Digestion system.docx

Mouth First stage of digestion Mastication (chewing) breaks food down and increases surface area. 3 pairs of glands Sublingual (tongue) Submandibular (jaw) Parotid (ear)- largest one Saliva 99.5% water/0.5% electrolytes and proteins Secreted by acinar cells. 2 types of acinar cells: Serous- pr...

Mouth First stage of digestion Mastication (chewing) breaks food down and increases surface area. 3 pairs of glands Sublingual (tongue) Submandibular (jaw) Parotid (ear)- largest one Saliva 99.5% water/0.5% electrolytes and proteins Secreted by acinar cells. 2 types of acinar cells: Serous- proteins in an isotonic fluid (produces amylase) Mucous- mucin proteins. Under control of autonomic nervous system. Key functions of saliva Lubrication- facilitates eating and speech. Digestion- amylase breaks down carbohydrates. Oral hygiene- contains lysosomes and washes away debris. Taste- solvent to allow molecules interact with taste receptors. Buffering- neutralises acids to protect teeth. The pharynx and oesophagus Pharynx- behind mouth and nasal cavity Takes food to the oesophagus and air to trachea. Oesophagus- straight tube surrounded by smooth muscle. Passes food to the stomach using peristalsis. Only secretion is mucus (no absorption) Stomach 3 key functions: Mixes and breaks food into chyme. Stores food and releases slowly into small intestine. Begins protein digestion through secretion of hydrochloric acid and enzymes. *No absorption happens in stomach* Gastroesophageal sphincter- prevents stomach contents entering the oesophagus. Pyloric sphincter- controls the amount of chyme entering the small intestine. Waves of peristaltic contractions churn food Secretes 2L of gastric juice daily. Stomach Mucosa Secretions of the stomach Exocrine Mucous cells- produces mucus. Chief cells- pepsinogen which digests proteins (inactive form of pepsin, if pepsin is released, it digests gastric pits). Parietal cells- hydrochloric acid and intrinsic factor (HCL activates pepsinogen into pepsin. Intrinsic factors aid vitamin B12 absorption) Endocrine Enterochromaffin cells- histamines G cells- gastrin (increases stomach acid secretion) D cells- somatostatin (decreases production of stomach acid) Stomach acid secretion When stimulated by gastrin, parietal cells from canaliculi (increases SA) Water in parietal cells breaks down to H+ and OH- H+ actively transported into lumen via H+/K+ ATPase. K+ passively leaks back into lumen via leak channels. OH- combines with CO2 via carbonic anhydrase to form bicarbonate. HCO3- exchanged for CL- at the basolateral membrane. CL- diffuses out of the luminal membrane via electrochemical gradient. Function of stomach acid Breaks down connective tissue. Activates pepsinogen to pepsin. Denatures proteins. Antimicrobial. Pancreas Secretes: Pancreatic juice into duodenum Alkaline fluid which neutralises chyme, enzymes such as amylase, lipase etc. Hormones into blood stream (endocrine system) Insulin, glucagon The biliary system (liver and gallbladder) Liver Processes fats, carbohydrates and proteins once absorbed at duodenum. Secretes bile (allows fats to be emulsified) Detoxification of waste Synthesis of plasma proteins Gallbladder Stores bile before release into duodenum Digestive tract 4 layers Serosa/mesentery (outer layer) Secretes fluid. Supports digestive system. Muscularis externa Smooth muscle Submucosa Flexibility/elasticity Mucosa Lines luminal surfaces Protective Highly folded in small intestine Small intestine Duodenum Receives chyme, bile and pancreatic enzymes. Jejunum Major part of absorption Ileum Absorption of vitamin B12 and bile. Small intestine secretes succus entericus. Water Salts Mucus Microvilli/brush border No digestive enzymes secreted. Fat broken by bile. Membrane bound enzymes completes carbohydrate and protein digestion. Large intestine 4 parts of the colon Ascending colon Transverse colon Deseing colon Sigmoid colon 2 main functions: Water and salt reabsorption Storage of ingestible material Types of diarrhoea Osmotic- too many solutes in the lumen Secretory- caused by agents that lead to water being secreted into lumen. Inflammatory- epithelial damage causing them to be replaced with cells lacking transporters. Rapid intestinal transit- the motility of the gut is increases not allowing sufficient time for absorption.