HUBS1416 Topic 1 Lecture The Digestive System & Its Accessory Organs PDF

School of Biomedical Science and Pharmacy College of Health, Medicine & Wellbeing The Digestive System & Its Accessory Organs Advanced Human Bioscience (HUBS1416) School of Biomedical Science and Pharmacy College of Health, Medicine & Wellbeing  Lecture Overview l...

School of Biomedical Science and Pharmacy College of Health, Medicine & Wellbeing The Digestive System & Its Accessory Organs Advanced Human Bioscience (HUBS1416) School of Biomedical Science and Pharmacy College of Health, Medicine & Wellbeing  Lecture Overview l Part 1: Purpose of the digestive system and obesity l Part 2: General action of the nervous system, the mouth, oesophagus and swallowing l Part 3: The stomach and small intestines, pancreas & gall bladder l Part 4: The liver & large intestines & summary of GIT system l Part 5: Metabolism, fate of nutrients, control of feeding & satiety Digestive System Gastrointestinal tract Accessory organs: liver gall bladder pancreas Food components l Macronutrients Ø components that supply energy Ø consumed in the greatest amounts Ø carbohydrates, lipids and proteins Ø have to be broken down to be absorbed and then used l Micronutrients Ø only required in tiny amounts Ø don’t provide energy, but are essential for normal function Ø include the vitamins & minerals l Water and gases Immediately used OR converted to adipose tissue for energy storage  You Are What You Eat (and what you do) 61.4% of the Australian population are either overweight or obese 25% of Australian children are currently overweight or obese (5% in 1960s) What is Obesity? BMI: Body Mass Index 25-30 kg/m2 = overweight >30 kg/m2 = obese The Global Obesity Problem What Causes Obesity? Unhealthy eating patterns: ↑ consumption of foods high in fat and energy and low in nutrients Energy Intake > Energy Expenditure ↓ in or lack of Genetic factors physical exercise Energy Expenditure Resting metabolic rate l ~ 60-75% of daily energy expenditure l homeostasis requires energy Thermic response of a meal l 10% of daily energy expenditure l energy cost of digestion, absorption, mobilisation and storage of food Physical activity l most variable component of energy expenditure (260 à 5000 kcals/day) l voluntary muscular activity (conscious and unconscious) Factors affecting metabolic rate Factor Effect on Metabolic rate Age Ü With increasing age Gender Higher in men Size Higher in bigger people Pregnancy, lactation, menstruation Û In these conditions Activity Û with increasing activity Body temperature Û with increasing temperature Thyroid hormone level Û With increasing thyroid hormone level Starvation Ü In this state Emotional states Û with stress, anxiety The many health consequences of obesity DON’T NEED TO KNOW DETAILS  The GI Tract l The tissue is essentially the same along entire length l Four layers, two of which are muscle layers Digestive System Processes l Contents are both mixed, liquidised and moved along the gastrointestinal tract by contractions of the smooth muscle of the gut wall l Chemical digestion occurs by means of enzymes l After digestion, absorption of the simple sugars, amino acids, fatty acids, vitamins, ions, etc, can occur Mixing and moving of gut contents Segmentation Peristalsis The Mouth l Mechanical digestion l Teeth l Incisors tear off a bite l Molars chew and grind l Tongue l Manipulates crushed food into a bolus What’s saliva for? l Salivary glands produce saliva, containing mucus, salivary amylase, lipase, antibodies and lysozyme l Mucus binds and moistens the crushed food l Amylase begins the digestion of the carbohydrates and lipase will begin the digestion of the fats  Swallowing has both voluntary & involuntary phases: Voluntary - tongue & cheeks push the bolus back towards the pharynx Involuntary - pharyngeal muscles propel the bolus into the oesophagus, it is pushed down the oesophagus by peristalsis  The Oesophagus Collapsible tube extending from the pharynx to the stomach, passing through the diaphragm Peristalsis in the oesophagus pushes the food down, & the lower oesophageal sphincter opens when the food touches it The food then passes into the stomach Lower Oesophagus oesophageal (cardiac) sphincter Stomach  The Stomach l Three layers of muscle: contraction of all layers gives a churning action l Sphincters at top and bottom keep the Cardiac Sphincter fluid in the stomach while churning inal tud cle i Long mus C l Contains strong hydrochloric acid to kill microbes Pyloric sphincter m ircu us la cl r e muscle Oblique l Protein digestion starts here (pepsin) l Food turns into chyme – a thick, partially digested fluid that enters the small intestine through the pyloric sphincter - a solid meal of mixed macromolecules will take around 5 hours to fully leave the stomach  Actions of Gastric Juice Various specialised cells in the stomach produce the components of gastric juice that each have specific functions: l Hydrochloric acid – l Mucus – l Pepsinogen – l Gastric lipase – l Intrinsic factor (IF) – The stomach needs protection from its own acid and protease Thick Viscous Alkaline  What stimulates the stomach? l The sight or smell of food The brain stimulates gastric activity via the parasympathetic division; causing acid, mucus and enzyme secretion l The presence of food in the stomach Causes the stomach to release the hormone gastrin; stimulates gastric juice production by the stomach and muscular contractions of the stomach  What inhibits the stomach? l The arrival of chyme in the small intestine Both the nervous and endocrine systems work to slow down gastric activity and reduce acidity when the chyme starts to come through into the duodenum Why do we only want small amounts of chyme at a time in the intestine?  Small intestine l There are three regions: duodenum, jejunum & ileum l It is 5-6m in length l Digestion is completed (duodenum) l Digested food is absorbed (primarily jejunum) l Requires secretions from l Gall bladder & Liver l Pancreas  The pancreas and gall bladder Gall bladder Bile duct Pancreas Pancreatic duct  What is in pancreatic juice? l Sodium bicarbonate (neutralises stomach acid) l Enzymes to digest (break down) all the major food types: l Amylase for carbohydrates l Lipase for lipids l Proteases for proteins l Nucleases for nucleic acids The proteases produced in the stomach and pancreas are in an inactive form which has to be activated in the GIT lumen – Why? Activation of proteases in the duodenum Ready to Intestine digest wall Enterokinase proteins  What’s in bile? l Bile salts These emulsify fats to allow the lipase to get at the fat droplets l Bile pigments e.g. bilirubin Play no role in digestion – they are excreted via this route. Bilirubin is a breakdown product of haem l Cholesterol Being excreted by this route & may also protect the duodenum from the bile salts  Mechanical digestion also occurs in the small intestine Segmentation mixes and breaks contents down Peristalsis moves contents along Arrival of chyme in the duodenum triggers hormonal responses that (with the nervous system) coordinate intestine and stomach function 1. Chyme enters the 6. CCK stimulates duodenum from the liver to increase stomach bile production 2. Fatty acids & amino acids in chyme entering 7. CCK causes the duodenum stimulates gall bladder to the release of contract & cholecystokinin (red dots) sphincter of Oddi from duodenal cells to relax à bile enters duodenum 3. Acid in chyme entering the duodenum stimulates release of secretin (yellow dots) from duodenal cells into bloodstream 4. CCK causes the exocrine pancreas to secrete enzyme-rich juice 5. Secretin causes exocrine pancreas to secrete bicarbonate-rich juice and inhibits further gastric acid secretion and gastric motility  Intestinal Mucosa l Internal surface area of small intestine is the size of a tennis court. Ø This surface area is achieved by lots of circular folds in internal lining, villi & microvilli l Contains Peyers patches: nodules of lymphatic tissue that protect the GI tract from pathogens that escape stomach Circular folds Peyers patches  The structure of the small intestine reflects its function Surface area is maximised by: Circular folds Microvilli Villi  Villi contain blood and lymph vessels to transport the absorbed nutrients Absorption of the products of digestion occurs in a number of ways: Ø diffusion Ø facilitated diffusion Epithelial cells with Ø active transport microvilli (brush border) Blood capillaries sugars and peptides are Lymph capillary absorbed into the blood (lacteal) along with water fats are absorbed into Mucus-secreting cells the lymphatic vessels (lacteals) Absorbed nutrients are carried from the small intestine straight to the liver via the hepatic portal vein  The liver receives the products of digestion for processing: l Detoxification/biotransformation l Storage of excess nutrients Glucose àglycogen, fat soluble vitamins l Synthesis of bile l Destruction of old red blood cells Large Intestine l Water is absorbed from the large intestine contents l Any carbohydrates are acted on by bacteria à production of gases  Faeces After absorption of most of the water, chyme becomes faeces It contains any insoluble fibre, dead and live microbes, epithelial cells shed from the tract and some fatty acids Mucus is secreted by the large intestine to act as a lubricant, allowing the faeces to move along  Water use and absorption in the GIT About 1.2 litres of water is taken in via food and drink, and another 7 litres of water are added to the gastrointestinal tract in the form of all the secretions required for digestion About 8 litres of water is absorbed back into the bloodstream from the gastrointestinal tract. Very little water is normally lost in faeces Summary - roles of the regions of the GIT Saliva Contains amylase and Mouth lipase. Also moistens Chewing and lubricates food breaks up food Stomach HCl for protection and enzyme activation. Pepsin activated. Churning to create chyme Large intestine Limited absorption Absorption of water and ions Small intestine Site of most chemical digestion and absorption. Proteases, nucleases, amylases and lipase delivered here from Slide 41 pancreas.  Summary - roles of the accessory organs Liver Makes bile. Converts glucose à glycogen for storage. Breaks down and builds up many biological molecules. Detoxifies Pancreas Produces digestive Gall bladder hormones trypsin & Stores and chymotrypsin (proteases), concentrates bile. amylases and lipase. Contracts to release Produces bicarbonate to bile into duct neutralise acid  Metabolism This is all of the chemical reactions occurring in all the cells of your body at any moment. These reactions use the nutrients gained from digestion to provide the energy needed for: Ø Homeostasis Ø Voluntary activity Ø Growth Ø Maintenance and repair Ø Secretion and contraction Metabolism There are two main types of metabolic reactions: l Anabolic: This is the building up of substances from simple to more complex ones. These reactions require energy input l Catabolic: This is the breaking down of complex substances into simpler ones. These reactions release energy Which nutrients for which functions? l For growth and repair, amino acids would be used in preference to fatty acids or glucose (growth & repair would involve making structural proteins) i.e. Amino acids, then lipids, then glucose l For energy to power all activities, glucose is used in preference to amino acids or fatty acids. i.e. Glucose, then lipids, then amino acids Not all nutrients provide the same amount of energy 1g carbohydrate provides: 17 kJ (4kcal) of energy 1g protein provides: 17 kJ (4kcal) of energy 1g fat provides: 38kJ (9kcal) of energy Can either be used straight away or stored Metabolic Turnover How is glucose utilised? Some glucose is converted to glycogen and stored Glucose leaves the liver in the hepatic vein It is used to produce energy for cells (ATP) It is used to maintain blood glucose level in the normal range (3.5-8 mmol/l) Glucose comes in from the intestine It is converted to glycogen and stored in muscles (in the hepatic portal vein) It is converted to fat and stored as adipose tissue  How are lipids utilised? l To make lots of ATP (one fatty acid molecule can yield 126 ATP molecules as compared to 1 glucose which yields about 32 ATP) l To be stored as fat l To make lipid components of cells (e.g. phospholipids for membranes, myelin for nerve cells)  How are amino acids utilised? l To produce new cellular proteins to replace old ones l Eg. enzymes, hormones & antibodies l Can be utilised in by the mitochondria to make ATP when there is not enough glucose or lipids l Excess amino acids can be processed by the liver (deamination) to keto acids so they can be stored for future energy usage – by conversion to lipids How are amino acids utilised? All our proteins are made from just 20 different amino acids Ten of these amino acids can be made in the body and do not have to be taken in as part of the diet. These are the non-essential amino acids The others cannot be made in the body, and have to be present in the diet – these are the essential amino acids What regulates our intake of food? The hypothalamus in the brain contains a feeding centre and a satiety centre Various signals will feed into either the feeding or the satiety centre, to drive us either to eat or to stop eating Factors feeding into the feeding and satiety centres Higher brain regions Hypothalamus Signals from fat tissue Signals from GIT Nutrients in blood Some of the hormonal signals which influence the hypothalamus & control feeding  Leptin Deficiency Leptin-deficient child aged 3. Same child aged 7 after treatment with Weight 42 Kg recombinant human leptin. Weight 32 Kg