Digestive System.pptx by maam Ayesha shamim

Biochemistry

1. Digestive System Kamran

2. OVERVIEW OF DIGESTIVE SYSTEM The Digestive System Consists of ; Long hollow muscular tube or canal or tract called gastrointestinal tract (GIT) : it is about 9 meters (30 ft ) long Accessory glands : include: Salivary glands Liver and gall bladder Pancreas

3. GIT consists of; Oral cavity or mouth Pharynx Esophagus Stomach Small intestine Large intestine Rectum Anus

4. Sphincters of GIT The intestinal tract is functionally divided into segments, by means of muscle rings known as sphincters , that restrict the flow of intestinal contents to optimize digestion and absorption These sphincters include the upper and lower esophageal sphincters, the pylorus that retards emptying of the stomach, the ileocecal valve that retains colonic contents (including large numbers of bacteria) in the large intestine, and the inner and outer anal sphincters

5. Function of GIT The alimentary tract provides the body with a continual supply of water, electrolytes, vitamins, and nutrients, which requires the following: (1) movement of food through the alimentary tract; (2) secretion of digestive juices and digestion of the food; (3) absorption of water, various electrolytes, vitamins, and digestive products; (4) circulation of blood through the gastrointestinal organs to carry away the absorbed substances; and (5) control of all these functions by local, nervous, and hormonal systems.

6. Structure of GIT Wall

7. The GIT wall is composed of four layers, each having different tissues and functions From the inside out , they are called: mucosa, submucosa , muscularis and serosa MUCOSA: The mucosa is made up of three layers: Lining the lumen of GI tract is a single layer of epithelial cells representing the barrier that nutrients must traverse to enter the body; this layer is responsible for most digestive, absorptive and secretory process Below the epithelium is a layer of loose connective tissue known as the lamina propria The lamina propria in turn is surrounded by a loose layer of muscle cells called muscularis mucosa

8. General Anatomy of GI Wall

9. The mucosae (singular: mucosa) are highly specialized in each part of the GIT in order to deal with different digestive tract conditions The most variation is seen in the epithelial tissue layer of the mucosa Esophagus: The mucosa of the esophagus consists of nonkeratinized stratified squamous epithelium Near the stomach, the mucosa of the esophagus also contains mucous glands Stomach: The stomach wall is lined internally with mucosa formed from simple columnar epithelium

10. The mucosa is turned into large folds, called rugae when stomach is empty and depressions called gastric pits The epithelia at different points in pits is thrown deep into mucosa to form gastric glands which deal with gastric secretions Small Intestine: Throughout the small intestine, the epithelium (particularly the ileum) is folded up into fingerlike projections called villi (0.5–1 mm) to maximizes the surface area available for nutrient absorption The villi contains blood vessels & lacteals (lymph vessel)

11. In intestine, between the villi are infoldings known as crypts The epithelial layer of the small intestinal mucosa consists of simple columnar epithelium that contains many types of cells Absorptive cells of the epithelium contain enzymes that digest food and possess microvilli which are projections of the apical (free) membrane of the absorptive cells; means greater surface area for absorption Also present in the epithelium are goblet cells , which secrete mucus

12. VILLI & CRYPT

13. Cells lining the crypts form the intestinal glands , or crypts of Lieberkühn and secrete intestinal juice The intestinal glands also contain paneth cells and enteroendocrine cells Paneth cells secrete lysozyme, a bactericidal enzyme, and are capable of phagocytosis; may have a role in regulating the microbial population in the small intestine Three types of enteroendocrine cells are found in the intestinal glands of the small intestine: S cells , CCK cells , and K cells , which secrete the hormones secretin, cholecystokinin (CCK) , and glucose-dependent insulinotropic peptide (GIP) respectively

14. Stem cells that give rise to both crypt and villus epithelial cells reside toward the base of the crypts and are responsible for completely renewing the epithelium every few days or so Daughter cells formed in the crypts from stem cells then migrate out onto the villi, where they are eventually shed and lost in the stool The lamina propria of the small intestinal mucosa contains areolar connective tissue and mucosa-associated lymphoid tissue (MALT) The microvilli in the apical cells are endowed with a dense glycocalyx (the brush border) that probably protects the cells to some extent from the effects of digestive enzymes

15. Some digestive enzymes are actually part of the brush border, being membrane-bound proteins; these so-called “brush border hydrolases” perform the final steps of digestion for specific nutrients. Large Intestine: Since major function of large intestine is to absorb water; the epithelium contains mostly absorptive and goblet cells similar to small intestine

16. Intestinal Structure

17. SUBMUCOSA: The submucosa lies under the mucosa and consists of dense irregular layer of connective tissue with large blood vessels, lymphatics and nerves It is relatively thick and supports mucosa It holds the glands and a nerve plexuses called Meissner’s plexus The absorbed elements that pass through the mucosa are picked up by the blood vessels of submucosa

18. MUSCULARIS: Around the sub-mucosa is the muscularis externa or muscularis propria consisting of two rings or layers of smooth muscles; an inner one oriented circumferentially and outer one longitudinally to the axis of the gut (the circular and longitudinal muscle layers, respectively) Between the two muscle layers is the layers of nerve plexus called myenteric or Auerbach’s plexus Esophagus: The muscularis of the superior third of the esophagus is skeletal muscle, the intermediate third is skeletal and smooth muscle, and the inferior third is smooth muscle

19. At each end of the esophagus, the muscularis becomes slightly more prominent and forms two sphincters—the upper esophageal sphincter (UES) , which consists of skeletal muscle, and the lower esophageal (cardiac) sphincter (LES) , which consists of smooth muscle and is near the heart The UES regulates the movement of food from the pharynx into the esophagus; the LES regulates the movement of food from the esophagus into the stomach; both prevent backflow of food Stomach: The stomach has a third layer, the inner oblique layer which helps churn the chyme in the stomach

20. Large Intestine: To match the needs of large intestine, in the layer of muscularis, portions of the longitudinal muscles are thickened, forming three conspicuous bands called the teniae coli ( teniae = flat bands) that run most of the length of the large intestine The teniae coli are separated by portions of the wall with less or no longitudinal muscle Tonic contractions of the bands gather the colon into a series of pouches called haustra (HAWS- tra = shaped like pouches; singular is haustrum), which give the colon a puckered appearance A single layer of circular smooth muscle lies between teniae coli In the colon , the muscularis externa is much thicker because the feces are large and heavy, requiring more force to push along

21. SEROSA: The outermost layer covering the wall of GIT is serosa which separates GI tract from other organs in the abdominal cavity It is composed of a secretory epithelial layer and an underneath connective tissue layer The epithelial layer known as mesothelium, consists of avascular flat nucleated cells (simple squamous epithelium) that produce the lubricating serous fluid The serous fluid has a consistency similar to thin mucus and provides lubrication to reduce friction The superficial layer of the esophagus is known as the adventitia which attaches the esophagus to surrounding structures

22. THE MUSCULATURE OF THE DIGESTIVE TRACT The smooth muscles of the digestive tract are generally organized in distinct layers. Two important muscle layers for motility in the lower esophagus and small and large intestine are the longitudinal and circular layers The two layers form the intestinal muscularis externa The stomach has an additional obliquely oriented muscle layer

23. GI Smooth Muscle Functions as a Syncytium The individual smooth muscle fibers in the GI tract are 200 to 500 μ m in length and 2 to 10 μ m in diameter, and they are arranged in bundles of as many as 1000 parallel fibers In the longitudinal muscle layer, the bundles extend longitudinally down the intestinal tract; in the circular muscle layer, they extend around the gut Within each bundle, the muscle fibers are electrically connected with one another through large numbers of gap junctions that allow low-resistance movement of ions from one muscle cell to the next and help to travel electrical signals more readily in the longitudinal direction than sideways

24. Each bundle of smooth muscle fibers is partly separated from the next by loose connective tissue However, the muscle bundles fuse with one another at many points, thus, representing a branching latticework of smooth muscle bundles This makes the muscle layer a syncytium, i.e., when an action potential is elicited anywhere within the muscle mass, it generally travels in all directions in the muscle The distance that it travels depends on the excitability of the muscle; sometimes it stops after only a few mm, and at other times it travels many cm or even the entire length and breadth of the intestinal tract A few connections also exist between the longitudinal and circular muscle layers, excitation of one of these layers often excites the other as well

25. GI Smooth Muscle: Circular Muscle and Longitudinal Muscle Longitudinal Muscle Thin Muscle Coat Contraction shortens intestine length & expands radius Innervated & activated by excitatory motor neurons Few gap junctions to adjacent fibers Extracellular Ca 2+ influx important in excitation-contraction coupling Circular Muscle Thick Muscle Coat Contraction increases intestine length & decreases radius Innervated by excitatory & inhibitory motor neurons Activated by myogenic pacemaker & excitatory motor neurons Many gap junctions to adjacent fibers Intracellular Ca 2+ release important in excitation-contraction coupling

26. THE ENTERIC NERVOUS SYSTEM (ENS) The ENS lies entirely in the wall of the gut, beginning in the esophagus and extending all the way to the anus as two layers of nerve plexus This highly developed ENS is especially important in controlling GI movements and secretion These two nerve plexus are intrinsic to GIT: the myenteric plexus ( Auerbach‘s plexus), between the outer longitudinal and middle circular muscle layers, and the submucous plexus (Meissner‘s plexus), between the middle circular layer and the mucosa Collectively, these neurons constitute the enteric nervous system

27. The system contains about 100 million sensory neurons, inter-neurons, and motor neurons in humans more than the entire spinal cord; so referred as “little brain” The myenteric plexus innervates the longitudinal and circular smooth muscle layers and is concerned primarily with motor control, The submucous plexus innervates the glandular epithelium, intestinal endocrine cells, and submucosal blood vessels and is primarily involved in the control of intestinal secretion.

28. EXTRINSIC INNERVATION The ENS in intestine receives a dual extrinsic innervation from the ANS, with parasympathetic cholinergic activity generally increasing the activity of intestinal smooth muscle; and sympathetic noradrenergic activity generally decreasing it while causing sphincters to contract The preganglionic parasympathetic fibers consist of about 2000 vagal efferents and other efferents in the sacral nerves and generally end on cholinergic nerve cells of the myenteric and submucous plexuses

29. The sympathetic fibers are postganglionic, but many of them end on postganglionic cholinergic neurons, where they inhibit Ach secretion by activating α2 presynaptic receptors via NE release Other sympathetic fibers appear to end directly on intestinal smooth muscle cells Still other fibers innervate blood vessels, where they produce vasoconstriction Besides, the intestinal blood vessels seems to have intrinsic innervation too by fibers of the ENS VIP and NO are among the mediators in the intrinsic innervation

30. Integration of sympathetic, parasympathetic and enteric nervous system

31. Oral/or Buccal Cavity The digestive tract begins with the oral cavity The oral cavity includes the mouth, teeth, tongue, and salivary glands The buccal cavity is composed of two separate regions; the vestibule-the area between the cheeks, teeth and lips, and the oral cavity proper which is mostly filled with the tongue It is lined by oral mucosa consisting of non-keratinized stratified squamous epithelium The oral cavity proper is formed by the cheeks, hard and soft palates, and tongue The cheeks form the lateral walls of the oral cavity

32. The palate is a wall or septum that separates the oral cavity from the nasal cavity, and forms the roof of the mouth The anterior portion of the palate called hard palate is made up of the maxillae and palatine bones The posterior portion of the palate is called soft palate; it is an arch-shaped muscular partition between the oropharynx and nasopharynx The posterior margin of the soft palate supports the uvula- a dangling fingerlike muscular structure that helps prevent food from entering the pharynx

33. During swallowing, the soft palate and uvula are drawn superiorly, closing off the nasopharynx and preventing swallowed foods and liquids from entering the nasal cavity At the posterior border of the soft palate, the oral cavity opens into the oropharynx The functions of the oral cavity include: Sensory analysis of material before swallowing Mechanical processing through the actions of the teeth, tongue and palatal surfaces Lubrication by mixing with mucus and salivary gland secretions; and Limited digestion of carbohydrates and lipids

34. Pharynx The pharynx is an anatomical funnel-shaped space present at the posterior portion of the buccal cavity It serve as a common passageway for solid food, liquids and air The pharynx is composed of skeletal muscle and lined by mucous membrane; there are 3 parts of pharynx: Nasopharynx Oropharynx Laryngopharynx Food normally passes through the oropharynx and laryngopharynx on its way to esophagus Both of the later 2 parts have a stratified squamous epithelium similar to oral cavity The muscular contractions of these areas help propel food into the esophagus and then into the stomach

35. Esophagus The esophagus is a collapsible hollow muscular tube, about 25 cm (10 in.) long and a diameter of about 2 cm(0.8 in.) The esophagus begins at the inferior end of the laryngopharynx in the neck and descends through the thoracic cavity posterior to trachea Passing along the mediastinum anterior to the vertebral column, it pierces the diaphragm through an opening called the esophageal hiatus The esophagus then empties into the stomach

36. Sometimes, part of the stomach protrudes above the diaphragm through the esophageal hiatus; this condition is termed a hiatus hernia The esophagus secretes mucus and transports food into the stomach

37. Deglutition or Swallowing The movement of food from the mouth into the stomach is achieved by the act of swallowing It is a complex process that can be initiated voluntarily but proceeds automatically once it begins On eating and drinking, swallowing is a conscious effort; it is also controlled at the subconscious level For example, swallowing occurs at regular intervals as saliva collects at back of the mouth; each day we swallow approximately 2400 times Deglutition is facilitated by the secretion of saliva and mucus and involves the mouth, pharynx, and esophagus

38. Swallowing occurs in three stages: (1) the voluntary stage, in which the bolus is passed into the oropharynx; also called buccal phase (2) the pharyngeal stage, the involuntary passage of the bolus through the pharynx into the esophagus; and (3) the esophageal stage, the involuntary passage of the bolus through the esophagus into the stomach In the buccal phase, the bolus is forced to the back of the oral cavity and into the oropharynx by the movement of the tongue upward and backward against the palate With the passage of the bolus into the oropharynx, the involuntary pharyngeal phase of swallowing begins in which the bolus stimulates receptors in the oropharynx

39. The impulses are send to the deglutition center in the medulla oblongata and lower pons of the brain stem The returning impulses cause the soft palate and uvula to move upward to close off the nasopharynx , which prevents swallowed foods and liquids from entering the nasal cavity In addition, the epiglottis closes off the opening to the larynx, which prevents the bolus from entering the rest of the respiratory tract; during this period, the respiratory centers are inhibited and breathing stops The bolus moves through the oropharynx and the laryngopharynx into the esophagus as UES relaxes

40. The esophageal stage of swallowing begins once the bolus enters the esophagus During this phase, peristalsis, a progression of coordinated contractions and relaxations of the circular and longitudinal layers of the muscularis , pushes the bolus onward The approach of bolus to the LES triggers its opening and then the bolus continues into the stomach Mucus secreted by esophageal glands lubricates the bolus and reduces friction The passage of solid or semisolid food from the mouth to the stomach takes 4 to 8 seconds; very soft foods and liquids pass through in about 1 second

41. Stomach The stomach performs 4 functions: Storage of ingested food Mechanical breakdown of ingested food Disruption of chemical bonds in food material through the action of acid and enzymes Production of intrinsic factor, a glycoproteins, needed for absorption of Vitamin B12 in intestine The food enters the stomach as a bolus but exit into the intestine as a viscous, highly acidic, soupy mixture of partially ingested food called chyme

42. The stomach has the shape of an expanded j The stomach has four main regions: the cardia , fundus, body, and pyloric part The cardia is the smallest part (within 3 cm of the junction between stomach and the esophagus) that surrounds the opening of the esophagus into the stomach The rounded portion superior to and to the left of the cardia is the fundus; It contacts the inferior, posterior surface of the diaphram Inferior to the fundus is the large central portion of the stomach, the body; it functions as a mixing tank for ingested food and secretion produced in the stomach

43. The Pylorus forms the sharp curve of the J and is divisible in 3 regions—the pyloric antrum , connects to the body of the stomach; the pyloric canal leads to the third region; the pylorus which in turn connects to the duodenum via a smooth muscle sphincter called the pyloric sphincter (valve) The concave medial border of the stomach is called the lesser curvature ; the convex lateral border is called the greater curvature Mechanical Digestion in the Stomach Several minutes after food enters the stomach, waves of peristalsis pass over the stomach every 15 to 25 seconds

44. Most waves begin at the body of the stomach and intensify as they reach the antrum Each peristaltic wave moves gastric contents from the body of the stomach down into the antrum , a process known as propulsion The pyloric sphincter normally remains almost, but not completely, closed Because most food particles in the stomach initially are too large to fit through the narrow pyloric sphincter, they are forced back into the body of the stomach, a process referred to as retropulsion

45. Another round of propulsion then occurs, moving the food particles back down into the antrum If the food particles are still too large to pass through the pyloric sphincter, retropulsion occurs This cycle of propulsion and retropulsion results in mixing of gastric contents with gastric acid to form a soupy chyme Once the food particles in chyme are small enough, they can pass through the pyloric sphincter, a phenomenon known as gastric emptying Gastric emptying is a slow process: only about 3 mL of chyme moves through the pyloric sphincter at a time Gastric transit time for food is 2—4 hours

46. Small Intestine Most digestion and absorption of nutrients occur in the small intestine For this reason, Its structure is specially adapted for these functions Its length alone provides a large surface area for digestion and absorption, and that area is further increased by circular folds, villi, and microvill The small intestine begins at the pyloric sphincter of the stomach and opens into the large intestine through ileocecal valve It averages 2.5 cm (1 in.) in diameter; its length is about 3 m (10 ft ) in a living person and about 6.5 m (21 ft ) in a cadaver due to the loss of smooth muscle tone after death

47. The small intestine is divided into three regions: Duodenum, the shortest region, about 25 cm (10 in.) The jejunum is the next portion and is about 1 m (3 ft ) The final and longest region of the small intestine, the ileum , measures about 2 m (6 ft ) and joins the large intestine through ileocecal valve Functions of the Small Intestine: Segmentations mix chyme with digestive juices and bring food into contact with mucosa for absorption; peristalsis propels chyme through small intestine Completes digestion of carbohydrates, proteins, and lipids; begins and completes digestion of nucleic acids Absorbs about 90% of nutrients and water that pass through digestive system

48. Chemical Digestion in Small Intestine Chyme entering the small intestine from the stomach contains partially digested carbohydrates, proteins, and lipids The completion of the digestion of carbohydrates, proteins, and lipids is a collective effort of pancreatic juice, bile, and intestinal juice in the small intestine The absorptive cells of the small intestine synthesize several digestive enzymes, called brush-border enzymes , and insert them in the plasma membrane of the microvilli Thus, some enzymatic digestion also occurs at the surface of the absorptive cells that line the villi Among the brush-border enzymes are four carbohydrate-digesting enzymes called α - dextrinase , maltase, sucrase , and lactase; proteindigesting enzymes called peptidases ( aminopeptidase and dipeptidase ); and two types of nucleotide-digesting enzymes, nucleosidases and phosphatases

49. Absorption in Intestine

50. Large Intestine The large intestine is the terminal portion of the GIT The medical specialty that deals with the diagnosis and treatment of disorders of the rectum and anus is called proctology Functions of the Large Intestine 1. Haustral churning, peristalsis, and mass peristalsis drive contents of colon into rectum 2. Bacteria in large intestine convert proteins to amino acids, break down amino acids, and produce some B vitamins and vitamin K 3. Absorption of some water, ions, and vitamins 4. Formation of feces 5. Defecation (emptying rectum)

51. Anatomy of Large Intestine The large intestine , which is about 1.5 m (5 ft ) long and 6.5 cm (2.5 in.) in diameter in living humans and it extends from the ileum to the anus Structurally, it is divided into four major regions, the cecum, colon, rectum, and anal canal The opening from the ileum into the large intestine is guarded by a fold of mucous membrane called the ileocecal sphincter (valve), which allows materials from the small intestine to pass into the large intestine Hanging inferior to the ileocecal valve is the cecum , a small pouch about 6 cm (2.4 in.) long Attached to the cecum is a twisted coiled tube, measuring about 8 cm (3 in.) in length, called the appendix

52. The cecum continues as a long tube called the colon which is divided into ascending, transverse, descending, and sigmoid portions The rectum is about 15 cm (6 in.) in length and is connected to the anal canal The last part of large intestine is called the anal canal The mucous membrane of the anal canal is arranged in longitudinal folds called anal columns that contain a network of arteries and veins The opening of the anal canal to the exterior, called the anus , is guarded by an internal anal sphincter of smooth muscle (involuntary) and an external anal sphincter of skeletal muscle (voluntary) Normally these sphincters keep the anus closed except during the elimination of feces.

53. Mechanical Digestion The passage of chyme from the ileum into the cecum is regulated by the action of the ileocecal sphincter The cecum continues as a long tube called the colon which is divided into ascending, transverse, descending, and sigmoid portions The rectum is about 15 cm (6 in.) in length and is connected to the anal canal The last part of large intestine is called the anal canal The mucous membrane of the anal canal is arranged in longitudinal folds called anal columns that contain a network of arteries and veins

54. The opening of the anal canal to the exterior, called the anus , is guarded by an internal anal sphincter of smooth muscle (involuntary) and an external anal sphincter of skeletal muscle (voluntary) Normally these sphincters keep the anus closed except during the elimination of feces. When the distension reaches a certain point, the walls contract and squeeze the contents into the next haustrum Peristalsis also occurs, although at a slower rate (3–12 contractions per minute) than in more proximal portions of the tract

55. A final type of movement is mass peristalsis , a strong peristaltic wave that begins at about the middle of the transverse colon and quickly drives the contents of the colon into the rectum Because food in the stomach initiates this gastrocolic reflex in the colon, mass peristalsis usually takes place three or four times a day, during or immediately after a meal Chemical Digestion The final stage of digestion occurs in the colon through the activity of bacteria that inhabit the lumen Mucus is secreted by the glands of the large intestine, but no enzymes are secreted Chyme is subjected to the action of bacteria, which ferment any remaining carbohydrates and release hydrogen, carbon dioxide, and methane gases

56. These gases contribute to flatus (gas) in the colon, termed flatulence when it is excessive Bacteria also convert any remaining proteins to amino acids and break down the amino acids into simpler substances: indole, skatole, H 2 S, and fatty acids Some indole & skatole eliminated gives specific odor to the feces Bacteria also decompose bilirubin to simpler pigments, including stercobilin, which gives feces their brown color Bacterial products that are absorbed in the colon include several vitamins needed for normal metabolism, among them some B vitamins and vitamin K

57. Defecation Mass peristaltic movements push fecal material from the sigmoid colon into the rectum The resulting distension of the rectal wall stimulates stretch receptors, which initiates a defecation reflex that results in defecation , the elimination of feces from the rectum through the anus The amount of bowel movements that a person has over a given period of time depends on various factors such as diet, health, and stress The normal range of bowel activity varies from two or three bowel movements per day to three or four bowel movements per week

58. Diarrhea is an increase in the frequency, volume, and fluid content of the feces caused by increased motility of and decreased absorption by the intestines When chyme passes too quickly through the small intestine and feces pass too quickly through the large intestine, there is not enough time for absorption Frequent diarrhea can result in dehydration and electrolyte imbalances Excessive motility may be caused by lactose intolerance, stress, and microbes that irritate the gastrointestinal mucosa Constipation refers to infrequent or difficult defecation caused by decreased motility of the intestines

59. Because the feces remain in the colon for prolonged periods, excessive water absorption occurs, and the feces become dry and hard Constipation may be caused by poor habits (delaying defecation), spasms of the colon, insufficient fiber in the diet, inadequate fluid intake, lack of exercise, emotional stress, and certain drugs A common treatment is a mild laxative, such as milk of magnesia, which induces defecation However, laxative are habit forming and adding fiber to the diet, increasing the amount of exercise, and increasing fluid intake are safer ways of controlling this common problem