Lecture 4: Structures of the Digestive System (BE226) PDF

BE226 Cell Biology Lecture 4 Structures Of The Digestive System Dr. Naomi Walsh School of Biotechnology The Digestive System  All living organisms must get nutrients from their environment in order to sustain life A nutrient is any organic or inorganic...

BE226 Cell Biology Lecture 4 Structures Of The Digestive System Dr. Naomi Walsh School of Biotechnology The Digestive System  All living organisms must get nutrients from their environment in order to sustain life A nutrient is any organic or inorganic substance that is taken in by an organism and used for survival, growth, development, tissue repair or reproduction Carbohydrates, proteins, lipids, nucleic acids, and vitamins These nutrients are used as raw materials for synthesising essential compounds, and broken down to provide energy that cells need to continue functioning 50,000,000 calories in your lifetime to keep trillions of the body’s cells performing Working with the lymphatic and cardiovascular system, the digestive system provides these essential nutrients in the forms of carbohydrates, lipids and proteins School of Biotechnology The Digestive System  The gastrointestinal (GI) system has six key functions: 1. Absorb required dietary nutrients 2. Secrete molecules capable of converting complex molecules to simpler ones 3. Digest molecules from complex molecules into simpler ones 4. Move the nutrients through the specialised compartments of the GI tract 5. Store the matter until function is executed 6. Excrete undigested materials/waste products School of Biotechnology The Digestive System School of Biotechnology The Digestive System  The major components of the digestive system are those organs which form the gastrointestinal (GI) tract Starts at the mouth and ends at the anus (9metres) Sometimes referred to as the alimentary canal While the GI tract is recognised as one structure, it is comprised of a number of structures, each with their own specialised function School of Biotechnology The Digestive System There are a number of accessory organs that assist in the breakdown, and absorption of nutrients from our diet Often have other functions, with their contribution to the digestive system secondary These exert their effects through secretions into the GI tract, or via ducts that enter such Food passes by them, not through them like the main digestive organs School of Biotechnology The Digestive System  GI tract is composed of multiple layers, each with their own distinctive function Depending on structure-function relations, a particular layer’s prominence changes along the length of the tract Stomach lining has longitudinal folds which disappear as the cavity fills, small intestine has permanent folds for maximum absorption Surrounded by layers of smooth muscle, neurons connective tissue, and blood vessels Highly adaptable School of Biotechnology The Digestive System  Mucosa: Innermost lining comprised of three layers Often arranged into villi (peaks) and crypts (troughs) Increase surface area/maximise contact between the mucosa and GI contents Epithelium is either simple or stratified cell layer that forms a continuous lining of the GI tract Simple – places of absorption Stratified – places of mechanical digestion Polarised Goblet cells – produce mucus Enteroendocrine cells – produce hormones Shed and replaced every 2-3 days School of Biotechnology The Digestive System  Mucosa: The lamina propria is loose connective tissue composed of elastin and collagen fibres Contains sensory nerves, blood and lymphatic vessels, and secretory glands Contains sensory nerves, blood and lymphatic vessels, and secretory glands The muscularis mucosa is two thin layers of smooth muscle One runs around the tract lumen One runs the length of the tract Contractions/relaxations of such controls the surface area by creating mucosal ridges and folds School of Biotechnology The Digestive System  Submucosa: A thicker layer with a similar composition to the lamina propria Incorporates blood vessels, lymphatic vessels and exocrine glands Gastric-releasing peptide – gastrin release Substance P – muscle contraction Enkephalins – muscle contraction The outer margin contains bundles of nerve fibres and neurons that collectively form a submucosal plexus Very important for communication in the enteric nervous system School of Biotechnology The Digestive System  Muscularis externa: Comprised of circular muscle, myenteric plexus, and longitudinal muscle layers Circular muscle is arranged in rings around the tract and can narrow/widen the tract Longitudinal muscle is arranged in parallel in the direction of the tract and can elongate/shorten the tract The enteric nervous system coordinates these layers to mix tract contents and move them from compartment to compartment Can create ‘gates’ or ‘sphincters’ in the tract The myenteric plexus coordinates these actions School of Biotechnology The Digestive System  Serosa: Outermost layer of connective tissue and a layer of epithelial cells Protective/structural layer The only portion not present in every portion of the GI tract The oesophagus does not have a serosa layer A fibrous sheath of collagen called the adventitia connects it directly to the abdominal walls School of Biotechnology The Digestive System  MOUTH Upper part of the GI tract plays a minimal role in nutrient absorption Prepares food, and transports it, to the site of absorption Mechanical Chemical Sensory Lubrication Teeth aid in cutting (incisors), tearing (canines) and grinding (premolars and molars) food Together with the jaw muscles, mastication or chewing breaks the food down into smaller pieces/increases the foods surface area The tongue grips and positions food for chewing Also contains taste buds School of Biotechnology The Digestive System  ~1000 glands in the oral cavity producing 1.5 litres of saliva Salivary glands each day Three pairs of salivary glands secrete into the oral cavity: Parotid – Produces salivary amylase for breaking down starch Sublingual – Produces mucus that acts as a buffer/lubricant Submandibular – Mixture of buffers, salivary amylase and mucus 99.4% water, 0.6% electrolytes, buffers, glycoproteins, antibodies, enzymes and waste Serous Cells Keeps cavity clean Duct Keeps pH neutral Reduces acids produced by bacteria Goblet Cells Controls bacteria numbers School of Biotechnology The Digestive System  Bolus is moved to the pharynx The Oesophagus Elastic fibres work with skeletal muscle to push the material into the oesophagus - swallow Oesophagus actively moves ingested material down towards the stomach Allow you to eat upside down 30 cm in length, 2 cm diameter at its widest point Mucosa and submucosa are packed into large folds that allow for expansion Muscle tone keeps the lumen closed except for when material is moving through it Also prevents air entering the GI tract (superior) and acid entering the oesophagus (inferior) School of Biotechnology The Digestive System  Smooth muscle layers in the muscularis mucosae known as pacesetters undergo a depolarisation Triggers a spontaneous wave of contraction This depolarisation ripples like a wave through the sheet of muscle Coordinated contractions propels food through the GI tract in a type of movement or motility known as peristalsis As to the bolus enters the oesophagus the circular muscles behind the bolus contract At the same time, the circular muscles ahead of the bolus relax The muscles cells running in the direction of the tract (longitudinal) ahead of the bolus then contract, shortening the adjacent segment This process repeating moves the bolus along the tract towards the stomach School of Biotechnology The Digestive System  Expandable tube-like organ The stomach Temporarily store ingested food Mechanically digest food Chemically digest food Ingested material and contents of the stomach combine to produce chyme When the stomach is relaxed it has longitudinal folds called rugae As volume increases the rugae flatten out allowing up to 50 times expansion Typical volume – 1-1.5 litres Additional muscular layers of the stomach churn the contents until the chyme can enter the small intestine School of Biotechnology The Digestive System  Single columnar epithelium line the entire stomach Creates a secretory sheet which lays down a carpet of mucus on the stomach wall Alkaline layer that protects against the acid and enzymes of the stomach School of Biotechnology The Digestive System  Single columnar epithelium line the entire stomach Creates a secretory sheet which lays down a carpet of mucus on the stomach wall Alkaline layer that protects against the acid and enzymes of the stomach Shallow depressions called gastric pits open onto the gastric surface High turnover of cells at the neck of the pits as cell layer is often shed and removed with the chyme Average life span of 3-7 days, less if alcohol or other factors are ingested Also contains glands that secrete acids and enzymes for gastric digestion 1.5 litres of pH 2 acid a day School of Biotechnology The Digestive System  The small intestine Longest part of the alimentary canal (10 ft long) Where chemical digestion is completed and 99% of nutrient absorption takes places The intestines are stabilised in place by the mesentery; connective tissue that connects blood vessels, nerve endings and lymphatic vessels Duodenum – mixes chyme with digestive juices from liver and pancreas Jejunum – Bulk of chemical digestion and absorption happens here Ileum – Regulates flow of material into the large intestine School of Biotechnology The Digestive System  The large intestine Terminal part of the alimentary canal Subdivided into four main regions: the cecum, the colon, the rectum, and the anus. ileocecal valve, controls the flow of chyme from the small intestine to the large intestine. Cecum - sac-like structure that is suspended inferior to the ileocecal valve. It is about 6 cm (2.4 in) long, receives the contents of the ileum, and continues the absorption of water and salts Colon- food residue travels up the ascending to the transverse colon, across the abdominal to the left colic flexure, down the descending colon, becomes s-shaped anal canal - located in the perineum, completely Rectum- structure follows a curved contour and has outside of the abdominopelvic cavity, opens to 3 lateral bends that create folds called rectal valves the exterior of the body at the anus. The anal – which help separate the faeces from gas to canal includes two sphincters. prevent simultaneous passage! School of Biotechnology The Digestive System  The primary exocrine functions of the pancreas are to neutralise acids and deliver enzymes for macronutrient The pancreas digestion Pancreatic duct delivers secretions from the pancreas to the duodenum School of Biotechnology The Digestive System  The primary exocrine functions of the pancreas are to neutralise acids and deliver enzymes for macronutrient digestion Pancreatic duct delivers secretions from the pancreas to the duodenum The acinar cells are the primary exocrine secretory cells Small clusters of these cells secrete their contents into intercalated ducts, which converge within the organ into the collecting duct Endocrine – secretes insulin and glucagon into the bloodstream to control blood sugar Secretes about 1000 ml of pancreatic juice a day Duodenum sends signal to the pancreas and regulates these activities School of Biotechnology The Digestive System The liver The liver secretes hepatic bile; a solution that contains bile acids, electrolytes, cholesterol, phospholipids and bilirubin Bile is secreted by hepatocytes into bile ductiles which until they converge into the common hepatic duct School of Biotechnology The Digestive System The liver Has over 200 functions Metabolic regulation – all blood leaving the digestive system flows to the liver Liver extracts nutrients and toxins before it gets circulated to the systemic regions Removes and stores excess nutrients, and corrects for deficiencies Regulates carbohydrates, lipids, amino acids, waste, vitamins, minerals, drug levels School of Biotechnology The Digestive System  The liver constantly produces bile but not in sufficient quantity to properly emulsify lipids in the small intestine Depending on demand, bile can get directed to the small intestine via the common bile duct, or to the gallbladder by the cystic duct Gallbladder is a pear-shaped organ that stores concentrated bile prior to secretion into the small intestine The bile stored in the gallbladder has the water removed over time increasing the overall salt concentration When stimulated (by CCK) the gallbladder contracts releasing via the common bile duct into the small intestine The higher the lipid content of the chyme, the more CCK that is produced, which regulates the amount of bile released School of Biotechnology The Digestive System  In the upper portion of the GI tract, food is now liquefied – ready to be absorbed Intestinal motility moves the chyme along, and mixes it with the enzymes and secretions from the accessory organs 12 waves/minute Epithelial cells add to the mix with enzymes, and importantly mucus Folds on the surface of the small intestine are permanent 800 folds – huge surface area Without – 4 square feet With – 2200 square feet School of Biotechnology The Digestive System  To facilitate absorption, the simple molecules require transportation across both the apical and basolateral membranes of the epithelium Each villus contains a dense network of capillaries for nutrient uptake in the lamina propria These vessels connect to a larger network located in the submucosa Delivered to the liver for filtration Fatty acids and lipids that are not able to be uptaken by the vasculature are taken by lymphatic system School of Biotechnology The Digestive System  To facilitate absorption, the simple molecules require transportation across both the apical and basolateral membranes of the epithelium Each villus contains a dense network of capillaries for nutrient uptake in the lamina propria These vessels connect to a larger network located in the submucosa Delivered to the liver for filtration Fatty acids and lipids that are not able to be up taken by the vasculature are taken by lymphatic system At the bases of the villi are the intestinal crypts Epithelial Paneth cells in the base of the pits Stem Defensive role, and regulate the flora of the gut Paneth Once the necessary items are extracted, the remainder needs to be eliminated from the body School of Biotechnology The Digestive System  Stores digestive wastes and reabsorbs water Length of 1.5 metres, and divided into three segments: Cecum – collects material and compacts it Colon – water absorption Rectum – elimination Less than 10% of absorption occurs in the large intestine Produce 1500 ml of material, remove about 200ml as faeces 75% water 5% bacteria 20% indigestible materials, dead cells Most of the microbiome lives here School of Biotechnology The Digestive System  Bile salts, vitamins, organic wastes are reabsorbed and transported by the blood to the liver No villi Pits are much deeper than the small intestine and are primary lined with goblet cells Goblet cells produce mucus Acts as a lubricant for faeces as it is dehydrated in the colon Arrives in the rectum and is stored until signals are sent to by the nervous system to remove the material School of Biotechnology The Digestive System  Regulatory mechanisms Initial regulation of digestive function occurs at the local level pH, volume, chemical composition of ingested contents influence the activity in that particular area of the tract Neural and endocrine regulatory mechanisms work to maintain the optimal conditions in the lumen needed for digestion and absorption. These regulatory mechanisms, which stimulate digestive Buffers, acids, enzymes released activity through mechanical and chemical activity, are controlled both extrinsically and intrinsically. For example, the volume of food in a region will stretch/distort the tract wall at that point Mechanosensors sense this and trigger a contraction to move the contents School of Biotechnology The Digestive System Neural Mechanisms Local factors only propel the bolus a few centimetres Neural mechanisms courtesy of the neurons located in the myenteric plexus lead to reflexes These trigger secretion events by glandular cells, or local peristalsis by smooth muscle Enteric nervous system has as many neurons as the spinal cord, and as many neurotransmitters as the brain Buffers, acids, enzymes released Extrinsic nerve plexuses orchestrate long reflexes, which involve the central and autonomic nervous systems and work in response to stimuli Short reflexes, on the other hand, are orchestrated by intrinsic nerve plexuses within the alimentary canal School of Biotechnology The Digestive System  Hormonal Control Cells of the digestive tract are capable of producing numerous hormones These hormones can enhance, or diminish, the sensitivity of smooth muscles cells to neural commands They are produced by The main hormone is Gastrin, secreted enteroendocrine cells and released into the by the stomach in response to food bloodstream to reach Hormones produced by the duodenum their target organs include secretin – stimulates a watery secretion of bicarbonate by the Buffers, acids, enzymes released pancreas Cholecystokinin (CCK), - stimulates the secretion of pancreatic enzymes and bile from the liver and release of bile from the gallbladder; gastric inhibitory peptide - inhibits gastric secretion and slows gastric emptying and motility. School of Biotechnology The Digestive System  Learning Outcomes Why we eat? How our diet affects cellular activity, and in turn tissue and organ function Anatomy of the Digestive System The different parts of the digestive tract, their features, and contribution to digestion The Intestinal Barrier Introduction to the structure and efficiency of such School of Biotechnology The Digestive System