Digestive System REVISED 092523 (Cueto) PDF

DIGESTIVE SYSTEM By : Marc Anthony Cueto MD Learning Objectives 1. Identify the detailed structure and landmarks of organs of digestion. 2. Provide an understanding on the organs of digestion. 3. Describe the mechanism of digestion from mastication down to removal of waste. Digestive System...

DIGESTIVE SYSTEM By : Marc Anthony Cueto MD Learning Objectives 1. Identify the detailed structure and landmarks of organs of digestion. 2. Provide an understanding on the organs of digestion. 3. Describe the mechanism of digestion from mastication down to removal of waste. Digestive System The digestive system has three main functions relating to food : digestion of food , absorption of nutrients from food , and elimination of solid food waste. Digestion is the process of breaking down food into components the body can absorb. It consists of two types of processes: mechanical digestion and chemical digestion. Mechanical digestion is the physical breakdown of chunks of food into smaller pieces. This type of digestion takes place mainly in the mouth and stomach. Chemical digestion is the chemical breakdown (bonds are broken) of large, complex food molecules into smaller, simpler nutrient molecules that can be absorbed by body fluids (blood or lymph). This type of digestion begins in the mouth and continues in the stomach but occurs mainly in the small intestine. Digestive System After food is digested, the resulting nutrients are absorbed. Absorptionis the process in which substances pass into the bloodstream or lymph system to circulate throughout the body. The absorption of nutrients occurs mainly in the small intestine. Anyremaining matter from food that is not digested and absorbed passes out of the body through the anus in the process of elimination. Digestive System 2 divisions of the digestive system: 1.alimentary canal – organs through which food passes: mouth, pharynx, esophagus, stomach, small intestine, large intestine and anus 2.accessory organs – structures that aid in digestion: teeth, salivary glands, liver, gallbladder, and pancreas Abdomen Abdomen ORAL CAVITY Oral Cavity The oral cavity represents the first part of the digestive tube. Its primary function is to serve as the entrance of the alimentary tract and to initiate the digestive process by salivation and propulsion of the alimentary bolus into the pharynx. It also serves as a secondary respiratory conduit, a site of sound modification for the production of speech, and a chemosensory organ. The mobility of the lips is also critical to speech production, whistling, singing, the playing of wind and brass musical instruments, expectoration, and human behavioral communication (eg, kissing, smiling, pouting, baring of teeth). Even minor disruptions in the function of the oral cavity can seriously jeopardize an individual’s quality of life. Oral Cavity The oral cavity is inferior to the nasal cavity. It has a roof and floor and lateral walls, opens onto the face through the oral fissure, and is continuous with the cavity of the pharynx at the oropharyngeal isthmus. The roof of the oral cavity consists of the hard and soft palate. The floor is formed mainly of soft tissues, which include a muscular diaphragm and the tongue. The lateral walls also referred to as the cheeks are muscular and merge anteriorly with the lips surrounding the oral fissure (anterior opening of the oral cavity) The posterior aperture of the oral cavity is the oropharyngeal isthmus, which opens into the oral part of the pharynx. Oral Cavity The oral cavity is separated into two regions by the upper and lower dental arches consisting of the teeth and the alveolar bone that supports them: The outer oral vestibule, which is horseshoe shaped, is between the dental arches and the deep surfaces of the cheeks and lips-the oral fissure opens into it and can be opened and closed by muscles of facial expression, and by movement of the lower jaw. The inner oral cavity proper is enclosed by the dental arches. Oral Cavity The degree of separation between the upper and lower arches is established by elevating and depressing the lower jaw or mandible at the temporomandibular joint. The oropharyngeal isthmus at the back of the oral cavity proper can be opened and closed by surrounding soft tissues, which include the soft palate and tongue. Tongue The tongue is a muscular structure that forms part of the floor of the oral cavity and part of the anterior wall of the oropharynx. Its anterior part is in the oral cavity and is somewhat triangular in shape with a blunt apex of tongue. The apex is directed anteriorly and sits immediately behind the incisor teeth. The root of tongue is attached to the mandible and the hyoid bone. The superior surface of the oral or anterior two-thirds of the tongue is oriented in the horizontal plane. Tongue The pharyngeal surface or posterior one-third of the tongue curves inferiorly and becomes oriented more in the vertical plane. The oral and pharyngeal surfaces are separated by a V-shaped terminal sulcus of tongue. This terminal sulcus forms the inferior margin of the oropharyngeal isthmus between the oral and pharyngeal cavities. At the apex of the V-shaped sulcus is a small depression (the foramen cecum of tongue), which marks the site in the embryo where the epithelium invaginated to form the thyroid gland. Tongue In some people a thyroglossal duct persists and connects the foramen cecum on the tongue with the thyroid gland in the neck. The thyroglossal duct is an embryological anatomical structure forming an open connection between the initial area of development of the thyroid gland and its final position. It is located exactly mid-line, between the anterior 2/3 and posterior 1/3 of the tongue. The thyroid gland starts developing in the oropharynx in the fetus and descends to its final position taking a path through the tongue, hyoid bone and neck muscles. The connection between its original position and its final position is the thyroglossal duct. This duct normally atrophies and closes off as the foramen cecum before birth but can remain open in some people. Thyroglossal duct cyst A thyroglossal cyst is a fibrous cyst that forms from a persistent thyroglossal duct. Thyroglossal cysts can be defined as an irregular neck mass or a lump which had developed from cells and tissues left over after the formation of the thyroid gland during developmental stages. Papillae The superior surface of the oral part of the tongue is covered by hundreds of papillae : filiform papillae are small cone-shaped projections of the mucosa that end in one or more points; fungiform papillae are rounder in shape and larger than the filiform papillae, and tend to be concentrated along the margins of the tongue; the largest of the papillae are the vallate papillae, which are blunt-ended cylindrical papillae invaginations in the tongue's surface-there are only about 8 to 12 vallate papillae in a single V shaped line immediately anterior to the terminal sulcus of tongue; foliate papillae are linear folds of mucosa on the sides of the tongue near the terminal sulcus of tongue. The papillae in general increase the area of contact between the surface of the tongue and the contents of the oral cavity. All except the filiform papillae have taste buds on their surfaces. Inferior surface of the tongue The undersurface of the oral part of the tongue lacks papillae, but does have a number of linear mucosal folds. A single median fold (the frenulum of tongue) is continuous with the mucosa covering the floor of the oral cavity, and overlies the lower margin of a midline sagittal septum, which internally separates the right and left sides of the tongue. On each side of the frenulum is a lingual vein, and lateral to each vein is a rough fimbriated fold. Pharyngeal surface The mucosa covering the pharyngeal surface of the tongue is irregular in contour because of the many small nodules of lymphoid tissue in the submucosa. These nodules are collectively the lingual tonsil. There are no papillae on the pharyngeal surface. Muscles of the tongue The bulk of the tongue is composed of muscle. The tongue is completely divided into a left and right half by a median sagittal septum composed of connective tissue. This means that all muscles of the tongue are paired. There are intrinsic and extrinsic lingual muscles. Except for the palatoglossus, which is innervated by the vagus nerve [X], all muscles of the tongue are innervated by the hypoglossal nerve [XII]. Intrinsic muscles of the tongue The intrinsic muscles of the tongue originate and insert within the substance of the tongue. They are divided into superior longitudinal, inferior longitudinal, transverse, and vertical muscles, and they alter the shape of the tongue by: lengthening and shortening it; curling and uncurling its apex and edges; and flattening and rounding its surface. Working in pairs or one side at a time the intrinsic muscles of the tongue contribute to precision movements of the tongue required for speech, eating, and swallowing. Extrinsic muscles of the tongue Extrinsic muscles of the tongue originate from structures outside the tongue and insert into the tongue. There are four major extrinsic muscles on each side, the genioglossus, hyoglossus, styloglossus, and palatoglossus. These muscles protrude, retract, depress, and elevate the tongue. Oral fissure and lips The oral fissure is the slit-like opening between the lips that connects the oral vestibule to the outside. It can be opened and closed, and altered in shape by the movements of the muscles of facial expression associated with the lips and surrounding regions, and by movements of the lower jaw (mandible). The lips are entirely composed of soft tissues. They are lined internally by oral mucosa and covered externally by skin. Externally, there is an area of transition from the thicker skin that covers the face to the thinner skin that overlies the margins of the lips and continues as oral mucosa onto the deep surfaces of the lips. Oral fissure and lips Blood vessels are closer to the surface in areas where the skin is thin and as a consequence there is a vermilion border that covers the margins of the lips. The vermilion border is the normally sharp demarcation between the lip (red colored) and the adjacent normal skin. It represents the change in the epidermis from highly keratinized external skin to less keratinized internal skin. It has no sebaceous glands, sweat glands, or hair. The upper lip has a shallow vertical groove on its external surface (the philtrum) sandwiched between two elevated ridges of skin. The philtrum and ridges are formed embryologically by fusion of the medial nasal processes. On the inner surface of both lips, a fold of mucosa (the median labial frenulum) connects the lip to the adjacent gum. The lips enclose the orbicularis oris muscle, neurovascular tissues, and labial glands. The small pea-shaped labial glands are between the muscle tissue and the oral mucosa and open into the oral vestibule. Oral fissure and lips A number of muscles of facial expression control the shape and size of the oral fissure. The most important of these is the orbicularis oris muscle, which encircles the orifice and acts as a sphincter. A number of other muscles of facial expression blend into the orbicularis oris or other tissues of the lips and open or adjust the contours of the oral fissure. These include buccinator, levator labii superioris, zygomaticus major and minor, levator anguli oris, depressor labii inferioris, depressor anguli oris, and platysma. Oropharyngeal isthmus The oropharyngeal isthmus is the opening between the oral cavity and the oropharynx. It is formed: laterally by the palatoglossal arches; superiorly by the soft palate; and and inferiorly by the sulcus terminalis of the tongue that divides the oral surface of the tongue (anterior two-thirds) from the pharyngeal surface (posterior one-third). The oropharyngeal isthmus can be closed by elevation of the posterior aspect of the tongue, depression of the palate, and medial movement of the palatoglossal arches toward the midline. Medial movement of the palatopharyngeal arches medial and posterior to the palatoglossal arches is also involved in closing the oropharyngeal isthmus. By closing the oropharyngeal isthmus, food or liquid can be held in the oral cavity while breathing. Teeth and gingivae The teeth are attached to sockets (alveoli) in two elevated arches of bone on the mandible below and the maxillae above (alveolar arches). If the teeth are removed, the alveolar bone is resorbed and the arches disappear. The gingivae (gums) are specialized regions of the oral mucosa that surround the teeth and cover adjacent regions of the alveolar bone. The different types of teeth are distinguished on the basis of morphology, position, and function. Teeth and gingivae In adults, there are 32 teeth, 16 in the upper jaw and 16 in the lower jaw. On each side in both maxillary and mandibular arches are two incisor, one canine, two premolar, and three molar teeth. the incisor teeth are the "front teeth" and have one root and a chisel-shaped crown, which "cuts"; the canine teeth are posterior to the incisors, are the longest teeth, have a crown with a single pointed cusp, and "grasp"; the premolar teeth (bicuspids) have a crown with two pointed cusps, one on the buccal (cheek) side of the tooth and the other on the lingual (tongue) or palatal (palate) side, generally have one root (but the upper first premolar next to the canine may have two), and "grind"; the molar teeth are behind the premolar teeth, have three roots and crowns with three to five cusps, and "grind." Teeth and gingivae Two successive sets of teeth develop in humans, deciduous teeth ("baby" teeth) and permanent teeth ("adult" teeth). The deciduous teeth emerge from the gingivae at between six months and two years of age. Permanent teeth begin to emerge and replace the deciduous teeth at around age six years, and can continue to emerge into adulthood. The 20 deciduous teeth consist of two incisor, one canine, and two molar teeth on each side of the upper and lower jaws. These teeth are replaced by the incisor, canine, and premolar teeth of the permanent teeth. The permanent molar teeth erupt posterior to the deciduous molars and require the jaws to elongate forward to accommodate them. ESOPHAGUS Esophagus The esophagus is a muscular tube passing between the pharynx in the neck and the stomach in the abdomen. It begins at the inferior border of the cricoid cartilage, opposite vertebra CVI, and ends at the cardiac opening of the stomach, opposite vertebra TXI. The esophagus descends on the anterior aspect of the bodies of the vertebrae, generally in a midline position as it moves through the thorax. As it approaches the diaphragm, it moves anteriorly and to the left, crossing from the right side of the thoracic aorta to eventually assume a position anterior to it. It then passes through the esophageal hiatus, an opening in the muscular part of the diaphragm, at vertebral level TX.  Esophagus The esophagus is a flexible, muscular tube that can be compressed or narrowed by surrounding structures at four locations : the junction of the esophagus with the pharynx in the neck; in the superior mediastinum where the esophagus is crossed by the arch of the aorta; in the posterior mediastinum where the esophagus is compressed by the left main bronchus; in the posterior mediastinum at the esophageal hiatus in the diaphragm. These constrictions have important clinical consequences. For example, a swallowed object is most likely to lodge at a constricted area. An ingested corrosive substance would move more slowly through a narrowed region, causing more damage at this site than elsewhere along the esophagus. Also, constrictions present problems during the passage of medical instruments.  Esophagus The upper esophageal sphincter, which is continuous with the inferior pharyngeal constrictor, controls the movement of food from the pharynx into the esophagus. Esophagus is unique, unlike any other organ in the body, it is made up of partly skeletal and partly smooth muscles. Upper part is entirely skeletal (2–4 cm), the middle, a mixture of skeletal and smooth muscle, and the lower part, 11 cm or so in length is entirely smooth. Rhythmic waves of peristalsis, which begin in the upper esophagus, propel the bolus of food toward the stomach. Meanwhile, secretions from the esophageal mucosa lubricate the esophagus and food. Food passes from the esophagus into the stomach at the lower esophageal sphincter (also called the gastroesophageal or cardiac sphincter). Esophagus Recall that sphincters are muscles that surround tubes and serve as valves, closing the tube when the sphincters contract and opening it when they relax. The lower esophageal sphincter relaxes to let food pass into the stomach, and then contracts to prevent stomach acids from backing up into the esophagus. Surrounding this sphincter is the muscular diaphragm, which helps close off the sphincter when no food is being swallowed. When the lower esophageal sphincter does not completely close, the stomach’s contents can reflux (that is, back up into the esophagus), causing heartburn or gastroesophageal reflux disease (GERD). Arterial supply and venous and lymphatic drainage of the Esophagus The arterial supply and venous drainage of the esophagus in the posterior mediastinum involve many vessels. Esophageal arteries arise from the thoracic aorta, bronchial arteries, and ascending branches of the left gastric artery in the abdomen. Venous drainage involves small vessels returning to the azygos vein, hemiazygos vein, and esophageal branches to the left gastric vein in the abdomen. Lymphatic drainage of the esophagus in the posterior mediastinum returns to posterior mediastinal and left gastric nodes. Innervation of the Esophagus Innervation of the esophagus, in general, is complex. Esophageal branches arise from the vagus nerves and sympathetic trunks. Peritoneum The digestive organs within the abdominal cavity are held in place by the peritoneum, a broad serous membranous sac made up of squamous epithelial tissue surrounded by connective tissue. It is composed of two different regions: the parietal peritoneum, which lines the abdominal wall, and the visceral peritoneum, which envelopes the abdominal organs. The peritoneal cavity is the space bounded by the visceral and parietal peritoneal surfaces. A few milliliters of watery fluid act as a lubricant to minimize friction between the serosal surfaces of the peritoneum. STOMACH  Stomach The stomach is a J-shaped dilated portion of the alimentary tract situated in the epigastric, umbilical and left hypochondriac regions of the abdominal cavity. The stomach is continuous with the oesophagus at the cardiac sphincter and with the duodenum at the pyloric sphincter. It has two curvatures. The lesser curvature is short, lies on the posterior surface of the stomach and is the downwards continuation of the posterior wall of the esophagus. Just before the pyloric sphincter, it curves upwards to complete the J shape. Where the esophagus joins the stomach the anterior region angles acutely upwards, curves downwards forming the greater curvature then slightly upwards towards the pyloric sphincter.  Stomach The stomach is divided into four regions: the cardia, which surrounds the opening of the esophagus into the stomach; the fundus of the stomach, which is the area above the level of the cardial orifice; the body of the stomach, which is the largest region of the stomach; and the pyloric part, which is divided into the pyloric antrum and pyloric canal and is the distal end of the stomach.  Stomach The most distal portion of the pyloric part of the stomach is the pylorus. It is marked on the surface of the organ by the pyloric constriction and contains a thickened ring of gastric circular muscle, the pyloric sphincter, that surrounds the distal opening of the stomach, the pyloric orifice. The pyloric orifice is just to the right of midline in a plane that passes through the lower border of vertebra LI (the transpyloric plane). Stomach Other features of the stomach include: the greater curvature, which is a point of attachment for the gastrosplenic ligament and the greater omentum; the lesser curvature, which is a point of attachment for the lesser omentum; the cardial notch, which is the superior angle created when the esophagus enters the stomach; and the angular incisure, which is a bend on the lesser curvature.  Greater Omentum The greater omentum is a large, apron-like, peritoneal fold that attaches to the greater curvature of the stomach and the first part of the duodenum. It drapes inferiorly over the transverse colon and the coils of the jejunum and ileum. Turning posteriorly, it ascends to associate with, and become adherent to, the peritoneum on the superior surface of the transverse colon and the anterior layer of the transverse mesocolon before arriving at the posterior abdominal wall. Usually a thin membrane, the greater omentum always contains an accumulation of fat, which may become substantial in some individuals. Greater Omentum The greater omentum prevents the parietal and visceral peritoneum of the abdominal cavity from adhering to each other. For example, it prevents the parietal peritoneum lining the anterior abdominal wall from sticking to the visceral peritoneum of the ileum. It is very mobile, and moves around the abdomen with rhythmic, peristaltic motion of the intestines. It can also adhere to an inflamed organ, such as the appendix, to protect the healthy organs in the abdomen. Because of this, the greater omentum is sometimes referred to as the ‘policeman of the abdomen’. Lesser Omentum The other two-layered peritoneal omentum is the lesser omentum. It extends from the lesser curvature of the stomach and the first part of the duodenum to the inferior surface of the liver. Stomach : Functions Temporary storage allowing time for the digestive enzymes, pepsins, to act. Chemical digestion — pepsins convert proteins to polypeptides. Mechanical breakdown — the three smooth muscle layers enable the stomach to act as a churn, gastric juice is added and the contents are liquefied to chime. Performs limited absorption of water, alcohol and some lipid-soluble drugs Non-specific defense against microbes — provided by hydrochloric acid in gastric juice. Preparation of iron for absorption further along the track — the acid environment of the stomach solubilizes iron salts, which is required before iron can be absorbed Production of intrinsic factor needed for absorption of vitamin B12 in the terminal ileum Regulation of the passage of gastric contents into the duodenum. When the chyme is sufficiently acidified and liquefied, the pyloric antrum forces small jets of gastric contents through the pyloric sphincter into the duodenum. Small Intestine The small intestine is the longest part of the gastrointestinal tract and extends from the pyloric orifice of the stomach to the ileocecal fold. This hollow tube, which is approximately 6 to 7 m long with a narrowing diameter from beginning to end, consists of the duodenum, the jejunum, and the ileum. Small Intestine The small intestine comprises three main sections continuous with each other: 1. The duodenum: It is about 25 cm long and curves around the head of the pancreas. Secretions from the gall bladder and pancreas are released into the duodenum through a common structure, the hepatopancreatic ampulla, and the opening into the duodenum is guarded by the hepatopancreatic sphincter (of Oddi). 2. The jejunum: It is the middle section of the small intestine and is about 2 meters long. 3. The ileum, or terminal section, is about 3 meters long and ends at the ileocaecal valve, which controls the flow of material from the ileum to the caecum, the first part of the large intestine, and prevents regurgitation. Small Intestine : Duodenum The first part of the small intestine is the duodenum. This C- shaped structure, adjacent to the head of the pancreas, is 20 to 25 cm long and is above the level of the umbilicus; its lumen is the widest of the small intestine. It is retroperitoneal except for its beginning, which is connected to the liver by the hepatoduodenal ligament, a part of the lesser omentum. Small Intestine : Duodenum The duodenum is divided into four parts : The superior part (first part) extends from the pyloric orifice of the stomach to the neck of the gallbladder, is just to the right of the body of vertebra LI, and passes anteriorly to the bile duct, gastroduodenal artery, portal vein, and inferior vena cava. Clinically, the beginning of this part of the duodenum is referred to as the ampulla or duodenal cap, and most duodenal ulcers occur in this part of the duodenum. Small Intestine : Duodenum The descending part (second part) of the duodenum is just to the right of midline and extends from the neck of the gallbladder to the lower border of vertebra LIII. Its anterior surface is crossed by the transverse colon, posterior to it is the right kidney, and medial to it is the head of the pancreas. This part of the duodenum contains the major duodenal papilla, which is the common entrance for the bile and pancreatic ducts, and the minor duodenal papilla, which is the entrance for the accessory pancreatic duct.  Small Intestine : Duodenum The inferior part (third part) of the duodenum is the longest section, crossing the inferior vena cava, the aorta, and the vertebral column. It is crossed anteriorly by the superior mesenteric artery and vein. The ascending part of the duodenum passes upward on, or to the left of, the aorta to approximately the upper border of vertebra LII and terminates at the duodenojejunal flexure. This duodenojejunal flexure is surrounded by a fold of peritoneum containing muscle fibers called the suspensory muscle (ligament) of duodenum (ligament of Treitz). Contracts to help the contents of the intestines move along. Helps anchor the duodenum (first part of the small intestine, just after the stomach). Is an important landmark in human anatomy, especially for surgeons. It forms the boundary between the upper GI tract to the lower GI tract and helps define what classifies as an upper or lower gastrointestinal bleed. Upper GI vs Lower GI tract The upper digestive tract includes the esophagus (the tube that connects the mouth to the stomach), the stomach, and the duodenum (the first part of the small intestine). The lower gastrointestinal tract or lower GI includes most of the small intestine, the large intestine, and the anus. Small Intestine : Jejunum The jejunum and ileum make up the last two sections of the small intestine. The jejunum represents the proximal two-fifths. It is mostly in the left upper quadrant of the abdomen and is larger in diameter and has a thicker wall than the ileum. Additionally, the inner mucosal lining of the jejunum is characterized by numerous prominent folds that circle the lumen (plicae circulares). Small Intestine : Ileum The ileum makes up the distal three-fifths of the small intestine and is mostly in the right lower quadrant. Compared to the jejunum, the ileum has thinner walls, fewer and less prominent mucosal folds (plicae circulares), shorter vasa recta, more mesenteric fat, and more arterial arcades. The ileum opens into the large intestine, where the cecum and ascending colon join together. Jejunum vs Ileum Large Intestine/Colon It is about 1.5 meters long, beginning at the caecum in the right iliac fossa and terminating at the rectum and anal canal deep in the pelvis. The general characteristics of most of the large intestine are: its large internal diameter compared to that of the small intestine; peritoneal-covered accumulations of fat (the omental appendices) are associated with the colon; the segregation of longitudinal muscle in its walls into three narrow bands (the taeniae coli), which are primarily observed in the cecum and colon and less visible in the rectum; and the sacculations of the colon (the haustra of the colon). Large Intestine/Colon The colon is divided into the caecum, ascending colon, transverse colon, descending colon, sigmoid colon rectum, and anal canal. The caecum/cecum This is the first part of the colon. It is a dilated region which has a blind end inferiorly and is continuous with the ascending colon superiorly. Just below the junction of the two, the ileocaecal valve opens from the ileum. The vermiform appendix is a fine tube, closed at one end, which leads from the caecum. It is usually about 13 cm long and has the same structure as the walls of the colon but contains more lymphoid tissue. The surface projection of the base of the appendix is at the junction of the lateral and middle one-third of a line from the anterior superior iliac spine to the umbilicus (McBurney’s point). People with appendicular problems may describe pain near this location.  Large Intestine/Colon The ascending colon The colon begins as the ascending colon, a retroperitoneal structure which ascends superiorly from the cecum. When it meets the right lobe of the liver, it turns 90 degrees to move horizontally. This turn is known as the right colic flexure (or hepatic flexure), and marks the start of the transverse colon. The transverse colon This is a loop of the colon which extends across the abdominal cavity in front of the duodenum and the stomach to the area of the spleen where it forms the splenic flexure and curves acutely downwards to become the descending colon. The descending colon This passes down the left side of the abdominal cavity then curves towards the midline. After it enters the true pelvis it is known as the sigmoid colon. The sigmoid colon This part describes an S-shaped curve in the pelvis then continues downwards to become the rectum. Rectum/Anal Canal It is a slightly dilated section of the colon about 13 cm long. It leads from the sigmoid colon and terminates in the anal canal. The anal canal is a short passage about 3.8 cm long in the adult and leads from the rectum to the exterior. Two sphincter muscles control the anus; the internal sphincter, consisting of smooth muscle fibers, is under the control of the autonomic nervous system and the external sphincter, formed by skeletal muscle, is under voluntary control. Colon, Rectum and Anal Canal : Functions Absorption The contents of the ileum which pass through the ileocaecal valve into the caecum are fluid, even though some water has been absorbed in the small intestine. In the large intestine absorption of water continues until the familiar semisolid consistency of feces is achieved. Mineral salts, vitamins, and some drugs are also absorbed into the blood capillaries from the large intestine. Microbial activity The large intestine is heavily colonized by certain types of bacteria, which synthesize vitamin K and folic acid. They include Escherichia coli, Enterobacter aerogenes, Streptococcus faecalis and Clostridium perfringens (welchii). Defecation Usually, the rectum is empty, but when a mass movement forces the contents of the sigmoid colon into the rectum the nerve endings in its walls are stimulated by a stretch. Defaecation involves involuntary contraction of the muscle of the rectum and relaxation of the internal anal sphincter. Contraction of the abdominal muscles and lowering of the diaphragm increase the intra-abdominal pressure (Valsalva’s maneuver) and so assist the process of defecation. Small Intestine vs Large Intestine EXTERNAL DIFFERENCES The small intestine is more mobile (except for the duodenum), whereas the ascending and the descending parts of the colon are fixed. The small intestine has a mesentery (except for the duodenum), whereas the large intestine is retroperitoneal (except for the transverse colon and sigmoid colon). The diameter of the full small intestine is smaller than that of the full large intestine. In the small intestine, the longitudinal muscle forms a continuous layer around the gut, whereas in the large intestine (except for the appendix, rectum, and anal canal), the longitudinal muscle forms three bands (the teniae coli). The small intestine has no fatty tags attached to its wall, whereas the large intestine has the appendices epiploicae. The wall of the small intestine is smooth, whereas the wall of the large intestine is sacculated. Small Intestine vs Large Intestine INTERNAL DIFFERENCES The mucous membrane of the small intestine has permanent folds ( the plicae circulares), whereas the large intestine does not. The mucous membrane of the small intestine has Peyer’s patches, whereas the large intestine has solitary lymph follicles. The mucous membrane of the small intestine has villi, whereas the large intestine does not. Mesenteries Mesenteries are peritoneal folds that attach viscera to the posterior abdominal wall. They allow some movement and provide a conduit for vessels, nerves, and lymphatics to reach the viscera and include: the mesentery—associated with parts of the small intestine, the transverse mesocolon—associated with the transverse colon, and the sigmoid mesocolon—associated with the sigmoid colon. All of these are derivatives of the dorsal mesentery. Mesentery The mesentery is a large, fan-shaped, double-layered fold of peritoneum that connects the jejunum and ileum to the posterior abdominal wall. Its superior attachment is at the duodenojejunal junction, just to the left of the upper lumbar part of the vertebral column. In the fat between the two peritoneal layers of the mesentery are the arteries, veins, nerves, and lymphatics that supply the jejunum and ileum. Transverse Mesocolon The transverse mesocolon is a fold of peritoneum that connects the transverse colon to the posterior abdominal wall. Its two layers of peritoneum leave the posterior abdominal wall across the anterior surface of the head and body of the pancreas and pass outward to surround the transverse colon. Between its layers are the arteries, veins, nerves, and lymphatics related to the transverse colon. Sigmoid Mesocolon The sigmoid mesocolon is an inverted, V-shaped peritoneal fold that attaches the sigmoid colon to the abdominal wall The Digestive Process 1. The start of the process - the mouth: The digestive process begins in the mouth. Food is partly broken down by the process of chewing and by the chemical action of salivary enzymes (these enzymes are produced by the salivary glands and break down starches into smaller molecules). A mouthful takes 30-60 seconds. 2. On the way to the stomach: the esophagus After being chewed and swallowed, the food enters the esophagus. The esophagus is a long tube that runs from the mouth to the stomach. It uses rhythmic, wave-like muscle movements (called peristalsis) to force food from the throat into the stomach. This muscle movement gives us the ability to eat or drink even when we're upside-down. For a medium-sized bolus, it takes about 5–8 seconds to reach the stomach. The Digestive Process 3. In the stomach The stomach is a large, sack-like organ that churns the food and bathes it in a very strong acid (gastric acid). Food in the stomach that is partly digested and mixed with stomach acids is called chyme. Emptying the stomach takes 2–6 hours. 4. In the small intestine After being in the stomach, food enters the duodenum, the first part of the small intestine. It then enters the jejunum and then the ileum (the final part of the small intestine). In the small intestine, bile (produced in the liver and stored in the gall bladder), pancreatic enzymes, and other digestive enzymes produced by the inner wall of the small intestine help in the breakdown of food. It takes 3–5 hours from entry to the duodenum to exit from the ileum. The Digestive Process 5. In the large intestine – After passing through the small intestine, food passes into the large intestine. In the large intestine, some of the water and electrolytes (chemicals like sodium) are removed from the food. Many microbes (bacteria like Bacteroides, Lactobacillus acidophilus, Escherichia coli, and Klebsiella) in the large intestine help in the digestion process. The first part of the large intestine is called the cecum (the appendix is connected to the cecum). Food then travels upward in the ascending colon. The food travels across the abdomen in the transverse colon, goes back down the other side of the body in the descending colon, and then through the sigmoid colon. Residence time in the colon ranges from 4–72 hours, with a normal average of 36 hours. 6. The end of the process - Solid waste is then stored in the rectum until it is excreted via the anus. ACCESORY ORGANS OF THE DIGESTIVE SYSTEM Salivary glands Salivary glands are glands that open or secrete into the oral cavity. Most are small glands in the submucosa or mucosa of the oral epithelium lining the tongue, palate, cheeks, and lips, and open into the oral cavity directly or via small ducts. In addition to these small glands are much larger glands, which include the paired parotid, submandibular, and sublingual glands. Parotid gland The parotid gland on each side is entirely outside the boundaries of the oral cavity in a shallow triangular-shaped trench formed by: the sternocleidomastoid muscle behind; the ramus of mandible in front; and superiorly, the base of the trench is formed by the external acoustic meatus and the posterior aspect of the zygomatic arch. Parotid gland The gland normally extends anteriorly over the masseter muscle, and inferiorly over the posterior belly of the digastric muscle. The parotid duct or Stensen’s duct passes anteriorly across the external surface of the masseter muscle and then turns medially to penetrate the buccinator muscle of the cheek and open into the oral cavity adjacent to the crown of the second upper molar tooth.  Submandibular gland The elongate submandibular glands are smaller than the parotid glands, but larger than the sublingual glands. Each is hook shaped : the larger arm of the hook is directed forward in the horizontal plane below the mylohyoid muscle and is therefore outside the boundaries of the oral cavity-this larger superficial part of the gland is directly against a shallow impression on the medial side of the mandible (submandibular fossa) inferior to the mylohyoid line; the smaller arm of the hook (or deep part) of the gland loops around the posterior margin of the mylohyoid muscle to enter and lie within the floor of the oral cavity where it is lateral to the root of the tongue on the lateral surface of the hyoglossus muscle. Submandibular gland The submandibular duct or Wharton’s duct emerges from the medial side of the deep part of the gland in the oral cavity and passes forward to open on the summit of a small sublingual caruncle (papilla) beside the base of frenulum of the tongue. The lingual nerve loops under the submandibular duct, crossing first the lateral side and then the medial side of the duct, as the nerve descends anteromedially through the floor of the oral cavity and then ascends into the tongue. Sublingual glands The sublingual glands are the smallest of the three major paired salivary glands. Each is almond shaped and is immediately lateral to the submandibular duct and associated lingual nerve in the floor of the oral cavity. Each sublingual gland lies directly against the medial surface of the mandible where it forms a shallow groove (sublingual fossa) superior to the anterior one- third of the mylohyoid line. The sublingual gland drains into the oral cavity via numerous small ducts (minor sublingual ducts or Ducts of Rivinus), which open onto the crest of the sublingual fold. Occasionally, the more anterior part of the gland is drained by a duct (major sublingual duct or Bartholin’s duct) that opens together with the submandibular duct on the sublingual caruncle.  Pancreas The pancreas lies mostly posterior to the stomach. It extends across the posterior abdominal wall from the duodenum, on the right, to the spleen, on the left. The pancreas is (secondarily) retroperitoneal except for a small part of its tail and consists of a head, uncinate process, neck, body, and tail. The head of the pancreas lies within the C-shaped concavity of the duodenum. Projecting from the lower part of the head is the uncinate process, which passes posterior to the superior mesenteric vessels. The neck of the pancreas is anterior to the superior mesenteric vessels. Posterior to the neck of the pancreas, the superior mesenteric and splenic veins join to form the portal vein. The body of the pancreas is elongate and extends from the neck to the tail of the pancreas. The tail of the pancreas passes between layers of the splenorenal ligament. Pancreas The pancreatic duct begins in the tail of the pancreas. It passes to the right through the body of the pancreas and, after entering the head of the pancreas, turns inferiorly. In the lower part of the head of the pancreas, the pancreatic duct joins the bile duct. The joining of these two structures forms the hepatopancreatic ampulla (ampulla of Vater), which enters the descending (second) part of the duodenum at the major duodenal papilla. Surrounding the ampulla is the sphincter of ampulla (sphincter of Oddi), which is a collection of smooth muscles. The accessory pancreatic duct empties into the duodenum just above the major duodenal papilla at the minor duodenal papilla. Exocrine Pancreas It consists of a large number of lobules made up of small alveoli, the walls of which consist of secretory cells. Each lobule is drained by a tiny duct and these unite eventually to form the pancreatic duct, which extends the whole length of the gland and opens into the duodenum. Just before entering the duodenum the pancreatic duct joins the common bile duct to form the hepatopancreatic ampulla. The duodenal opening of the ampulla is controlled by the hepatopancreatic sphincter (of Oddi). The function of the exocrine pancreas is to produce pancreatic juice containing enzymes that digest carbohydrates, proteins and fats. Endocrine Pancreas Distributed throughout the gland are groups of specialized cells called the pancreatic islets (of Langerhans). Alpha cells – glucagon Beta cells – insulin Delta cells – somatostatin PP cells – pancreatic polypeptide The islets have no ducts so the hormones diffuse directly into the blood. The function of the endocrine pancreas is to secrete the hormones insulin and glucagon, which are principally concerned with control of blood glucose levels. Function of the Pancreas As part of the exocrine system, the pancreas secretes enzymes that work in tandem with bile from the liver and gallbladder to help break down substances for proper digestion and absorption. Enzymes produced by the pancreas for digestion include: lipase to digest fats amylase to digest carbohydrates chymotrypsin and trypsin for digesting proteins The pancreas produces enzymes as soon as food reaches the stomach. These enzymes travel through a series of ducts until they reach the main pancreatic duct. The main pancreatic duct meets the common bile duct, which carries bile from the gallbladder and liver towards the duodenum. This meeting point is called the ampulla of Vater. Bile from the gallbladder and enzymes from the pancreas are released into the duodenum to help digest fats, carbohydrates, and proteins so they can be absorbed by the digestive system. Function of the Pancreas As part of the endocrine system, the pancreas secretes two main hormones that are vital to regulating glucose (also known as blood sugar) level: Insulin. The pancreas secretes this hormone to lower blood glucose when levels get too high. Glucagon: The pancreas secretes this hormone to increase blood glucose when levels get too low. Balanced blood glucose levels play a significant role in the liver, kidneys, and even brain. Proper secretion of these hormones is important to many bodily systems, such as your nervous system and cardiovascular system. Liver The liver is the largest gland in the body, weighing between 1 and 2.3 kg. It is situated in the upper part of the abdominal cavity occupying the greater part of the right hypochondriac region, part of the epigastric region and extending into the left hypochondriac region. Its upper and anterior surfaces are smooth and curved to fit the undersurface of the diaphragm - diaphragmatic surface ; its posterior surface is irregular in outline – visceral surface. The liver is attached to the anterior abdominal wall by the falciform ligament and, except for a small area of the liver against the diaphragm (the bare area), the liver is almost completely surrounded by visceral peritoneum. The bare area of the liver is a part of the liver on the diaphragmatic surface where there is no intervening peritoneum between the liver and the diaphragm. Liver The liver is divided into right and left lobes by the falciform ligament anterosuperiorly and the fissure for the ligamentum venosum and ligamentum teres on the visceral surface. The right lobe of the liver is the largest lobe, whereas the left lobe of the liver is smaller. The quadrate and caudate lobes are described as arising from the right lobe of the liver but functionally are distinct. Gall Bladder The gallbladder is 8–10 cm (~3–4 in) long and is nested in a shallow area on the posterior aspect of the right lobe of the liver. This muscular sac stores, concentrates, and, when stimulated, propels the bile into the duodenum via the common bile duct. It is divided into three regions. The fundus is the widest portion and tapers medially into the body, which in turn narrows to become the neck. The neck angles slightly superiorly as it approaches the hepatic duct. The cystic duct is 1–2 cm (less than 1 in) long and turns inferiorly as it bridges the neck and hepatic duct. Gall Bladder The simple columnar epithelium of the gallbladder mucosa is organized in rugae, similar to those of the stomach. There is no submucosa in the gallbladder wall. The wall’s middle, muscular coat is made of smooth muscle fibers. When these fibers contract, the gallbladder’s contents are ejected through the cystic duct and into the bile duct. Visceral peritoneum reflected from the liver capsule holds the gallbladder against the liver and forms the outer coat of the gallbladder. The gallbladder’s mucosa absorbs water and ions from bile, concentrating it by up to 10-fold. Bile The cells of the liver produce about 800 to 1,000 milliliters (about 27 to 34 fluid ounces) of bile every day. Bile is a yellow, brownish or olive-green liquid that helps our body digest fats. The liver cells secrete the bile into small canals that lead to the common bile duct. From there, a smaller duct branches off and leads to the gallbladder. The common bile duct ends at the small intestine. The bile produced by the liver flows directly into the small intestine during a meal. Between meals, when there's no fat that needs to be digested, most of the bile flows into the gallbladder instead, where it is concentrated and stored. The gallbladder usually holds about 30 to 80 milliliters (about 1 to 2.7 fluid ounces) of fluid. When we eat fatty foods, the gallbladder contracts and squeezes bile through the bile duct. The bile is mixed into the semi-digested food in the small intestine. Bile is mainly made up of water, but also has bile salts, cholesterol, certain fats (lecithin) and bile pigments in it. The most important bile pigment, bilirubin, is made when red blood cells are broken down in the liver. Bilirubin is what makes urine yellow and stool brown. Bile salts break down larger fat globules in food into small droplets of fat. Smaller fat droplets are easier for the digestive enzymes from the pancreas to process and break down. The bile salts also help the cells in the bowel to absorb these fat droplets. Duct System for the Bile The duct system for the passage of bile extends from the liver, connects with the gallbladder, and empties into the descending part of the duodenum. The coalescence of ducts begins in the liver parenchyma and continues until the right and left hepatic ducts are formed. These drain the respective lobes of the liver. The two hepatic ducts combine to form the common hepatic duct, which runs near the liver, with the hepatic artery proper and portal vein in the free margin of the lesser omentum. As the common hepatic duct continues to descend, it is joined by the cystic duct from the gallbladder. This completes the formation of the common bile duct. The bile duct continues to descend, passing posteriorly to the superior part of the duodenum before joining with the pancreatic duct to enter the descending part of the duodenum at the major duodenal papilla Cholelithiasis THANK YOU !