Gastrointestinal Tract Functions Quiz

Section 6: Gastrointestinal Physiology

• Kim E. Barrett and Helen E. Raybould authored the book
• The book covers functional anatomy and general principles of regulation in Gastrointestinal Tract

Chapter 27: Functional Anatomy and General Principles of Regulation in the Gastrointestinal Tract

• The Gastrointestinal (GI) tract is comprised of the alimentary tract from the mouth to the anus, and also includes associated glands. Its function is to absorb nutrients and water, and eliminate waste products. Major processes are motility, secretion, digestion, and absorption.
• The GI tract is exposed to external environment, thus it has a defense system against microorganisms.
• The GI tract contains many functional segments: Mouth, pharynx, esophagus, stomach, duodenum, jejunum, ileum, colon, rectum, and anus. Associated with these are glands like Brunner's glands, which empty secretions into the lumen.
• Other associated organs, such as salivary glands and pancreas, empty secretions into the gut lumen.

Chapter 28: The Cephalic, Oral, and Esophageal Phases of the Integrated Response to a Meal

• This chapter details the processes in the GI tract in the early stages of the integrated response to a meal.
• The cephalic phase occurs before ingestion, driven by anticipation, smells, sights, sounds of food. This results in increased parasympathetic activity.
• The oral phase is triggered when food is in the mouth. The responses in this phase (primarily salivary secretion), are identical to the cephalic phase.
• The esophageal phase occurs when food moves from mouth to esophagus. This is mainly to prepare the GI tract for digestion and absorption, which involves swallowing.

Chapter 29: The Gastric Phase of the Integrated Response to a Meal

• This chapter details the processes in the GI tract when food is in the stomach.
• The main function of the stomach is to act as a temporary reservoir for the meal and initiate protein digestion via acid and pepsinogen secretion.
• The stomach is divided into the cardia, corpus, and antrum, with the latter two sections used in a proximal and distal functional sense.
• The gastric mucosa is lined with gastric pits (the opening of ducts). These glands include regions like the isthmus, necks with mucous neck cells, and parietal cells, chief cells, and enterochromaffin-like (ECL) cells that secrete histamine and D cells that secrete somatostatin.
• Stomach secretions protect the mucosa (mucus, HCO3), and initiate protein digestion. Components are acid (HCl), pepsinogen, mucus, intrinsic factor, and HCO3.
• Gastric motility is regulated by endocrine (gastrin, somatostatin), paracrine, and neural pathways.

Chapter 30: The Small Intestinal Phase of the Integrated Response to a Meal

• This chapter covers digestion and absorption in the small intestine.
• The small intestine plays a major role in assimilation due to its large surface area (folds, villi, microvilli)
• Digestion occurs via many secretions:
  • Pancreatic juice: contains many proteolytic enzymes in their inactive forms. This is crucial for protein digestion, and these enzymes are activated at the appropriate time/site. Also contains bicarbonate to neutralize acid.
  • Bile: produced in the liver and stored in the gallbladder. It is a critical component for lipid digestion, providing detergents (bile acids) to increase lipid solubility.
• The rates of gastric emptying are dependent on the type of meal consumed. Liquids empty more rapidly than solids.
• Many factors regulate gastric emptying:
  • Distension
  • Nutrients.
  • Osmolarity
  • These result in changes in pancreatic secretion, gallbladder contraction, relaxation of sphincters of Oddi, regulation of gastric emptying, and inhibition of gastric acid secretion.

Chapter 31: The Colonic Phase of the Integrated Response to a Meal

• The large intestine is the most distal part of the Gastrointestinal tract and includes the cecum, the ascending, transverse, descending portions of the colon, rectum and anus
• Its primary roles include digestion/absorption of unabsorbed nutrients; reabsorption of water and electrolytes; storage of waste; and the microbiome.
• Motility of the large intestine is largely designed for mixing and retention, rather than propulsion. Thus, its most distinctive feature is the production of haustra (sacculations or pockets).
• The large intestine also stores and releases indigestible materials.
• Key neural/hormonal signals and feedback loops regulate colonic function.
• The microbiota in the colon interact with substances in the intestinal tract and play a major role in metabolism and host defense.

Chapter 32: Transport and Metabolic Functions of the Liver

• The liver is a major metabolic center, involved in carbohydrate, lipid, and protein metabolism.
• The liver plays a crucial role in detoxification of various substances.
• The liver contains specialized cell types that form cords around sinusoids, allowing efficient exchange of blood and metabolites.
• Bile is a key product of liver function, involved in lipid digestion and excretion.
• Ammonia is processed into urea in the liver and excreted in the urine.
• The liver is a crucial processing point for many substances (drugs, nutrients, and wastes) before they enter circulation or are excreted.