These pages have been left in this location as a service to the numerous websites around the world which link to this content. The original authors are no longer at the University of Leeds, and the former Centre for Human Biology became the School of Biomedical Sciences which is now part of the Faculty of Biological Sciences.
Introductory Anatomy: Digestive System
Dr D.R.Johnson, Centre for Human Biology
The digestive system prepares food for use by hundreds of millions of body cells. Food when eaten cannot reach cells (because it cannot pass through the intestinal walls to the bloodstream and, if it could would not be in a useful chemical state. The gut modifies food physically and chemically and disposes of unusable waste. Physical and chemical modification (digestion) depends on exocrine and endocrine secretions and controlled movement of food through the digestive tract.
Mouth Food enters the digestive system via the mouth or oral
cavity, mucous membrane lined. The lips (labia) protect its outer
opening, cheeks form lateral walls, hard palate and soft palate
form anterior/posterior roof. Communication with nasal cavity
behind soft palate. Floor is muscular tongue. Tongue has bony
attachments (styloid process, hyoid bone) attached to floor of
mouth by frenulum.
In leaving the mouth a bolus of food must cross the respiratory
tract (trachea is anterior to oesophagus) by a complicated mechanism
known as swallowing or deglutination which empties the mouth
and ensures that food does not enter the windpipe.
The oesophagus (about 10") is the first part of the digestive
tract proper and shares its distinctive structure. Basic tissue
layers of the gut are
C shaped, left side abdominal cavity (because liver is on right). Cardioesophageal sphincter guarding entrance from oesophagus is of doubtful anatomical integrity (though functionally the diaphragmatic pinch cock serves). Pyloric sphincter guarding the outlet is much better defined. Fundus, body and pylorus recognised as distinct regions. Stomach secretes both acid and mucus (for self protection). Surface area increased by rugae. Serves as a temporary store for food which is also churned by muscular layers (three here) to form chyme, creamy substance voided via pyloric sphincter to duodenum
First part of small intestine. C shaped 10" long and curves around head of pancreas and entry of common bile duct (accessory organs of digestion, pancreas, liver see below). Chemical degradation of small controlled amounts of food controlled by pyloric sphincter begins here, enzymes secreted by pancreas and duodenum itself aided by emulsifying bile (which also lowers pH). Duodenal ulcers caused by squirting of acid stomach contents into duodenal wall opposite sphincter.
Jejunum (8 feet) and ileum (12 feet) continue degenerative process. Surface area increased by plica circulares (circular folds) carrying villi: cells of villi carry microvilli. Each villus has a capillary and a lacteal (lymphatic capillary) Absorption of digested foodstuffs is via these to the rich venous and capillary drainage of the gut. Towards the end of the small intestine accumulations of lymphoid tissue (Peyer's patches) more common. Undigested residue of food is rich in bacteria.
Jejunum terminates at caecum. Caecum is small saclike evagination,
important in some animals as a repository for bacteria/other
organisms able to digest cellulose. A blind ending appendix may
give trouble (appendicitis) if infected. The large intestine
has three longitudinal muscle bands (taenia coli) with bulges
in the wall (haustra) between them. These may evaginate in the
elderly to become diverticuli and infected in diverticulitis.
Accessory digestive organs
Three pairs, parotid, submandibular, sublingual. Mumps begins as infective parotitis in the parotid glands in the cheek. The others open into the floor of the mouth. Saliva is a mixture of mucus and serous fluids, each produced to various extents in various glands. Also contains salivary amylase, (starts to break down starch) lysozyme (antibacterial) and IgA antibodies. In some mammals (and snakes!) saliva may be poisonous, quietening down living prey.
Endocrine and exocrine gland. Exocrine part produces many enzymes which enter the duodenum via the pancreatic duct. Endocrine part produces insulin, blood sugar regulator.
Liver and gallbladder
Bile, a watery greenish fluid is produced by the liver and secreted via the hepatic duct and cystic duct to the gall bladder for storage, and thence on demand via the common bile duct to an opening near the pancreatic duct in the duodenum. It contains bile salts, bile pigments (mainly bilerubin, essentially the non-iron part of haemoglobin) cholesterol and phospholipids. Bile salts and phospholipds emulsify fats, the rest are just being excreted. Gallstones are usually cholesterol based, may block the hepatic or common bile ducts causing pain, jaundice.
Multifunctional: important in this context since the capillaries of the small intestine drain fat and other nutrient rich lymph into it via the hepatic portal system.