The Digestive System PDF

The digestive system 1 Functions of the digestive system The digestive system performs 6 basic processes: – Ingestion: taking in food – Secretion: water, acid, buffers and enzymes – Mixing and propulsion: mix food and secretions and move materials –...

The digestive system 1 Functions of the digestive system The digestive system performs 6 basic processes: – Ingestion: taking in food – Secretion: water, acid, buffers and enzymes – Mixing and propulsion: mix food and secretions and move materials – Digestion: break down food into nutrient molecules – Absorption: entrance of the nutrient molecules into the bloodstream – Defecation: removal of indigestible remains 2 Organs of the digestive system Two groups of organs – alimentary canal or gastrointestinal (GI) tract – accessory digestive organs GI tract – continuous muscular tube that extends from mouth to anus through the ventral body cavity – about 9 m – food pass through it and broken down – provide space for digestion and absorption Accessory organs – are related to GIT – produce saliva, bile and digestive enzymes that contribute to the breakdown of foodstuffs 3 4  Organs The organs of the GIT – Mouth – Pharynx – Esophagus – Stomach – Small and large intestine The accessory digestive organs – Teeth – Tongue – Salivary glands – Gallbladder – Liver – Pancreas 5  Histology of the GI tract Each part of the alimentary tract has a highly specialized function but the basic structure of the tube is the same throughout its length From the esophagus to the anal canal, the walls of every organ of the GIT is made up of the same four basic layers or tunics 6 Cont., From internal to external the four layers are – Mucosa – Submucosa – Muscularis Externa – Serosa Each tunic contains a predominant tissue type that plays a specific role in food breakdown 8  Mucosa Mucous membrane that lines the lumen of GI tract Functions – Secretion of mucus, digestive enzymes and hormones – Absorption of digestion end products to blood and lymph – Protective barrier Consists of three sub layers – epithelium – lamina propria – muscularis mucosae 9  Epithelium Non-keratinized stratified squamous in mouth, pharynx, esophagus and anal canal (protective) Simple columnar epithelium with mucus secreting goblet cells in stomach and intestine (secretion and absorption) Cell types – Absorptive cells – Exocrine cells: secrete mucus and fluid – Enteroendocrine cells: secrete hormones 10  Lamina propria Loose areolar connective tissue Contains many blood and lymphatic vessels Its capillaries nourish the epithelium and absorb digested nutrients Supports epithelium and binds it to muscularis mucosae Its isolated lymph nodules are part of the mucosa associated lymphatic tissue (MALT); defense against pathogens Large collections of lymph nodules occur at strategic locations; pharynx (tonsils) and appendix 11  Muscularis mucosae Is a scant layer of smooth muscle cells that produces local movements and folding of the mucosa The twitching/movement out of place of this muscle layer dislodges/to remove food particles that have adhered to the mucosa In the small intestine, it throws the mucosa into a series of small folds that immensely increase its surface area 12  Basic mucosal forms Protective: in oral cavity, pharynx, esophagus and anal canal – Epithelium is stratified squamous Secretory: in stomach – Consists of tubular glands Absorptive: small intestine – Mucosa forms villi Absorptive and protective: lines large intestine – Arranged into tubular glands with cells specialized for water absorption and mucus-secreting cells 13  Submucosa Composed of moderately dense collagenous tissue Binds mucosa to muscularis Contains many blood vessels, lymphatic vessels, lymph nodules, glands, and nerve fibers Consists of submucosal (Meissner’s) plexus – formed by nerve fibres and ganglion cells – controls glandular secretion Its rich elastic fibers enables the stomach to regain its normal shape after storing a large meal 14  Muscularis Externa Main muscle coat Generally, consists of inner circular muscle and outer longitudinal muscle layers Responsible for segmentation and peristalsis In mouth, pharynx, superior and middle esophagus, and external anal sphincter contains skeletal muscle The rest contains smooth muscle Between the two layers are myenteric (Auerbach’s) plexus It mixes and propels foodstuffs along the digestive tract In several places along the GI tract, the circular layer thickens to form sphincters – Sphincters act as valves to prevent backflow and control food passage from one organ to the next 15  Serosa Is a protective outermost layer of interaperitoneal organ In abdominal cavity it is termed as visceral peritoneum Formed of areolar connective tissue lined by simple squamous epithelium (mesothelium) In the esophagus, which is located in thoracic cavity, the serosa is replaced by an adventitia The adventitia is a fibrous connective tissue that binds the esophagus to surrounding structures Retroperitoneal organs have both a serosa (on the side facing the peritoneal cavity) and an adventitia (on the side abutting the dorsal body wall) 16 Stomach: Microscopic Anatomy The stomach wall exhibits the four tunics of most of the alimentary canal but its muscularis and mucosa are modified for the special roles of stomach – The muscularis externa has an extra oblique layer of muscle that enables it to mix and churn food – The epithelium lining the stomach mucosa is simple columnar epithelium composed entirely of goblet cells, which produce a protective coating of mucus 17 18  Stomach: mucosa Tubular glandular form Epithelium: dotted with millions of deep gastric pits, which lead to the gastric glands – Branched tubular glands empty into gastric pit – Collectively produce gastric juice – Found throughout the stomach but vary depending on site – The glands of the stomach body are substantially larger and produce the majority of the stomach secretions 19  – 4 main secretory cells: Mucous neck cells Parietal cells Chief cells Enteroendocrine cells Lamina propria: loose CT with small lymphoid aggregation Muscularis mucosae: lie beneath gastric glands  Stomach: mucosa Mucus neck cells – In upper part of gland – Produce a different type of mucus from that secreted by the mucus secreting cells of the surface epithelium – The special function of this unique mucus is not yet understood 21  Stomach: mucosa Parietal cells – In the middle section of the glands – Scattered among the chief cells – Secrete hydrochloric acid (HCl) and intrinsic factor Intrinsic factor is required for absorption of B12 in the small intestine 22  Stomach: mucosa Chief cells – Occur mainly in the basal regions of the gastric glands – Produce pepsinogen, inactive form of the protein-digesting enzyme pepsin Pepsinogen is activated by HCl – Also secrete small amounts of lipases 23 Stomach: mucosa Enteroendocrine cells – In base of glands – Release a variety of hormones directly into the lamina propria Reverse polarity – secrete hormones into the blood space rather than the GI lumen – These products diffuse into capillaries and ultimately influence several digestive system target organs; – which regulate stomach secretion and mobility 24 Mucosal Barrier Gastric juice is a highly concentrated acid Under such harsh conditions the stomach must protect itself from self digestion by a mucosal barrier – Bicarbonate rich mucus cover the stomach wall – Epithelial cells are joined by tight junctions – Glandular cells are impermeable to HCl – Surface epithelium is replaced every 3 to 6 days 25  Other layers of stomach Submucosa – relatively loose, infiltrated by lymphoid cells Muscularis – has 3 layers – comprises inner circular and outer longitudinal but inner circular is reinforced by a further innermost oblique layer in body – enables the stomach to churn and mix the food into chyme – circular muscle layer thickened at the pylorus to form pyloric sphincter Serosa – thin & covered by mesothelium 26 Gastroduodenal junction At pyloric sphincter mucosa change from glandular to villous arrangement Pyloric sphincter consists of thickened circular layer 27 Small Intestine Major digestive organ In the small intestine, usable food is finally prepared for its journey into the cells of the body Here digestion is completed and virtually all absorption occurs However, this vital function cannot be accomplished without the aid of secretions from the liver (bile) and pancreas (digestive enzymes) 28 29 30 Small Intestine: Microscopic Anatomy The four tunics of the digestive tract are modified in the small intestine by variations in mucosa and submucosa The small intestine is highly adapted for nutrient absorption Its length provides a huge surface area for absorption 31  There are three structural modifications which increase the surface area for absorption – Plicae circulares – Villi – Microvilli Structural modifications increase the intestinal surface area tremendously – the surface area of the small intestine is equal to 200 m2 Most absorption occurs in the proximal part of the small intestine, with these structural modifications decreasing toward the distal end 32 33  Circular folds or plicae circularis – deep permanent folds of the mucosa and submucosa – nearly 1 cm tall – the folds force chyme to spiral through the lumen, slowing its movement – and allowing time for full nutrient absorption 34  Small Intestine: mucosa The epithelium of the mucosa is simple columnar epithelium serving as absorptive The cells are bound by tight junctions and richly endowed with microvilli Also present are many mucus-secreting goblet cells Scattered among the epithelial cells of the wall are T cells called intraepithelial lymphocytes – provide an immunological component Scattered enteroendocrine cells are the source of secretin and cholecystokinin 35  Enterocyte  Predominant type  Tall columnar with basal nuclei  Involved in digestion and absorption  Specialised for absorption of nutrients across luminal membrane to the basal membrane,  and from there into capillaries or lacteals  Luminal surface is covered by mucus which protect against auto digestion  At apex of cells are many microvilli – Protrusion of cell membrane – Increase surface area – Constitute striated border of light microscopy  Short life-span of a few days 36 Goblet cells Scattered among enterocytes Less abundant in duodenum and increase towards ileum Produce mucus 37  Villi Finger like projections of the mucosa; about 1 mm tall Give a velvety texture to the mucosa The epithelial cells of the villi are chiefly absorptive columnar cells Central core of lamina propria contains capillary bed and a wide lymphatic capillary called lacteal Digested food is absorbed through the epithelial cells into both the capillary blood and the lacteal Small smooth muscle in villi allows change in shape and size Villi become gradually narrower and shorter along the length of the small intestine 38 39 Microvilli Tiny projections of the plasma membrane of the absorptive cells of the mucosa It gives the mucosal surface a fuzzy appearance sometimes called brush border Beside increasing the absorptive surface area, the plasma membrane of the microvilli bear brush border enzymes – these enzymes complete the final stages of digestion of carbohydrates and proteins in the small intestine 40  Crypts of Lieberkuhn – Between villi the mucosa is studded with pits that lead into tubular intestinal glands called intestinal crypts or crypts of Lieberkuhn – The epithelial cells that line these crypts secrete intestinal juice Intestinal juice is a watery mixture containing mucus that serves as a carrier fluid for absorption of nutrients from chyme – Paneth cells Specialized secretory cells located deep on the crypts Secrete lysozyme; antibacterial enzyme The number of crypts decreases along the length of the wall of the small intestine, but the number of goblet cells becomes more abundant 41 42  M (microfold) cells – Specialized epithelial cells overlying lymphoid follicles of Peyer’s patches – Characterized by numerous membrane invaginations – Endocytose antigen and transport to lymphoid cells – Basement membrane under M cells is discontinuous to facilitate transit 43 Stem cells – The various epithelial cells arise from rapidly dividing stem cells at the base of the crypts – Stem cells divide in intestinal crypts and migrate up to the villi to replace damaged and dying cells – the ‘epithelial escalatory’ – The daughter cells gradually migrate up the villi where they are shed from the villus tips – In this way the villus of the epithelium is renewed every three to six days 44 Small Intestine: submucosa Typical areolar connective tissue Contains both individual and aggregated lymphoid follicles (Peyer’s patches) Peyer’s patches – increase in abundance toward the end of the small intestine, – reflecting the fact that the large intestine contains huge numbers of bacteria that must be prevented from entering the bloodstream 45 Small Intestine: submucosa A set of elaborated mucus-secreting duodenal glands (Brunner’s glands) is found in the submucosa of the duodenum only – produce an alkaline (bicarbonate-rich) mucus that helps neutralize the acidic chyme moving in from the stomach – When this protective mucus barrier is inadequate, the intestinal wall is eroded and duodenal ulcers results 46 Small Intestine: muscularis & serosa The muscularis is typical and bilayered The external intestinal surface is covered by visceral peritoneum (serosa) except for the bulk of the duodenum, which is retroperitoneal and has an adventitia 47 Liver and Gallbladder Liver and gallbladder are accessory organs associated with the small intestine Functions of liver – Detoxification – Destruction of spent RBCs – Synthesis of bile – Synthesis of plasma proteins – Metabolic activities The gallbladder is a storage site for bile 48 Liver: Microscopic Anatomy Stroma External surface is invested by thin collagenous capsule=Glisson’s capsule Thick at hilum, surround vessels and ducts to interior Fine meshwork of reticular fibers radiate from this connective tissue, support liver cells 49 Liver: Microscopic Anatomy Liver lobule Liver parenchymal cells (hepatocytes) are arranged into structural & functional units called liver lobules, around one million 50  Each lobule is roughly hexagonal in shape Bounded by thin septa of collagenous tissue At angles of lobule are portal tracts; Terminal braches of portal vein and hepatic artery and bile duct Center of lobule is a centrolobular venule (central vein)  Liver: Microscopic Anatomy Liver parenchyma – Hepatocytes or liver cells are organized to radiate out from a central vein running the length of the longitudinal axis of the lobule – Hepatocytes form flat anastomosing plates – Plates are directed from periphery of lobules to its center 52 – Spaces between plates contain capillaries=liver sinusoids Sinusoids are lined by discontinuous layers of cells which do not rest on basement membrane Endothelial cells are separated from hepatocytes by a narrow space = space of Disse, which drain to lymphatics  Liver Lobule At each of the six corners of a lobule is a portal triad so named because three basic structures are always present there: A branch of – hepatic artery – portal vein – bile duct 54 Liver Lobule Blood comes from the hepatic artery (20%) and portal vein (80%) – The hepatic artery supplies oxygen rich arterial blood to the liver – The hepatic vein carries blood laden with nutrients from the digestive viscera A bile duct carry secreted bile toward the common bile duct and ultimately to the duodenum 55 Liver Lobule Hepatocytes form plates of one-cell thick, divided by sinusoidal blood channels The portal triad supplies the nutrient and oxygen-rich blood for processing and is the route for the drainage of bile 56  Sinusoids drain into the thin-walled central vein Central vein  sublobular veins  collecting veins  hepatic veins  circulation Bile travels in the opposite direction to sinusoidal blood between hepatocyte layers Composition of blood entering the lobule modified by hepatocytes and macrophages  Liver Sinusoid Between the hepatocyte plates are enlarged, very leaky capillaries, the liver sinusoids Blood from both hepatic portal vein and hepatic artery percolates from the triad regions through these sinusoids and empties into the central vein From the central vein blood eventually enters the hepatic veins, which drain the liver, and empty into the inferior vena cava Inside the sinusoids are star shaped hepatic macrophages, also called Kupffer cells, which remove debris such as bacteria and worn-out blood cells 58 Hepatocyte ~ 80% of the mass of the liver Metabolic factories – form and secrete bile – store glycogen and buffer blood glucose – synthesize urea – metabolize cholesterol and fat – synthesize plasma proteins – detoxify drugs and poisons – process several steroid hormones and vitamin D 59 Hepatocyte Extensive rough endoplasmic reticulum – Protein synthesis Smooth ER – For hormone processing and detoxification Golgi body and lysosomes – for the formation of bile Mitochondria – For oxidation Microvilli into the Space of Disse Large nucleus 60  Hepatocytes are exposed on each side to sinusoids, which are lined by a discontinuous layer of cells via gaps in the sinusoid lining, the space of disse is continuous with the sinusoid lumen thus bathing hepatocyte surface with blood Numerous microvilli extend from hepatocytes into the space of Disse, increasing surface area for metabolic exchange 61  Hepatic vasculature Liver receives blood from 2 sources – Portal vein (80%) - carries oxygen poor nutrient rich blood from abdominal viscera – Hepatic artery (20%) - supplies oxygen rich blood Portal vein system – Portal vein branches and send portal venule to portal triads – Portal venules branch into distributing veins that run around periphery of lobule 62  – From distributing veins small inlet venules empty into sinusoids – Sinusoids converge in center of lobule to form central vein – Central vein leaves lobule and merge with sub lobular veins – Sub lobular veins converge to form hepatic veins Arterial system – Hepatic artery branch to form interlobular arteries – Interlobular arteries form inlet arterioles that ends in sinusoids NB: arterial and venous blood mixes in sinusoids 63  Biliary system – Liver cells secrete bile into bile canaliculi situated between plasma membranes of adjacent hepatocytes – The canalicular system drains to bile collecting ducts – Collecting ducts merge and form trabecular ducts which emerge from liver as right and left hepatic ducts – The two hepatic ducts join to form common hepatic duct which join cystic duct to form common bile duct  Liver regeneration Liver cells have extraordinary capacity for regeneration Loss of hepatic tissue triggers cell division and restore original mass Regenerated tissue is similar to the removed but if there is repeated damage, regeneration And production of CT occurs simultaneously which results in disorganization of liver structure=cirrhosis 65  Gallbladder: microscopic anatomy Mucosa – In non distended state is thrown into many folds – Lined by simple columnar epithelium – In neck region, epithelium invaginate and form mucous glands Submucosa – Loose, rich in elastic fibers, blood and lymph vessels Muscular layer – Thin, fibers are disposed obliquely Serosa/adventitia – Binds superior surface to liver (adventitia) – Opposite surface is lined by serosa (peritoneum) Cystic duct – Wall formed into twisted mucosa covered folds=spiral valve 66 of Heister  Bile flow 800 ml of bile is secreted by the hepatocytes into the bile canaliculi each day The canaliculi flow into the bile ductules in the portal triad The bile ductules join to flow into the right and left hepatic ducts These join to form the common hepatic duct The cystic duct from the gall bladder joins the common hepatic duct outside the liver to form the common bile duct Bile flows through the ampulla of Vater when the sphincter of Oddi relaxes and flows into the duodenum 67  Pancreas Mixed gland, both exocrine and endocrine Exocrine portion – forms the bulk of the gland – secretes enzyme rich fluid - pancreatic juice break down all categories of foodstuffs Endocrine tissue – forms islets of Langerhans – scattered throughout exocrine tissue – secrete hormone 68 Exocrine pancreas Pancreatic acini Made up of irregular clusters of pyramidal secretory cells, the apices of which surround a central lumen Cells are typical protein secreting cells Acini are surrounded by basement membrane supported by reticular fibers Between acini are CT with capillaries 69  Endocrine pancreas Islets of Langerhans – Isolated clumps of endocrine cells scattered in exocrine tissue – Vary in size and numerous in tail – Composed of clumps of secretory cells + fine collagenous network + fenestrated capillaries – Delicate capsule surround islets – Cells 2 main types – Glucagon secreting cells – Insulin secreting cells 70  Duct system – Lumen of acinus drain into intercalated ducts – Intercalated ducts drain into intralobular ducts – Intralobular ducts drain into interlobular ducts – Pancreatic juice drains from the pancreas via the centrally located main pancreatic duct – The pancreatic duct generally fuses with the bile duct just as it enters the duodenum – A smaller accessory pancreatic duct empties directly into the duodenum 71  Large Intestine: Microscopic anatomy Ileo-caecal junction – Abrupt transition in lining of ileo-caecal valve from villiform pattern in small intestine to glandular form in large intestine Mucosa Cells types – Absorptive cells – Mucus secreting goblet cells 72  Mucosa arranged in closely packed tubular glands Folded in non distended state but no plicae circularis Above anal valve mucosa forms longitudinal folds= anal columns (column of Morgagni) – The anal sinuses are recesses between the anal columns which exude mucus when compressed by feces – This aids in the emptying of the canal  Glands – Extends to muscularis mucosae – Separated by thin lamina propria – Muscularis mucosae extend into lamina propria, contraction facilitate mucus expulsion – Goblet cells dominate in base, luminal surface lined by columnar absorptive cells Lamina propria – Contain numerous blood and lymphatic vessels and lymphoid aggregations that extend to submucosa – Contains plexus of veins which dilate and varicose producing hemorrhoids 74  Muscularis mucosae – Prominent, contraction prevent clogging of glands and enhance expulsion of mucus Muscularis – Thick – Inner circular and outer longitudinal – Longitudinal layer forms 3 separate bands=teniae coli Serosa – In intraperitoneal portion characterized by small pendulous protuberances filled with adipose tissue=appendices epiploicae 75  Recto-anal junction – Rectal mucosa at this junction undergoes an abrupt transition to stratified squamous epithelium in anal canal reflect greater abrasions – Muscularis layers are larger for its expulsive role – At anal sphincter the stratified squamous epithelium undergoes a gradual transition to skin 76 Large Intestine: Microscopic anatomy The wall of the large intestine differs in several ways from that of the small intestine – The colon mucosa is simple columnar epithelium except in the anal canal – Because most food is absorbed before reaching the large intestine, – there are no circular folds, no villi, and no cells that secrete digestive enzymes – Thicker mucosa, deeper crypts, very high numbers of goblet cells Lubricating mucus produced by goblet cells eases the passage of feces and protects the intestinal wall from irritating acids and gases released by resident bacteria in the colon 77 Large Intestine: Microscopic anatomy In contrast to the more proximal regions of the large intestine, teniae coli and haustra are absent in the rectum and anal canal Consistent with its need to generate strong contractions to perform its expulsive role, the rectum’s muscularis muscle layers are complete and well developed 78  The enteric nervous system The gut’s brain with its own distinct habits and rhythms It influences motor, endocrine and secretory function of the gut as well as blood vessel tone It receives impulses from the extrinsic autonomic nervous system and gives information back to the autonomic nervous system on the degree of stretch and the contents of the intestine 79  The intrinsic enteric nervous system is connected to chemoreceptors, osmoreceptors and mechanical receptors in the mucosa – The stomach “knows” when you have had a fatty meal and if you have had a fatty meal it delays emptying the meal – You know when you feel full after a meal. – You know when you are constipated The enteric nervous system has been called the little brain The intrinsic enteric nervous system is found in two layers 1. Submucosal plexus immediately below the inner circular muscle layer 2. Outer myenteric plexus between the two muscle layers 81  Summary Parasympathetic (generally stimulates) via the vagus (esophagus to proximal colon) or via pelvic nerves (distal large intestine) Sympathetic (generally inhibits) T8-L2. Myenteric Submucosal plexus plexus Enteric Nervous System Smooth Endocrine Secretory Blood muscle cells cells vessels 82