Understanding the alimentary canal and its vital functions
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Three Primary Functions of Digestion
Mechanical & Chemical Breakdown
Complex food material taken into the mouth must be digested or broken down as it travels through the gastrointestinal tract. Digestive enzymes speed up chemical reactions, breaking down complex proteins to amino acids, complicated sugars to simple sugars like glucose, and large fat molecules to fatty acids and glycerol.
Nutrient Absorption
The digested food must be absorbed into the bloodstream by passing through the walls of the small intestine. This crucial step ensures that nutrients reach cells throughout the body to provide energy and support vital functions.
Waste Elimination
The elimination of solid waste materials that cannot be absorbed into the bloodstream occurs through the final portions of the digestive tract, completing the digestive cycle.
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The Oral Cavity: Gateway to Digestion
The alimentary canal begins with the oral cavity or mouth. The lips surround the opening whilst the cheeks form the walls of this oval-shaped cavity. The hard palate forms the anterior portion of the roof of the mouth, and the muscular soft palate lies posterior to it.
Rugae are irregular ridges in the mucous membrane covering the anterior portion of the hard palate. Hanging from the soft palate is the uvula, a small soft tissue that aids in producing sounds and speech.
The tongue extends across the floor of the oral cavity, with muscles attaching it to the lower jaw bone. It moves food during mastication (chewing) and deglutition (swallowing). Papillae, small raised areas on the tongue, contain taste buds sensitive to the chemical nature of foods.
Key Structures
- Lips and cheeks forming cavity walls
- Hard and soft palate
- Uvula for speech production
- Tongue with papillae and taste buds
- Tonsils as lymphatic filters
- 32 permanent teeth
- Three pairs of salivary glands
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Dental Architecture
There are 32 permanent teeth in the entire oral cavity—16 on the lower jaw and 16 on the upper jaw. The gums are made of fleshy tissue and surround the sockets of the teeth.
Central Incisor
Front cutting teeth
Lateral Incisor
Side cutting teeth
Canine
Pointed, dog-like teeth
First Premolar
Grinding teeth
Second Premolar
Additional grinding teeth
First Molar
Large grinding teeth
Second Molar
Additional molars
Third Molar
Wisdom teeth
Tonsils are masses of lymphatic tissue located in depressions of the mucous membranes on both sides of the oropharynx. They act as filters to protect the body from microorganisms and produce lymphocytes, white blood cells able to fight disease.
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Salivary Glands and Digestive Enzymes
Parotid Gland
Located near the ear, this exocrine gland produces saliva containing important digestive enzymes.
Submandibular Gland
Positioned below the jaw, it contributes significantly to saliva production.
Sublingual Gland
Located under the tongue, it releases saliva through narrow ducts into the oral cavity.
Three pairs of salivary glands surround the oral cavity. These exocrine glands produce saliva, a fluid containing important digestive enzymes. Narrow ducts carry the saliva into the oral cavity on each side of the mouth.
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The Pharynx and Esophagus
Pharynx (Throat)
The pharynx is a muscular tube about 5 inches long, lined with mucous membrane. It serves as a common passageway for air travelling from the nose to the windpipe and food travelling from the oral cavity to the oesophagus.
When swallowing (deglutition) occurs, a flap of tissue called the epiglottis covers the trachea so that food cannot enter and become lodged there.
Oesophagus
The oesophagus is a 9- to 10-inch muscular tube extending from the pharynx to the stomach. Rhythmic contractions of muscles in the wall propel food towards the stomach.
Peristalsis is this involuntary, progressive, rhythm-like contraction of the oesophagus and other gastrointestinal tubes.
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The Stomach: Chemical Processing Centre
Food passes from the oesophagus into the stomach, a crucial organ for both mechanical and chemical digestion.
Fundus
Upper portion of the stomach where food initially enters
Body
Middle section where most digestion occurs
Pylorus
Lower portion leading to small intestine
Rings of muscles called sphincters control the openings into and from the stomach. The cardiac sphincter relaxes and contracts to move food from the oesophagus into the stomach, whilst the pyloric sphincter allows food to leave when sufficiently digested. Folds in the mucous membrane lining called rugae contain digestive glands that produce enzymes and hydrochloric acid.
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The Small Intestine: Absorption Powerhouse
The small intestine (small bowel) extends for 20 feet from the pyloric sphincter to the first part of the large intestine. It has three distinct parts, each playing a vital role in digestion and absorption.
Duodenum
Only 1 foot in length, receives food from the stomach as well as bile from the liver and gallbladder, and pancreatic juice from the pancreas. Enzymes and bile help digest food before it passes into the jejunum.
Jejunum
About 8 feet long, this middle section connects the duodenum to the ileum. The name comes from Latin jejunus meaning empty, as this part was always empty when examined after death.
Ileum
About 11 feet long, this final section attaches to the first part of the large intestine. From Greek eilos meaning twisted, as it appeared twisted when viewed at necropsy.
Millions of tiny, microscopic projections called villi line the walls of the small intestine. The tiny capillaries in the villi absorb the digested nutrients into the bloodstream and lymph vessels.
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The Large Intestine: Final Processing
The large intestine extends from the end of the ileum to the anus. It is divided into four parts: caecum, colon, sigmoid colon, and rectum.
Caecum
A pouch on the right side that attaches to the ileum by the ileocaecal valve. The appendix hangs from the caecum.
Ascending Colon
Extends from the caecum to the undersurface of the liver, where it turns left.
Transverse Colon
Passes horizontally to the left towards the spleen, then turns downward.
Descending Colon
Travels downward on the left side of the abdomen.
Sigmoid Colon
Shaped like an S (sigma), leads into the rectum.
Rectum & Anus
The rectum terminates in the lower opening of the gastrointestinal tract, the anus.
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Accessory Organs: Liver, Gallbladder & Pancreas
Three important additional organs of the digestive system play crucial roles in the proper digestion and absorption of nutrients.
Liver
Located in the right upper quadrant (RUQ) of the abdomen, the liver manufactures bile—a thick, yellowish-brown, sometimes greenish fluid containing cholesterol, bile acids, and bile pigments including bilirubin. The liver continuously releases bile, which travels down the hepatic duct to the cystic duct. The normal adult liver weighs about 2½-3 pounds.
Gallbladder
A pear-shaped sac under the liver that stores and concentrates bile for later use. After meals, in response to food in the stomach and duodenum, the gallbladder contracts, forcing bile out the cystic duct into the common bile duct, which joins with the pancreatic duct before entering the duodenum.
Pancreas
Both an exocrine and endocrine organ. As an exocrine gland, it produces pancreatic juices filled with enzymes (amylase and lipase) to digest food. As an endocrine gland, it secretes insulin, a hormone needed to help release sugar from the blood and carry glucose into cells for energy.
Essential Digestive System Vocabulary
Absorption
Passage of materials through the walls of the intestine into the bloodstream
Alimentary Canal
The digestive tract; aliment means food
Amino Acids
Small substances that are the building blocks of proteins, produced when proteins are digested
Amylase
Enzyme from the pancreas to digest starch
Bile
Digestive juice made in the liver and stored in the gallbladder that physically breaks up large fat globules
Bilirubin
Pigment released by the liver in bile, produced from the destruction of haemoglobin
Deglutition
Swallowing
Emulsification
Physical process of breaking up large fat globules into smaller globules
Enzyme
A chemical that speeds up reactions between substances
Peristalsis
Rhythm-like contractions of the tubes of the alimentary tract
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Common Digestive Symptoms
Understanding the signs and symptoms of digestive system disorders is crucial for early detection and treatment.
Achlorhydria
Lack of hydrochloric acid in the stomach, possibly indicating chronic gastritis or gastric carcinoma
Anorexia
Lack of appetite (-orexia = appetite)
Ascites
Abnormal accumulation of fluid in the abdomen
Dysphagia
Difficulty or painful swallowing
Gastro-oesophageal Reflux
Solids and fluids return to the mouth from the stomach
Jaundice
Yellow-orange colouration of the skin due to high bilirubin levels in the blood
Stool-Related
- Constipation: Difficult, delayed elimination
- Diarrhoea: Loose, watery stools
- Melaena: Black, tarry stools with blood
- Steatorrhoea: Fat in the faeces
Pain & Discomfort
- Colic: Acute abdominal pain from spasms
- Nausea: Unpleasant sensation with tendency to vomit
- Flatus: Gas expelled through the anus
Blood-Related
- Haematochezia: Bright red blood from the rectum
- Melaena: Blood-containing faeces
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Pathological Conditions: Oral Cavity & Teeth
Aphthous Stomatitis
Inflammation of the mouth with small ulcers
Dental Caries
Tooth decay (caries means decay)
Herpetic Stomatitis
Inflammation of the mouth by herpes virus infection. Commonly called fever blisters or cold sores
Oral Leukoplakia
White plaques or patches on the mucosa of the mouth
Periodontal Disease
Inflammation and degeneration of gums, teeth, and surrounding bone; also called pyorrhoea
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Gastrointestinal Tract Disorders
The gastrointestinal tract can be affected by various pathological conditions ranging from functional disorders to structural abnormalities.
Achalasia
Failure of sphincter muscles of the lower oesophagus to relax during swallowing
Anal Fistula
Abnormal tube-like passageway near the anus, which may communicate with the rectum
Colonic Polyposis
Polyps (small growths) protrude from the mucous membrane of the colon
Colorectal Cancer
Carcinoma of the colon or rectum, or both
Crohn's Disease
Chronic inflammation of the intestinal tract affecting any part from mouth to anus
Diverticula
Abnormal side pockets (outpouchings) in the intestinal wall
Oesophageal Varices
Swollen, twisted veins around the distal end of the oesophagus
Haemorrhoids
Swollen, twisted, varicose veins in the rectal region
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Additional GI Tract Conditions
Structural Disorders
- Hernia: Protrusion of an organ through the muscle normally containing it
- Ileus: Intestinal obstruction
- Intussusception: Telescoping of the intestines
- Volvulus: Twisting of the intestine upon itself
Inflammatory Conditions
- Dysentery: Painful, inflamed intestines
- Irritable Bowel Syndrome: Diarrhoea, constipation, pain, and bloating associated with stress
- Ulcer: Open sore or lesion of epithelial tissue
- Ulcerative Colitis: Chronic inflammation of the colon with ulcers
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Liver, Gallbladder & Pancreas Disorders
Cirrhosis
Chronic disease of the liver with degeneration of liver cells. This progressive condition can result from various causes including alcohol abuse, viral hepatitis, and fatty liver disease.
Gallstones (Cholelithiasis)
Hard collections of bile form in the gallbladder and bile ducts. These can cause severe pain and may require surgical intervention if they obstruct bile flow.
Pancreatitis
Inflammation of the pancreas. Can be acute or chronic, often caused by gallstones or excessive alcohol consumption, leading to severe abdominal pain.
Viral Hepatitis
Inflammation of the liver caused by virus. There are three major types: Hepatitis A, B, and C, each with different transmission methods and treatment approaches.
Laboratory Tests & Clinical Procedures
Liver Function Tests
SGOT/AST
Serum glutamic oxaloacetic transaminase, also called aspartic acid transaminase. Enzyme levels elevated when liver cells are damaged.
SGPT/ALT
Serum glutamic pyruvic transaminase, also called alanine transaminase. High levels especially indicative of liver disease.
Alkaline Phosphatase
Enzyme test on serum. Increased levels found in liver disease, cancers, and other abnormal conditions.
Serum Bilirubin
High levels produce jaundice. Also known as the icterus index.
Stool Analyses
Stool Culture: Faeces placed in growth medium to test for microorganisms present in large numbers or abnormally.
Stool Guaiac or Hemoccult Test: Determines presence of blood in faeces, an important screening test for colon cancer. Guaiac is a chemical from tree wood that reacts with occult blood.
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X-Ray & Imaging Procedures
Barium Enema (Lower GI Series)
Technicians inject barium sulphate, a contrast medium, by enema into the rectum, and x-rays are taken of the rectum and colon.
Barium Swallow (Upper GI Series)
Barium sulphate is swallowed and x-rays are taken of the oesophagus, stomach, and small intestine. In a small bowel follow-through, sequential pictures are taken as barium passes through.
CT (CAT) Scan of Abdomen
Computed tomography takes a series of x-ray pictures processed by computer to show cross-sectional images of internal organs. Iodine-based dyes and barium can visualise blood vessels and the gastrointestinal tract.
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Advanced Diagnostic Procedures
Endoscopic Retrograde Cholangiopancreatography (ERCP)
Contrast material is injected via endoscope into pancreatic and bile ducts whilst x-rays are taken. The endoscope passes through the mouth, oesophagus, and stomach into the duodenum. Retrograde means dye is injected backward to normal flow.
Abdominal Ultrasonography
Sound waves are beamed into the abdomen, and a record is made of the echoes as they bounce off the abdominal viscera.
Liver Scan
Radioactive material is injected intravenously and taken up by liver cells. A scintiscan records uptake of radioactive material by the liver cells.
Gastrointestinal Endoscopy
A flexible fibreoptic tube is placed through the mouth or anus to visualise parts of the GI tract. Examples: oesophagogastroduodenoscopy, colonoscopy, sigmoidoscopy, proctoscopy.
Liver Biopsy
A needle is inserted percutaneously into the liver, and a sample of tissue is removed for microscopic examination. Useful in diagnosing cirrhosis, chronic hepatitis, and cancerous tumours. The average sample is less than 1 inch long.
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Additional Clinical Procedures
Nasogastric Intubation
A nasogastric tube (NG tube) is passed through the nose into the stomach and upper region of the small intestine. The procedure is used to remove fluid postoperatively and to obtain gastric or intestinal contents for analysis.
Paracentesis (Abdominocentesis)
Surgical puncture to remove fluid from the abdomen (peritoneal cavity). This procedure is done to remove fluid from a patient with ascites and for diagnostic purposes.
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The Digestive System