The Digestive System
23.1 Overview of the Digestive System
Anatomy of the Digestive System
The human gastrointestinal tract refers to the stomach and intestine, and sometimes to all the structures from the mouth to the anus.
Digestive Processes and Functions of the Digestive System
Digestion is necessary for absorbing nutrients from food and occurs through two processes: mechanical and chemical digestion.
Relationship of Digestive System Organs
The upper digestive tract consists of the esophagus, stomach, while the lower tract includes the small intestine and the large intestine.
23.2 Nervous System of the Digestive System
Enteric Nervous System
The enteric nervous system (ENS) is a subdivision of the autonomic nervous system (ANS) that directly controls the gastrointestinal system.
Autonomic Nervous System
The autonomic nervous system is a part of the peripheral nervous system that controls visceral functions such as heart rate and digestion.
Gastrointestinal Reflex Pathways
The digestive system functions via a system of long reflexes (CNS), short reflexes (ENS), and reflexes from GI peptides working together.
Relationship of Digestive Organs to Peritoneum
The peritoneum, the serous membrane that forms the lining of the abdominal cavity, covers most of the intra-abdominal organs.
23.4 Layers of the GI Tract
The mucosa is the innermost layer of the GI tract that is composed of simple epithelium cells and it is the absorptive and secretory layer.
The submucosa is a dense irregular layer of connective tissue with large blood vessels, lymphatics, and nerves that supports the mucosa.
The muscularis is responsible for segmental contractions and peristaltic movement in the GI tract.
Serosa consists of a secretory epithelial layer and a thin connective tissue layer that reduce the friction from muscle movement.
23.5 Organs of the Digestive System
The mouth is the first portion of the alimentary canal that receives and mechanically breaks down food and produces saliva.
The pancreas, liver, and gallbladder are the essential associated organs of the digestive system.
The pharynx is part of the digestive and respiratory systems and consists of three main parts: nasopharynx, oropharynx, and laryngopharynx.
The esophagus is a muscular tube through which food passes from the pharynx via peristalsis to the stomach.
Digestion in the Mouth, Pharynx, and Esophagus
Digestion is the mechanical and chemical breakdown of food into smaller components that are more readily absorbed into the bloodstream.
Gross Anatomy of the Stomach
The stomach is divided into four sections, each of which has different cells and functions.
Microscopic Anatomy of the Stomach
The layers of the stomach produce mucous to protect itself, enyzmes to break down the food for digestion, and muscles to 'churn' the food.
Stomach Digestive Properties
The movement and the flow of chemicals into the stomach is controlled by the autonomic nervous system and various digestive system hormones.
23.7 Small Intestine and Associated Structures
Anatomy of Small Intestine
The small intestine is the part of the gastrointestinal tract where much of the digestion and absorption of food takes place.
Histology of the Small Intestine
The small intestine wall has four layers: the outermost serosa, muscularis, submucosa, and innermost mucosa.
The Liver and Gallbladder
The liver makes bile, which is essential for the digestion of fats, and the gallbladder stores the bile until it is needed.
Anatomy of the Liver and Gallbladder
The liver is located in the abdomen, it has four lobes; the gallbladder is located under the liver, it is a hollow organ that stores bile.
Histology of Liver and Gallbladder
Hepatocytes are the main tissue cells of the liver; the gallbladder contains the mucosa, muscularis, perimuscular, and serosa layers.
Liver Blood Supply
In the hepatic portal system, the liver receives a dual blood supply from the hepatic portal vein and hepatic arteries.
Bile is a fluid produced by the liver that aids the process of digestion and absorption of lipids in the small intestine.
Functions of the Liver
The liver is thought to be responsible for up to 500 separate functions.
The pancreas is a gland organ in the digestive and endocrine systems.
Anatomy of the Pancreas
The pancreas lies in the epigastrium or upper central region of the abdomen and can vary in shape.
Histology of the Pancreas
The pancreas serves digestive and endocrine functions, and it is composed of two types of tissue: islets of Langerhans and acini.
Pancreatic fluid contains digestive enzymes that help to further break down the carbohydrates, proteins, and lipids in the chyme.
Small Intestine Digestive Processes
The small intestine uses different enzymes and processes to digest proteins, lipids, and carbohydrates.
23.8 Large Intestine
Anatomy of the Large Intestine
The large intestine absorbs water from remaining indigestible food matter and compacts feces prior to defecation.
Histology of Large Intestine
The large intestine has taeniae coli, and invaginations (the intestinal glands) as opposed to the small intestines.
The largest bacteria ecosystem in the human body is in the large intestine, where it plays a variety of important roles.
Digestive Processes of the Large Intestine
In the large intestine, a host of microorganisms known as "gut flora" help digest remaining food matter and create vitamins.
Absorption and Feces Formation in the Large Intestine
The large intestine absorbs water from the chyme and stores feces until they can be defecated.
Defecation is a combination of voluntary and involuntary processes with enough force to remove waste material from the digestive system.
23.9 Chemical Digestion
Mechanism of Chemical Digestion
Chemical digestion is the process of breakdown of large macronutrients into smaller molecules by enzyme-mediated hydrolysis.
Chemical Digestion of Carbohydrates, Proteins, Lipids, Nu...
Chemical breakdown of macromolecules contained in food is completed by various enzymes produced in the digestive system.
Absorption in the Small Intestine
Absorption of nutrients occurs partially by diffusion through the wall of the small intestine.
Absorption of Monosaccharides, Amino Acids, Dipeptides, T...
Glucose, amino acids, fats, and vitamins are absorbed in the small intestine via the action of hormones and electrolytes.
23.11 Phases of Digestion
The cephalic phase of gastric secretion occurs even before food enters the stomach via neurological signals.
The gastric phase is a period in which swallowed food activates gastric activity in the stomach.
The intestinal phase occurs in the duodenum, responds to arriving chyme, and moderates gastric activity via hormones and nervous reflexes.
Hormones of the Digestive System
There are five main hormones that aid and regulate the digestive system in mammals.
23.12 Development of the Digestive System
Digestive System Development
The digestive system is an endoderm-derived structure that begins developing about the fourth week of embryogenesis.
Aging and the Digestive System
Aging can result in changes of the digestive system due to decreased nerve sensitivity, loss of muscle, and increased infection rate.
23.13 Digestive System Disorders and Clinical Cases
Malabsorption of Nutrients
Malabsorption is a state arising from abnormality in absorption of food nutrients across the gastrointestinal (GI) tract.
Peritonitis is an inflammation of the peritoneum, usually caused by an infectious organism that is introduced into the abdominal cavity.
Mumps was a common childhood viral disease, but widespread vaccination has now made it rare in developed countries.
Root Canal Therapy
Endodontic therapy is the treatment for the pulp of a tooth which results in the protection of the decontaminated tooth.
Gastroesophageal Reflux Disease
Gastroesophageal reflux disease (GERD) is chronic mucosal damage caused by stomach acid coming up from the stomach into the esophagus.
Pylorospasm and Pyloric Stenosis
Pyloric stenosis (infantile hypertrophic pyloric stenosis) causes severe projectile non-bilious vomiting in the first few months of life.
Vomiting, known medically as emesis, is the forceful expulsion of the contents of one's stomach through the mouth and sometimes the nose.
Pancreatitis and Pancreatic Cancer
Pancreatic cancer is the uncontrolled growth of malignant cells in tissues forming the pancreas.The most common type is adenocarcinoma.
Jaundice is a yellowish pigmentation of the skin and the sclera of the eyes caused by increased levels of bilirubin in the blood.
A gallstone is a crystalline concretion formed within the gallbladder by accretion of bile components.
Lactose intolerance is the inability to digest lactose, a sugar found in milk, due to a lack of the enzyme lactase.
Absorption of Alcohol
Alcohol is metabolized (by alcohol dehydrogenase enzymes made in the liver) when it is absorbed into the bloodstream.
Appendicitis is an inflammation of the appendix generally requiring surgical removal since if it ruptures, it can lead to peritonitis.
Polyps in the Colon
Colon polyps, abnormal growths of tissue in the colon, are of concern due to their potential for harboring cancerous cells.
Fecal occult blood (FOB) refers to not-visibly-apparent blood in the feces that may indicate problems with the gastrointestinal tract.
Dietary fiber interacts with the contents of the gastrointestinal tract, and it changes how other nutrients and chemicals are absorbed.
The appendix is a blind-ended tube connected to the cecum and located near the junction of the small intestine and the large intestine.
X-ray studies of the GI Tract
Upper GI series, also upper gastrointestinal (GI) tract radiography, is a radiologic examination of the upper gastrointestinal tract.
Dental caries cause demineralization of the hard tissues and destruction of the organic matter of the tooth.
Plaque-induced inflammatory lesions make up the vast majority of periodontal diseases, which are divided into peridontitis or gingivitis.
Peptic Ulcer Disease
A peptic ulcer, also known as peptic ulcer disease, is an erosion in the wall of the stomach, duodenum, or esophagus.
Diverticulosis is the condition of having outpocketings in the colon wall due to weaknesses of the muscle layer, called diverticula.
Colorectal cancer, commonly known as bowel cancer, is a cancer from uncontrolled malignant cell growth in the colon, rectum, or appendix.
Hepatitis is the inflammation of the liver. Causes include viruses, bacterial infections, alcohol, autoimmune disorders, drugs, and toxins.