Gross Anatomy Of The Digestive System - Lecture notes, lecture 2 PDF

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BMedSci Year 1: Module: Digestive System Lecture 2: Gross Anatomy of the Digestive System Overview       

Digestive System situated within thorax and abdominopelvic cavity Head and Neck Region comprises of oral cavity, pharynx, and beginning of oesophagus Oesophagus to posterior thoracic cavity Largest part of digestive system is the abdominopelvic cavity Abdominopelvic cavity lasts 2cm of oesophagus, the stomach, small intestine, large intestine, rectum and anal canal Small intestine consists of duodenum, jejunum and ileum Large intestine consists of caecum and colon

Food  

Food is ingested and masticated in oral cavity Food is propelled through tubular digestive system where digested, absorbed into profuse blood supply (at alimentary canal) and unwanted fragments are excreted

Oral Cavity           

Digestion starts at the oral cavity  food is ingested, lubricated, chewed and tasted Teeth and temporomandibular joint moved by mastication muscles Tongue, cheek, lips and salivary glands supplied by cranial nerves Salivary glands secrete fluid to initiate digestion, lubricate food and help bolus formation Sublingual gland lies in oral cavity Submandibular and parotid salivary glands lie outside oral cavity They both empty their secretions into the cavity via ducts A discrete bolus is formed by action of tongue against soft palate Bolus is swallowed when tongue pushes it backwards Reflex movements make sure that bolus enters oesophagus and not respiratory tract (nasopharynx or larynx) There are lymphoid aggregations in the oral cavity – for example, the tonsil and oropharyngeal isthmus are in the back of the tongue and assist in immunological defence of oral cavity and pharynx

Oropharynx       

Oropharynx lies behind oral cavity Reflex activity elevates oropharynx to receive bolus and fluid from swallowing Oropharynx then recoils as the three stacked constrictor muscles propel the food down toward the oesophagus Oesophagus conveys bolus via peristalsis down neck and thorax to stomach (by thoracic organs) Oblique angle of entry of oesophagus to cardiac region of stomach + orientation of diaphragmatic fibres form a sling around the oesophagus This creates a sphincteric mechanism which prevents gastric reflux into the oesophagus A hiatus hernia may occur is this system does not work properly

Stomach  

Stomach is distensible sac covered in peritoneum under left diaphragm Stomach relates with spleen, pancreas, duodenum, left kidney and blood vessels

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Lesser sac of peritoneum lies between stomach and its bed – allows distension and mobility of the stomach Gastric fluid contains acid and enzymes  secreted into lumen as bolus enters stomach Acid is mixed with incoming food by muscular walls of stomach  produce chime Neural controls (vagus nerve) of tone of pyloric sphincters – this opens to allow chime to enter duodenum which is a short- C-shaped pat of small intestine Second part of duodenum receives pancreatic and bile juice – this neutralises acidic chime and enables further digestions Jejunum and Ileum are suspended from posterior abdominal wall by mesentery (peritoneum) – maintains nervous, vascular and lymphatic supply of tissue

Small and Large Intestine    

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Small intestine ends as ileum enters caecum in right iliac fossa Appendix vermiformis (vestigial [degenerative] structure in humans) hangs off end of caecum Appendix is cause of pain if inflamed (appendicitis) Ascending colon (retroperitoneal) becomes transverse colon at hepatic fixture – this has mesentery and is mobile at splenic flexure – then at descending coon it is retroperitoneal again Colon is highly absorptive of water and vitamins and produces of microbial fermentation Large intestine continues as sigmoid colon – runs into pelvis becoming the rectum  anal canal Smooth muscle of anal canal = internal sphincter of anus – under ANS control External anal sphincter (skeletal muscle) under somatic control – learned in childhood so that excretion at sociably acceptable time

The Liver and Pancreas       

Pancreas secretes alkaline solution of digestive proenzymes These aren’t activated until they reach duodenum Endocrine Islets of Langerhans secrete insulin and glucagon into blood stream Liver lies under right dome of diaphragm and extends across midline – protected by ribs and costal margins Liver is responsible for bile synthesis – bile stored and concentrated in gall bladder and secreted into duodenum via common bile duct Liver receives venous drainage of GI tract (stomach to rectum) and modulaes this blood for good systemic circulation Liver functions are also detoxification, regulation of glucose concentration and protein synthesis and breakdown

Blood Supply     

3 major branches of abdominal aorta supply stomach and intestines Coeliac trunk (below diaphragm) supplies lower oesophagus, stomach, first part of duodenum, spleen, liver and pancreas Superior mesenteric artery supplies second part of duodenum to transverse colon Inferior mesenteric artery supplies transverse colon to anal canal Venous drainage passes to liver via hepatic portal vein

Enteric Nervous System

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Maintains peristaltic waves and other reflexes ANS modulates ENS PNS promotes digestion, absorption and peristalsis 2

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SNS shuts down sphincters, decreases peristalsis and diverts blood from alimentary tract to skeletal and cardiac muscle tissue

Notes from Lecture Mouth   

Oesophagus Lips to isthmus of fauces Ingestion, fragmentation and moistening for swallowing Speech, facial expression, sensory reception and respiration

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Tongue    

Very mobile – involves intrinsic and extrinsic muscles Sensitive to touch and taste Anterior 2/3 in oral cavity – controlled by CN 12 Posterior 1/3 in pharynx – where the circumvallate papillae are

Salivary Glands 

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The parotid gland has serous watery fluid, goes from the cheek to the vestibule and controlled by CN 9 The submandibular gland is watery fluid which goes to the duct to the submandibular papilla, controlled by CN 7 The sublingual gland is in the mouth controlled by CN 7

Palate   

Pharynx      

Stomach    

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12cm long muscular tube Is not involved in peristalsis Has constrictors: superior, middle and inferior Also has air coming in The epiglottis is a flap covering the trachea to close of the larynx The laryngeal goes up when swallowing

Fragments completed and digestion is initiated The cardia and pylorus are fixed and the remained is mobile-shape The pylroci sphincter controls the onwards passage The folds in the stomach are known as rugae which allow the stomach muscle to stretch The gadtric epithelium are simple, columnar epithelia which has a thick shiny mucous Gastric acid also contains bleach

Small Intestine   

Very hard (top of mouth) As you move back to the soft part, this is important in swallowing This is stratified squamous epithelia which is not keratinized

25-30cm (The teeth to the stomach is 40cm) It opens up for bolus It is behind the trachea If there is hypertrophy of the heart, this pushes back to oesophagus

This is the site of absorption The large surface area includes the plachae circulares, villi and microvilli It is made up of the duodenum, jejunum and ileum (suspended on the mesentery)

Pancreas   

The pancreas pushes out proenzymes and bicarbonates Proenzymes  duct  duodenum  neutralise chime Bile is usually green which allows lipid absorption

Large Intestine  

The appendix is attached here There is haustrations (small patches), appendices epiploicae (fat pads for energy) and taenae coli (ribbons of smooth muscle)

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