Gastrointestinal Tract: Anatomy & Physiology

Gastrointestinal Tract

1. GASTRO INTESTINAL TRACT Presented By- Shruti Richa Assistant Professor Central Institute of Pharmacy

2. Digestive System The gastrointestinal tract ( digestive tract , digestional tract , GI tract , GIT , gut , or alimentary canal ) is an organ system which takes in food, digests it to extract and absorb energy and nutrients, and expels the remaining waste as feces. E. g. mouth, esophagus, stomach, and intestines Gastrointestinal means pertaining to the stomach and intestines and a tract is a collection of related anatomical structures or a series of connected body organs.

3. Digestive System Facts Digestion is the process by which food is broken down into smaller pieces so the body can use them to build and nourish cells and to provide energy. Digestion involves the mixing of food, its movement through the digestive tract (also known as the alimentary canal), and the chemical breakdown of larger molecules into smaller molecules. Every piece of food eaten has to be broken down into smaller nutrients that the body can absorb, which is why it takes hours to fully digest food. The digestive system is made up of the digestive tract. This consists of a long tube of organs that runs from the mouth to the anus and includes the esophagus, stomach, small intestine, and large intestine, together with the liver, gallbladder, and pancreas, which produce important secretions for digestion that drain into the small intestine. The digestive tract in an adult is about 30 feet long.

4. Functions/ Steps/ Process of Digestive System To achieve the goal of providing energy and nutrients to the body, six major functions take place in the digestive system: Ingestion Secretion Mixing and movement Digestion Absorption Excretion

5. Accessory organs GI tract Upper GI tract Lower GI tract Teeth , Tongue , Accessory organs Salivary glands, Liver , Gallbladder , and pancreas.

6. Accessory organs Accessory organs of the digestive system include the teeth, tongue, salivary glands, liver, gallbladder, and pancreas. These are the organs which works as accessory organ to perform the functions of digestion but is not directly involved or participates in digestion.

7. Mouth Food begins its journey through the digestive system in the mouth, it is also known as oral cavity . Inside the mouth are many accessory organs that aid in the digestion of food—the tongue, teeth, and salivary glands. Teeth chop food into small pieces, which are moistened by saliva before the tongue and other muscles push the food into the pharynx.

8. Teeth The teeth are 32 small hard organs found along the anterior and lateral edges of the mouth. The teeth are designed for cutting and grinding food into smaller pieces.

9. Tongue The tongue is located on the inferior portion of the mouth just posterior and medial to the teeth. It is a small organ made up of several pairs of muscles covered in a thin, bumpy, skin-like layer. The taste buds on the surface of the tongue detect taste molecules in food and connect to nerves in the tongue to send taste information to the brain. The tongue also helps to push food toward the posterior part of the mouth for swallowing.

10. Salivary Glands The salivary glands are accessory organs that produce a watery secretion known as saliva. Saliva helps to moisten food and begins the digestion of carbohydrates. The body also uses saliva to lubricate food as it passes through the mouth, pharynx, and esophagus. Surrounding the mouth are 3 sets of salivary glands.

11. In healthy individual, between 0.5 and 1.5 liters of saliva are produced every day . The secretion of saliva (salivation) is mediated by parasympathetic stimulation

12. Composition of Saliva Saliva is mainly water. In fact, it’s 97-99.5% water which makes it hypo osmotic. Generally, saliva is a bit acidic (6.75-7.00), but the pH can vary. Its solutes include electrolytes (mainly sodium, potassium, chloride, and bicarbonate); the digestive enzymes like: salivary amylase and lingual lipase; the proteins mucin , IgA , and lysozyme ; metabolic wastes (uric acid, urea). When dissolved in water, the glycoprotein mucin forms thick mucus that lubricates the oral cavity and hydrates foodstuffs. Saliva protects against microorganisms because it has: IgA antibodies Lysozyme- a bactericidal enzyme (it inhibits bacterial growth in the mouth) Defensins - defensins function as cytokines and call defensive cells (lymphocytes) into the mouth.

13. Parotid glands The two parotid glands are major salivary glands. It has taken from two words: Par=near , otid =ear It is the largest salivary gland, which secrete saliva to facilitate mastication and swallowing, and amylase to begin the digestion of starches . It is the serous type of gland which secretes ptyalin. It enters the oral cavity via the parotid duct ( Stensen duct ). They produce 20% of the total salivary content in the oral cavity .

14. Submandibular glands The submandibular glands (previously known as submaxillary glands) are a pair of major salivary glands located beneath the lower jaws, superior to the digastric muscles . The secretion produced is a mixture of both serous fluid and mucus, and enters the oral cavity via the submandibular duct or Wharton duct . Approximately 65-70% of saliva in the oral cavity is produced by the submandibular glands, even though they are much smaller than the parotid glands . This gland can usually be felt as it is in the superficial cervical region and feels like a rounded ball. It is located about two fingers above the Adam's apple and about two inches apart under the chin.

15. Sublingual glands The sublingual glands are a pair of major salivary glands located inferior to the tongue, anterior to the submandibular glands. The secretion produced is mainly mucous in nature; however, it is categorized as a mixed gland. Unlike the other two major glands, the ductal system of the sublingual glands does not have intercalated ducts and usually does not have striated ducts either, so saliva exits directly from Rivinus ducts . Approximately 5% of saliva entering the oral cavity comes from these glands .

16. Minor salivary glands There are 800 to 1,000 minor salivary glands located throughout the oral cavity within the submucosa & oral mucosa, the lateral parts of the hard palate, and the floor of the mouth or between tongue. They are 1 to 2 mm in diameter and unlike the major glands, they are not encapsulated by connective tissue. A minor salivary gland may have a common excretory duct with another gland, or may have its own excretory duct. Their secretion is mainly mucous in nature.

17. Von Ebner's glands Von Ebner's glands are glands found in circumvallate papillae on the dorsal surface of the tongue. They secrete a purely serous fluid that begins lipid hydrolysis. They also facilitate the perception of taste through secretion of digestive enzymes and proteins . The arrangement of these glands around the circumvallate papillae provides a continuous flow of fluid over the great number of taste buds lining the sides of the papillae, and is important for dissolving the food particles to be tasted.

18. Nerve supply in salivery gland Salivary glands are innervated, either directly or indirectly, by the parasympathetic and sympathetic arms of the autonomic nervous system. Parasympathetic stimulation evokes a copious flow of saliva. In contrast, sympathetic stimulation produces either a small flow, which is rich in protein, or no flow at all. Parasympathetic innervation to the salivary glands is carried via cranial nerves. The parotid gland receives its parasympathetic input from the glossopharyngeal nerve (CN IX) via the otic ganglion , [ while the submandibular and sublingual glands receive their parasympathetic input from the facial nerve (CN VII) via the submandibular ganglion.These nerves release acetylcholine and substance P, which activate the IP3 and DAG pathways respectively.

19. Control of Salivation The minor salivary glands secrete saliva continuously, keeping the mouth optimally moist. When food enters, the major glands activate and large amounts of saliva pour out. The average human being produces around 1500ml of saliva per day. Salivation is controlled by the parasympathetic division of the autonomic nervous system. When food is ingested, chemoreceptors and mechanoreceptors in the mouth send signals to the salivatory nuclei in the brain stem to the pons and medulla. As a result, parasympathetic nervous system activity increases. Impulses sent by motor fibers in the facial (VII) and glossopharyngeal (IX) nerves dramatically increase the output of watery saliva. The chemoreceptors are activated the most by acidic foods and liquids (vinegar, pickles, ect ). The mechanoreceptors are activated by almost any type of mechanical stimulus in the mouth (chewing).

20. Pharynx The pharynx, or throat, is a funnel-shaped tube connected to the posterior end of the mouth. The pharynx is responsible for the passing of masses of chewed food from the mouth to the esophagus. The pharynx also plays an important role in the respiratory system, as air from the nasal cavity passes through the pharynx on its way to the larynx and eventually the lungs . Because the pharynx serves two different functions, it contains a flap of tissue known as the epiglottis that acts as a switch to route food to the esophagus and air to the larynx .

21. Liver and Gallbladder The liver is a roughly triangular accessory organ of the digestive system located to the right of the stomach, just inferior to the diaphragm and superior to the small intestine. The liver weighs about 3 pounds and is the second largest organ in the body. The liver has main function to produce bile and its secretion into the small intestine. Bile is a dark-green-to-yellowish-brown fluid produced by the liver that aids the digestion of lipids in the small intestine. Bile  is produced continuously by the liver (liver  bile ) and stored and concentrated in the gallbladder.

22. Liver is the largest gland (weighing an average of 1500 g). It lies under the diaphragm in the right upper abdomen and mid-abdomen and extends to the left upper abdomen. The liver has the general shape of a prism or wedge, with its base to the right and its apex to the left (see the image below). It is pinkish brown in color. The gallbladder is a small, pear-shaped organ located just posterior to the liver. The gallbladder is used to store and recycle excess bile from the small intestine so that it can be reused for the digestion of meals

23. Location This is useful for remembering the anatomical relations of the liver: Anterior to the liver is the anterior abdominal wall and ribcage. Superior to the liver is the diaphragm (separating the abdominal cavity from the thoracic cavity) Posterior to the liver are the oesophagus , stomach, gallbladder, first part of the duodenum (the foregut-derived organs).

24. Structure The entire liver is covered by a fibrous layer, known as  Glisson’s capsule . The ligaments and surface depressions of the liver divide it into four lobes. It is divided into a right lobe and left lobe by the attachment of the falciform ligament . There are two further ‘accessory’ lobes that arise from the right lobe, and are located on the visceral surface of liver: The caudate lobe is located on the upper aspect of the visceral surface. The quadrate lobe is located on the lower aspect of the visceral surface. Between the caudate and quadrate lobes is a deep fissure, known as the porta hepatis . It transmits all the vessels, nerves and ducts entering or leaving the liver.

25. T he cells of the liver (known as hepatocytes) are arranged into lobules . These are the structural units of the liver. Each lobule is hexagonal-shaped, and is drained by a venule in its centre , called a central vein . At the periphery of the hexagon are three structures collectively known as the portal triad: Arteriole – a branch of the hepatic artery entering the liver. Venule – a branch of the hepatic portal vein entering the liver. Duct – branch of the bile duct leaving the liver. The portal triad also contains  lymphatic vessels and vagus nerve (parasympathetic) fibres .

26. Pancreas The pancreas is a large gland located just inferior and posterior to the stomach. It is about 6 inches long and shaped like short, lumpy snake with its “head” connected to the duodenum and its “tail” pointing to the left wall of the abdominal cavity. The pancreas secretes digestive enzymes into the small intestine to complete the chemical digestion of foods.

27. Process Mouth and Salivary Glands Digestion begins in the mouth, where chemical and mechanical digestion occurs. Saliva or spit, produced by the salivary glands (located under the tongue and near the lower jaw), is released into the mouth. Saliva begins to break down the food, moistening it and making it easier to swallow. A digestive enzyme i . e. amylase in the saliva begins to break down the carbohydrates (starches and sugars). One of the most important functions of the mouth is chewing. Chewing allows food to be mashed into a soft mass that is easier to swallow and digest later. Movements by the tongue and the mouth push the food to the back of the throat for it to be swallowed. A flexible flap called the epiglottis closes over the trachea (windpipe) to ensure that food enters the esophagus and not the windpipe to prevent choking.

28. Upper gastrointestinal tract The upper gastrointestinal tract consists of the buccal cavity, pharynx, esophagus , stomach, and duodenum. The exact demarcation between the upper and lower tracts is the suspensory muscle of the duodenum. This delineates the embryonic borders between the foregut and midgut , and is also the division commonly used by clinicians to describe gastrointestinal bleeding as being of either "upper" or "lower" origin. Upon dissection, the duodenum may appear to be a unified organ, but it is divided into four segments based upon function, location, and internal anatomy. The four segments of the duodenum are as follows (starting at the stomach, and moving toward the jejunum): bulb, descending, horizontal, and ascending. The suspensory muscle attaches the superior border of the ascending duodenum to the diaphragm. The suspensory muscle is an important anatomical landmark which shows the formal division between the duodenum and the jejunum, the first and second parts of the small intestine, respectively.This is a thin muscle which is derived from the embryonic mesoderm.

29. Esophagus The esophagus is a muscular tube connecting the pharynx to the stomach that is part of the  upper gastrointestinal tract . It carries swallowed masses of chewed food along its length. At the inferior end of the esophagus is a muscular ring called the lower esophageal sphincter or cardiac sphincter. The function of this sphincter is to close of the end of the esophagus and trap food in the stomach.

30. Once food is swallowed, it enters the esophagus, a muscular tube that is about 10 inches long. The esophagus is located between the throat and the stomach. Muscular wavelike contractions known as peristalsis push the food down through the esophagus to the stomach. A muscular ring (cardiac sphincter) at the end of the esophagus allows food to enter the stomach, and, then, it squeezes shut to prevent food and fluid from going back up the esophagus.

31. Stomach The stomach is a muscular sac that is located on the left side of the abdominal cavity, just inferior to the diaphragm . In an average person, the stomach is about the size of their two fists placed next to each other. This major organ acts as a storage tank for food so that the body has time to digest large meals properly. The stomach also contains hydrochloric acid and digestive enzymes that continue the digestion of food that began in the mouth.

32. Position The stomach is located in the superior aspect of the abdomen. It lies in the epigastric , protected by the lower portion of the rib cage . The stomach , part of the gastrointestinal tract, is a digestive organ located between the esophagus and the duodenum . It has a ‘J’ shape, and features a lesser and greater curvature. The anterior and posterior surfaces are smoothly rounded with a peritoneal covering . The exact size, shape and position of the stomach can vary from person to person.

33. Fig1: Curvatures of stomach Fig2: Parts of stomach

34. Greater and Lesser Curvatures The structure of the stomach are curved, forming the lesser and greater curvatures: Greater curvature  – It forms the long, convex, lateral border of the stomach. Arising at the cardiac orifice, it arches backwards and passes inferiorly to the left. It curves to the right. The short gastric arteries and the right and left gastro-arteries have supply to the greater curvature. Lesser curvature  – It forms the shorter, concave, medial surface of the stomach. The most inferior part of the lesser curvature, indicates the junction of the body and pyloric region. The lesser curvature gives attachment to the hepatogastric ligament and is supplied by the left gastric artery and right gastric branch of the hepatic artery.

35. Structure The stomach has four main regions; the cardia , fundus, body and pylorus: Cardia Fundus Stomach Body Pylorus

36. Cardia  – It surrounds the superior opening of the stomach. The cardia is where the contents of the oesophagus empty into the stomach. The cardia is defined as the region following the "z-line" of the gastroesophageal junction. Near the cardia is the lower oesophageal sphincter. Fundus  – It is the rounded portion superior to and left of the cardia . It means "bottom" is formed by the upper curvature of the organ. Body  – It is the large central portion inferior to the fundus. Pylorus  – It connects the stomach to the duodenum . "gatekeeper") is the lower section of the organ that facilitates emptying the contents into the small intestine.

37. Blood supply Schematic image of the blood supply to the human stomach: left and right gastric artery, left and right gastroepiploic artery and short gastric artery . The lesser curvature of the human stomach is supplied by the right gastric artery inferiorly, and the left gastric artery superiorly, which also supplies the cardiac region. The greater curvature is supplied by the right gastroepiploic artery inferiorly and the left gastroepiploic artery superiorly. The fundus of the stomach, and also the upper portion of the greater curvature, is supplied by the short gastric artery which arises from the splenic artery.

38. Lower gastrointestinal tract The lower gastrointestinal tract includes small intestine and the large intestine. In human anatomy, the intestine ( bowel , or gut ) is the segment of the gastrointestinal tract extending from the pyloric sphincter of the stomach to the anus. The intestines are a long, continuous tube running from the stomach to the anus. Most absorption of nutrients and water happen in the intestines. The intestines include the small intestine, large intestine, and rectum.

39. In humans, the small intestine is further subdivided into the duodenum, jejunum and ileum while the large intestine is subdivided into the cecum, colon, rectum, and anal canal . The small intestine (small bowel) is about 20 feet long and about an inch in diameter. Its job is to absorb most of the nutrients from what we eat and drink. The large intestine (colon or large bowel) is about 5 feet long and about 3 inches in diameter. The colon absorbs water from wastes, creating stool. As stool enters the rectum, nerves there create the urge to defecate.

40. Small Intestine The small intestine is a long, thin tube about 1 inch in diameter and about 10 feet long that is part of the  lower gastrointestinal tract . It is located just inferior to the stomach and takes up most of the space in the abdominal cavity. The entire small intestine is coiled like a hose and the inside surface is full of many ridges and folds. These folds are used to maximize the digestion of food and absorption of nutrients. By the time food leaves the small intestine, around 90% of all nutrients have been extracted from the food that entered it.

41. Small intestine The small intestine begins at the duodenum and is a tubular structure, usually between 6 and 7 m long. Its mucosal area in an adult human is about 30 m 2 . The small intestine is so called because its lumen diameter is smaller than that of the large intestine, although it is longer in length than the large intestine. Its main function is to absorb the products of digestion (including carbohydrates, proteins, lipids, and vitamins) into the bloodstream. There are three major divisions: Duodenum Small intestine Jejunum Ileum

42. Duodenum A short structure (about 20–25 cm long) which receives chyme from the stomach, together with pancreatic juice containing digestive enzymes and bile from the gall bladder. The digestive enzymes break down proteins, and bile emulsifies fats into micelles. The duodenum contains Brunner's glands, which produce a mucus-rich alkaline secretion containing bicarbonate. These secretions, in combination with bicarbonate from the pancreas, neutralizes the stomach acids contained in the chyme .

43. Jejunum This is the midsection of the small intestine, connecting the duodenum to the ileum. The duodenum continues into the jejunum at the duodenojejunal junction. It is about 2.5 m long, and contains the circular folds, and villi that increase its surface area. Products of digestion (sugars, amino acids, and fatty acids) are absorbed into the bloodstream here.

44. Ileum The final section of the small intestine. It is about 3 m long, and contains villi similar to the jejunum. It absorbs mainly vitamin B12 and bile acids, as well as any other remaining nutrients.

45. Large intestine The large intestine also called the colon, consists of the cecum, rectum, and anal canal. It also includes the appendix, which is attached to the cecum. The colon is further divided into: Cecum (first portion of the colon) and appendix Ascending colon (ascending in the back wall of the abdomen) Right colic flexure (flexed portion of the ascending and transverse colon apparent to the liver) Transverse colon (passing below the diaphragm) Left colic flexure (flexed portion of the transverse and descending colon apparent to the spleen) Descending colon (descending down the left side of the abdomen) Sigmoid colon (a loop of the colon closest to the rectum) Rectum Anus The main function of the large intestine is to absorb water. The area of the large intestinal mucosa of an adult human is about 2 m 2 .

46. Digestive System Physiology The digestive system is responsible for taking whole foods and turning them into energy and nutrients to allow the body to function, grow, and repair itself. The six primary processes of the digestive system include: Ingestion of food Secretion of fluids and digestive enzymes Mixing and movement of food and wastes through the body Digestion of food into smaller pieces Absorption of nutrients Excretion of wastes

47. Ingestion The first function of the digestive system is ingestion, or the intake of food. The mouth is responsible for this function, as it is the orifice through which all food enters the body. The mouth and stomach are also responsible for the storage of food as it is waiting to be digested. This storage capacity allows the body to eat only a few times each day and to ingest more food than it can process at one time.

48. Secretion In the course of a day, the digestive system secretes around 7 liters of fluids. These fluids include saliva, mucus, hydrochloric acid, enzymes, and bile. Saliva moistens dry food and contains salivary amylase, a digestive enzyme that begins the digestion of carbohydrates. Mucus serves as a protective barrier and lubricant inside of the GI tract. Hydrochloric acid helps to digest food chemically and protects the body by killing bacteria present in our food. Enzymes are like tiny biochemical machines that disassemble large macromolecules like proteins, carbohydrates, and lipids into their smaller components. Finally , bile is used to emulsify large masses of lipids into tiny globules for easy digestion.

49. Mixing and Movement The digestive system uses 3 main processes to move and mix food: Swallowing : Swallowing is the process of using smooth and skeletal muscles in the mouth, tongue, and pharynx to push food out of the mouth, through the pharynx, and into the esophagus. Peristalsis : Peristalsis is a muscular wave that travels the length of the GI tract, moving partially digested food a short distance down the tract. It takes many waves of peristalsis for food to travel from the esophagus, through the stomach and intestines , and reach the end of the GI tract. Segmentation : Segmentation occurs only in the small intestine as short segments of intestine contract like hands squeezing a toothpaste tube. Segmentation helps to increase the absorption of nutrients by mixing food and increasing its contact with the walls of the intestine.

50. Digestion Digestion is the process of turning large pieces of food into its small component. Mechanical digestion is the physical breakdown of large pieces of food into smaller pieces. This mode of digestion begins with the chewing of food by the teeth and is continued through the muscular mixing of food by the stomach and intestines. Bile produced by the liver is also used to mechanically break fats into smaller globules. The food is also being chemically digested as larger and more complex molecules are being broken down into smaller molecules that are easier to absorb. Chemical digestion begins in the mouth with salivary amylase in saliva splitting complex carbohydrates into simple carbohydrates. The enzymes and acid in the stomach continue chemical digestion, but the bulk of chemical digestion takes place in the small intestine. The pancreas secretes pancreatic juice, which is capable of digesting lipids, carbohydrates, proteins and nucleic acids. By the time food has left the duodenum , it has been reduced to its chemical building blocks—fatty acids, amino acids, monosaccharides , and nucleotides.

51. Absorption Once food has been reduced to its building blocks, it is ready for the body to absorb. Absorption begins in the stomach with simple molecules like water and alcohol being absorbed directly into the bloodstream. Most absorption takes place in the walls of the small intestine, which are densely folded to maximize the surface area in contact with digested food. Small blood and lymphatic vessels in the intestinal wall pick up the molecules and carry them to the rest of the body. The large intestine is also involved in the absorption of water and vitamins B and K before feces leave the body.

52. Excretion The final function of the digestive system is the excretion of waste in a process known as defecation. Defecation removes indigestible substances from the body so that they do not accumulate inside the gut.

53. Physiology of Saliva It Cleanses the mouth. It helps moisten and compact food into a round mass called a bolus. It contains enzymes that begin the chemical breakdown  of starch. It dissolves food chemicals so they can be “tasted ”.

54. Physiology of Liver The liver regulates most chemical levels in the blood. It excretes a product called bile. This bile helps carry away waste products from the liver. All the blood leaving the stomach and intestines passes through the liver. The liver processes this blood and breaks down, balances, and creates the nutrients and also metabolizes drugs into forms that are easier to use for the rest of the body or that are nontoxic. Production of bile, which breaks down fats in the small intestine during digestion Production of certain proteins for blood plasma Production of cholesterol and special proteins to help carry fats through the body Conversion of excess glucose into glycogen for storage (glycogen can later be converted back to glucose for energy) and to balance and make glucose as needed  Regulation of blood levels of amino acids, which form the building blocks of proteins The liver stores iron. Conversion of poisonous ammonia to urea. Clearing the blood of drugs and other poisonous substances. Regulating blood clotting. Resisting infections by making immune factors (coagulation factors) and removing bacteria from the bloodstream. It helps in clearance of bilirubin , also from red blood cells. If there is an accumulation of bilirubin , the skin and eyes turn yellow. 

55. Stomach as nutrition sensor The human stomach can "taste" sodium glutamate using glutamate receptors and this information is passed to the lateral hypothalamus and limbic system in the brain through the vagus nerve . The stomach can also sense, independently of tongue and oral taste receptors, glucose, carbohydrates, proteins , and fats . This allows the brain to link nutritional value of foods to their tastes.

56. References https://www.emedicinehealth.com/anatomy_of_the_digestive_system/article_em.htm https://en.wikipedia.org/wiki/Human_digestive_system www.innerbody.com/image/digeov.html

57. q UERY sESSION