BIOL1XX8_2024_L16_Digestive System (1).pdf

The Digestive System Dr. Hong Dao Nguyen School of Life and Environmental Sciences Learning objectives Describe the anatomy and functions of structures within the digestive system Describe the layers of the gastrointestinal tract and specialised modifications in the stomach and small intest...

The Digestive System Dr. Hong Dao Nguyen School of Life and Environmental Sciences Learning objectives Describe the anatomy and functions of structures within the digestive system Describe the layers of the gastrointestinal tract and specialised modifications in the stomach and small intestine Describe the digestive enzymes and their activities. Salivary and pancreatic amylase Lingual and pancreatic lipase Pepsin Gastric lipase Maltase Explain the importance of the slow delivery of chyme from the stomach to the small intestine. Analyse the role of different digestive secretions in the processes of protection, chemical digestion, and absorption and predict the potential impacts of inadequate or excess secretion. Understand how nutrients are absorbed for use in the body. Describe the waste products from our digestive system and how these are made. Digestive system anatomy: overview Gastrointestinal tract (alimentary canal) Accessory organs Teeth Oral cavity Tongue Pharynx Salivary glands Oesophagus Stomach Liver Gallbladder Pancreas Small intestine Large intestine Image modified from Wikimedia Commons Layers of the GI tract Oesophagus to anal canal has 4 distinct layers Serosa/Adventitia Serosa Muscularis externae Muscularis Submucosa Submucosa Longitudinal muscle Circular muscle Mucosa Mucosa Muscularis mucosae Lumen Lumen Epithelium Image modified from Wikimedia Commons  Digestive system functions: overview Ingestion Secretion Motility Digestion Intake of food into the Cells of the GI tract and Mixing and propulsion Breakdown of food into mouth accessory glands resulting from contractions absorbable components and relaxation of muscle Acid, buffers, enzymes, fibres Chemical hormones, mucus Mechanical Absorption Protection Elimination Mainly in small intestine Mucus & acidic fluid protect Removal of wastes as faeces against pathogens Undigested material, bacteria, old Molecules pass into blood or cells lining GI tract lymph  Organic compounds Carbohydrates Proteins Carbon, hydrogen, oxygen Amino acids Monosaccharides e.g. glucose, fructose Dipeptide = two amino acids joined Disaccharides e.g. sucrose, lactose, maltose Chains of amino acids = polypeptides Proteins are polypeptides that are folded Polysaccharides e e.g. starch, cellulose, into functional structure glycogen Lipids Nucleic acids Includes fats, waxes, steroids, fat-soluble vitamins Nucelotides Triglycerides (glycerol attached to 3 fatty acid RNA, DNA chains) Fatty acids = chains of hydrocarbons Digestion Mechanical digestion involves the physical breakdown of food into smaller particles Chemical digestion assisted via enzymatic activity to form absorbable compounds What we consume What can be absorbed Carbohydrates Monosaccharides (mostly starches) (mostly glucose) Proteins Amino acids Lipids Fatty acids (mostly triglycerides) Digestive system anatomy: overview Gastrointestinal tract (alimentary canal) Accessory organs Teeth Oral cavity Tongue Pharynx Salivary glands Oesophagus Stomach Liver Gallbladder Pancreas Small intestine Large intestine Image modified from Wikimedia Commons Oral cavity: anatomy Accessory organs include teeth, tongue and salivary glands 3 major pairs of salivary glands Parotid – Parotid Sublingual – Submandibular Submandibular – Sublingual Image from Wikimedia Commons Oral cavity: function Ingestion Location for food enter body which will eventually turn into bolus (ball of food) Digestion Teeth is used for mechanical digestion via mastication Chemical digestion via action of two enzymes in saliva - Salivary amylase digests starch to maltose - Lingual lipase digests triglycerides Oral cavity: function Secretion Saliva secreted by salivary glands - 99% water, 1% salts and proteins - Mucus lubricates food Protection Oral cavity provides an ideal habit for microbes Salvia has anti-microbial properties – mucins bind and aggregate microbes to prevent colonisation – antibodies Pharynx Anatomy: funnel-shaped tube of skeletal muscle Function: – Shared by two organ systems (digestive and respiratory systems) – During swallowing, structures close to ensure bolus moves to the oesophagus Oesophagus: anatomy Muscular tube ~25 cm long Upper: skeletal muscle Middle: mixed Lower: smooth muscle Image from Wikimedia Commons Oesophagus: function Motility Propulsion of bolus to stomach via peristalsis Peristalsis: continuous waves of contraction and relaxation of muscles Video: https://youtu.be/o18UycWRsaA Stomach: anatomy J-shaped chamber 4 main regions Gastro-oesophageal sphincter Oesophagus – The most distal portion is the pylorus Muscularis (pyloric region) Rugae increase surface area 3 layers of muscles 2 sphincters (muscular values) Pyloric sphincter prevent backflow Duodenum Pylorus – Gastro-oesophageal sphincter – Pyloric sphincter Image from Wikimedia Commons Stomach: function Mechanical digestion & motility Peristaltic contractions ~3/min push bolus towards pylorus (propulsion) Contractions strengthen towards the pylorus, grinding bolus into chyme Pylorus allows 1/10 of chyme it holds to pass through pyloric valve per contraction Contractions also close pyloric value and remaining contents are propelled back (retropulsion) Marieb & Hoehn, Human Anatomy & Physiology  Stomach: unique anatomical features Mucosa extends towards the submucosa to columns of specialized secretory cells called gastric glands Muscularis externa has 3 muscle layers (G cell) Image from Wikimedia Commons Stomach: function Special cells of the gastric gland secrete components of gastric juice - These have important roles in chemical digestion, protection and absorption Components of gastric juice Gastrin: hormone that stimulates secretion of HCl and pepsinogen Pepsinogen - Activated to pepsin in the presence of HCl - Pepsin chemically digests proteins into peptides in the stomach HCl - Denatures proteins, converts pepsinogen to pepsin - Kills microbes Bicarbonate-rich mucous: protects stomach wall from protein digestion and corrosion from HCl Gastric lipase: digests triglycerides into fatty acids and monoglycerides Intrinsic factor: needed for transportation & absorption of vitamin B12 in small intestine Small intestine: anatomy 3-6m in length 3 regions – Duodenum, 20-25 cm, closest to stomach – Jejunum, ~2.5 m – Ileum, ~3 m, closest to the large intestine Joins large intestine at ileocecal sphincter Specialised features increase surface area for absorption – Villi: finger-like projections of mucosa layer into the lumen – Microvilli: hair-like projections of enterocytes (absorptive cells) Image from Wikimedia Commons Small intestine: function Chemical digestion Largely occurs in the duodenum All forms of organic compounds are digested in the small intestine – carbohydrates, proteins/peptides, lipids and nucleic acids Most digestive enzymes are secreted from pancreas Brush border enzymes: on microvilli Image modified from Wikimedia Commons  Accessory organs Liver Largest gland Secretes bile – Bile salts: emulsifies fats, facilitates absorption of fat and cholesterol – Bilirubin: pigment that is a waste product from the breakdown of red blood cells in liver. Gut microbes further breaks down bilirubin to other compounds and eliminated in faeces Gallbladder Stores and concentrates bile by absorbing water and ions Pancreas Endocrine cells releases insulin and glucagon Exocrine cells secrete pancreatic juice – Water – Bicarbonate ions: neutralizes HCl from stomach – Pancreatic proteases – Pancreatic amylase, lipases, nucleases Images from Wikimedia Commons Small intestine: function Chemical digestion Enzyme Source Substrates Products Pancreatic amylase Pancreatic juice Starch Maltose, maltotriose Trypsin (activated from Pancreatic juice Proteins Peptides trypsinogen) Pancreatic lipase Pancreatic juice Triglycerides emulsified by bile Fatty acids, monoglycerides salts Ribonuclease Pancreatic juice RNA Nucleotides Deoxyribonuclease Pancreatic juice DNA Nucleotides Sucrase Brush border enzymes Sucrose Glucose, fructose Lactase Brush border enzymes Lactose Glucose, galactose Maltase Brush border enzymes Maltose Glucose Dipeptides Brush border enzymes Dipeptides Amino acids Image modified from Wikimedia Commons Small intestine Motility Peristalsis moves chyme through intestinal tract, ~16 contractions/min Segmentation the most common motion in the jejunum - Localised contractions and relaxations move chyme back and forth a few centimetres - Promotes mixing, absorption of nutrients and water Video: https://youtu.be/GdNtRom-Pvs Small intestine: function Absorption in the jejunum Products of carbohydrate and protein digestion (95%) Electrolytes (NA+, Cl-, HCO3−) (80%) Water (90%) Vitamins Absorption in ileum Water-insoluble products of lipid digestion, fat-soluble vitamins, cholesterol form micelles with bile salts - Components in micelle transferred to enterocytes (absorptive cells) - Lipids would ‘float’ in chyme in the absence of bile and be eliminated in faeces Large intestine: anatomy ~1.5 m in length 4 major regions - caecum, colon, rectum, anal canal Internal anal sphincter (involuntary) External anal sphincter (voluntary) Image from Wikimedia Commons Large intestine: function Digestion Some digestion by resident bacteria - Insoluble carbohydrates broken down via fermentation  short-chain fatty acids Absorption Absorbs remaining water and electrolytes (Na+, Cl-) Absorbs vitamins synthesised by resident bacteria (vitamin B complexes, vitamin K) Large intestine: function Motility Haustral contractions in ascending and transverse colon - Occurs every ~30min and each contraction lasts ~1min - Similar to segmentation in the small intestine but less frequent and slower - Aids with mixing and water absorption Peristalsis occurs 3-4 times/day Allows for mass movement of content towards rectum Elimination Formation of solid or semi-solid material (faeces) Faecal material stored in rectum until enough accumulates to initiate defacation reflex Expelling of faeces via anus Absorption of water Stomach is almost impermeable to water Small intestine is highly permeable to water Colon is permeable to water but less so than the small intestine Water transport can move between lumen and blood Q: Small amounts of chyme enters the small intestine from the stomach with each peristaltic contraction. Chyme entering the small intestine is also normally hypertonic. Which of the following best explains the importance of the rate of delivery of chyme from the stomach to the small intestine? A. Slow delivery of chyme into the small intestine allows more time for water absorption from the lumen of the stomach to blood. B. Rapid movement of chyme out of the stomach enhances water absorption from the lumen of the large intestine to blood C. Rapid delivery of chyme prevents rapid movement of water from intestinal lumen to blood D. Slow delivery of chyme prevents rapid movement of water from blood to the intestinal lumen Learning objectives Describe the anatomy and functions of structures within the digestive system - Gastrointestinal tract: oral cavity, pharynx, oesophagus, stomach, small intestine, large intestine - Accessory organs: teeth, salivary glands, liver, gallbladder, pancreas - Other structures: sphincters Describe the layers of the gastrointestinal tract and specialised modifications in the stomach and small intestine - mucosa, submucosa, muscularis externa, serosa/adventitia - Gastric glands, villi, microvilli Describe the function of digestive enzymes. - Salivary and pancreatic amylase; lingual, gastric and pancreatic lipase; pepsin; maltase Discuss the importance of the slow delivery of chyme from the stomach to the small intestine. Analyse the role of different digestive secretions in the processes of protection, chemical digestion, and absorption and predict the potential impacts of inadequate or excess secretion. - Gastric juice: gastrin, pepsinogen, HCl, bicarbonate ions, - Bile, pancreatic juice Understand how nutrients and water are absorbed. - Products of carbohydrate, protein and lipid digestion Describe the waste products from our digestive system, how these are made and how they are eliminated - Bilirubin - Faeces