Digestive System

• Digestive System


  • Organs are structured to carry out one or more of these functions: Ingestion of food and water (Mouth), Mechanical digestion of food (Mouth, Stomach, Small intestine), Chemical digestion (Mouth, stomach, small intestine), Movement of food along the alimentary canal (mouth, pharynx, oesophagus, stomach, intestines, rectum and anus), Absorption of digested food and water into blood and lymph (small and large intestines), Elimination of material that is not absorbed (anus)
• Digestion


  A process by which carbohydrates, protein and fat molecules are broken down to products small enough to be absorbed into the blood and cells
• Mechanical digestion


  • Physical breakdown of food particles into smaller particles, Occurs in the mouth, stomach and small intestine, Purpose is to breakdown into smaller particles to increase surface area to allow chemical digestion to occur more effectively, It does not alter the chemical makeup of the food
• Chemical digestion


  • Chemicals break down large molecules into smaller molecules, Small enough to be absorbed into bloodstream, Achieved by enzyme action
• Mechanical digestion


  Physical breakdown of food into smaller pieces to increase surface area for more effective chemical breakdown
• Chemical digestion


  Chemicals break down large complex molecules into smaller simpler molecules which can be absorbed into the bloodstream
• Alimentary canal


  Another name for the digestive system, Continuous tube which runs from the mouth to the anus, Together with associated organs it makes up the digestive system
• Mouth


  • Ingestion - Intake of food, Food held for short while to commence digestion, Mechanical digestion (physical breakdown) - Jaw and teeth break food into smaller pieces (mastication) increases surface area for chemical digestion, Tongue (muscular organ) positions food for chewing mixes food with saliva, forms food into a bolus for swallowing, Moves upwards and backwards pushing food towards pharynx (swallowing), Water in saliva moistens food, mucins soften and lubricate food, Chemical digestion - Food is mixed with saliva (from salivary glands) containing amylase which breaks down starch to maltose, Bicarbonate ions in saliva neutralize acidic foods
• Teeth


  • Incisors - biting and cutting, Canines - tearing, Molars/Premolars - crushing and grinding
• Oesophagus


  • Connects pharynx to stomach, Double layer of muscle - Circular muscle contracts behind food forming a constricting wave pushing the food down into the stomach in process called peristalsis, Lubricated by mucus from inner lining
• Peristalsis


  Wave like contraction of the muscular wall to push contents along parts of the alimentary canal
• Stomach


  • Oesophagus passes through the diaphragm to the stomach, Has large folds that fill with food, 3 muscle layers - circular, longitudinal and oblique (additional muscle layer) continue mechanical breakdown and mixing of food, No nutrient absorption due to layer of mucus, Gastric juice secreted by mucosa lining at gastric glands located in gastric pits contains Hydrochloric acid, Mucus, Digestive enzymes, Hydrochloric acid activated protein digestion, Rugae wavy pits of the stomach, Mechanical digestion - Muscular contraction churns food and mixes it with gastric juice and forms chyme, Chemical digestion - Gastric juice containing enzymes (pepsin) begins digestion of protein to break bonds between the amino acids, Propulsion - Peristaltic waves move from the pylorus, Grinding - The most vigorous peristalsis and mixing action occur close to the pylorus, Retropulsion - The pyloric end of the stomach acts as a pump that delivers small amounts of chyme into the duodenum, simultaneously forcing most of its contained material back into the stomach, Nutrients not absorbed - due to thick mucus lining, Some alcohol and drugs (aspirin) can be absorbed, Pyloric sphincter is a thickening of circular muscle, Constricts = stopping the food from leaving, Relaxes = peristalsis = food into small intestine
• Small intestine


  • Duodenum - first part of the small intestine, most chemical digestion occurs here, Chemical digestion - Pancreatic juice by pancreas enters duodenum from the pancreas through bile duct, Neutralises the acid coming from the stomach (alkaline medium), It also helps in the digestion of food using a range of enzymes like Pancreatic Amylase, Pancreatic protease, Pancreatic lipases, Nucleases, Intestinal juice by glands in lining contains Intestinal amylase, intestinal proteases and intestinal lipases, Mechanical digestion - Bile by the liver and stored in the gall bladder, Emulsification not change in chemistry, Bile salts emulsify fats breaking it down into tiny droplets, Increases surface area for lipases to act upon, No digestive enzymes, Jejunum - middle section, allows selective absorption of carbohydrates and proteins, Ileum - final part of small intestine, vitamin B12, bile salts and any remaining products are absorbed, Mechanical digestion in small intestine - Segmentation - contractions of circular muscle narrow intestines which help break up bolus and mixes it with the juices and bile
• Villus absorption


  • Simple sugars / amino acids/water and water soluble vitamins which enter blood capillary travel to the liver by the hepatic vein, Some removed or processed before going to other cells, Fatty acids and glycerol recombine to form fats in the lacteal, Transported in lymph system which eventually empties into the blood through the upper part of the chest
• Large intestine


  • 1.5m long and larger in diameter to the small intestine, No villi and no digestive juices, Mucus secreted into the intestine, Slow movement, takes 18-24 hours for material to pass through, Made up of caecum, colon, rectum and anus, Appendix attaches to caecum, Large amount of water absorption, wastes become more solid, Bacteria works to break down remaining organic compounds, Some bacteria produce vitamins, Vitamins and mineral nutrients absorbed into blood
• Elimination


  • Faeces contains water, undigested food material (eg; cellulose), bacteria, bile pigments (give faeces its colour) and remains of cells which have broken away from internal lining of the alimentary canal, Faeces is pushed along the rectum by peristalsis, Excretion is the removal of metabolic wastes (most of faeces is not metabolic waste), Defaecation is better defined as elimination
• Egestion


  The act of discharging unusable or undigested material from the body via the digestive system
• Rectum and Anus


  • Rectum - Forms last part of the colon, About 12cm in length, Continuous with colon, Links to anal canal and anus, Usually empty as faeces do not usually collect for any length of time, Anus - has external sphincter (skeletal muscle and voluntary in nature) and internal sphincter (smooth muscle and involuntary), Sphincter only open to pass faeces - defecation
• Digestive enzymes


  • Amylase - Digestive enzyme that acts on starch in food, breaking it down into smaller carbohydrate molecules, Produced in salivary glands and pancreas, Protease - Enzymes that breaks down protein into amino acids, Produced in the stomach and pancreas, Lipase - Enzyme that breaks down dietary fats into smaller molecules called fatty acids and glycerol, Produced in liver and pancreas