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Digestive System

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The digestive system is an entire array of organs that work together in a process known as digestion. It is also known collectively as the alimentary canal and functions to process food that is placed in the mouth and that passes through the digestive tract where it is broken down, processed, absorbed and utilized as energy that fuels the body's daily functions, before the excess dietary mass and any toxins or waste products that are produced through metabolic processes are excreted through the anus. This article aims to create an overview of this complex system by discussing step by step where the food bolus goes and which organs are directly and indirectly involved in the digestive process. Organs that are indirectly involved are known as the accessory digestive organs. Lastly, a few common pathological conditions of the digestive tract will be mentioned along with their clinical symptoms.

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The Digestive Pathway

Hormones are rhythmically secreted that cause the human body to feel hunger. This sensation of needing to feed oneself induces the phenomenon of eating, whereby food is manually placed into the mouth. This is the first stage of digestion. The food is bitten and torn off by the teeth in bite sized amounts and then ground down into swallowable portions that are lubricated with saliva. Hormones, the smell of food and the masticatory movements increase the flow of saliva that contains digestive enzymes which initialize the breakdown of food's microstructure. After a couple of seconds the food bolus is swallowed and is rhythmically pumped down the esophagus via contractions known as peristalsis. Once it reaches the lower border of the esophagus, the gastroesophageal sphincter opens and the bolus enters the stomach. Here the bolus is broken down by the acid of the stomach and the rhythmic contractions that mix it with the gastric juices. Once the bolus has been sufficiently processed, the gastroduodenal or pyloric sphincter opens and the acidic chyme enters the duodenum, where it is neutralized with basic compounds that are secreted from the pancreatic duct and fat is broken down by the biliary secretions of the gallbladder. Absorption of the digested nutrients occurs from the duodenum up until the end of the small intestine and is transferred in the blood to the liver or the kidneys for processing. The large intestine absorbs excess water and the unwanted food mass is held in the colon until a sufficient amount is collected for excretion. Excess fluid that has been processed by the kidney is collected in the urinary bladder and exits the body during urination.

The Alimentary Canal and the Accessory Organs

The alimentary canal is the actual pathway that the food bolus travels along. It directly comes into contact with the bolus at all times and is generally tubelike in structure. The accessory organs are those that indirectly affect the food bolus and are active during the digestive process. The parts of the alimentary canal are as follows:

  • The Oral Cavity
  • The Pharynx
  • The Esophagus
  • The Stomach
  • The Duodenum
  • The Small Intestine: including the jejunum and the ileum
  • The Large Intestine: including the appendix, the ascending colon, the transverse colon, the descending colon, the sigmoid colon and the rectum.

The accessory organs of the digestive tract are as follows:

  • The Salivary Glands: secrete digestive enzymes in the oral cavity and lubricate the food bolus
  • The Spleen and the Tonsils: provide immune protection against noxious substances and byproducts from the food bolus
  • The Lymph Nodes: governs lymphatic drainage of the gastrointestinal tract in its entirety
  • The Liver: filters the blood, processes all the absorbed nutrients taken from the food bolus, stores nutrients and produces bile
  • The Gallbladder: stores the bile acid that is secreted into the duodenum for lipid breakdown
  • The Pancreas: secretes digestive enzymes into the duodenum and basic compounds to neutralize the chyme as well as secreting insulin into the blood to increase the glucose uptake into cells

Pathology

GERD or gastroesophageal reflux disease is a disorder that is very common in patients of all ages. It is the chronic irritation of the esophagus, pharynx and sometimes even the larynx due to regurgitation of or the overproduction of gastric acid back up the alimentary canal. This can happen due to a gastroesophageal hernia or an insufficient lower esophageal sphincter. Antacids usually combat the symptoms. Acute erosive gastritis is the focal damage to the gastric mucosa that is accompanied by acute inflammation, necrosis and hemorrhage. It is caused by nonsteroidal antiinflammatory drugs (NSAIDs), cigarette smoking, heavy and chronic alcohol consumption, a burn injury such as a curling ulcer and a brain injury that can cause a cushing ulcer. Crohn’s disease is an autoimmune disease of the small intestine. Its a chronic inflammatory condition that prevents the absorption of nutrients and is most common in younger people in their second or third decade of life. In severe cases it can lead to the growth of a carcinoma in the small intestine or the colon.

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Show references

References:

  • Frank H. Netter, MD, Atlas of Human Anatomy, Fifth Edition, Saunders - Elsevier, Chapter 4 Abdomen, Subchapters 24 to 31.
  • Neil S. Norton, Ph.D. and Frank H. Netter, MD, Netter’s Head and Neck Anatomy for Dentistry, 2nd Edition, Elsevier Saunders, Chapter 22 Introduction to the Upper Limb, Back, Thorax and Abdomen, Page 556 to 614.
  • The Digestive System and how it works. National Digestive Diseases Information Clearinghouse (NDDIC). September 18, 2013.
  • Kimball Johnson, MD. Your digestive system. WebMD. July 25, 2012.
  • Arthur S. Schneider and Philip A. Szanto, Board Review Series Pathology, 1st Edition, Wolters Kluwer - Lippincott, Williams and Wilkins, Chapter 15 Gastrointestinal Tract, Page 220 to 223.

Author:

  • Dr. Alexandra Sieroslawska

Illustrators:

  • Esophagus in situ - Yousun Koh 
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