Digestive System Essay -- essays research papers

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... ...a mixture of mucus and serous fluids, each produced to various extent in various glands. Also contains salivary amylase, (starts to break down starch) (antibacterial) and antibodies. In some mammals (and snakes!) saliva may be poisonous, quieting down living prey. Pancreas 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 hemoglobin) cholesterol and phospholipids. Bile salts and phosholipds emulsify fats, the rest are just being excreted. Gallstones are usually cholesterol based, may block the hepatic or common bile ducts causing pain, jaundice. Liver Multi functional: 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.