Alimentary Canal Journey PDF

A Journey through the Alimentary Canal The Human Alimentary Canal – the digestive system The beginning of the alimentary canal Nasal Cavity Soft Palate Tongue Bolus of food Salivary duct...

A Journey through the Alimentary Canal The Human Alimentary Canal – the digestive system The beginning of the alimentary canal Nasal Cavity Soft Palate Tongue Bolus of food Salivary duct Epiglottis Salivary gland Trachea Oesophagus  Oesophagus Diaphragm Liver Cardiac sphincter Gall Bladder Bile duct Pyloric Stomach sphincter Pancreas Pancreatic duct Duodenum Small intestine Ileum Colon Caecum Appendix Large intestine Alimentary canal in the Rectum abdominal cavity. Anal sphincter  The digestive system A muscular tube in which food is moved by peristalsis, stored, digested, absorbed and egested. Why have such a system? – Most food we eat contains large molecules eg starch, proteins. – We can only absorb small molecules (eg glucose, amino acids) into our blood stream so that it can be taken to all our body cells.  Heterotrophic nutrition Ingestion Food taken into the alimentary Digestion Large insoluble molecules broken down into small soluble ones, mechanically and chemically. Absorption Small molecules absorbed into the bloodstream Small molecules used by cells to build useful Assimilation larger molecules such as proteins Undigested food is removed as faeces. Different Egestion from excretion because these molecules have never been inside cells. NB Autotrophic nutrition (‘self feeding’) is carried out by organisms that photosynthesise – plants. Which enzymes are involved?  The Mouth Mechanical Digestion by the teeth helps to increase the surface area of the food for chemical digestion by enzymes. Salivary amylase secreted by the salivary glands breaks down starch into maltose. The tongue forms the food into a ball called a Bolus which is moved down the oesophagus into the stomach by a process called peristalsis. tigerteeth Prominent canines  Prominent incisors at the front of the mouth Large molars for chewing grass  Peristalsis Food is squeezed Oesophagus down the oesophagus by peristalsis. Circular muscles just behind the bolus (ball of food) ______. Circular muscles just in front of the bolus ______.  Functions of The Stomach Storage (up to 2 hrs) gives time for digestion to occur. Chemical Digestion of Proteins by pepsin into peptides. (Pepsin works optimally at pH 2) Mechanical Digestion of food by strong muscular contractions and relaxation which churns the food. Disinfection of food due to the strongly acidic environment (pH2), which kills most microbes. Walls of the stomach contain Gastric Pits:  Gastric Pits line the stomach Lumen of Stomach Glands within the walls of the gastric Gastric Pit pits secrete: – Mucus as a lubricant – Hydrochloric Acid to kill bacteria and provide low pH. – Pepsin to break down proteins into peptides. (works optimally in acidic conditions) Muscular Wall  Digestion in the Small Intestine Food exits the stomach via a sphincter muscle which relaxes periodically. This mixture called chyme is mixed with pancreatic juices (from the pancreas), which contains enzymes such as: Amylase (Starch to Maltose) Maltase (Maltose to glucose) Lipase (Fats to Fatty Acids & Glycerol) These enzymes work best at pH 7. Therefore, the stomach contents must be neutralised, which is one of the key functions of Bile.  Bile Produced by the Liver and stored in the Gall Bladder. Bile contains: Sodium Hydrogen Carbonate which neutralises acids. Bile Salts which emulsify fats (turn large droplets into much smaller ones). Bile pigments which are excreted in our faeces. Bile Contains NO enzymes, so Bile does NOT digest fat.  Digestion of fats Large droplets of fat are emulsified by bile salts into millions of tiny droplets. Emulsification Lipase then digests the fats into fatty acids and glycerol. The fatty acids and glycerol Lipase can then be absorbed into digestion the blood stream via lacteals Individual lipid molecules are digested into fatty Acids and Glycerol Absorption in the Small intestine Once large food molecules have been digested by enzymes into smaller ones, they can be absorbed into the bloodstream. Monosaccharides, Amino Acids, Water, Vitamins and minerals can be absorbed directly into the blood. Fatty Acids and glycerol are absorbed into vessels called lacteals which eventually feed into the blood vessels.  The Small Intestine Approx. 5 metres long Walls are covered in villi – finger like projections over the whole surface. Individual cells on the villi are covered in microvilli, which increase the surface area for absorption. It also has a very good blood supply (capillaries) to take away the absorbed food. Villi – upto a mm long  Longitudinal section through a single villus Lacteals for absorbing fats Blood capillaries for absorbing amino acids and sugars Infoldings of Villi give large surface area for absorption Microvilli – thousandths of a mm long Diagrammatic view of the surface of villi  Adaptations of the small intestine Adaptation Function Very long – 5metres Increased Time & surface area for digestion and absorption to occur Villi Increased SA Microvilli Further increased SA Excellent blood supply Digested food products can be via capillaries absorbed and transported to the liver rapidly. Lacteals Absorb fatty acids and glycerol Thin walls – one cell Absorption into blood can be rapid thick as short distance to travel.  Absorbed food Once in the blood stream, it travels via the hepatic portal vein to the Liver. Here, foods are either: – Stored (eg excess glucose as glycogen) – Broken down to release heat energy – Broken down and detoxified or – Passed to the rest of the body.  The Large Intestine Undigested food travels into the large intestine. Water and salts are absorbed. Approx. 30% of faeces consists of bacteria which live here. Faeces are stored until they are egested via the anal sphincter. NB high fibre diets are associated with a low incidence of Bowel Cancer.  Some links between diet and disease High sugar – tooth decay High fat/energy intake and low exercise – obesity Obesity – arthritis, diabetes, high blood pressure and heart disease High salt – high blood pressure High cholesterol – heart disease Low fibre – Bowel Cancer