THE DIGESTIVE SYSTEM
TYPES OF DIGESTIVE SYSTEMS Ingested food may be stored or first subjected to physical fragmentation Chemical digestion occurs next Hydrolysis reactions liberate the subunit molecules Products pass through gut s epithelial lining into the blood (absorption) Wastes are excreted from the anus 2
VERTEBRATE DIGESTIVE SYSTEMS Consists of a tubular gastrointestinal tract and accessory organs Mouth and pharynx entry Esophagus delivers food to stomach Stomach preliminary digestion Small intestine digestion and absorption Large intestine absorption of water and minerals Cloaca or rectum expel waste 3
Copyright The McGraw-Hill Companies, Inc. Permission required for reproduction or display. Oral cavity Salivary glands Salivary gland Pharynx Esophagus Liver Gallbladder Cecum Appendix Stomach Pancreas Small intestine Large intestine Rectum Anus 4
VERTEBRATE DIGESTIVE SYSTEMS Accessory organs Liver Produces bile Gallbladder Stores and concentrates bile Pancreas Produces pancreatic juice Digestive enzymes and bicarbonate buffer 5
MOUTH AND TEETH Inside the mouth, the tongue mixes food with saliva Moistens and lubricates the food Contains salivary amylase, which initiates the breakdown of starch Salivation is controlled by the nervous system Tasting, smelling, and even thinking or talking about food stimulate increased salivation 6
MOUTH AND TEETH Swallowing Starts as voluntary action Continued under involuntary control When food is ready to be swallowed, the tongue moves it to the back of the mouth Soft palate seals off nasal cavity Elevation of the larynx (voice box) pushes the glottis against the epiglottis Keeps food out of respiratory tract 7
Mouth and Teeth Copyright The McGraw-Hill Companies, Inc. Permission required for reproduction or display. Pharynx Air Larynx Trachea Esophagus Hard palate Tongue Soft palate Epiglottis 1. As food moves to the back of the mouth, the soft palate seals off the nasal cavity. 2. During swallowing, the larynx rises and is sealed off by the epiglottis. This forces the bolus into the esophagus and prevents entry into the trachea. As the bolus moves into the esophagus the larynx relaxes. 8
THE ESOPHAGUS Muscular tube connecting the esophagus to the stomach Actively moves a bolus through peristalsis Swallowing center in brain stimulates successive one-directional waves of contraction Sphincter opens to allow food to enter stomach Humans lack a true sphincter here 9
Peristalic movement Copyright The McGraw-Hill Companies, Inc. Permission required for reproduction or display. Esophagus Relaxation Contraction Food Bolus Relaxation 10
THE STOMACH Saclike portion of tract Convoluted surface allows expansion Contains 3 rd layer of smooth muscles for mixing food with gastric juice 3 kinds of secretory cells Mucus-secreting cells Parietal cells Secrete HCl and intrinsic factor (for vitamin B12 absorption) Chief cells Secrete pepsinogen (inactive form of pepsin) 11
The Stomach Copyright The McGraw-Hill Companies, Inc. Permission required for reproduction or display. Stomach Duodenum Esophagus Esophageal sphincter Pyloric sphincter Serosa Mucosa Muscularis Longitudinal Circular Oblique 12
The Stomach (Cont.) Copyright The McGraw-Hill Companies, Inc. Permission required for reproduction or display. Gastric pit Gastric pit Submucosa Mucosa Mucous cell Chief cell Parietal cell Muscularis Oblique Circular Longitudinal Serosa Gastric glands 13
THE STOMACH Low ph in the stomach helps denature food proteins Activates pepsin and keeps it functioning No significant digestion of carbohydrates or fats occurs Absorption of some water Mixture of partially digested food and gastric juice is called chyme Peptic ulcer commonly caused by bacteria (H. pylori) Leaves the stomach through the pyloric sphincter to enter the small intestine 14
THE SMALL INTESTINE About 4.5 m long small diameter Consists of duodenum, jejunum, and ileum Receives Chyme from stomach Digestive enzymes and bicarbonate from pancreas Bile from liver and gallbladder 15
Small intestine Copyright The McGraw-Hill Companies, Inc. Permission required for reproduction or display. Villus Microvilli Epithelial cell Lacteal Cell membrane Villi Mucosa Submucosa Muscularis Serosa Capillary Lymphatic duct Vein Artery Ron Boardman/ Stone/Getty Images Epithelial wall is covered with villi Villi are covered by microvilli Greatly increase surface area Microvilli participate in digestion and absorption Brush border enzymes within microvilli (maltase, sucrase, lactase, peptidase) Many adults lack the enzyme lactase Have lactose intolerance 2 µm 16
ACCESSORY ORGANS Pancreas Pancreatic fluid is secreted into the duodenum through the pancreatic duct Enzymes Trypsin and chymotrypsin proteins into smaller polypeptides Pancreatic amylase polysaccharides into shorter sugars Lipase fats into free fatty acids and monoglycerides Bicarbonate neutralizes acidic chyme 17
ACCESSORY ORGANS Liver Body s largest internal organ Secretes bile Bile pigments (waste products from haemoglobin) and bile salts (for emulsification of fats) Biliruben (orange/yellow), Biliverdin (green) Gallbladder Stores and concentrates bile Arrival of fatty food in the duodenum triggers a neural and endocrine reflex that stimulates the gallbladder to contract, causing bile to be transported through the common bile duct and injected into the duodenum 18
Copyright The McGraw-Hill Companies, Inc. Permission required for reproduction or display. Pancreatic islet (of Langerhans) b cell From liver a cell Common bile duct Pancreas Gallbladder Pancreatic duct Duodenum 19
ABSORPTION Amino acids and monosaccharides are transported through epithelial cells to blood Blood carries these products to the liver via the hepatic portal vein Fatty acids and monoglycerides diffuse into epithelial cells Reassembled into triglycerides and then chylomicrons Enter the lymphatic system and later join the circulatory system Almost all fluid reabsorbed in small intestine 20
ABSORPTION Copyright The McGraw-Hill Companies, Inc. Permission required for reproduction or display. Protein Carbohydrate Bile salts Fat globules (triglycerides) Lumen of small intestine Amino acids Monosaccharides Emulsified droplets Free fatty acids, monoglycerides Resynthesis of triglycerides Epithelial cell of intestinal villus Transport protein Blood capillary Transport protein Lymphatic capillary Chylomicron Triglycerides get protein cover a. b. 21
THE LARGE INTESTINE ( Much shorter than small intestine, but has larger diameter Small intestine empties directly into the large intestine at a junction where two vestigial structures, cecum and appendix, remain No digestion occurs Function to reabsorb water, remaining electrolytes, and vitamin K Prepare waste for expulsion 22
Copyright The McGraw-Hill Companies, Inc. Permission required for reproduction or display. Ascending portion of large intestine Ileocecal valve Last portion of small intestine Cecum Appendix 23
THE LARGE INTESTINE Many bacteria live and reproduce within the large intestine Feces compacted and passed to rectum Feces exit anus Smooth muscle sphincter (involuntary) Striated muscle sphincter (voluntary) 24
FOOD ENERGY Ingestion of food serves two primary functions 1. Source of energy 2. Source of raw material Basal metabolic rate (BMR) Minimal amount of energy consumed under defined resting conditions Continued ingestion of excess food energy results primarily in accumulation of fat 25
REGULATION OF FOOD INTAKE Control mechanism links food intake to energy balance Leptin peptide hormone Key to appetite control Produced by adipose tissue Leptin receptor located in hypothalamus Reduced leptin signals brain to intake food (vagus nerve) Research on leptin in humans ongoing 26
REGULATION OF FOOD INTAKE Other hormones involved in the control of feeding and energy include Insulin, GIP, and CCK, which signal satiety Ghrelin which stimulates food intake 27
Copyright The McGraw-Hill Companies, Inc. Permission required for reproduction or display. Reproduction and Growth Appetite and Feeding High levels of leptin and insulin reduce appetite. Low levels increase appetite. Low levels of leptin can inhibit reproduction and growth. Hypothalamus Energy and Expenditure High levels of leptin and insulin increase energy expenditure. Efferent Afferent Long Term Circulating levels of leptin and insulin are proportional to body fat. High body fat leads to high levels of these hormones. Leptin ( ) Insulin ( ) ( ) GIP CCK ( + ) Ghrelin Short Term CCK and GIP are produced in response to feeding and act to limit food intake. Ghrelin stimulates feeding. 28
ESSENTIAL NUTRIENTS Animal cannot manufacture these for itself but are necessary for health and so must be obtained in the diet Vitamins Humans, apes, monkeys, and guinea pigs have lost the ability to synthesize ascorbic acid (vitamin C) Amino acids humans require 9 Long-chain unsaturated fatty acids Vertebrates can synthesize cholesterol, a key component of steroid hormones, but some carnivorous insects cannot Minerals 29
30
REGULATION OF THE DIGESTIVE TRACT Gastrointestinal activities are coordinated by the nervous and endocrine systems Nervous system stimulates salivary and gastric secretions in response to sight, smell, and consumption of food In the stomach, proteins stimulate the release of gastrin Triggers the secretion of HCl and pepsinogen from the gastric glands 31
REGULATION OF THE DIGESTIVE TRACT Enterogastrones or duodenal hormones Inhibit stomach contractions and prevent additional chyme from entering duodenum Cholecystokinin (CCK), secretin, and gastric inhibitory peptide (GIP) All inhibit gastric motility and secretions CCK also stimulates gallbladder contraction and pancreatic enzyme secretion Secretin also stimulates the secretion of pancreatic bicarbonate 32
Copyright The McGraw-Hill Companies, Inc. Permission required for reproduction or display. Stomach Liver ph Proteins Gastrin GIP ( ) ( + ) ( + ) Chief cells Parietal cells Pepsin HCl ( + ) Bile Pancreas Enzymes Bicarbonate Acinar cells Gallbladder ( + ) ( + ) CCK Secretin Duodenum 33
34
ACCESSORY ORGAN FUNCTION Liver Chemically modifies the substances absorbed from the digestive tract before they reach the rest of the body Ingested alcohol and other drugs are taken into liver cells and metabolized Removes toxins, pesticides, and carcinogens, converting them to less toxic forms Regulates levels of steroid hormones Produces most proteins found in plasma 35
ACCESSORY ORGAN FUNCTION Regulation of blood glucose After a carbohydrate-rich meal Insulin stimulates removal of excess blood glucose by liver and skeletal muscles (glycogen) When blood glucose levels decrease Glycogenolysis glucagon stimulates liver to break down glycogen to release glucose into blood Gluconeogenesis liver converts other molecules into glucose if fasting continues 36
Copyright The McGraw-Hill Companies, Inc. Permission required for reproduction or display. Metabolism Eating carbohydraterich meal Increasing blood glucose Fasting or exercise Decreasing blood glucose Pancreatic Islets Insulin secretion Glucagon secretion Pancreatic Islets Insulin secretion Glucagon secretion Formation of glycogen (in liver) and fat (in adipose tissue) Breakdown of glycogen (in liver) and fat (in adipose tissue) 37