Gastrointestinal Tract Anatomy

Anatomy of the Gastrointestinal Tract

• This lecture is part of the PHMD 141 course.
• The lecture is delivered by Dr. Kevin Adutwum-Ofosu from the Department of Anatomy, CHS at UG.

Expected Learning Outcomes

• Students should be able to identify different parts of the gastrointestinal tract.
• Students should be able to describe the anatomy of its different parts.
• Students should be able to describe its functional design.
• Students should be able to describe the functional role of its different parts.

Activities in the Digestive System

• The digestive system involves ingestion, eating, and drinking.
• It involves propulsion, digestion (mechanical and chemical), absorption of nutrients, and elimination of waste.

Parts of the Gastrointestinal Tract

• Main parts include the oral cavity, oro-pharynx, esophagus, stomach, duodenum, liver, gall bladder, pancreas, jejenum, ileum, and large bowel.
• The oral cavity contains teeth, tongue, palate, and salivary glands.
• The esophagus is approximately 10 inches (25 cm) long.
• The duodenum is approximately 10 inches (25 cm) long.
• The jejenum is 8 feet (2.5 meters) long.
• The ileum is 12 feet (3.6 meters) long.
• The large bowel is 5 feet (1.5 meters) long.

Design of the Gastrointestinal Tract

• The GI tract is largely tubular from the oesophagus to the anal canal.
• The wall is comprised of four layers: Mucosa, Submucosa, Muscularis externa, and Tunica adventitia or serosa.
• The muscle layer typically has inner circular and outer longitudinal layers, although exceptions exist.
• The lumen is lined by epithelium.
• Different parts have different types of epithelia that are either protective, secretive or absorptive.

Role of the Oral Cavity

• Teeth are for mastication and softening food.
• Speech occurs in the oral cavity.
• The tongue is for taste, general sensation, deglutition and speech.
• The palate is for taste and deglutition.

Role of the Salivary Glands

• Salivary glands include the parotid, submandibular, and sublingual glands.
• Functions include lubrication, moistening food, and secretion of salivary amylase to break down starch to glucose.
• Salivary glands secrete lysozymes for antibacterial action and release antibodies like immunoglobulin A to protect the mucosa.

The Oesophagus

• It spans from the pharynx to the stomach, specifically from C6 to T11 vertebrae.
• It is about 25 cm (10 inches) in length and 1.5 to 2 cm (about 1 inch) in breadth.
• It begins on the right posterior to the trachea, descends posterior to the arch of aorta and base of the heart.
• It deviates to the left before passing through the diaphragm to join the stomach.
• Its superior third has only skeletal muscles, the middle third has a mix of skeletal & smooth muscles, while the inferior third has only smooth muscle.

Functions of the Stomach

• The stomach serves as a receptacle for food and fluids, with a capacity of 2-4 litres.
• It is involved with protein digestion, converting pepsinogen to pepsin at a pH of 1-2.
• The stomach facilitates absorption of water and electrolytes, as well as lipid-soluble substances such as alcohol, aspirin and caffeine.
• It produces the intrinsic factor necessary for absorption of vitamin B12.
• The stomach propels food into the duodenum.

Gastric Secretions

• Hydrochloric acid is secreted by parietal cells, activating pepsinogen and inactivating ingested bacteria.
• Mucus provides lubrication and protection.
• Pepsinogen, secreted by chief cells, is an enzyme, which initiates protein digestion as pepsin.
• Gastrin, a hormone that stimulates acid secretion via parietal cells and also increases gastric motility.
• Intrinsic factor, a glyco-protein, aids in the absorption of vitamin B12 in the ileum.

Functions of the Duodenum

• The duodenum receives food from the stomach and regulates food entry.
• It secretes hormones and completes processes of digestion of proteins and carbohydrates.
• The duodenum begins the process of absorption, and primarily absorbs iron.

Duodenal Gland Secretions

• Brunner's glands secrete mucus rich alkaline secretions, deactivating pepsinogen and protecting the duodenum from gastric secretions.
• Secretin stimulates bicarbonate secretion from the liver and pancreas to regulate pH, and triggers insulin secretion to regulate glucose.
• Cholecystokinin is a peptide hormone responsible for the digestion of fat and contributes to digestion of proteins.
• Urogastrone inhibits secretion of acid and pepsinogen.

Functions of the Pancreas

• The pancreas has exocrine and endocrine functions.
• Exocrine functions include producing digestive juices containing alkaline fluid (bicarbonates).
• Digestive juices contrain chymotrypsin (breakdown of proteins and polypeptides), lipase (hydrolysis of lipids), and amylase (digestion of starch).
• Endocrine functions, performed by the islets of Langerhans, include hormone production, secretion of insulin (lowers blood glucose level).
• Endocrine functions include secretion of glucagon (increases blood sugar level), and somatostatin (inhibits release of growth hormone and thyroid stimulating hormone).

Parts of the Liver Lobes

• Liver lobes include the right lobe, left lobe, caudate lobe, and quadrate lobe.

Functions of the Liver

• The liver synthesizes plasma proteins like albumin, globulin, and fibrinogen.
• It is involved in glucose metabolism.
• The liver is responsible for deamination of amino acids form urea and conversion of ammonia to urea.
• The liver is involved in the formation of blood clotting factors I, II, IV, V, VI and VII.
• The liver has a role in protein and lipid synthesis/cholesterol.
• It is involved in the storage of glucose, iron, and vitamins A, D, E, K and B12.
• The liver breaks down haemoglobin to iron and bile and is involved in detoxification/metabolism of drugs.

The Gall Bladder

• The gall bladder is attached to the liver and receives bile.
• Bile is stored and concentrated there.
• The gall bladder secretes bile for fat digestion through the common bile duct into the duodenum.
• Associated clinical problems include gall stones, gall bladder infections, and cancer of the gall bladder.

Jejunum and Ileum

• They make up the 2nd and 3rd parts of the small intestines.
• They extend from the duodenojejunal junction (left side of L2 vertebra) to the ileocaecal junction (right side of sacroiliac joint).

Jejunum and Ileum: Anatomical Differences

• The jejunum is 2.5 meters (8 feet) long and the Ileum is 3.6 meters (12 feet) long.
• The jejunum mainly occupies the left upper and central abdomen, while the ileum mainly occupies the central and right lower abdomen and pelvis.
• The jejunum has a thicker wall and wider lumen (usually empty), while the ileum has a thinner wall and smaller lumen.
• The jejunum has less mesenteric fat and the Ileum has abundant mesenteric fat.
• The jejunum has circular folds that are large, tall and closely packed.
• The ileum has circular folds that are short and sparse (absent in distal ileum).
• The jejunum has fewer lymphoid nodules (Peyer’s patches), while the ileum has many lymphoid nodules.

Jejunum and Ileum: Functions

• The jejunum functions in the absorption of products of digestion (glucose, amino acids and fatty acids).
• It absorbs water-soluble vitamins (the B series & vitamin C, except B12), remaining iron and sodium.
• The ileum functions in the absorption of B12 (aided by intrinsic factor), fat soluble vitamins (A, D, K & E), bile salts and water.
• The ileum is involved with immunologic functions (Peyer's patches).

Parts of the Large Bowel

• Parts include the caecum, ascending colon, transverse colon, descending colon, sigmoid colon, rectum, and anal canal.

Functions of the Large Bowel

• It is involved with the storage of waste matter and the absorption of water and electrolytes.
• The large bowel is important for the formation of faeces.
• It also involved in the absorption of some vitamins such as Vitamin K.
• It is important for the propulsion of waste material to the anus and is involved with defecation.

Summary

• The functional design of the gastrointestinal tract
• The parts of the gastrointestinal tract
• The functional role of the different parts of the gastrointestinal tract
• Clinical correlates