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Mechanical and Chemical Breakdown of Ingested Food Most food is solid and in the form of large complex Talk to your neighbor. Tell them you think itand is important that we inert molecules which arewhy insoluble chemically break foodreadily into smaller pieces. (not usable) As food was synthesized by other organisms, it contains materials not suitable for human tissue these need to be separated and removed Large molecules need to be broken down into smaller molecules that can be readily absorbed across membranes and into cells Small molecules can be reassembled into new products (e.g. amino acids can be reassembled to make new proteins) Digestion - breakdown of food into particles/molecules small enough to pass into the blood stream. 1. 2 types of digestion ◦ 2. 3. mechanical (Mastication) chemical (Enzymes) Absorption of nutrients into the blood stream Elimination of indigestible nutrients There are two major groups of organs that comprise the human digestive system: Alimentary Canal: Contains organs through which the food actually passes (esophagus, stomach, small intestine, large intestine, etc.) Accessory Organs: Organs that assist in digestion but no food passes through them (liver, pancreas, gall bladder, salivary glands, etc.) Alimentary Canal Accessory Organs Enzymes are biological catalysts which speed up the rate of a chemical reaction (e.g. digestion) by lowering the activation energy Enzymes allow digestive processes to occur at body temperature and at sufficient speed to meet the organism's survival requirements Enzymes are specific for a given substrate and so can allow digestion of certain molecules to occur independently of others Mechanical breakdown of nutrients begins in the mouth by chewing (Mastication). ◦ The purpose of chewing is to increase the surface area of food. Chemical breakdown of starch also begins in mouth. Starch is converted into glucose by salivary amylase (secreted by the salivary glands) This mixture of food and saliva is made into a mass of food called a bolus and then pushed into the pharynx by the tongue which triggers involuntary swallowing. The esophagus is a muscular tube whose muscular contractions (peristalsis) propel food to the stomach. ◦ Peristalsis – the wave action of muscle that moves food through the digestive system. Oral Phase: mastication ◦ Food is prepared into a food bolus (pellet of food) Pharyngeal Phase: involuntary part (reflex) ◦ Food is pushed further into the pharynx and esophagus. Involuntary closure of larynx prevents food from going “down the wrong pipe” Esophageal Phase ◦ Food is passed through esophagus to stomach ◦ upper sphincter prevents food from being regurgitated; lower sphincter prevents acid and stomach contents from traveling backwards Functions: ◦ a storage bin, holding a meal in the upper portion and releasing it a little at a time into the lower portion for processing Pyloric sphincter- valve that controls flow of partially digested food into the small intestine ◦ a food mixer, the strong muscles contract and mash the food into a sticky ◦ slushy mass; a sterilizing system, where there cells in the stomach produce an acid which kills germs ◦ digestive tub, the stomach produces digestive fluid which splits and cracks the chemicals (mainly proteins) in food to be distributed as fuel for the body Acids (hydrochloric acid) create a low pH environment (pH~1-2) that denatures proteins, while proteases like pepsin hydrolyze large proteins The stomach also releases a hormone (gastrin) that regulates stomach secretions The stomach turns food into a creamy paste called chyme Specialized Mucosa of the Stomach Simple columnar epithelium Mucous neck cells – produce a sticky alkaline mucus Gastric glands – secrete gastric juice Chief cells – produce protein-digesting enzymes (pepsinogens) Parietal cells – produce hydrochloric acid Endocrine cells – produce gastrin Copyright © 2003 Pearson Education, Inc. publishing as Benjamin Cummings Slide Structure of the Stomach Mucosa Gastric pits formed by folded mucosa Glands and specialized cells are in the gastric gland region Copyright © 2003 Pearson Education, Inc. publishing as Benjamin Cummings Slide Structure of the Stomach Mucosa Figure 14.4b, c Copyright © 2003 Pearson Education, Inc. publishing as Benjamin Cummings Slide The small intestine is where usable food substances (e.g. nutrients) are absorbed into the bloodstream The pancreas and gall bladder (via the bile duct) both secrete substances into the small intestine to aid in digestion The small intestine is lined with smooth muscle to allow for the mixing and moving of digested food products (via segmentation and peristalsis) It also contains small pits (crypts of lieberkuhn) that secrete intestinal juices The small intestine contain infoldings called villi, to increase surface area and optimise the rate of absorption Subdivisions of the Small Intestine “Dogs Just Itch!” Duodenum Attached to the stomach Curves around the head of the pancreas Jejunum Attaches anteriorly to the duodenum Ileum Extends from jejunum to large intestine Copyright © 2003 Pearson Education, Inc. publishing as Benjamin Cummings Slide The small intestine is lined with villi and microvilli The purpose of villi and microvilli are to increase surface area in the small intestines. Increase the speed at which nutrients are absorbed ◦ The small intestines a 6 meter long tube has a surface area of 300 square meters or the surface area of a 500-600m long tube. Each villus has a capillary network supplied by a small artery. Absorbed nutrients pass through the microvilli into the capillary (blood stream), usually by passive transport (diffusion). The large intestine is made up by the cecum, appendix, colon, and rectum. Digested food is pushed from the small intestines into the large intestine. No digestion takes place in the large intestines Water, and salts are absorbed, the remaining contents form feces (mostly cellulose, bacteria, bilirubin). Bacteria in the large intestine, such as E. coli, produce vitamins (including vitamin K) that are absorbed. The large intestine absorbs water and dissolved minerals from the indigestible food residues, and by doing so converts what remains from a fluid state into a semi-solid feces The feces is stored in the rectum and eliminated out the anus There are 3 parts to the Colon 1. Ascending Colon 2. Transverse Colon 3. Descending Colon Accessory Digestive Organs Salivary glands Teeth Pancreas Liver Gall bladder Copyright © 2003 Pearson Education, Inc. publishing as Benjamin Cummings Slide Salivary Glands Saliva-producing glands Parotid glands – located anterior to ears Submandibular glands Sublingual glands Copyright © 2003 Pearson Education, Inc. publishing as Benjamin Cummings Slide Saliva Mixture of mucus and serous fluids Helps to form a food bolus Contains salivary amylase to begin starch digestion Dissolves chemicals so they can be tasted Copyright © 2003 Pearson Education, Inc. publishing as Benjamin Cummings Slide Teeth The role is to masticate (chew) food Humans have two sets of teeth Deciduous (baby or milk) teeth 20 teeth are fully formed by age two Copyright © 2003 Pearson Education, Inc. publishing as Benjamin Cummings Slide Teeth Permanent teeth Replace deciduous teeth beginning between the ages of 6 to 12 A full set is 32 teeth, but some people do not have wisdom teeth Copyright © 2003 Pearson Education, Inc. publishing as Benjamin Cummings Slide Pancreas Produces a wide spectrum of digestive enzymes that break down all categories of food Enzymes are secreted into the duodenum Alkaline fluid introduced with enzymes neutralizes acidic chyme Endocrine products of pancreas Insulin Glucagons Copyright © 2003 Pearson Education, Inc. publishing as Benjamin Cummings Slide When the acidic chyme is pushed out of the stomach into the small intestines. It stimulates the pancreas to send pancreatic juice, which neutralizes the chyme, begins digestions of carbohydrates, lipids and continues digestion of protein. Pancreatic juice also contains Lipase which digested emulsified Lipids Liver Largest gland in the body Located on the right side of the body under the diaphragm Consists of four lobes suspended from the diaphragm and abdominal wall by the falciform ligament Connected to the gall bladder via the common hepatic duct Copyright © 2003 Pearson Education, Inc. publishing as Benjamin Cummings Slide Role of the Liver in Metabolism Several roles in digestion Detoxifies drugs and alcohol Degrades hormones Produce cholesterol, blood proteins (albumin and clotting proteins) Plays a central role in metabolism Copyright © 2003 Pearson Education, Inc. publishing as Benjamin Cummings Slide Gall Bladder Sac found in hollow fossa of liver Stores bile from the liver by way of the cystic duct Bile is introduced into the duodenum in the presence of fatty food Gallstones can cause blockages Copyright © 2003 Pearson Education, Inc. publishing as Benjamin Cummings Slide The Liver makes Bile and stores it in the gall bladder. The Gall bladder sends bile to the small intestine when the acidic chyme from the stomach. Bile contains bile salts, which emulsify fats, making them susceptible to enzymatic breakdown. The liver also stores excess glucose in the form of glycogen. Processes of the Digestive System 1. Digestion • There are three major parts to digestion: • Digestion Part 1 •Swallowing: process of using smooth and skeletal muscles in the mouth, tongue, and pharynx to push food out of the mouth, through the pharynx, and into the esophagus. Copyright © 2003 Pearson Education, Inc. publishing as Benjamin Cummings Slide Digestion Part 2 ◦ Peristalsis: a muscular wave that travels the length of the GI tract, moving partially digested food a short distance down the tract. It takes many waves of peristalsis for food to travel from the esophagus, through the stomach and intestines, and reach the end of the GI tract. Digestion Part 3 ◦ Segmentation: occurs only in the small intestine as short segments of intestine contract like hands squeezing a toothpaste tube. Segmentation helps to increase the absorption of nutrients by mixing food and increasing its contact with the walls of the intestine. Processes of the Digestive System Mechanical digestion Mixing of food in the mouth by the tongue Churning of food in the stomach Segmentation in the small intestine Bile produced by liver breaks fats into smaller globules Copyright © 2003 Pearson Education, Inc. publishing as Benjamin Cummings Slide Processes of the Digestive System Chemical Digestion Begins in mouth (saliva), continues in stomach (enzymes and acids), most occurs in small intestine (pancreatic juices secreted into S.I. by pancreas) Enzymes break down food molecules into their building blocks Each major food group uses different enzymes Carbohydrates are broken to simple sugars (mouth) Proteins are broken to amino acids (stomach) Fats are broken to fatty acids and alcohols (small intestine) Water is degraded into hydrogen and oxygen Copyright © 2003 Pearson Education, Inc. publishing as Benjamin Cummings Slide Processes of the Digestive System 2. Absorption End products of digestion are absorbed in the blood or lymph Begins in stomach, ends in large intestine Stomach: water and alcohol Small intestine: most absorption takes place here Large intestine: water and vitamins B and K Copyright © 2003 Pearson Education, Inc. publishing as Benjamin Cummings Slide 3. Defecation/Excretion Elimination of indigestible substances as feces Controlled voluntarily but must be accomplished on a regular basis The following compounds are not absorbed, but rather defecated ◦ Bile pigments (bilirubin) ◦ Epithelial cells of the intestinal lining ◦ Lignin (found in root vegetables, wheat, and berry seeds) ◦ Cellulose (found in bran, legumes, nuts, peas, roots, cabbage and apple skins) ◦ Human flora / bacteria Processes of the Digestive System Figure 14.11 Copyright © 2003 Pearson Education, Inc. publishing as Benjamin Cummings Slide Control of Digestive Activity Relationship between Digestive System and Nervous system: Mostly controlled by reflexes via the parasympathetic division Chemical and mechanical receptors are located in organ walls that trigger reflexes Copyright © 2003 Pearson Education, Inc. publishing as Benjamin Cummings Slide
