Gastrointestinal System 1 Lecture Notes PDF

LECTURE 6 Gastrointestinal System 1 Reference: Amerman; Chapters 22 and 23 Dr. Nancy Haydar School of Science [email protected] 1 2  Objectives Describe the layers, movements and functions of...

LECTURE 6 Gastrointestinal System 1 Reference: Amerman; Chapters 22 and 23 Dr. Nancy Haydar School of Science [email protected] 1 2  Objectives Describe the layers, movements and functions of the gastrointestinal tract Describe the functional anatomy of the: – Mouth – Oesophagus – Stomach – Small intestine  Gastrointestinal Tract The continuous muscular digestive tube that winds through the body digesting and absorbing foodstuffs. Organs include: – Mouth, pharynx, oesophagus, stomach, small intestine (duodenum, jejunum and ileum), large intestine (ascending, transverse, descending and sigmoid colon), rectum and anus Accessory digestive organs: – Teeth, tongue, salivary glands, liver, gallbladder and pancreas  Functions Ingestion Mastication (mouth) – Mechanical Food and chemical breakdown Mechanical breakdown Pharynx Ingestion of food – Swallowing and Chewing (mouth) Esophagus Churning (stomach) Propulsion propulsion Segmentation Swallowing (small intestine) (oropharynx) Digestion – Mechanical and Peristalsis Digestion (esophagus, chemical breakdown of food stomach, particles into pieces that can pass small intestine, large intestine) through cell membranes Stomach Absorption Absorption – Transfer of nutrients Lymph from the gut to the blood circulation vessel Excretion – Expulsion of Small undigested material intestine Large Blood intestine vessel Mainly H2O Faeces Anus Defaecation © 2013 Pearson Education, Inc.  GIT (Alimentary Canal). Mouth (oral cavity) Parotid gland Tongue* Sublingual gland Salivary Submandibular gland glands* Esophagus Pharynx Stomach Pancreas* (Spleen) Liver* Gallbladder* Transverse colon Duodenum Descending colon Small Jejunum intestine Ileum Ascending colon Large Caecum Sigmoid colon intestine Rectum Appendix Anus Anal canal  Layers of the GIT 1. Mucosa lines the lumen; composed of: – Epithelium (different types in different locations → mostly simple columnar epithelium with mucus-secreting cells). – Lamina propria (loose areolar connective tissue with capillaries for nourishment of the mucosa and absorption) – Muscularis mucosae (smooth muscle that produces local movements of mucosa) 2. Submucosa: Dense connective tissue; blood/lymph vessels; glands and the submucosal plexus (neural network) – controls gland secretion 3. Smooth muscle/Muscularis Externa responsible for segmentation and peristalsis; also contains myenteric plexus (neural network) – controls motility 4. Serosa: Visceral peritoneum Layers of the GIT (Peritoneum) Visceral peritoneum Layers of the GIT  Movement of the GIT Peristalsis Segmentation Adjacent segments of GI tract organs Nonadjacent segments of GI tract alternately contract and relax, which organs alternately contract and relax, moves food along the tract distally. moving the food forward then backward. Food mixing occurs.  Peritoneum Serous epithelial membrane that covers the internal walls of the abdominal cavity and the outer surface of the abdominal organs. Two layers: - Parietal peritoneum – Covers internal abdominal walls - Visceral peritoneum – Covers outer surface of abdominal organs Peritoneal cavity – Space between parietal and visceral peritoneum Peritoneal folds - Omentum (suspends stomach) - Mesentery (suspends SI) - Mesocolon (suspends LI) © 2013 Pearson Education, Inc.  Greater Omentum Two layers of visceral peritoneum - descends from stomach Pass in front of small intestines, it then turns upwards and backwards and ascends to transverse colon. The two layers separate and enclose the transverse colon. Contains adipose tissue, which in obese people accumulates in considerable quantity. Like an apron which covers the intestines. © 2013 Pearson Education, Inc. Greater Omentum © 2013 Pearson Education, Inc.  Mesentary and Mesocolon Two layers of visceral peritoneum against each other Attach small and large intestines to the posterior abdominal wall Allow freedom of movement in the abdominal cavity. Hold viscera in proper relationship to each other. Prevent the intestines from becoming twisted and tangled by changes in body position and by its own contractions. Provides passage of blood vessels and nerves that supply digestive tract. Contain lymph nodes and vessels. © 2013 Pearson Education, Inc. Mesentary and Mesocolon  Intraperitoneal and Retroperitoneal Organs Retroperitoneal – Behind the parietal peritoneum Intraperitoneal – Almost entirely covered in visceral peritoneum. Infraperitoneal – Below the parietal peritoneum  Divisions of the GIT Embryologically: – Foregut Celiac Trunk* Mouth → First half of duodenum – Midgut Superior Mesenteric Artery Second half of duodenum → proximal half of large intestines – Hindgut Distal half of large intestine → rectum Inferior Mesenteric Artery *supplies foregut abdominal organs only Proper hepatic artery IVC Short Break  Mouth Mouth – oral cavity Main parts: – Cheeks – Hard and soft palate (form roof) – Uvula – Tongue – Teeth Oral cavity is lined with stratified squamous epithelium which is kept moist by saliva. Mouth  Salivary Glands Saliva - Produced by salivary glands (parotid, submandibular and sublingual); contain enzyme salivary amylase (starts starch breakdown) Salivation is triggered by taste, smell, sight or thought of food. Salivation is stimulated by the parasympathetic nervous system via CN VII - facial nerve and CN IX -glossopharyngeal nerve.  Teeth Primarily composed of dentin (similar to bone but composed of calcified connective tissue). Three parts: Crown, neck and root Dentin of the crown covered by hard enamel; dentin of the root covered by softer cementum. Inside the tooth is pulp cavity and pulp (vessels and nerves) – cavity extends into root canals. Alveolar processes of upper and lower jaws are covered by gingiva (gums) Teeth are fixed to bony sockets of the alveolar processes of the jaws with many periodontal ligaments.  Teeth 20 appear ~6-24 months  Incisors (8 total) - Chisel-shaped for biting and cutting  Canines (4 total) - Fang-like for tearing  Premolars (8 total) - Broad for crushing and grinding  Molars (12 total) - Broad for crushing and grinding 32 appear ~6-12 years 3rd molars (wisdom) appear ~17-25 years Teeth  Tongue Muscular organ made of intrinsic muscles that change shape of tongue; numerous extrinsic muscles alter tongue’s position Participates in food mixing during chewing, bolus formation and initiation of swallowing; also Epiglottis essential for speech and taste Palatopharyngeal arch Sublingual glands produce enzyme Palatine tonsil lingual lipase Lingual tonsil Tongue papillae: Palatoglossal – Filiform: Whitish, give the tongue arch roughness and provide friction. Terminal sulcus – Fungiform: Reddish, scattered Foliate papillae over the tongue. Taste buds. Vallate papilla – Circumvallate (vallate): V-shaped Medial sulcus row in back of tongue. of the tongue Tongue also contains lingual tonsil Dorsum of tongue (immune tissue). Fungiform papilla Filiform papilla  Digestion in the Mouth Mechanical digestion (mastication or chewing) – Breaks food into smaller pieces. – Mixes pieces of food with saliva. Chemical digestion refers to breakdown of food through enzyme activity (or acid → stomach). – The process is called hydrolysis – breakdown of complex molecules into simpler ones with the addition of a molecule of water. – The main enzyme in the mouth is amylase – begins starch digestion into simpler sugars. This digestion stops in the stomach due to the high acidity (low pH) of the stomach which neutralises amylase.  Oesophagus Muscular tube that connects pharynx and stomach Located in the mediastinum – Anterior to the vertebral column – Posterior to the trachea – Posterior to the heart Passes through diaphragm at an opening called the oesophageal hiatus Function: Transports food into the stomach during swallowing. – Swallowing is aided by the presence of saliva – Peristalsis moves the bolus → stomach. Upper and lower oesophageal muscular sphincters control passage of food; normally closed, open during swallowing.  Oesophagus Four layers: – Mucosa – Submucosa – Muscle layer – Outer connective tissue covering (adventitia) → blends with surrounding connective tissue.  Stomach Parts: Cardia, fundus, body, pyloric antrum, pyloric canal and pylorus (opening into duodenum with circular pyloric sphincter muscle) Borders: Lesser curvature (medial) and greater curvature (lateral) Location: Located under diaphragm in the left upper abdominal area  Stomach Stomach wall layers: − Mucosa - Simple columnar epithelium; has large folds known as rugae that make the surface rough (to increase the mixing & mechanical breakdown effects) − Submucosa- Containing submucosal plexus − Smooth muscle - Several layers, containing myenteric plexus − Serosa – Smooth visceral peritoneum Gastric Mucosa  Gastric Mucosa Composed of simple columnar epithelium that folds inside the wall containing numerous gastric glands opening on mucosal surface via gastric pits. Surface Mucous and Mucous Neck Cells produce mucus and are the most numerous in the gastric mucosa Parietal cells produce hydrochloric acid (HCl) and intrinsic factor (protein required for absorption of vitamin B12) Chief cells produce digestive enzymes; pepsinogen and gastric lipase G cells are endocrine – produce hormone gastrin → stimulates release of gastric juice and ↑ gastric motility  Digestion in the Stomach Mechanical digestion – Achieved through peristaltic movements (mixing waves) – Food turned into thick soupy liquid called chyme Chemical digestion – Hydrochloric acid from parietal cells: Kills microbes; denatures (unfolds) proteins from food making enzymatic proteolysis easier; transforms pepsinogen (inactive enzyme) into pepsin, inactivates salivary amylase. – Pepsin (proteolytic enzyme): Breaks down protein chains into smaller peptide fragments Most effective in the very acidic environment (pH=2)  Regulation of Gastric Secretion and Motility Cephalic phase – Starts with sight, smell, taste, thought of food – Parasympathetic activation leads to stimulation of gastric secretion and motility (via cranial nerve X – vagus) Gastric phase – Activated via stretch receptors and chemoreceptors (monitor pH) in the stomach after the food comes in – Outcomes: increased peristalsis (mixing), release of gastrin → increased gastric secretions Intestinal phase – Enterogastric reflex and hormones (enterogastrones - secretin, cholecystokinin and vasoactive intestinal peptide) decrease stomach secretions, motility and emptying.  Gastric Motility and Empyting PYLORIC VALVE PYLORIC VALVE PYLORIC VALVE SLIGHTLY OPENED CLOSED SLIGHTLY OPENED  Regulation of Gastric Emptying As chyme enters duodenum → intestinal phase – Receptors respond to stretch and chemical signals. – Enterogastric reflex occurs (inhibits gastric motility) and enterogastrones (inhibits gastric acid production). Carbohydrate-rich chyme moves quickly through duodenum. Fatty chyme remains in duodenum 6 hours or more. © 2013 Pearson Education, Inc.  Small Intestine ~6m long; From pyloric sphincter to ileocaecal valve Major organ of digestion and absorption Subdivisions: – Duodenum Retroperitoneal – Jejunum Attached posteriorly by mesentery – Ileum attached posteriorly by mesentery Motility: Mostly segmentation with intermittent peristalsis  Duodenum (1st part of SI) Shortest part of the small intestine ~25 cm Curves around head of pancreas Receives the bile duct (from gall bladder and liver) and main pancreatic duct (from pancreas). Brunner's glands secrete alkaline mucus to neutralize acidic chyme © 2013 Pearson Education, Inc.  Jejunum and Ileum (2nd and 3rd part of SI) Jejunum – Extends from duodenum to ileum – About 2.5 m long Ileum – Extends from jejunum to caecum (1st part of LI) – Joins large intestine at ileocaecal valve – About 3.6 m long  Vessels and Nerves of SI Superior mesenteric artery (branch of the abdominal aorta) brings oxygen-rich, nutrient-poor blood supply Veins (carrying nutrient-rich, oxygen-poor blood) drain into superior mesenteric veins → hepatic portal vein → liver Vagus nerve (parasympathetic/rest and digest) and sympathetics (fight or flight) from thoracic splanchnic nerves serve small intestine  Histology of SI Lined with simple columnar epithelium specialised for absorption (called Enterocytes) Contains numerous mucus-producing cells (Goblet cells) Contains Peyer's patches which protect against bacteria Structures that increase surface area: – Circular folds of mucosa (plicae circulares) Permanent folds (~1 cm deep) that force chyme to slowly spiral through lumen → nutrient absorption – Villi Extensions (~1 mm high) of mucosa with capillary bed and lymphatic vessels (lacteals) for absorption – Microvilli Microscopic extensions of cell membrane on the upper surface Microvilli form so-called brush border which contain some digestive enzymes (for digesting carbohydrates and proteins) Histology of Small Intestine  Intestinal Crypts Intestinal crypt epithelium renewed every 2-4 days Contain secretory cells that produce intestinal juice – 1- 2L secreted daily in response to distension or irritation of mucosa – Slightly alkaline; isotonic with blood plasma – Largely water; enzyme-poor (enzymes of SI only in brush border); contains mucus – Facilitates transport and absorption of nutrients Enteroendocrine cells → enterogastrones (inhibits gastric motility/secretions) Intraepithelial lymphocytes (IELs) – Release cytokines that kill infected cells Paneth cells – Secrete antimicrobial agents (defensins and lysozyme) Stem cells divide to produce crypt cells  Digestion in Small Intestines Mechanical digestion – Bile (from liver/gall bladder), peristalsis and segmentation Chemical digestion – Enzymes from pancreas and small intestine brush border enzymes  Next Week (Week 7) MONDAY/TUESDAY/WEDNESDAY/THURSDAY Practical 5: Gastrointestinal System 1 Topics covered: – Abdominopelvic Cavity – Organs of the Gastrointestinal Tract (up to the ileum) – Layers of the GIT – Peritoneum and Peritoneal Cavity – Blood Supply and Drainage of the Abdominal Gastrointestinal Organs Complete before practical: – Read or listen to Gastrointestinal lecture 1 (week 6) – Work through your practical notes using the pre-practical activities and answer the short answer questions – Reminder: Print your practical notes and come with lots of questions! MONDAY https://www.youtube.com/watch?v=-1aZj6v6dxc Lecture 7: Gastrointestinal 2 Pre-reading: Amerman Chapter 22 and 23 50