Human Biology Topic 4 Digestion PDF

The Digestive System and Body Metabolism Copyright © 2003 Pearson Education, Inc. publishing as Benjamin Cummings  Digestion Stages Humans, like all animals, use holozoic nutrition, which consists of these stages: ingestion - taking large pieces of food into the body...

The Digestive System and Body Metabolism Copyright © 2003 Pearson Education, Inc. publishing as Benjamin Cummings  Digestion Stages Humans, like all animals, use holozoic nutrition, which consists of these stages: ingestion - taking large pieces of food into the body digestion - breaking down the food by mechanical and chemical means absorption - taking up the soluble digestion products into the body's cells assimilation - using the absorbed materials Egestion/defication - eliminating the undigested material Note Egestion is elimination of material from the body cavity and Excretion is elimination of substances from within body cells.  Organs of the digestive system There are two main groups: 1. The alimentary canal—which consist of the following organs: · Mouth Anus Pharynx · Esophagus Stomach · Small intestine Large intestine 2. Accessory organs– which includes: · Liver Teeth Pancreas · Salivary glands Gall bladder Organs of the Digestive System Figure 14.1 Copyright © 2003 Pearson Education, Inc. publishing as Benjamin Cummings Slide Organs of the Digestive System Figure 14.1 Copyright © 2003 Pearson Education, Inc. publishing as Benjamin Cummings Slide  What happens in the mouth? The teeth, tongue and chewing action break up the food physically which increases surface area, and they form it into a ball or bolus. The salivary glands secrete saliva, which contains water to dissolve soluble substances, mucus for lubrication, lysozymes to kill bacteria and amylase to digest starch. The food bolus is swallowed by an involuntary reflex action through the pharynx (the back of the mouth). During swallowing the trachea is blocked off by the epiglottis to stop food entering the lungs.  Teeth The role is to masticate (chew) food · Humans have two sets of teeth: 1. Deciduous (baby or milk) teeth · 20 teeth are fully formed by age two 2. 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 Classification of Teeth Figure 14.9 Copyright © 2003 Pearson Education, Inc. publishing as Benjamin Cummings Slide  Regions of a Tooth · Crown – exposed part · Outer enamel · Dentin · Pulp cavity · Neck · Region in contact with the gum · Connects crown to root · Root · Periodontal membrane attached to the bone · Root canal carrying blood vessels and nerves Copyright © 2003 Pearson Education, Inc. publishing as Benjamin Cummings  Pharynx (throat) Function · Serves as a passageway for air and food · Food is propelled to the esophagus by two muscle layers · Longitudinal inner layer · Circular outer layer · Food movement is by alternating contractions of the muscle layers (peristalsis)  Esophagus This is a simple tube through the thorax, which connects the mouth to the rest of the gut. No digestion takes place. There is a thin epithelium, no villi, a few glands secreting mucus, and a thick muscle layer, which propels the food by peristalsis and empties it into the stomach. The oesophagus is a soft tube that can be closed, unlike the trachea, which is a hard tube, held open by rings of cartilage. Passageway for food only (respiratory system branches off after the pharynx)  Stomach Anatomy After the oesophagus, food enters the stomach at the cardioesophageal sphincter. · The stomach is an expandable bag where the food is stored for up to a few hours. There are three layers of muscle to churn the food into a liquid called chyme. · This is gradually released into the small intestine by the pyloric sphincter, a region of thick circular muscle that acts as a valve. · The mucosa of the stomach wall has no villi, but numerous gastric pits leading to gastric glands. These secrete gastric juice, which contains: hydrochloric acid (pH 1) to kill bacteria (the acid does not help digestion, in fact it hinders it by denaturing most enzymes); mucus to lubricate the food and to line the epithelium to protect it from the acid; and the enzymes pepsin and rennin to digest proteins.  Stomach Anatomy Figure 14.4a Copyright © 2003 Pearson Education, Inc. publishing as Benjamin Cummings Slide  Specialized Mucosa of the Stomach · Simple columnar epithelium · Mucous neck cells – produce a sticky alkaline mucus which neutralizes HCL · Gastric glands – secrete gastric juice · Chief cells – produce protein-digesting enzymes (pepsinogens) · Parietal cells – produce hydrochloric acid 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  Small Intestine · The body’s major digestive organ (1” X 23ft) · Site of nutrient absorption into the blood · Muscular tube extending form the pyloric sphincter to the ileocecal valve · Suspended from the posterior abdominal wall by the mesentery · Subdivisions include: (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  Chemical Digestion in the Small Intestine Figure 14.6 Copyright © 2003 Pearson Education, Inc. publishing as Benjamin Cummings Slide Chemical Digestion in the Small Intestine  Villi of the Small Intestine · Fingerlike structures formed by the mucosa · Give the small intestine more surface area · Small projections of the plasma membrane · Found on absorptive cells Figure 14.7a  Digestion in the Small Intestine · Pancreatic enzymes play the major digestive function (continued) · Responsible for fat digestion (lipase) · Digest nucleic acids (nucleases) · Alkaline content neutralizes acidic chyme, etc. Copyright © 2003 Pearson Education, Inc. publishing as Benjamin Cummings Slide  Absorption in the Small Intestine · Water is absorbed along the length of the small intestine · End products of digestion · Most substances are absorbed by active transport through cell membranes · Lipids are absorbed by diffusion · Substances are transported to the liver by the hepatic portal vein or lymph Copyright © 2003 Pearson Education, Inc. publishing as Benjamin Cummings Slide Large Intestine · Larger in diameter, but shorter than the small intestine · Functions: · Absorption of water · Eliminates indigestible food from the body as feces · Does not participate in digestion of food · Goblet cells produce mucus to act as a lubricant Copyright © 2003 Pearson Education, Inc. publishing as Benjamin Cummings Slide Large Intestine Figure 14.8 Copyright © 2003 Pearson Education, Inc. publishing as Benjamin Cummings Slide  Structures of the Large Intestine · Cecum – saclike first part of the large intestine · Appendix · Accumulation of lymphatic tissue that sometimes becomes inflamed (appendicitis) · Hangs from the cecum · Colon Ascending Transverse · Descending S-shaped sigmoidal · Rectum · Anus – external body opening Copyright © 2003 Pearson Education, Inc. publishing as Benjamin Cummings Slide  Food Breakdown and Absorption in the Large Intestine · No digestive enzymes are produced · Resident bacteria digest remaining nutrients · Produce some vitamin K (help blood clot and bone productrion and B (several uses) · Release gases · Water and vitamins K and B are absorbed · Remaining materials are eliminated via faeces Copyright © 2003 Pearson Education, Inc. publishing as Benjamin Cummings Slide  Propulsion in the Large Intestine · Sluggish peristalsis · Mass movements · Slow, powerful movements · Occur three to four times per day · Presence of faeces in the rectum causes a defecation reflex · Internal anal sphincter is relaxed · Defecation occurs with relaxation of the voluntary (external) anal sphincter. Copyright © 2003 Pearson Education, Inc. publishing as Benjamin Cummings Slide  Accessory organs: 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 Copyright © 2003 Pearson Education, Inc. publishing as Benjamin Cummings Slide  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 · Connected to the gall bladder via the common hepatic duct Copyright © 2003 Pearson Education, Inc. publishing as Benjamin Cummings Slide Bile · Produced by cells in the liver · Composition · Bile salts · Bile pigment (mostly bilirubin from the breakdown of hemoglobin) · Cholesterol · Phospholipids · Electrolytes 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 Processes of the Digestive System · Ingestion – getting food into the mouth · Propulsion – moving foods from one region of the digestive system to another · Peristalsis – alternating waves of contraction · Segmentation – moving materials back and forth to aid in mixing Figure 14.12 Copyright © 2003 Pearson Education, Inc. publishing as Benjamin Cummings Slide 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 · Chemical Digestion · Enzymes break down food molecules into their building blocks · Each major food group uses different enzymes · Carbohydrates are broken to simple sugars, glucose · Proteins are broken to amino acids · Fats are broken to fatty acids and alcohols Copyright © 2003 Pearson Education, Inc. publishing as Benjamin Cummings Slide Processes of the Digestive System · Absorption · End products of digestion are absorbed in the blood or lymph · Food must enter mucosal cells and then into blood or lymph capillaries · Defecation · Elimination of indigestible substances as feces Copyright © 2003 Pearson Education, Inc. publishing as Benjamin Cummings Slide Processes of the Digestive System Copyright © 2003 Pearson Education, Inc. publishing as Benjamin Cummings Slide