2- DIGESTIVE SYSTEM

DIGESTIVE SYSTEM

It is formed by the GASTROINTESTINAL TRACT and the ACCESSORY GLANDS.         GASTROINTESTINAL TRACT:  (8m long)         It starts in the mouth and ends in the anus.                                                                                  It has a inner cell layer (MUCOUS MEMBRANE) protected by a lubricant substance (MUCUS)                            The mucosa contain glands that secrete protective mucus and digestive juices.                                           Surrounded by muscular layers that contracts to push food down the digestive tract                                    It has 3 muscular layers; a longitudinal one, a circular one and an oblique one (in the stomach)                  Parts: ORAL CAVITY , PHARYNX, OESOPHAGUS, STOMACH, SMALL INTESTINE, LARGE INTESTINE    ACCESSORY GLANDS : organs that discharge their secretion in the digestive system:       SALIVARY GLANDS --> release saliva in the mouth       LIVER (upper right abdomen) --> releases bile into the intestine       PANCREAS (behind the stomach)  --> release pancreatic juices into the intestine           

DIGESTION

Process by which foods are transformed into simpler substances (nutrients) that can be used by cells. There are mechanical and chemical processes in the digestion.    Mechanical : chewing and muscular action of different organs (crush and mix food with digestive juices)    Chemical : digestive enzymes that break down the substances found in food into simpler components.    MOUTH     - SALIVATION       Saliva is a watery liquid released by the salivary glands when we smell, see or place food in our mouths.             Salivary glands are located in the tongue and on the sides of the mouth.        Functions :         1) starts the digestion of the starch molecules transforming them into simpler sugars by using AMYLASE               (starch is in bread, potatoes, rice ...)        2) Destroy some bacteria by using LYSOZYME (prevent infections and cavities)        3) Eases the bolus into the pharynx and oesophagus by lubricate the food with MUCIN (mucous substance)      - CHEWING      Mechanical process by which foods are ground and crushed into smaller chunks to facilitate their digestion.      Teeth are responsible for grinding and crushing. They are formed by calcium and fluorine compounds so           they are very hard.      There are different teeth that fulfils a different function :                                                                        INCISOR (4, cut), CANINE (2, tear), PROMOLAR (4) and MOLAR (6) (crush and crumble)                Kids only have 20 baby teeth / milk teeth                                                                                  Adult have 32 teeth  (16 in each jaw)      - sWALLOWING      Bolus : small rounded mass of chewed food mixed with saliva.      Once the bolus if formed, the tongue pushes it towards the pharynx, from there goes to the oesophagus           which ends in the stomach.      Stages:         - bolus driven to the botton of the oral cavity when the tongue pushes itself against the palate.                    - A series of reflexes makes the palate close the nasal passage (we cannot breathe), larynx rise and              the epiglotis close its upper orifice (this prevent food from entering the respiratory tract), entering the            bolus in the pharynx.                                                                                                      - Another reflex helps the bolus to pass into the oesophagus.  

DIGESTION (2)

 STOMACH  The downward movement of the bolus in the oesophagus is performed by PERISTALTIV MOVEMENTS (Contraction    of 2 layers of muscle of its walls)  At the end of the oesophagus is the stomach (separated by a valve called CARDIA, that prevents reflux)             Connecting the stomach and the small intestine is another valve called PYLORUS.  When the bolus enters the stomach, its muscles begin to contract, mixing the bolus with GASTRIC JUICES secreted   by the glands of the inner layer of the stomach (3 to 4 hours remain the bolus in the stomach)        GASTRIC JUICES       - PEPSIN (make the protein digestion, breaking that proteins down to PEPTIDES)       - HYDROCHLORIC ACID (activate pepsin , support pepsin by disintegration food fibers, destroy bacteria that                                    could be present in food)   After food has mixed with gastric juices , it is transformed into CHYME (more fluid substance) The 3 muscular layers of the stomach mix the chyme (by performing massaging motions) --> the mucus protects the stomach from the effects of hydrochloric acid. When the chyme reaches a high level of acidity , the pylorus opens and the chyme enters the small intestine. (fats and proteins takes longer to enter the small intestine)

DIGESTION (3)

 SMALL INTESTINE  Tube (6 or 7 m) located between the pylorus  and the ILEOCECAL VALVE, where the large intestine begins.  Sections :     - DUODENUM (25CM, shaped like a horseshoe)                                                                              - JEJUNUM (middle section, longest part)                                                                                      - ILEUM (final section that ends int the large intestine)     After the pylorus opens, the chyme enters the small intestine.    Once in the duodenum, chyme mixes with secretions released there from two organs: LIVER AND PANCREAS      PANCREAS      Glandular organ located in the abdomen behind the stomach, with 2 functions :         - produces INSULIN AND GLUCAGON (hormones) that regulates the quantities of glucose in blood.         - secrete PANCREATIC JUICES release through the AMPULLA OF VATER (liver ducts also join there)                    Pancreatic juices contains ENZYMES that digest carbohydrates, fats and proteins, and SODIUM                      BICARBONATE that neutralises chyme´s acidity protecting the intestinal cells.        LIVER      Organ located in the right upper part of the abdomen that performs functions related to digestive and             excretory processes (captures and destroys most toxic molecules that enter the body , alcohol, drugs ...)           Liver produces BILE, released into the duodenum through the Ampulla of Vater.       This bile, as the pancreatic juices, is not secreted continuosly, but it is stored in the gallbladder, and it is           only released when food enters the duodenum.       Bile DOES NOT CONTAIN ENZYMES. It contains BILE SALTS, that intervene in the emulsification of fats,             transforming them into small drops, that help enzymes from pancreatic and intestinal juices to break them         down.    In the small intestine, apart from bile and pancreatic juices, the small intestine´s mucous glands secrete INTESTINAL JUICES, similar to pancreatic juices, that contain enzymes capable of digesting all types of molecules. There the chyme is transformed in CHYLE.   FOOD   ------>  BOLUS  --------------> CHYME -------------------> CHYLE          SALIVA              GASTRIC J.          PANCREATIC J.                                                                                                               BILE                                                                                                                           INTESTINAL J. FOOD ABSORPTION Passage of nutrients to our blood , so they can be distributed across the organism. They go through the small intestine´s mucosa (villi and microvilli), entering the blood capillaries, except the fat digestion products that are absorved by the lymphatic capillaries.   FOOD TO NUTRIENTS   STARCH  ---------------------------------------------------> MALTOSE  -----------------------------> GLUCOSE                       AMYLASE (saliva, pancreatic j., intestinal j.)                   MALTASE (intestinal j.)   SUCROSE ------------------------------> FRUCTOSE AND GLUCOSE                                                                    SUCRASE (intestinal j.)    FATS -------------------------------------------------> GLYCERINE AND FATTY ACIDS                                                LIPASE (intestinal and pancreatic j.)   PROTEINS ---------------------> PEPTIDE -------------------------> SHORT PEPTIDE -----------------> AMINOACIDS                PEPSIN (gastric j.)            TRYPSIN (pancreatic j.)                    EREPSIN (intestinal j.)   These nutrients must be distributed to the tissues when cell will use them to obtain energy or make new molecules.

DIGESTION (4)

 LARGE INTESTINE  End part of the digestive tract (1,5m), with a larger circumference than small intestine, and with no villi or microvilli.  Parts:     - CECUM (first section, shaped like a bag with a narrow extention, APPENDIX.     - COLON (it has symbiotic bacteria of the intestinal flora ; B12 and K vitamins are produced there)           It can be ascending colon , transverse colon and descending colon.     - RECTUM (final section that ends in the anus) Once nutrients have been ingested in the small intestine (passing to the blood), the remaining food waste enters  the large intestine through the ILEOCECAL VALVE. By the absorption of most water and minerals faeces compact to form stools (they contain food waste, bacteria and mucosa cells) Undigested food waste advances by peristalsis and is expelled to the outside through the anux by defecation.