Digestive System

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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...
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