Digestive System PDF

Digestive system By: Fikre Bayu (MSc) Digestive system  General objectives  Trace the path of food through the alimentary canal, and describe the general structure and function of each organ mentioned  Explain the four layers that form the wall of the GIT  List the major processes...

Digestive system By: Fikre Bayu (MSc) Digestive system  General objectives  Trace the path of food through the alimentary canal, and describe the general structure and function of each organ mentioned  Explain the four layers that form the wall of the GIT  List the major processes that occur during digestion  Describe the wall of the small intestine and relate its anatomy to nutrient absorption  Name the hormones produced by the alimentary canal that help control digestive secretions  Name five accessory organs of digestion  Describe the location, anatomy, and functions of the teeth, tongue, pancreas, the liver and the gallbladder  Draw the major subdivisions of the anterior abdominal wall  Introduction  The alimentary canal also called the gastrointestinal (GI) tract, is the muscular digestive tube that winds through the body, extending from the mouth to the anus  The organs of the alimentary canal are the mouth, pharynx, esophagus, stomach, small intestine (small bowel), and large intestine (large bowel), the last of which leads to the terminal opening, or anus  In a cadaver, the alimentary canal is about 9m (30 feet) long, but in a living person it is considerably shorter because of its muscle tone  Cont.…  The accessory digestive organs are the teeth and tongue, plus the gallbladder and various large digestive glands (salivary glands, liver and pancreas) that lie external to and are connected to the alimentary canal by ducts The accessory digestive glands secrete saliva, bile and digestive enzymes, all of which contribute to the breakdown of foodstuffs Processes of Digestion  The organs of the digestive system perform the following six essential food processing activities:  ingestion,  Propulsion  Mechanical digestion  chemical digestion  Absorption  defecation Divisions of the anterior abdominal wall The parasagittal planes are the midclavicular lines; the transverse planes are the subcostal plane superiorly and the transtubercle plane inferiorly Abdominal regions Abdominal quadrants Appendix Peritoneum and peritoneal cavity  The digestive organs in the abdominopelvic cavity all develop surrounded by peritoneum and the peritoneal cavity  The visceral peritoneum covers the external surfaces of most digestive organs and continuous with the parietal peritoneum, which lines the body wall  Between the visceral and parietal peritonea is the peritoneal cavity, a slit like potential space between the digestive organs and the abdominal body wall  The peritoneal cavity contains a lubricating serous fluid that is secreted by the peritoneum and allows the digestive organs to glide easily along one another and along the body wall as they move during digestion Cont.…  Mesentery  A mesentery (double layer of peritoneum) is a sheet of two serous membranes fused back to back that extends to the digestive organs from the body wall  Mesenteries hold the organs in place, are sites of fat storage, and provide a route by which circulatory vessels and nerves reach the organs in the peritoneal cavity  Some mesenteries are called “ligaments” even though these peritoneal sheets are not the same as the fibrous ligaments that interconnect bones  Most mesenteries are dorsal mesenteries, extending dorsally from the alimentary canal to the posterior abdominal wall  Such as greater omentum, gastrosplenic ligament and splenorenal ligament, gastrophrenic ligament Cont.… Cont.…  Cont.…  In the superior abdomen, however, a ventral mesentery extends ventrally from the stomach and liver to the anterior abdominal wall  Such as falciform ligament and lesser omentum) Summary of Intraperitoneal and Secondarily Retroperitoneal Digestive Organs in the Abdomen and Pelvis  Anatomy and histology of GIT  Each part of the alimentary tract has a highly specialized function but the basic structure of the tube is the same throughout its length  From the esophagus to the anal canal, the walls of every organ of the GIT is made up of the same four basic layers  From internal to external the four layers are 1. Mucosa 2. Submucosa 3. Muscularis externa 4. Serosa/adventitia  Each tunic contains a predominant tissue type that plays a specific role in food breakdown Cont.…  1. Mucosa  The innermost layer is the mucosa, or mucous membrane  More complex than other mucous membranes in the body, the typical digestive mucosa contains three sublayers: (1) a lining epithelium, (2) a lamina propria, and (3) a Muscularis mucosae Function – Secretion of mucus, digestive enzymes and hormones – Absorption of digestion end products to blood and lymph – Protective barrier  Epithelium Non-keratinized stratified squamous in mouth, pharynx, esophagus and anal canal (protective) Simple columnar epithelium with mucus secreting goblet cells in stomach and intestine (secretion and absorption)  Cell types – Absorptive cells: absorb nutrients – Exocrine cells: secrete mucus and fluid – Enteroendocrine cells: secrete hormones  Lamina propria  Loose areolar or reticular connective tissue  Contains many blood and lymphatic vessels  Its capillaries nourish the epithelium and absorb digested nutrients  Supports epithelium and binds it to muscularis mucosae  Its isolated lymph nodules are part of the mucosa associated lymphatic tissue (MALT); defense against pathogens  Large collections of lymph nodules occur at strategic locations; pharynx (tonsils) and appendix  Muscularis mucosae Is a scanty layer of smooth muscle cells that produces local movements and folding of the mucosa The twitching of this muscle layer dislodges food particles that have adhered to the mucosa In the small intestine, it throws the mucosa into a series of small folds that immensely increase its surface area  2. Submucosa Composed of moderately dense collagen fibers Binds mucosa to muscularis externa Contains many blood vessels, lymphatic vessels, lymph nodules, glands, and nerve fibers Consists of submucosal (Meissner’s) plexus – formed by nerve fibres and ganglion cells – controls glandular secretion Its rich elastic fibers enables the stomach to regain its normal shape after storing a large meal  3. Muscularis externa  Main muscle coat  consists of inner circular muscle and outer longitudinal muscle layers  In several places along the GI tract, the circular layer thickens to form sphincters – Sphincters act as valves to prevent backflow and control food passage from one organ to the next  Responsible for segmentation and peristalsis  In the mouth, pharynx, superior and middle esophagus, and external anal sphincter contains skeletal muscle  The rest contains smooth muscle  Between the two layers are myenteric (Auerbach’s) plexus  4. Serosa/Adventitia  Serosa is a protective outermost layer of Intraperitoneal organ  Formed of areolar connective tissue lined by simple squamous epithelium (mesothelium)  In the esophagus, which is located in thoracic cavity, the serosa is replaced by an adventitia  The adventitia is a fibrous connective tissue that binds the esophagus to surrounding structures  Retroperitoneal organs have both a serosa (on the side facing the peritoneal cavity) and an adventitia (on the side abutting the dorsal body wall)  Mouth and associated organs  Food enters the alimentary canal through the mouth, where it is chewed by teeth, manipulated by the tongue, and moistened with saliva  The mouth/oral cavity is a mucosa-lined cavity whose boundaries are the lips anteriorly, the cheeks laterally, the palate superiorly, and the tongue inferiorly  Its anterior opening is the oral orifice  Posteriorly, the mouth borders the fauces of the oropharynx  The mouth is divided into the vestibule and oral cavity proper  The vestibule is the slit between the teeth and the cheeks/lips  The oral cavity proper is the region of the mouth that lies internal to the teeth Cont.…  Histology of mouth The walls of the oral cavity consist of just a few layers of tissue an internal mucosa made of an epithelium and lamina propria only, a thin Submucosa in some areas, and an external layer of muscle or bone The lining of the mouth, a thick stratified squamous epithelium, protects it from abrasion by sharp pieces of food during chewing On the tongue, palate, lips, and gums this epithelium may show slight keratinization, which provides extra protection against abrasion  Cont.…  The lips/labia and the cheeks, which help keep food inside the mouth during chewing, are composed of a core of skeletal muscle covered by skin  Whereas the cheeks are formed largely by the Buccinator muscles, the orbicularis oris muscle forms the bulk of the lips  The lips are thick flaps extending from the inferior boundary of the nose to the superior boundary of the chin  The region of the lip where one applies lipstick or lands a kiss is called the transition part, a zone where the highly keratinized skin meets the oral mucosa  This region is poorly keratinized and translucent, so it derives its reddish color from blood in the underlying capillaries  The labial frenulum is a median fold that connects the internal aspect of each lip to the gum  Palate  The palate, which forms the roof of the mouth, has two distinct parts: the hard palate anteriorly and the soft palate posteriorly  The bony hard palate forms a rigid surface against which the tongue forces food during chewing  The muscular soft palate is a mobile flap that rises to close off the nasopharynx during swallowing  Laterally, the soft palate is anchored to the tongue by the palatoglossal arches and to the wall of the oropharynx by the palatopharyngeal arches  These two folds form the boundaries of the fauces, the arched area of the oropharynx that contains the palatine tonsils  Tongue  The tongue, which occupies the floor of the mouth is predominantly a muscle constructed of interlacing fascicles of skeletal muscle fibers  During chewing, the tongue grips food and constantly repositions it between the teeth  Tongue movements also mix the food with saliva and form it into a compact mass called a bolus then, during swallowing, the tongue moves posteriorly to push the bolus into the pharynx  In speech, the tongue helps to form some consonants Finally, it houses most of the taste buds  Cont.…  The tongue has both intrinsic and extrinsic muscle fibers  The intrinsic muscles, which are confined within the tongue and are not attached to bone, have fibers that run in several different planes  These intrinsic muscles change the shape of the tongue, for example rolling the tongue, but do not change its position  The extrinsic muscles extend to the tongue from bones of the skull and the hyoid bone  These extrinsic muscles alter the position of the tongue  They protrude it, retract it, and move it laterally  The tongue is divided by a median septum of connective tissue, and both halves contain identical groups of muscles  Cont.…  A fold of mucosa on the undersurface of the tongue, the lingual frenulum secures the tongue to the floor of the mouth and limits its posterior movements  Individuals in which the lingual frenulum is abnormally short or extends exceptionally far anteriorly are said to be “tongue tied,” and their speech is distorted because movement of the tongue is restricted  This congenital condition, called (fused tongue), is corrected surgically by snipping the frenulum  The dorsal surface of the tongue is covered with three major types of peg like projections of the mucosa (filiform, fungiform, foliate and vallate papillae)  The terms papillae and taste buds are not synonymous  foliate, fungiform and vallate papillae contain the taste buds  Cont.… 1. Filiform Papillae  have an elongated conical shape  are quite numerous and are present over the entire surface of the tongue  their epithelium, which does not contain taste buds, is keratinized stratified squamous epithelium 2. Fungiform Papillae  resemble mushrooms in that they have a narrow stalk and a smooth-surfaced, dilated upper part  these papillae, which contain scattered taste buds on their upper surfaces, are irregularly interspersed among the filiform papillae Tongue papillae  Cont.… 3. Foliate Papillae – poorly developed in humans – They consist of two or more parallel ridges and furrows on the dorsolateral surface of the tongue – contain many taste buds 4. Vallate (or circumvallate) papillae  are the least numerous and largest lingual papillae  With diameters of 1 - 3 mm, 7 to 12 circular vallate papillae  form a V-shaped line just before the terminal sulcus  Ducts from several serous salivary (von Ebner) glands empty into the deep groove that surrounds each vallate papillae  These glands also secrete a lipase that prevents the formation of a hydrophobic film over the taste buds that would hinder their function  Cont.... The posterior third of the tongue, which lies in the oropharynx, not in the mouth, is covered not with papillae but with the bumpy lingual tonsil  Teeth  The teeth lie in sockets (alveoli) in the gum covered margins of the mandible and maxillae  We masticate or chew by raising and lowering the mandible and by moving it from side to side while using the tongue to position food between the teeth  In the process, the teeth tear and grind the food, breaking it into smaller fragments  During their lifetime, humans have two sets of teeth  By age 21, the primary dentition, called the deciduous teeth has been replaced by the permanent dentition Human Deciduous and Permanent Teeth  Cont.…  At about 6 months after birth, the lower central incisors become the first of the deciduous teeth to appear  Additional pairs of teeth erupt at varying intervals until all 20 deciduous teeth have emerged, by about 2 years of age  As the deep lying permanent teeth enlarge and develop, the roots of the deciduous teeth are resorbed until these teeth loosen and fall out, typically between the ages of 6 and 12 years  Generally, by the end of adolescence, all permanent teeth have erupted except for the third molars (also called wisdom teeth), which emerge between the ages of 17 and 25 years  There are 32 permanent teeth in a full set, but in some people the wisdom teeth are either completely absent or fail to erupt  Cont.…  Teeth are classified according to their shape and function as incisors, canines, premolars and molars  The chisel-shaped incisors are adapted for nipping off pieces of food, and the cone-shaped canines(eyeteeth) specialized for tear and pierce  The premolars(bicuspids) and molars have broad crowns with rounded cusps(surface bumps) for grinding food  The molars “millstones” have four or five cusps and are the best grinders  During chewing, the upper and lower molars lock repeatedly together, the cusps of the uppers fitting into the valleys in the lowers, and vice versa  Generates tremendous crushing forces  Cont.…  The dental formula is a shorthand way of indicating the numbers and relative positions of the different classes of teeth in the mouth The formula for the permanent dentition (two incisors, one canine, two premolars, and three molars) is written as follows:  Cont.… The upper teeth are innervated by the superior alveolar nerves, branches of the maxillary division of the trigeminal nerve (cranial nerve V) The lower teeth are innervated by the inferior alveolar nerves, branches of the mandibular division of the trigeminal nerve  The arterial supply for the teeth comes via the superior and inferior alveolar arteries, which are branches of the maxillary artery from the external carotid artery  Tooth structure Each tooth has two main regions, the exposed crown and the root(s)in the socket  These regions meet at the neck near the gum line The surface of the crown, which bears the forces of chewing, is covered by a layer of enamel, the hardest substance in the body, that is 0.96–1.6 mm thick Enamel lacks cells and vessels, and 99% of its mass consists of densely packed hydroxyapatite crystals (the same calcium salts found in bone) arranged in force resisting rods or prisms oriented perpendicular to the tooth’s surface  Cont.…  Dentine underlies the enamel cap and forms the bulk of the tooth  This is a bonelike tissue with mineral and collagen components, but it is harder than bone and lacks internal blood vessels  Dentin contains unique radial striations called dentinal tubules  Cont.…  The pulp cavity in the center of the tooth, is filled with dental pulp, a loose connective tissue containing the tooth’s vessels and nerves  Pulp supplies nutrients for the tooth’s hard tissues  The part of the pulp cavity in the root is the root canal; the opening into the root canal at the tip of each root is the apical foramen  When a tooth is damaged by a blow or by a deep cavity, the pulp may die and become infected  In such cases, root canal therapy must be performed  In this procedure, all of the pulp is drilled out, and the pulp cavity is sterilized and filled with an artificial, inert material before the tooth is capped  Cont.…  The external surface of the tooth root is covered by a calcified connective tissue called cementum  Essentially a bone layer, cementum attaches the tooth to the periodontal ligament “around the tooth”) or periodontium  The periodontal ligament is continuous with the gum, or gingiva at the neck of the tooth  Cont.…  Dental cavities “rottenness” result from a gradual demineralization of the enamel and dentine by bacterial action  The decay process begins with the accumulation of dental plaque, a film of sugar, bacteria and other debris that adheres to the teeth  Metabolism of the trapped sugars by the bacteria produces acids, which dissolve the calcium salts from the teeth  Once the salts have leached out, the remaining organic matrix is broken down by protein-digesting bacterial enzymes  Frequent brushing helps prevent tooth decay by removing plaque  The salivary glands  The salivary glands produce saliva, a complex mixture of water, ions, mucus, and enzymes that performs many functions: It moistens the mouth, dissolves food chemicals so that they can be tasted, wets food, and binds the food together into a bolus, and its enzymes begin the digestion of carbohydrates  Saliva contains a bicarbonate buffer that neutralizes the acids that are produced by oral bacteria and that initiate tooth decay  Additionally, it contains bactericidal enzymes, antiviral substances, antibodies, and a cyanide compound, all of which kill harmful oral microorganisms  Saliva also contains proteins that stimulate the growth of beneficial bacteria to outcompete harmful bacteria in the mouth  All salivary glands are compound tubuloalveolar glands  Small intrinsic salivary glands are scattered within the mucosa of the tongue, palate, lips and cheeks  Saliva from these glands keeps the mouth moist at all times  By contrast, large extrinsic salivary glands, which lie external to the mouth but connect to it through their ducts secrete saliva only during eating or anticipation of a meal, causing the mouth to water  These paired extrinsic glands are the parotid, submandibular and sublingual glands Major salivary glands  Parotid gland  The largest extrinsic gland  True to its name (par near; otid the ear), it lies anterior to the ear, between the masseter muscle and the skin  Its parotid duct runs parallel to the zygomatic arch, penetrates the muscle of the cheek, and opens into the mouth lateral to the second upper molar tooth  Because the branches of the facial nerve run through the parotid gland on their way to the muscles of facial expression, surgery on this gland can lead to facial paralysis  Submandibular gland  The submandibular gland, which is about the size of a walnut, lies along the medial surface of the mandibular body, just anterior to the angle of the mandible  is a branched tubuloacinar gland with secretory portions containing both mucous and serous cells  Most of the secretory units in this gland are serous acinar, with about 10% consisting of mucous tubules capped with serous cells. Such caps are called serous demilunes  Its duct raises the mucosa of the floor of the mouth and opens directly lateral to the tongue’s lingual frenulum Cont.…  Sublingual gland  The sublingual gland lies in the floor of the oral cavity, inferior to the tongue  mucous cells predominate, with serous cells only present in demilunes on mucous tubules  Its 10 to 12 ducts open into the mouth, directly superior to the gland  Pharynx  From the mouth, swallowed food passes posteriorly into the oropharynx and then the laryngopharynx both of which are passageways for food, fluids, and inhaled air  The histology of the pharyngeal wall resembles that of the mouth  The oropharynx and laryngopharynx are lined by non keratinized stratified squamous epithelium, which protects them against abrasion  The external muscle layer consists primarily of three (superior, middle, and inferior) pharyngeal constrictor muscles of swallowing that encircle the pharynx and partially overlap one another  The pharyngeal constrictors are skeletal muscles, as swallowing is a voluntary action, innervated by the vagus nerve (CN X)  Cont.…  The internal muscles of the pharynx include Stylopharygeus, Palatopharyngeus, Salpingopharyngeus  These muscles elevate the larynx and pharynx in swallowing and during speaking  the inferior constrictor muscle prevents air from entering the esophagus during breathing  Esophagus  is a muscular tube (25cm) whose function is to transport food from the pharynx to the stomach  has the same major layers as the rest of the digestive tract  lined by non-keratinized stratified squamous epithelium with stem cells scattered throughout the basal layer  In the lamina propria of the region near the stomach are groups of glands, the esophageal cardiac glands, which also secrete mucus  In the submucosa are groups of small mucus-secreting glands, the esophageal glands, secretions of which facilitate the transport of food and protect the mucosa Cont.…  Stomach  The J-shaped organ and the widest part of the alimentary canal, is a temporary storage tank in which food is churned and turned into a paste called chyme  The stomach also starts the breakdown of food proteins by secreting pepsin, a protein-digesting enzyme that can function only under acidic conditions, and hydrochloric acid, a strong acid that destroys many harmful bacteria in the food  Although most nutrients are absorbed in the small intestine, some substances are absorbed through the stomach, including water, electrolytes, and some drugs (aspirin and alcohol)  Cont.…  The stomach extends from the esophagus to the small intestine  The stomach lies in the superior left part of the peritoneal cavity, in the left hypochondriac, epigastric and umbilical regions of the abdomen  It is directly inferior to the diaphragm and anterior to the spleen and pancreas  Its upper part is hidden behind the left side of the liver  Although the stomach is anchored at both ends by esophageal and intestinal attachments, it is quite mobile in between  Cont.…  The main regions of the stomach are:  The cardiac region (“near the heart”), is a ring-shaped zone encircling the cardiac orifice at the junction with the esophagus  The fundus (the stomach’s dome) is tucked under the diaphragm  The large midportion of the stomach, the body, ends at the funnel-shaped pyloric region, composed of the wider pyloric antrum(“cave”) and the narrower pyloric canal  The pyloric region ends at the terminus of the stomach, the pylorus(“gatekeeper”) containing the pyloric sphincter, which controls the entry of chyme into the intestine  Cont.…  The convex left surface of the stomach is its greater curvature, and the concave right margin is the lesser curvature  The greater and lesser omenta, mesenteries that connect to the stomach that are named for their attachment to these curvatures  The stomach’s structure accounts for its great distensibility—it easily holds 1.5 liters of food and has a maximum capacity of about 4 liters (1 gallon)  The internal surface of the empty stomach contains numerous longitudinal folds of mucosa called rugae ( “wrinkles”) which flatten as the stomach fills; the resulting expansion in volume accommodates the increasing quantity of food within the stomach  Vessels and nerves of stomach  The stomach is innervated by sympathetic fibers that derive from the thoracic splanchnic nerves by way of the celiac plexus, and by parasympathetic fibers that derive from the vagus  The stomach contains no submucosal nerve plexus, so the myenteric plexus innervates its mucosa as well as its Muscularis externa  The arteries to the stomach arise from the celiac trunk and include the right and left gastric, short gastric, and right and left gastroepiploic arteries; the corresponding veins drain into the portal, splenic and superior mesenteric veins  Stomach: Microscopic Anatomy  The stomach wall exhibits the four tunics of most of the alimentary canal but its muscularis externa and mucosa are modified for the special roles of stomach – The muscularis externa has an extra oblique layer of muscle that enables it to mix and churn food – The epithelium lining the stomach mucosa is simple columnar epithelium composed entirely of goblet cells, which produce a protective coating of mucus  Cont.…  The surface of the stomach mucosa is dotted with millions of cup-shaped gastric pits, which open into tubular gastric glands  Surface mucous cells invariably line the gastric pits, but the cells lining the gastric glands vary among the different regions of the stomach  In the pyloric and cardiac regions the cells of the glands are primarily mucous cells  In the fundus and body, by contrast, the gastric glands contain three types of secretory cells: mucous neck cells, parietal (oxyntic) cells, and chief (zymogenic) cells  Cont.… 1. Mucus neck cells – In upper part of gland – Produce a different type of mucus from that secreted by the mucus secreting cells of the surface epithelium – The special function of this unique mucus is not yet understood  Cont.… 2. Parietal (Oxyntic) cells – In the middle section of the glands – Scattered among the chief cells – Secrete hydrochloric acid (HCl) and intrinsic factor Intrinsic factor is required for absorption of B12 in the small intestine 3. Chief (Zymogen) cells – Occur mainly in the basal regions of the gastric glands – Produce pepsinogen, inactive form of the protein- digesting enzyme pepsin Pepsinogen is activated by HCl – Also secrete small amounts of lipases  Cont.… 4. Enteroendocrine cells – In the base of glands – Release a variety of hormones directly into the lamina propria  Reverse polarity – secrete hormones into the blood space rather than the GI lumen – These products diffuse into capillaries and ultimately influence several digestive system target organs which regulate stomach secretion and mobility  Small intestine  The small intestine is the longest part of the alimentary canal and the site of most enzymatic digestion and virtually all absorption of nutrients  Most digestive enzymes that operate within the small intestine are secreted not by the intestine, but by the pancreas  During digestion, the small intestine undergoes active segmentation movements, shuffling the chyme back and forth and thereby maximizing its contact with the nutrient-absorbing mucosa  Peristalsis propels chyme through the small intestine in about 3–6 hours  Cont.…  The small intestine is a convoluted tube that runs from the pyloric sphincter, in the epigastric region of the abdomen, to the first part of the large intestine, in the lower right quadrant  It is shorter in living people (2.7–5 meters) than in preserved cadavers (6–7 meters), where loss of muscle tone and the effects of preservatives have caused it to lengthen  Cont.…  The small intestine has three subdivisions are the duodenum “twelve finger widths long”, the jejunum “empty”, and the ileum “twisted intestine”), which contribute 5%, almost 40%, and almost 60% of the length of the small intestine respectively  Whereas most of the C-shaped duodenum lies secondarily retroperitoneal, the jejunum and ileum form sausage like coils that hang from the posterior abdomen by the mesentery and are framed by the large intestine  The jejunum makes up the superior left part of this coiled intestinal mass, whereas the ileum makes up the inferior right part  Cont.…  Even though the duodenum is the shortest subdivision of the small intestine, it has the most features of interest  It receives digestive enzymes from the pancreas via the main pancreatic duct and bile from the liver and gallbladder via the bile duct  These ducts enter the wall of the duodenum where they form a bulb called the hepatopancreatic ampulla “flask from the liver and pancreas”)  This ampulla opens into the duodenum via a mound called the major duodenal papilla  Entry of bile and pancreatic juice into the duodenum is controlled by sphincters of smooth muscle that surround the hepatopancreatic ampulla and the ends of the pancreatic and bile ducts  Nerves and blood vessels of SI The small intestine is innervated by parasympathetic fibers from the vagus and by sympathetic fibers from the thoracic splanchnic nerves, both relayed through the superior mesenteric (and celiac) plexus  Its arterial supply comes primarily via the superior mesenteric artery The veins run parallel to the arteries and typically drain into the superior mesenteric vein; from there, the nutrient-rich venous blood drains into the hepatic portal vein, which carries it to the liver  Cont.…  There are three structural modifications which increase the surface area for absorption (Plicae circularis, Villi and Microvilli  Structural modifications increase the intestinal surface area tremendously  the surface area of the small intestine is equal to 200 m2  Cont.… 1. Absorptive cells: contain many mitochondria because the uptake of digested nutrients is an energy-demanding process  They also contain an abundant endoplasmic reticulum, which assembles the newly absorbed lipid molecules into lipid-protein complexes called chylomicrons  Once made, the chylomicrons enter the lacteal capillaries, so it is in this form that absorbed fat enters the circulation 2. Goblet cells: secrete onto the internal surface of the intestine a coat of mucus that lubricates the chyme and forms a protective barrier that prevents enzymatic digestion of the intestinal wall  Cont.… 3. Enteroendocrine cells: secrete several hormones, which signal the gallbladder to release stored bile and the pancreas to secrete digestive enzymes and a bicarbonate rich juice to neutralize the acidic chyme entering the duodenum  Cont.…  Between the villi, the mucosa contains invaginations called intestinal crypts, or crypts of Lieberkühn  The epithelial cells that line these crypts secrete intestinal juice, a watery liquid that mixes with chyme in the intestinal lumen  Undifferentiated epithelial cells lining the intestinal crypts renew the mucosal epithelium by dividing rapidly and moving continuously onto the villi  These are among the most quickly dividing cells of the body, completely renewing the inner epithelium of the small intestine every 3–6 days  Such rapid replacement is necessary because individual epithelial cells cannot withstand the destructive effects of the digestive enzymes in the intestinal lumen for long  Intestinal crypts also contain mature Paneth cells  These epithelial cells secrete enzymes that destroy certain bacteria and may help determine which kinds of bacteria live in the intestinal lumen  Cont.…  The small intestine contains many areas of lymphoid tissue  Mucosa-associated lymphoid tissue (MALT) is found in the mucosal layer throughout the intestine, and aggregated lymphoid nodules (Peyer’s patches) are located in the submucosa of the ileum  The submucosa of the small intestine is a typical connective tissue. In the duodenum only, it contains a set of compound tubular duodenal glands (Brunner’s glands), whose ducts open into the intestinal glands  These glands secrete an alkaline, bicarbonaterich mucus that helps neutralize the acidity of the chyme from the stomach and contributes to the protective layer of mucus on the inner surface of the small intestine  Large intestine  Large intestine extends from the ileocecal junction to the anus & is about 1.5m long  During the 12–24 hours that this residue remains in the large intestine, little additional breakdown of food occurs, except for the small amount of digestion performed by the many bacteria living there  Even though the large intestine absorbs these few remaining nutrients, its main function is to absorb water and electrolytes from the digested mass, resulting in semisolid feces  Propulsion through the large intestine is sluggish and weak, except for mass peristaltic movements, which pass over the colon a few times a day to force the feces powerfully toward the rectum  Parts of Large intestine  Cecum is about 6cm  Appendix is about 10 cm  Ascending colon is about 25 cm  Descending colon is about 25 cm  Transverse colon is about 50 cm  Sigmoid colon is about 14 cm  Rectum is about 12 cm  Anal canal is about 8 cm  Cont.…  Over most of its length, the large intestine exhibits three special features: teniae coli, haustra, and epiploic appendages  Teniae (taeniae) coli “ribbons of the colon” are three longitudinal strips, spaced at equal intervals around the circumference of the cecum and colon  They are thickenings of the longitudinal layer of the muscularis externa, which is thin except at these sites  Because the teniae maintain muscle tone, they cause the large intestine to pucker into sacs, or haustra  Epiploic appendages “membrane-covered”), also called omental appendices, are fat-filled pouches of visceral peritoneum that hang from the intestine  Their significance is unknown  Position of Appendix  Normal (Most incident)  Rtrocecal (64%)  Pelvic (32%)  Abnormal (least incident)  Subcecal (5%)  Perileal (1%)  Postileal (0.5%)  Colon  Has distinct regions: ascending colon, transverse colon, descending colon, and sigmoid colon  The transverse and sigmoid portions are anchored via mesenteries called mesocolons  The sigmoid colon joins the rectum  The anal canal, the last segment of the large intestine, opens to the exterior at the anus  Rectum  The Rectum In the pelvis, the sigmoid colon joins the rectum which descends along the inferior half of the sacrum in a secondarily retroperitoneal position  The rectum has no Teniae coli; instead, its longitudinal muscle layer is complete and well developed, so that it can generate strong contractions for defecation Even though the word rectum means “straight,” the rectum actually has several tight bends  Internally, these bends are represented as three transverse folds of the rectum, or rectal valves which prevent feces from being passed along with gas  Anal canal  The last subdivision of the large intestine is the anal canal  About 3 cm long, it begins where the rectum passes through the Levator ani, the muscle that forms the pelvic floor  A portion of the levator ani is responsible for maintaining the anorectal angle, an acute angle between the anus and the rectum that contributes to fecal continence  The anal canal lies entirely external to the abdominopelvic cavity in the perineum  Internally, the superior half of the anal canal contains longitudinal folds of mucosa called anal columns  These columns contain the terminal portions of the superior rectal artery and vein (the hemorrhoidal vessels)  Cont.…  Neighboring anal columns join each other inferiorly at crescent- shaped transverse folds called anal valves  The pockets just superior to these valves are anal sinuses, which release mucus when they are compressed by feces, providing lubrication that eases fecal passage during defecation  held shut by two anal sphincters:  internal anal sphincter of smooth muscle Cont.…  external anal sphincter of skeletal muscle  The horizontal line along which the anal valves lie is called the pectinate “comb-shaped” line  Because the mucosa superior to this line is innervated by visceral sensory fibers, it is relatively insensitive to pain while Inferior to the pectinate line, however, the mucosa is sensitive to pain because it is innervated by somatic nerves  Histology of large intestine  The wall of the large intestine resembles that of the small intestine in some ways and differs from it in others  The internal surface of the colon is lined by a simple columnar epithelium containing the same cell types as in the small intestine  Goblet cells are more abundant in the large intestine, for they secrete large amounts of lubricating mucus that eases the passage of feces toward the end of the alimentary canal  The absorptive cells take in water and electrolytes  Villi are absent, which reflects the fact that fewer nutrients are absorbed in the large intestine  Intestinal crypts are present as simple tubular glands containing many goblet cells  Finally, undifferentiated stem cells occur at the bases of the intestinal crypts, and epithelial cells are fully replaced every week  Blood vessels and nerves of LI  The first half of the large intestine to a point two-thirds of the way along the transverse colon is supplied by the superior mesenteric vessels the rest by inferior mesenteric branches  Its sympathetic innervation is from the superior mesenteric and celiac ganglia and plexuses, and its parasympathetic innervation is from the vagus nerve  The distal half of the large intestine, up to the proximal portion of the rectum, is supplied by the inferior mesenteric vessels  The lower rectum and the anal canal are served by rectal branches of the internal iliac vessels  The sympathetic innervation of the distal half of the large intestine is via the inferior mesenteric and hypogastric plexuses, and the parasympathetic innervation is from the pelvic splanchnic nerves  The final part of the anal canal below the pectinate line is innervated by somatic nerves, such as the pudendal nerve  Defecation  The rectum is usually empty and the anal sphincters contracted  When feces are squeezed into the rectum by mass peristaltic movements, the stretching of the rectal wall initiates the defecation reflex  Mediated by the sacral spinal cord, this parasympathetic reflex signals the walls of the sigmoid colon and rectum to contract and the anal sphincters to relax  If one decides to delay defecation, the reflexive contractions end, and the rectum relaxes  Another mass movement occurs a few minutes later, initiating the defecation reflex again and so on, until one chooses to defecate or the urge to defecate becomes unavoidable  Liver The ruddy liver is the largest gland in the body, weighing about 1.4 kg (3 pounds) in an average adult Amazingly versatile, it performs over 500 functions: – Detoxification – Destruction of spent RBCs – Synthesis of bile – Synthesis of plasma proteins – Metabolic activities  Cont.…  Its digestive function is to produce bile, a green alkaline liquid that is stored in the gallbladder and secreted into the duodenum  Bile salts emulsify fats in the small intestine; that is, they break up fatty nutrients into tiny particles  The liver also performs many metabolic functions:  It picks up glucose from nutrient-rich blood returning from the alimentary canal and stores this carbohydrate as glycogen for subsequent use by the body; it processes fats and amino acids and stores certain vitamins; it detoxifies many poisons and drugs in the blood; and it makes the blood proteins. Almost all of these functions are carried out by a type of cell called a Hepatocyte or simply a liver cell  Gross anatomy of liver  The liver lies inferior to the diaphragm in the right superior part of the abdominal cavity filling much of the right hypochondriac and epigastric regions and extending into the left hypochondriac region  It lies almost entirely within the rib cage, which protects this highly vascular organ from blows that could rupture it  The liver is shaped like a wedge, the wide base of which faces right and the narrow apex of which lies just inferior to the level of the left nipple  Cont.…  Other important structures on the liver’s visceral surface are the gallbladder and the inferior vena cava, which lie to the right of the quadrate and caudate lobes, respectively  The inferior vena cava receives the hepatic veins carrying blood out of the liver  Several structures pass through the liver’s fissure  Lying in the fissure’s inferior half is the ligamentum teres (teres round), or round ligament  This cordlike ligament, the remnant of the umbilical vein in the fetus, ascends to the liver from the navel, within the inferior margin of the falciform ligament  Additionally, the superior half of the liver’s fissure contains the ligamentum venosum a cordlike remnant of the ductus venosum of the fetus  Cont.…  The liver has two surfaces: the diaphragmatic and visceral surfaces  The diaphragmatic surface faces anteriorly and superiorly, whereas the visceral surface faces posteroinferiorly  Even though most of the liver is covered with a layer of visceral peritoneum, the superior part, called the bare area, is fused to the diaphragm and is therefore devoid of peritoneum  Cont.…  The liver has a right lobe and a left lobe, which traditionally were considered to be divided by the falciform ligament on the anterior part of the diaphragmatic surface and the fissure on the visceral surface  The falciform ligament is a ventral mesentery that binds the liver to the anterior abdominal wall, and the fissure is a deep groove in the same sagittal plane as the falciform ligament  Two other lobes, the quadrate lobe and the caudate lobe, are visible on the visceral surface just to the right of the fissure  Anatomically considered part of the right lobe, these lobes are physiologically considered part of the left lobe, with which they share nerves and vessels Blood vessels and nerves of liver  An important area near the center of the visceral surface is the porta hepatis “gateway to the liver”, where most of the major vessels and nerves enter and leave the liver  The right and left branches of the hepatic portal vein, which carry nutrient-rich blood from the stomach and intestines, enter the porta hepatis, as do the right and left branches of the hepatic artery carrying oxygen-rich blood to the liver  The right and left hepatic ducts, which carry bile from the respective liver lobes, exit from the porta hepatis and fuse to form the common hepatic duct, which extends inferiorly toward the duodenum  Autonomic nerves reach the liver from the celiac plexus and consist of both sympathetic and parasympathetic (vagal) fibers  Histology of liver Stroma  External surface is invested by thin collagenous capsule=Glisson’s capsule  Thick at the hilum, surround vessels and ducts to interior  Fine meshwork of reticular fibers radiate from this connective tissue, support liver cells  Histology of liver Liver parenchyma – Liver parenchymal cells (hepatocytes) are arranged into structural & functional units called liver lobules ; around one million – Spaces between plates contain capillaries=liver sinusoids  Sinusoids are lined by discontinuous layers of cells which do not rest on basement membrane  Endothelial cells are separated from hepatocytes by a narrow space = space of Disse, which drain to lymphatics – At angles of lobule are portal triads terminal braches of portal vein and hepatic artery and bile duct – Center of lobule is a centrolobular venules (central vein)  Hepatocytes  80% of the mass of the liver  Metabolic factories – form and secrete bile – store glycogen and buffer blood glucose – synthesize urea – metabolize cholesterol and fat – synthesize plasma proteins – detoxify drugs and poisons – process several steroid hormones and vitamin D  Hepatocytes Extensive rough endoplasmic reticulum – Protein synthesis Smooth ER – For hormone processing and detoxification Golgi body and lysosomes – for the formation of bile Mitochondria – For oxidation Microvilli into the Space of Disse Large nucleus  Anatomy of Gall bladder Is a pear-shaped sac lying on the visceral surface of the right lobe of the liver in a fossa between the right and quadrate lobes Fundus of gallbladder: a rounded end which may project from the inferior border of the liver Body of gallbladder: a major part in the fossa which may be against the transverse colon and the superior part of the duodenum Neck of gallbladder: a narrow part with mucosal folds forming the spiral fold  Cont.…  Stores and concentrates bile by absorbing its water and ions  Releases bile via the cystic duct, which flows into the bile duct  Histology of Gall bladder 1. Mucosa – In non distended state is thrown into many folds – Lined by simple columnar epithelium – In neck region, epithelium invaginate and form mucous glands 2. Submucosa – Loose, rich in elastic fibers, blood and lymph vessels 3. Muscular layer – Thin, fibers are disposed obliquely 4. Serosa/adventitia – Binds superior surface to liver (adventitia) – Opposite surface is lined by serosa (peritoneum)  Pancreas Mixed gland, both exocrine and endocrine  It has three parts; the head, body and tail It is a retroperitoneal organ  Endocrine tissue – Forms Islets of Langerhans – scattered throughout exocrine tissue – secrete hormone  Exocrine portion – forms the bulk of the gland – secretes enzyme rich fluid - pancreatic juice break down all categories of foodstuffs  Cont.…  Pancreatic acini – Made up of irregular clusters of pyramidal secretory cells, the apices of which surround a central lumen – Cells are typical protein secreting cells – Acini are surrounded by basement membrane supported by reticular fibers – Between acini are CT with capillaries  Pancreatic duct  The pancreatic duct begins in the tail of the pancreas  In the lower part of the head of pancreas, the pancreatic duct joins the bile duct known as hepatopancreatic ampulla (ampulla of Vater), which enters the descending 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 muscle  The accessory pancreatic duct empties into the minor duodenal papilla  Both accessory and main pancreatic duct communicate to each other Cont.…  Biliary tract – Liver cells secrete bile into bile canaliculi situated between plasma membranes of adjacent hepatocytes – The canalicular system drains to bile collecting ducts – Collecting ducts merge and form trabecular ducts which emerge from liver as right and left hepatic ducts – The two hepatic ducts join to form common hepatic duct which join cystic duct to form common bile duct Thank you for your attention! 126