a message that activates the glands that secrete digestive juices into the lumen, or it may mean the stimulation of muscles within the alimentary canal, thereby activating peristalsis and segmentation that move food along the intestinal tract. The walls of the entire alimentary canal are embedded with nerve plexuses that interact with the central nervous system and other nerve plexuses—either within the same digestive organ or in different ones.

These interactions prompt several types of reflexes. Extrinsic nerve plexuses orchestrate long reflexes, which involve the central and autonomic nervous systems and work in response to stimuli from outside the digestive system. Short reflexes, on the other hand, are orchestrated by intrinsic nerve plexuses within the alimentary canal wall. These two plexuses and their connections were introduced earlier as the enteric nervous system.

Short reflexes regulate activities in one area of the digestive tract and may coordinate local peristaltic movements and stimulate digestive secretions. For example, the sight, smell, and taste of food initiate long reflexes that begin with a sensory neuron delivering a signal to the medulla oblongata. The response to the signal is to stimulate cells in the stomach to begin secreting digestive juices in preparation for incoming food.

Physiology of the Gastrointestinal Tract Physiology of the Gastrointestinal Tract
Physiology of the Gastrointestinal Tract Physiology of the Gastrointestinal Tract
Physiology of the Gastrointestinal Tract Physiology of the Gastrointestinal Tract
Physiology of the Gastrointestinal Tract Physiology of the Gastrointestinal Tract
Physiology of the Gastrointestinal Tract Physiology of the Gastrointestinal Tract

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