Digestive System
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Purpose
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
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. Posterior exit from mouth guarded by a ring of palatine/lingual tonsils. Enlargement sore throat, tonsillitis. Food is first processed (bitten off) by teeth, especially the anterior incisors. Suitably sized portions then retained in closed mouth and chewed or masticated (especially by cheek teeth, premolars, molars) aided by saliva Ducted salivary glands open at various points into mouth. This process involves teeth (muscles of mastication move jaws) and tongue (extrinsic and intrinsic muscles). Mechanical breakdown, plus some chemical (ptyalin, enzyme in saliva). Taste buds allow appreciation, also sample potential hazards (chemicals, toxins)
Swallowing
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.
Swallowing involves co-ordinated activity of tongue, soft palate pharynx and oesophagus. The first (buccal) phase is voluntary, food being forced into the pharynx by the tongue. After this the process is reflex. The tongue blocks the mouth, soft palate closes off the nose and the larynx rises so that the epiglottis closes off the trachea. Food thus moves into the pharynx and onwards by peristalsis aided by gravity. If we try to talk whilst swallowing food may enter the respiratory passages and a cough reflex expels the bolus.
Oesophagus
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
1. mucosa. Innermost, moist lining membrane. Epithelium (friction resistant stratified squamous in oesophagus, simple beyond) plus a little connective tissue and smooth muscle.
2. submucosa. Soft connective tissue layer, blood vessels, nerves, lymphatics 3. Muscular externa. Typically circular inner layer, longitudinal outer layer of smooth muscle
4. Serosal fluid producing single layer.
Stomach
C shaped, left side abdominal cavity (because liver is on right). Cardio esophageal 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. Fungus, body and pylorus recognized as distinct regions. Stomach secretes both acid and mucus (for self protection). Surface area increased by rug.. 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
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.
Small Intestine
Jejunum (8 feet) and ileum (12 feet) continue degenerative process. Surface area increased by plica circulars (circular folds) carrying: cells