The Digestive System PDF

Y SYSTEMS THE BOD THE DIGESTIVE SYSTEM  AN AT OM Y O F DIGEST IV E SY ST EM The digestive system in the human body processes food and liquids. It consists of the digestive tract, a long tube that runs from the mouth to the anus, also known as the gastrointestinal...

Y SYSTEMS THE BOD THE DIGESTIVE SYSTEM  AN AT OM Y O F DIGEST IV E SY ST EM The digestive system in the human body processes food and liquids. It consists of the digestive tract, a long tube that runs from the mouth to the anus, also known as the gastrointestinal (GI) tract or alimentary canal. The accessory organs, which include the tongue, teeth, salivary glands, liver, gallbladder, and pancreas, help by producing and releasing fluids into the digestive tract to aid digestion.  AN AT OM Y O F DIGEST IV E SY ST EM The digestive tract and associated accessory organs include the following: Oral cavity: Includes the tongue, teeth, and salivary glands (accessory organs). Pharynx: Connects the mouth to the esophagus. Esophagus: A muscular tube that transports food to the stomach. Stomach: Breaks down food with digestive acids and enzymes. Small intestine: Divided into the duodenum, jejunum, and ileum, with the liver, gallbladder, and pancreas as accessory organs that assist in digestion. Large intestine: Includes the cecum, colon, rectum, anal canal, and ends at the anus for waste elimination.  IO NS O F T HE FUNCT DIGEST IVE SYS TE M THE SIX MAJOR FUNCTIONS OF THE DIGESTIVE SYSTEM ARE INGESTION AND MASTICATION, PROPULSION AND MIXING, SECRETION, DIGESTION, ABSORPTION, AND ELIMINATION. 1. Ingestion and Mastication. Ingestion is the intake of solids or liquids. The normal route of ingestion is through the oral cavity. Mastication is the process by which the teeth chew food in the mouth to begin the process of digestion.  IO NS O F T HE FUNCT DIGEST IVE SYS TE M 2. Propulsion and Mixing. Propulsion is the movement of food from one end of the digestive tract to the other. Mixing is the movement of food back and forth in the digestive tract, without forward movement. A. Swallowing, or deglutition, moves liquids or a soft mass of food and liquid, called a bolus, from the oral cavity into the esophagus  IO NS O F T HE FUNCT DIGEST IVE SYS TE M B. Peristalsis- Peristaltic waves are muscular contractions consisting of a wave of relaxation of the circular muscles in front of the mass of undigested food (now called chyme). C. Mass movements - the contractions that move material in the distal parts of the large intestine to the anus. Mixing contractions blend food with digestive fluids in the stomach and small intestine. These contractions aid with mechanical digestion. There are two major types of mixing contractions:  IO NS O F T HE FUNCT DIGEST IVE SYS TE M a. Mixing waves - are gentle contractions in the stomach that churn the food with gastric secretions. Ingested food is stored and mixed in the stomach, from where it is slowly released into the small intestine as chyme. b. Segmental contractions - mix food particles with digestive secretions in the small intestine.  IO NS O F T HE FUNCT DIGEST IVE SYS TE M 3. Secretion - Secretions in the digestive tract play crucial roles in processing food. Mucus lubricates and protects the digestive tract lining, while water in secretions helps liquefy food for easier digestion and absorption. The liver produces bile to emulsify lipids, aiding their digestion and absorption. Enzymes from the oral cavity, stomach, small intestine, and pancreas break down large food molecules into smaller ones that can be absorbed by the intestinal wall.  NS O F T HE FUNCTIO DIGESTIVE SYS TE M 4. Digestion - involves breaking down large organic molecules into their smaller components. Mechanical digestion includes chewing and mixing food, while chemical digestion uses enzymes to further break down molecules. Carbohydrates are reduced to monosaccharides, proteins to amino acids, and triglycerides to fatty acids and glycerol. Minerals and water are absorbed without being digested, and vitamins are absorbed in their original form without digestion, as their structure can affect their function.  IO NS O F T HE FUNCT DIGEST IVE SYS TE M 5. Absorption - the process of moving molecules from the digestive tract into the blood or lymphatic system. The method of absorption depends on the type of molecule and can occur through diffusion, facilitated diffusion, active transport, symport, or endocytosis. 6. Elimination - the removal of waste products from digestion. In the large intestine, water and salts are absorbed, converting the material from a liquid to a semisolid form known as feces. These feces are stored in the distal large intestine and are expelled from the body through defecation.  HISTOLOGY Overview of the Digestive Tract Layers These layers are present throughout the digestive tract, from the esophagus to the anus. The digestive tract consists of four major tunics (layers): SEROSA OR THE MUCOSA SUBMUCOSA MUSCULARIS ADVENTITIA LAYER LAYER LAYER LAYER  MUCOUS EPITHELIUM: SA LAYER THE MUCO Nonkeratinized stratified squamous epithelium in parts of the tract (e.g., mouth, esophagus) and simple columnar epithelium in others. THE INNERMOST LAYER OF THE DIGESTIVE TRACT. CONSISTS OF THREE SUB-LAYERS: The epithelial layer forms the surface of the mucosa. Epithelial cells secrete thick, gel-like mucus that protects against irritants. This layer is named "mucosa" due to its mucus production. Cells can arrange in one or multiple layers, either columnar or brick-like in structure. High cell turnover rate allows for frequent replacement and removal of invasive particles. Some epithelial cells have cilia, tiny hairs that help clear foreign substances.  SA LAYER THE MUCO 2. Lamina Propria: Loose connective tissue layer THE INNERMOST LAYER OF THE DIGESTIVE TRACT. CONSISTS OF THREE SUB-LAYERS: middle layer of the mucosa. Composed of structural proteins, nerves, and veins. Supplies blood to the epithelium and binds it to the smooth muscle below. Nerves adjust to muscle movements, helping to change the shape of the epithelium. Contains immune cells that destroy pathogens.  SA LAYER THE MUCO 3. Muscularis Mucosae: Thin outer layer of smooth muscle. THE INNERMOST LAYER OF THE DIGESTIVE TRACT. CONSISTS OF THREE SUB-LAYERS: The deepest layer of the mucosa, made of smooth muscle. Thickness varies throughout the digestive tract, most active in the stomach. Provides a motor function that keeps the mucosa in motion. Aids in stretching, contracting, and cleansing by keeping cilia in motion.  The Mucosa Layer Functions of the Mucosa Layer Contains intestinal glands and crypts that extend into the lamina propria. It provides a barrier against foreign particles Specialized cells within the mucosa: 1. Mechanoreceptors: Detect mechanical changes involved in peristalsis. 2. Chemoreceptors: Detect chemical composition of food. THE SUBM UCOSA Thick connective tissue beneath the mucosa. LAYER Contains: 1. Nerves 2. Blood vessels 3. Lymphatic vessels 4. Small glands Houses the enteric nervous system (ENS), which controls certain digestive functions THE MUSC ULARIS Consists of smooth muscle fibers in two layers: a. Inner circular layer b. Outer longitudinal layer LAYER Responsible for peristaltic movement of food through the digestive tract. Between the two muscle layers is a second portion of the enteric nervous system called the: 1. Myenteric plexus – controls the motility of intestinal tract. 2. Interstitial cells – promotes rhythmic contractions of smooth muscle along the digestive tract.  THE SERO SA OR Outermost layer of the digestive tract. IA LAY ER ADVENTIT Serosa: Present when the digestive tract is covered by peritoneum. Adventitia: Found where there is no peritoneum (e.g., esophagus), composed of connective tissue that merges with surrounding structures.  Types of Glands in the Digestive Tract There are three major types of glands associated with the digestive system: Unicellular mucous glands in the mucosa. These glands secrete mucus to protect and lubricate the lining of the digestive tract. Multicellular glands in the mucosa and submucosa. They perform functions like secreting digestive enzymes, acid, and mucus to aid in digestion and protect the tissues from damage. Multicellular accessory glands (e.g., pancreas) outside the digestive tract. These glands produce and release large amounts of enzymes, bile, and other digestive juices that are essential for proper digestion and absorption of nutrients. REGULATION OF THE DIGESTIVE SYSTEM is controlled by elaborate nervous and chemical mechanisms that regulate the movement, secretion, absorption and elimination processes.  DIGESTIVE SYSTEM NERVOUS REGULATION: primarily controlled by the enteric nervous system (ENS), a division of the autonomic nervous system. The ENS is an extensive network of neurons located within the walls of the digestive tract and is composed of two main plexuses: the submucosal plexus and the *myenteric plexus*. This system contains more neurons than the spinal cord and functions through local reflexes to control digestive processes like peristalsis, mixing movements, and blood flow.  DIGESTIVE SYSTEM THREE MAJOR TYPES OF NEURONS: The ENS has three major types of neurons: 1. Enteric sensory neurons: detect changes in the chemical composition or mechanical conditions, like stretching of the digestive tract wall. 2. Enteric motor neurons: control smooth muscle contractions and glandular secretions. 3. Enteric interneurons: connect sensory and motor neurons to coordinate local reflexes.  DIGESTIVE SYSTEM NERVOUS REGULATION: While the ENS can function independently, it often works with the central nervous system (CNS), particularly through parasympathetic innervation (via the vagus nerves) and to a lesser degree through sympathetic nerves. CNS reflexes can influence digestive activity in response to external stimuli, like the sight or smell of food, which can increase saliva and pancreatic fluid secretion.  DIGESTIVE SYSTEM NERVOUS REGULATION: A clinical example highlighting the importance of the ENS is Hirschsprung disease (megacolon), caused by the absence of enteric neurons in the large intestine due to mutations in the RET gene. This results in poor intestinal motility and severe constipation.  DIGESTIVE SYSTEM 1. Neurotransmitters: CHEMICAL REGULATION: - Acetylcholine Stimulates digestive tract motility and secretions. - Norepinephrine Inhibits motility and secretions. - Serotonin Also stimulates digestive tract motility. It is produced both by neural release and by endocrine cells in the digestive tract. Over 95% of the body’s serotonin is found here. Increased serotonin levels, due to medications or chemotherapy, can lead to nausea by stimulating the vomiting center in the brain. Serotonin receptor blockers, like ondansetron (Zofran®), are used to treat this nausea.  DIGESTIVE SYSTEM CHEMICAL REGULATION: 2. Hormones: - Gastrin and secretin: These are secreted by endocrine cells in the digestive system and are carried through the bloodstream to target organs, influencing their function. Overall, these chemical signals play a crucial role in regulating the digestive process by modulating motility, secretions, and other digestive functions.  ANATOMY CLASS PERITONEUM The abdominal cavity is lined by a continuous serous membrane called the peritoneum, which reduces friction between organs through its lubricating serous fluid.  PERITONEUM THE MESENTERY a double layer of epithelial tissue supports and holds the abdominal organs in place. It is attached to the posterior abdominal wall and is divided into several regions: 1. Mesentery of the small intestine: Connects the jejunum and ileum to the posterior wall. 2. Right mesocolon: Extends from the small intestine mesentery to the transverse colon.  PERITONEUM THE MISERY: 3. Transverse mesocolon: Connects the transverse colon. 4. Left mesocolon: Connects the splenic flexure to the mesosigmoid. 5. Mesosigmoid: Attaches to the sigmoid colon. 6. Mesorectum: Anchors the rectum.  PERITONEUM THE MESENTERY Historically thought to be a fragmented structure, the mesentery is now recognized as a continuous organ. It is divided into two domains: the mesenteric domain (where abdominal digestive organs are embedded) and the nonmesenteric domain (containing structures like the kidneys, which are retroperitoneal).  PERITONEUM THE MESENTERY The mesentery also includes the lesser omentum, connecting the stomach and duodenum to the liver and diaphragm, and the greater omentum, extending from the stomach over the intestines. The greater omentum can store fat and lymphocytes and is highly mobile, forming a pocket called the omental bursa between its layers.  VE ORGANS DIGESTI ORAL CAVITY The mouth is the first organ in the digestive system. It serves several essential functions: Ingestion: Food enters the mouth, allowing the initial stage of digestion. Mechanical Digestion (Chewing): Teeth break down large food pieces into smaller ones. Chemical Digestion: Saliva initiates digestion by breaking down starch into sugar.  ORAL CAVITY The oral cavity is divided into two regions: 1. The vestibule, the space between the lips or cheeks and the teeth. 2. The oral cavity proper, which lies medial to the teeth. It is lined with nonkeratinized stratified squamous epithelium to protect against abrasion.  ORAL CAVITY Lips, Cheeks, and Palate The lips and cheeks are crucial for mastication, speech, and food manipulation. The lips, or labia, form the anterior boundary of the vestibule and are primarily composed of the orbicularis oris muscle and connective tissue. Their skin is less keratinized and more transparent than other body skin, with underlying blood vessels giving the lips a reddish tint.  ORAL CAVITY LIPS The epithelium of the lips is continuous with the nonkeratinized stratified squamous epithelium of the oral mucosa. Each lip has a central mucosal fold called the labial frenulum, which attaches it to the gingiva within the vestibule.  ORAL CAVITY CHEEKS The cheeks form the lateral walls of the oral cavity and are lined with nonkeratinized stratified squamous epithelium and covered by skin. Each cheek contains the buccinator muscle, which helps flatten the cheek against the teeth, and the buccal fat pad, which facilitates the movement of mastication muscles. The buccal fat pads can vary significantly with weight changes over a person's lifetime.  ORAL CAVITY PALATE The roof of the oral cavity is the palate, which separates the oral and nasal cavities, preventing food from entering the nasal cavity during chewing and swallowing. The palate has two parts: the anterior, bony hard palate, and the posterior, nonbony soft palate, composed of skeletal muscle and connective tissue. The uvula is a projection from the soft palate. The fauces, the posterior boundary of the oral cavity, leads to the pharynx, and the palatine tonsils are located in the lateral walls of the fauces.  ORAL CAVITY Lips, Cheeks, and Palate Oral Cavity, Tongue, and Teeth The lips and cheeks are crucial for ORAL speech, mastication, CAVITY and food manipulation. The oral cavity Theislips, or labia, where form liquid andthe anterior boundary of the solid food is ingested. The tonguevestibule and are primarily composed of the orbicularis comprises the body, which has oris muscle and connective tissue. Their papillae with taste buds, and the skin is less keratinized and more root, which contains transparent the body than other lingual tonsil. skin, with The mouth includes underlying bothgiving blood vessels permanent the lips a and deciduous teeth,reddish with tint. a universal numbering and lettering system used for identification.  ORAL CAVITY TONGUE The tongue is a large, muscular organ that occupies most of the oral cavity when the mouth is closed. It is divided by a groove called the terminal sulcus into two parts: the body and the root. The body, located in the oral cavity, is relatively free except for its attachment to the floor of the mouth via the lingual frenulum. It is covered by nonkeratinized stratified squamous epithelium and papillae, some of which contain taste buds. The root, found in the oropharynx, contains scattered taste buds and the lingual tonsil.  ORAL CAVITY The tongue's muscles are categorized as intrinsic or extrinsic. Intrinsic muscles, located within the tongue, alter its shape for movements like flattening and elevating. Extrinsic muscles, located outside the tongue but attached to it, enable movements such as protrusion, retraction, and side-to-side motion. The tongue aids in food manipulation during chewing, contributes to swallowing, hosts taste buds, and speech. Removal of part or all of the tongue due to conditions like cancer can impact chewing and swallowing, though speech ability may remain relatively intact.  ORAL CAVITY TEETH Teeth, collectively known as the dentition, are essential for chewing and speaking. Adults typically have 32 teeth, divided into two dental arches: the maxillary (upper) and mandibular (lower). Each arch is further divided into four quadrants: right-upper, left-upper, right-lower, and left-lower. Each quadrant contains one central and one lateral incisor, one canine, two premolars, and three molars. The third molars, or wisdom teeth, often emerge in late teens or early twenties but may become impacted if there’s insufficient space  ORAL CAVITY Teeth fall into two categories: permanent (secondary) teeth and deciduous (primary or milk) teeth. Deciduous teeth appear between 6 months and 2 years, and are gradually replaced by permanent teeth from around age 5 to 11. A tooth consists of three parts: 1. Crown: The visible part covered by enamel and containing one or more cusps 2. Neck: The narrow area between the crown and root. 3. Root: The portion embedded in the jawbone, anchoring the tooth.  ORAL CAVITY The tooth's central portion includes the pulp cavity, filled with blood vessels, nerves, and connective tissue. The root canal, a part of the pulp cavity within the root, connects to the outside through the apical foramen. The pulp cavity is encased in dentin, a calcified tissue, while the enamel covers the crown, protecting it from abrasion and acids.  ORAL CAVITY Molar Tooth in Place in the Alveolar Bone A tooth consists of a crown, a neck, and a root. The root is covered with cementum, and the tooth is held in the socket by periodontal ligaments. Nerves and vessels enter and exit the tooth through the apical foramen.  ORAL CAVITY The root of the tooth is covered by a bonelike substance called cementum, which helps anchor the tooth to the periodontal ligament within the jawbone. Teeth are secured in their sockets (alveoli) along the alveolar processes of the mandible and maxilla. These sockets and the surrounding alveolar processes are covered by gingiva (gums), which consists of dense fibrous connective tissue and stratified squamous epithelium. Periodontal ligaments secure the teeth in the alveoli, providing stability.  ORAL CAVITY Several dental conditions can affect tooth and supporting structures: Dental Caries: Tooth decay caused by bacterial acids that erode enamel. Since enamel is nonliving and cannot regenerate, it requires fillings to repair damage. Advanced decay affecting the pulp can lead to a toothache and may necessitate a root canal procedure, where the pulp is removed.  ORAL CAVITY Gingivitis: Inflammation of the gingiva, often due to plaque buildup from food particles. It can progress to periodontal disease if not managed by proper oral hygiene. Periodontal Disease: This condition involves inflammation and destruction of the periodontal ligaments, gingiva, and alveolar bone, and is a leading cause of tooth loss in adults. It often results in halitosis (bad breath).  ORAL CAVITY Maintaining good oral hygiene is essential to prevent these conditions and ensure dental health. Mastication Food is chewed by teeth to increase its surface area for better digestion. Incisors and canines cut and tear, while premolars and molars crush and grind. Four muscles—temporalis, masseter, medial pterygoid, and lateral pterygoid—control jaw movements. The mastication reflex, managed by the medulla oblongata and influenced by the cerebrum, coordinates chewing by adjusting muscle contractions and relaxation in response to food presence.  ORAL CAVITY The cerebrum also modulates the rate and intensity of chewing.  ORAL CAVITY Salivary Glands: Major Salivary Glands: 1. Parotid Glands: Largest, produce mostly watery saliva. Located near the ear, with ducts opening near the second upper molar. 2. Submandibular Glands: Located below the mandible, produce a mix of serous and mucous secretions. Ducts open beside the tongue's frenulum. 3. Sublingual Glands: Smallest of the large glands, primarily produce mucous secretions. Located below the tongue with multiple small ducts opening into the oral cavity.  ORAL CAVITY Small Salivary Glands: Found in the tongue (lingual), palate (palatine), cheeks (buccal), and lips (labial) Saliva: Saliva is composed of fluid and proteins and has three main roles  ORAL CAVITY Roles: 1. Moistens the oral cavity for speech and taste. 2. Provides protective functions: Washes the oral surface to prevent bacterial infection. Contains bicarbonate ions to neutralize acids from bacteria. Contains lysozyme and immunoglobulin A for antibacterial protection. Mucous protects the digestive tract from irritation and digestion. 3. Begins the process of digestion.  ORAL CAVITY Approximately 1–1.5 liters of saliva are secreted daily, with the serous portion from parotid and submandibular glands providing moisture, and mucous secretions from submandibular and sublingual glands offering lubrication.  ORAL CAVITY Digestive Functions of Saliva Enzymes: Salivary Amylase: Breaks down starches into disaccharides like maltose and isomaltose, contributing to a sweet taste. Only about 3-5% of carbohydrates are digested in the mouth due to the brief time food spends there and the cellulose covering in plant foods.  ORAL CAVITY Lingual Lipase: Initiates lipid digestion, though this is minimal. SALIVARY SECRETION REGULATION: Parasympathetic Nervous System: Predominantly stimulates saliva production via facial (VII) and glossopharyngeal (IX) cranial nerves in response to stimuli like tactile sensations and sour tastes.  ORAL CAVITY Sympathetic Nervous System: Also stimulates salivary glands, but less so than the parasympathetic system. Brain: Higher brain centers can increase saliva secretion in response to food-related stimuli such as odors and hunger.  VE ORGANS DIGESTI SWALLOWING The esophagus ensures the smooth movement of food from the mouth to the stomach, allowing for efficient digestion and nutrient absorption. Unlike the stomach and small intestine, the esophagus does not secrete digestive enzymes. Its primary function is to facilitate the passage of food.  SWALLOWING 1. PHARYNX ANATOMY AND FUNCTION: The pharynx is a funnel-shaped tube that connects the mouth and nasal passages to the esophagus and larynx. It is divided into three regions.  PHARYNX NASOPHARYNX: Location: Behind the nasal cavity. Function: Part of the respiratory system, not directly involved in swallowing. Special feature: Lined with ciliated pseudostratified columnar epithelium, helps in trapping and moving particles from the air.  PHARYNX OROPHARYNX: Location: Extends from the soft palate to the epiglottis. Function: Acts as a passageway for food and air; plays a significant role in digestion. Special feature: Lined with nonkeratinized stratified squamous epithelium, providing protection against food abrasion.  PHARYNX LARYNGOPHARYNX: Location: Below the oropharynx, extends from the hyoid bone to the esophagus. Function: Transports food to the esophagus. Special feature: Shares the passageway for both food and air.  PHARYNGEA L MUSCLES: There are three key muscles in the pharynx responsible for the movement of food: 1. Superior pharyngeal constrictor. 2. Middle pharyngeal constrictor. 3. Inferior pharyngeal constrictor. These muscles work in a coordinated manner to push the food bolus down from the pharynx to the esophagus.  EPIGLOTTIS: A leaf-shaped flap made of elastic cartilage. Function: It covers the entrance to the larynx during swallowing to prevent food or liquid from entering the trachea (windpipe), thereby preventing choking.  SWALLOWING 2. ESOPHAGUS STRUCTURE: The esophagus is a muscular tube around 25 cm (10 inches) long that transports food from the pharynx to the stomach.  ESOPHAGUS Esophagus Layers (From Inside Out): 1. Mucosa: Inner lining of the esophagus made up of stratified squamous epithelium, which is resistant to abrasion from food. Contains glands that secrete mucus, lubricating food to ease its passage. 2. Submucosa: Contains connective tissue, blood vessels, and nerves. Has glands that produce mucus for additional lubrication. 3. Muscularis: Responsible for the peristaltic movements of the esophagus. Three regions of muscle composition: Upper third: Skeletal muscle (voluntary control). Middle third: Mixed skeletal and smooth muscle. Lower third: Smooth muscle (involuntary control). 4. Adventitia: Outer layer of the esophagus, made up of connective tissue that anchors the esophagus to surrounding structures.  ESOPHAGUS Esophageal Sphincters: ·Upper esophageal sphincter (UES): Located at the top of the esophagus. Controls the entry of food from the pharynx into the esophagus. Relaxes during swallowing to allow the bolus to pass. ·Lower esophageal sphincter (LES): Located at the junction between the esophagus and the stomach. Prevents the backflow of stomach acids (reflux) into the esophagus.  SWALLOWING 3. PHASES OF SWALLOWING (DEGLUTITION): Swallowing involves three distinct phases, coordinating over 20 muscles from the mouth, pharynx, and esophagus.  PH A S E S O F S W A LLO W IN G (DE GL U TIT IO N ): Phase 1: Voluntary Phase (Conscious): · Description: The process of swallowing begins voluntarily in the mouth. The tongue collects food, forms a bolus (small mass), and presses it against the hard palate. The bolus is then moved posteriorly to the oropharynx. · Mechanism: Once the food enters the oropharynx, the swallowing process becomes involuntary, transitioning into the pharyngeal phase.  PH A S E S O F S W A LLO W IN G (DE GL U TIT IO N ): Phase 2: Pharyngeal Phase (Involuntary): · Description: This is a reflexive phase initiated when the food touches the back of the oropharynx. During this phase, several actions occur to protect the airway and ensure smooth passage of the bolus into the esophagus. · Key Actions: The soft palate elevates to close off the nasopharynx, preventing food from entering the nasal cavity. The epiglottis folds down over the larynx, blocking the respiratory tract to prevent choking. The vocal cords and vestibular folds close, further protecting the airway. The pharyngeal constrictors (superior, middle, and inferior muscles) contract in sequence, pushing the bolus into the esophagus. The upper esophageal sphincter relaxes, allowing the bolus to enter the esophagus  PH A S E S O F S W A LLO W IN G (DE GL U TIT IO N ): Phase 2: Pharyngeal Phase (Involuntary): · Neural control: This phase is mediated by the trigeminal (V), glossopharyngeal (IX), vagus (X), and accessory (XI) cranial nerves. The swallowing center is located in the medulla oblongata. · Timing: The pharyngeal phase is rapid, lasting about 1-2 seconds.  PH A S E S O F S W A LLO W IN G (DE GL U TIT IO N ): Phase 3: Esophageal Phase (Involuntary): ·Description: This phase involves the transportation of food from the esophagus to the stomach via peristalsis (wave-like contractions of the esophageal muscles). ·Key Actions: Peristaltic waves push the bolus downward toward the stomach. Gravity also helps move the bolus, although peristalsis can function independently of gravity (e.g., swallowing upside down). The lower esophageal sphincter relaxes, allowing food to enter the stomach. ·Time taken: Typically takes 5-8 seconds for the bolus to reach the stomach. ·Final Action: Once the bolus enters the stomach, the lower esophageal sphincter closes, preventing backflow (acid reflux).  VE ORGANS DIGESTI STOMACH The Body's Mixing Bowl Location and Description: Positioned in the upper left part of the abdomen. Acts as a stretchy, expandable chamber. Primary Function: Stores and mixes food with digestive juices. Prepares food for further breakdown in digestion. Structure and Adaptability: Can shift shape and size like a flexible balloon. Adjusts based on food intake and body posture. Importance: Converts food into fuel for the body.  F STOMACH ANATOMY O The stomach is divided into four distinct regions, each playing a key role in the digestive process. 1. Cardia: 3. Body: Location: Where the esophagus meets Location: The largest region, curving to form the greater and lesser curvatures. the stomach. Function: Main site for the mixing and churning of Function: Controlled by the lower food with gastric juices. esophageal sphincter, preventing 4. Pylorus: stomach contents from refluxing back Location: The funnel-shaped region connecting to into the esophagus. the small intestine. Function: Comprises the pyloric antrum and pyloric 2. Fundus: canal. The pyloric sphincter regulates the passage of Location: Dome-shaped area, superior food into the small intestine. and to the left of the cardia. Hypertrophic Pyloric Stenosis: Function: Serves as a temporary A condition in infants where the pyloric sphincter storage area for food and gases thickens, blocking the stomach from emptying produced during digestion. properly into the small intestine, causing digestive issues.  F STOMACH ANATOMY O  OGY OF STOMACH HISTOL The stomach's histology reveals a complex structure designed for efficient digestion. Here's a breakdown of its layers and cell types: 1. Serosa: The outermost layer, also known as the visceral peritoneum, consists of simple squamous epithelium and connective 3. Submucosa and Mucosa: tissue. Rugae: Large folds in the submucosa and mucosa that allow the stomach to 2. Muscularis: This layer has three sub- stretch as it fills. These folds flatten layers: out as the stomach expands. Outer Longitudinal Layer Middle Circular Layer Mucosa: Lined with simple columnar Inner Oblique Layer: Unique to the epithelium and contains gastric pits, stomach, this layer helps generate which are openings to gastric glands. strong contractions to break down food. In some regions, like the fundus, these layers blend together.  OGY OF STOMACH HISTOL The stomach's histology reveals a complex structure designed for efficient digestion. Here's a breakdown of its layers and cell types: 4. Types of Epithelial Cells: Surface Mucous Cells: Protect the stomach lining from acid and digestive enzymes by producing alkaline mucus and forming tight junctions to prevent damage. They are rapidly replaced if damaged. Mucous Neck Cells: Located near the openings of gastric glands, they secrete mucus. Chief Cells: Secrete pepsinogen, which converts to Parietal Cells: Produce hydrochloric acid and pepsin, and gastric lipase, which digests fats. intrinsic factor. Endocrine Cells: Produce hormones and regulatory factors. Enterochromaffin-like Cells: Release histamine to stimulate acid secretion. Gastrin Cells: Secrete gastrin, which stimulates acid production. Somatostatin Cells: Release somatostatin, which inhibits gastrin and insulin secretion.  F STOMACH SECRETION O Once food enters the stomach, it combines with stomach secretions to form a semifluid substance known as chyme (KIME; juice). While the stomach primarily functions to store and mix chyme, some digestion and absorption do occur, although these are not its main roles. The stomach secretes several important substances: 1. Hydrochloric Acid (HCl): Secreted by parietal cells in the gastric glands, hydrochloric acid lowers the stomach's pH to between 1 and 3. The proton pump, a key player in this process, actively transports H+ ions into the stomach lumen, creating a highly acidic environment. This acid: - Kills bacteria ingested with food. - Denatures proteins, making them easier for digestive enzymes to break down. - Provides the optimal pH for pepsin to function and stops carbohydrate digestion by inactivating salivary amylase.  F STOMACH SECRETION O The production of hydrochloric acid involves: 1. Initial Reaction: H+ ions, derived from CO2 and water, enter the parietal cell from the serosal surface (opposite the gastric pit lumen). 2. Formation of Carbonic Acid: Inside the cell, the enzyme carbonic anhydrase catalyzes the reaction of CO2 with water to form carbonic acid. 3. Dissociation of Carbonic Acid: Carbonic acid partially dissociates into H+ ions and HCO3− (bicarbonate). 4. Ion Exchange and Alkaline Tide: H+ ions are pumped into the stomach lumen, while HCO3− moves into the extracellular fluid, exchanging with Cl− through an antiporter in the cell membrane. Cl− then enters the cell, causing an increase in blood pH in veins leaving the stomach, known as the alkaline tide, which occurs after meals. 5. Proton Pump Inhibitors: Drugs that block the proton pump lower gastric acid levels. The pump actively transports H+ against a concentration gradient, while Cl− diffuses out through ion channels. 6. Balancing Charges: The diffusion of Cl− into the gastric gland duct helps balance the positively charged H+ ions, reducing the energy required for transporting H+ against both concentration and electrical gradients.  F STOMACH SECRETION O  F STOMACH SECRETION O NOTE: Hydrochloric acid in the stomach kills most ingested bacteria, but some, like Helicobacter pylori (H. pylori), can resist it. H. pylori is the main cause of peptic ulcers, which occur when gastric acid damages the lining of the stomach, duodenum, or esophagus.  F STOMACH SECRETION O 2. Intrinsic Factor: This glycoprotein, also secreted by parietal cells, binds with vitamin B12, facilitating its absorption in the ileum of the small intestine. Vitamin B12 is crucial for DNA synthesis and red blood cell production. Its deficiency can lead to pernicious anemia and neurological issues due to its role in maintaining myelin in the peripheral nervous system. 3. Pepsinogen: Released by chief cells as an inactive enzyme, pepsinogen is converted into the active enzyme pepsin by hydrochloric acid. Pepsin, which functions optimally at a pH of 3 or less, breaks down proteins into smaller peptides. Chief cells also secrete gastric lipase, which digests lipids in the acidic environment of the stomach. 4. Mucus: Surface mucous cells and mucous neck cells secrete a thick, alkaline mucus that forms a protective layer over the stomach lining. This mucus helps to: - Lubricate the stomach lining. - Protect the epithelial cells from damage by acidic chyme and pepsin. - Increase mucus secretion in response to mucosal irritation.  MACH SECRETION OF STO REGULATION The stomach produces approximately 2–3 liters of gastric secretions (gastric juice) daily. The amount and type of food entering the stomach and small intestine significantly influence the quantity of these secretions. Typically, around 700 mL of gastric juice is secreted during a meal. Stomach secretions are regulated through both nervous and hormonal mechanisms, including the hormones gastrin, secretin, and cholecystokinin, and the paracrine messenger histamine.  MACH SECRETION OF STO REGULATION This regulation occurs in three distinct phases: cephalic, gastric, and intestinal. 1. Cephalic Phase ("Get Started!"): - This phase is triggered by the brain in response to the anticipation of food, even before it reaches the stomach. Stimuli such as the sight, smell, and taste of food, or even thoughts of food, stimulate the medulla oblongata. - Action potentials from the medulla travel via the vagus nerve to the stomach, activating parasympathetic neurons in the enteric nervous system (ENS). - These neurons release acetylcholine, which increases the secretory activity of parietal and chief cells, and stimulates the release of gastrin and histamine. - Gastrin travels through the bloodstream to stimulate more hydrochloric acid and pepsinogen secretion. Histamine also enhances hydrochloric acid production by acting on parietal cells. - The combined effect of acetylcholine, histamine, and gastrin results in increased acid secretion, with histamine having the most significant impact. Histamine blockers can reduce acid levels.  MACH SECRETION OF STO REGULATION CEPHALIC PHASE ("GET STARTED!"):  MACH SECRETION OF STO REGULATION 2. Gastric Phase ("Go For It!"): - This phase begins once food enters the stomach. The primary stimuli are stomach distension and the presence of amino acids and peptides. - Stomach distension activates mechanoreceptors, leading to reflexes that increase gastric secretions through acetylcholine release, similar to the cephalic phase. - Partially digested proteins, alcohol, and caffeine further stimulate gastrin secretion. - When the stomach pH drops below 2, a negative feedback mechanism inhibits additional gastric secretion to prevent excessive acid production.  MACH SECRETION OF STO REGULATION GASTRIC PHASE ("GO FOR IT!"):  MACH SECRETION OF STO REGULATION 3. Intestinal Phase ("Slow Down!"): - This phase inhibits gastric secretions and is triggered by acidic chyme entering the duodenum. - When chyme’s pH drops below 2 or contains lipid digestion products, secretin and cholecystokinin are released. - Secretin inhibits gastric secretions by affecting both parietal and chief cells. - Cholecystokinin is released in response to fatty acids and lipids, and to a lesser extent, protein digestion products, inhibiting gastric secretions. - Nervous control also plays a role through the enterogastric reflex, which reduces gastric secretions. This reflex is activated by duodenal distension, irritating substances, low pH, and hypertonic or hypotonic solutions.  MACH SECRETION OF STO REGULATION INTESTINAL PHASE ("SLOW DOWN!"):  ENTS OF STOMACH MOVEM 1. Stomach Filling: When food enters the stomach, the rugae (folds) flatten, allowing the stomach to expand up to 20 times its normal volume. This expansion happens with a minimal increase in pressure due to a reflex controlled by the medulla oblongata, which reduces muscle tone. Additionally, smooth muscle can stretch without increasing tension, further minimizing pressure as the stomach fills  ENTS OF STOMACH MOVEM 2. Mixing of Stomach Contents: Ingested food is mixed with stomach secretions to form chyme. Two types of movements aid digestion: Mixing Waves: These are weak contractions that occur every 20 seconds, moving chyme toward the pyloric sphincter. Peristaltic Waves: Less frequent but stronger, these contractions push the more fluid chyme toward the pyloric sphincter, while solid food is pushed back toward the body of the stomach for further digestion. Approximately 80% of stomach contractions are mixing waves, while 20% are peristaltic waves, ensuring thorough mixing and movement of chyme  ENTS OF STOMACH MOVEM 3. Stomach Emptying: Factors Influencing Time: The time food remains in the stomach depends on its type and volume. Liquids exit within minutes, generally completing the process within 2 hours. A typical meal typically exits in 3-4 hours. Pyloric Sphincter: Normally, the pyloric sphincter remains partially closed due to mild tonic contraction. Peristaltic contractions, known as the pyloric pump, push small amounts of chyme through the sphincter into the duodenum. Motility and Emptying: Increased stomach motility enhances emptying. In an empty stomach, strong peristaltic contractions (tetanic contractions) can occur for 2-3 minutes, particularly when blood glucose levels are low, causing hunger pangs. Hunger Pangs: These sensations typically begin 12-24 hours after eating, with their intensity peaking within 3-4 days if no food is consumed, then gradually weakening.  MACH EMPTYING: EGULATION OF STO R Too Fast: If the stomach empties too quickly, digestion and absorption efficiency are compromised, and acidic contents may damage the duodenum. Too Slow: If the stomach empties too slowly, it can lead to decreased digestion and absorption in the small intestine and potential stomach wall damage. Neural Mechanisms: The distension of the stomach wall stimulates both gastric secretion and motility, enhancing stomach emptying. Hormonal and Neural Controls: The duodenum releases hormones like cholecystokinin and triggers the enterogastric reflex, which inhibit gastric motility and prevent the relaxation of the pyloric sphincter. This reduces the rate of stomach emptying, ensuring proper digestion and protection of the duodenum.  VOMITING Vomiting is a protective mechanism that expels harmful or toxic substances from the stomach. It involves a complex reflex coordinated by the vomiting center in the medulla oblongata. The process includes: 1. Deep Breath: A deep breath is taken to increase intra-abdominal pressure. 2. Elevation of Hyoid Bone and Larynx: The hyoid bone and larynx are elevated, opening the upper esophageal sphincter. 3. Closure of the Larynx: The opening of the larynx is closed to prevent aspiration. 4. Elevation of Soft Palate: The soft palate rises to block the connection between the oropharynx and nasopharynx. 5. Contraction of Diaphragm and Abdominal Muscles: The diaphragm and abdominal muscles contract forcefully, increasing intra-abdominal pressure. 6. Relaxation of Lower Esophageal Sphincter: The lower esophageal sphincter relaxes to allow the contents to pass. 7. Expulsion: Gastric contents are forcefully ejected through the esophagus and out of the mouth.  VE ORGANS DIGESTI SMALL INTESTINE The small intestine's main functions include breaking down food, absorbing nutrients, and moving the intestinal contents along the digestive tract. Specifically, the small intestine absorbs carbohydrates, proteins, and fats. It plays a vital role in digestion, ensuring efficient nutrient absorption and waste elimination.  SMALL INTESTINE THREE KEY FEATURES TO MAXIMIZE NUTRIENT ABSORPTION: 1. Circular Folds (Plicae Circulares): These folds increase the surface area by running perpendicular to the digestive tract’s length. 2. Villi: Tiny, finger-like projections on the mucosa, each with its own blood and lymphatic capillaries. 3. Microvilli: Microscopic extensions on the surface of villi that further expand the surface area and form the brush border, enhancing nutrient absorption.  SMALL INTESTINE ANATOMY AND HISTOLOGY OF THE DUODENUM: The mucosa of the small intestine is lined with simple columnar epithelium featuring four main cell types: 1. Absorptive Cells: Equipped with microvilli, these cells produce digestive enzymes and absorb nutrients. 2. Goblet Cells: Produce mucus to protect the intestinal lining. 3. Granular Cells (Paneth Cells): Help protect the intestine from bacteria. 4. Endocrine Cells: Produce hormones like secretin and cholecystokinin that regulate digestion. These cells originate in intestinal glands (crypts of Lieberkühn) at the base of the villi, with absorptive and goblet cells migrating to the villi surface and being shed over time. Granular and endocrine cells stay in the glands. The duodenum, the shortest segment of the small intestine, curves around the pancreas and ends in a junction with the jejunum. It has two important projections, the major and minor duodenal papillae, where ducts from the liver and pancreas empty into the digestive tract  SMALL INTESTINE JEJUNUM AND ILEUM: The jejunum and ileum, which follow the duodenum in the small intestine, have a similar structure but differ in some ways as you move from the duodenum to the ileum. Specifically, the diameter, wall thickness, circular folds, and number of villi decrease from the jejunum to the ileum. The ileum contains lymphatic nodules called Peyer’s patches, which help initiate immune responses against harmful microorganisms. At the end of the ileum, it connects to the large intestine at the ileocecal junction, which includes the ileocecal sphincter and the ileocecal valve. These structures control the movement of intestinal contents from the ileum into the cecum of the large intestine and ensure it moves in one direction.  SMALL INTESTINE SECRECATION OF SMALL INTESTINE: The small intestine secretes mucus, electrolytes, and water to aid digestion and protect its lining: 1. Mucus: Produced by duodenal glands, intestinal glands, and goblet cells. It shields the intestinal wall from acidic chyme and digestive enzymes. Mucus production is triggered by nerve signals, hormones like secretin, and irritation of the intestinal lining. 2. Electrolytes and Water: Secreted by the intestinal epithelium to keep chyme in a liquid state, which helps digestive enzymes from both the pancreas and the small intestine function effectively. Brush-Border Enzymes: Located on the microvilli of the intestinal lining, these enzymes include disaccharidases (which break down sugars) and peptidases (which digest proteins). Their large surface area ensures efficient digestion and absorption of nutrients into the bloodstream or lymphatic system.  SMALL INTESTINE MOVEMENT IN THE SMALL INTESTINE: Movement in the small intestine involves two key processes: 1. Segmental Contractions: These mix the chyme (partially digested food) within the intestine, helping to thoroughly combine it with digestive enzymes. 2. Peristaltic Contractions: These are wave-like movements that push the chyme along the digestive tract. They usually move only short distances but can sometimes travel the entire length of the intestine. Peristaltic contractions often continue from those in the stomach and travel at about 1 cm per minute. It typically takes 3-5 hours for chyme to move from the stomach to the junction with the large intestine.  SMALL INTESTINE REGULATION OF MOVEMENT: Local Stimuli: Stretching of the intestinal wall and the presence of certain substances like acids or digestion products stimulate muscle contractions. Local Reflexes: Integrated within the enteric nervous system (ENS), these reflexes manage the intestine's response to stimuli. Parasympathetic Nerve Influence: This can increase motility but is less significant compared to its role in the stomach. Ileocecal Sphincter: - This muscle at the junction between the ileum (small intestine) and cecum (large intestine) normally remains slightly contracted. - When peristaltic waves reach it, the sphincter relaxes to allow chyme to enter the large intestine. - However, if the cecum becomes distended, it triggers a reflex that causes the sphincter to contract more intensely. - This helps slow down the movement of chyme, allowing for more digestion and absorption in the small intestine and preventing backflow into the ileum. ACCESSORY ORGANS  VE ORGANS DIGESTI LIVER The liver, the largest organ in the body, performs essential functions within the digestive system. The liver continually produces bile, aiding fat digestion and nutrient absorption. It processes toxins and removes them from the blood. The liver creates substances necessary for blood clotting after injury. It helps maintain healthy blood sugar levels.  LIVER LOCATION & STRUCTURE Largest internal organ, weighing about 1.36 kg (3 pounds). Located in the right-upper quadrant of the abdomen, beneath the diaphragm. Consists of four lobes: 1. Right lobe 2. Left lobe 3. Caudate lobe 4. Quadrate lobe The right and left lobes are separated by the falciform ligament.  LIVER PORTA HEPATIS Area on the liver's inferior surface where blood vessels, nerves, bile ducts, and lymphatic vessels enter and exit. Blood Supply: Hepatic portal vein and hepatic artery. Bile Flow: Exits via the right and left hepatic ducts.  LIVER BILE FLOW PATHWAY 1. Right and left hepatic ducts merge to form the common hepatic duct. 2. Cystic duct from the gallbladder joins the common hepatic duct to form the common bile duct. 3. Bile can flow from the gallbladder through the cystic duct into the common bile duct or back into the gallbladder. 4. The common bile duct joins the pancreatic duct at the hepatopancreatic ampulla, which then empties into the duodenum at the major duodenal papilla. 5. The accessory pancreatic duct empties pancreatic secretions into the duodenum at the minor duodenal papilla. HISTOLOGY OF THE LIVER  LIVER COVERINGS The liver is covered by a connective tissue capsule and visceral peritoneum, except for the "bare area" on the diaphragmatic surface, surrounded by the coronary ligament.  LIVER STRUCTURE The liver is divided into hexagon-shaped hepatic lobules, which are surrounded by connective tissue septa. Each lobule has a portal triad at its corners, consisting of: 1. Hepatic portal vein 2. Hepatic artery 3. Hepatic duct  LIVER STRUCTURE The central vein is located in the center of each lobule, collecting blood as it exits the lobule. Central veins converge to form hepatic veins that drain into the inferior vena cava.  LIVER HEPATIC CORD Hepatic cords are strings of hepatocytes (liver cells) radiating from the central vein. Hepatocytes process nutrients from blood and produce bile.  LIVER HEPATIC SINUSOIDS Blood channels between hepatic cords lined with thin, irregular squamous endothelium. They contain: 1. Sparse endothelial cells 2. Kupffer cells (phagocytic cells) LIVER BILE CANALICULI Small channels between hepatocytes where bile is collected.  LIVER BLOOD AND BILE FLOW 1. Oxygenated blood from the hepatic artery and nutrient-rich blood from the hepatic portal vein enter hepatic sinusoids. 2. Hepatocytes in hepatic cords process the blood, which then drains into central veins and eventually into hepatic veins and the inferior vena cava. 3. Bile produced by hepatocytes enters bile canaliculi, which converge into hepatic ducts. Bile exits the liver via the left and right hepatic ducts. Fetal Remnants: In the fetus, special vessels bypass liver sinusoids. These remnants are seen as the round ligament (ligamentum teres) and the ligamentum venosum in adults.  LIVER FUNCTION OF THE LIVER 1. Bile Production: Produces bile essential for digestion and excretion of waste products. 2. Nutrient Storage: Stores vitamins and minerals, as well as glucose in the form of glycogen. 3. Nutrient Processing: Converts nutrients from the digestive tract into forms that can be used or stored. 4. Detoxification: Breaks down and detoxifies harmful substances, including drugs and alcohol. 5. Synthesis of New Molecules: Synthesizes proteins such as albumin and clotting factors, and produces various other molecules crucial for bodily functions.  LIVER BILE PRODUCTION FOR DIGESTION AND EXCRETION 1. Blood Flow: Blood enters the liver through the hepatic portal vein and hepatic artery at the porta hepatis. These vessels branch into the portal triads within the liver lobules. Blood then flows into the central vein of each lobule. Central veins converge to form hepatic veins, which drain into the inferior vena cava and return blood to the heart. 2. Bile Flow: Bile is produced by hepatocytes in the liver lobules. Bile exits the lobules via bile canaliculi and flows into the hepatic duct branches of the portal triad. From the hepatic ducts, bile travels out of the liver through the right and left hepatic ducts at the porta hepatis.  LIVER FUNCTIONS Neutralizes Stomach Acid: Bile’s alkaline pH neutralizes acidic chyme from the stomach. Emulsifies Lipids: Bile salts break down fats into smaller droplets for digestion by lipase. Excretory Role: Bile pigments, such as bilirubin, are waste products from hemoglobin breakdown, contributing to fecal color.  LIVER STORAGE OF NUTRIENTS Nutrient Storage: Hepatocytes store glucose (as glycogen), lipids, vitamins (A, B12, D, E, K), copper, and iron. Blood Glucose Regulation: Hepatocytes manage blood glucose levels by removing excess glucose and releasing it as needed to maintain homeostasis.  LIVER PROCESSING OF NUTRIENTS Nutrient Interconversion: The liver converts excess amino acids into ATP, lipids, and glucose. It also synthesizes phospholipids and hydroxylates vitamin D to its active form. Conversion of Dietary Substances: Dietary fats are processed into phospholipids, and vitamin D is modified to regulate calcium levels.  LIVER DETOXIFICATION Toxin Alteration: The liver converts harmful substances into less toxic forms. Ammonia from amino acid metabolism is converted to urea. Kupffer Cells: These phagocytic cells in the liver sinusoids remove worn-out blood cells, bacteria, and debris from the blood.  LIVER SYNTHESIS OF NEW MOLECULES Plasma Proteins: The liver produces essential proteins, including albumins, fibrinogen, globulins, heparin, and clotting factors. Cholesterol Synthesis: The liver is a major site for cholesterol production, important for cell membranes and steroid hormone synthesis.  VE ORGANS DIGESTI GALLBLADDER The gallbladder is a small, pear-shaped organ located beneath the liver. Its primary role in the digestive system is to store and concentrate bile produced by the liver. Bile is a sticky, yellow-green digestive fluid produced by the liver and stored in the gallbladder. Its primary function is to break down fats into fatty acids during digestion.  GALLBLADDER GALLBLADDER ANATOMY AND FUNCTION Structure: The gallbladder is a sac-like organ approximately 8 cm long and 4 cm wide, located on the inferior surface of the liver. Inner Mucosa: Folded into rugae to allow expansion. Muscularis: Smooth muscle layer enabling contraction. Serosa: Outer covering.  GALLBLADDER FUNCTION Bile Storage and Concentration: Stores 40–70 mL of bile secreted by the liver. During storage, water and electrolytes are absorbed, concentrating bile salts and pigments up to 5– 10 times. Bile Release: Bile is released into the small intestine via the cystic duct when the gallbladder contracts, stimulated primarily by cholecystokinin and secondarily by vagal stimulation.  GALLBLADDER GALLSTONE Formation: Gallstones are insoluble aggregates, often resulting from excess cholesterol precipitating in the gallbladder. Effects: Gallstones can block the cystic duct, impeding bile release and causing digestive issues. If they obstruct the pancreatic duct, they may further disrupt digestion and require surgical removal.  VE ORGANS DIGESTI PANCREAS The pancreas is located behind the stomach; it performs two key functions: it produces enzymes that break down sugars, fats, proteins, and starches during digestion. The pancreas releases hormones into the bloodstream. These chemical messengers help regulate blood sugar levels, stimulate stomach acids, and control appetite and stomach emptying.  PANCREAS The pancreas is a dual-function organ located behind the stomach, with its head within the duodenum's curvature and its body and tail extending to the spleen. It has both endocrine and exocrine functions.  PANCREAS ENDOCRINE FUNCTION The pancreas contains the islets of Langerhans, which produce three key hormones: insulin, glucagon, and somatostatin. Insulin and glucagon regulate blood sugar, while somatostatin controls their secretion and may inhibit growth hormone production.  PANCREAS EXOCRINE FUNCTION The pancreas has acinar cells that produce digestive enzymes. These enzymes travel through a series of ducts into the small intestine to aid in digestion.  PANCREAS PANCREATIC JUICE COMPONENTS 1. Aqueous component: Rich in bicarbonate ions (HCO₃⁻), it neutralizes stomach acid in the small intestine. 2. Enzymatic component: Contains enzymes like trypsin, chymotrypsin, and carboxypeptidase (for proteins), pancreatic amylase (for carbohydrates), and lipase (for lipids).  PANCREAS The release of pancreatic juice is regulated by both neural and hormonal signals. Parasympathetic stimulation increases secretion, while hormones like secretin and cholecystokinin are triggered by acidic chyme and fatty acids in the duodenum, respectively. These hormones help control the release of bicarbonate and enzyme-rich juice.  VE ORGANS DIGESTI LARGE INTESTINE The large intestine, also known as the colon, follows the small intestine and extends to the anal canal, where food waste exits the body. The large intestine performs several essential functions, such as absorbing water and electrolytes, forming stool, facilitating bacterial fermentation, and protecting against infections.  LARGE INTESTINE SECREATION OF LARGE INTESTINE: 1. Mucus Production: The large intestine primarily secretes mucus, produced by goblet cells and crypts in the mucosa. This mucus lubricates the intestinal wall and helps fecal matter stay together. Increased mucus secretion occurs in response to tactile stimuli, irritation, and parasympathetic stimulation. 2. Feces Composition: Feces consist of water, solid substances (like undigested food), microorganisms, and sloughed-off epithelial cells. Diarrhea is characterized by frequent, watery stools. 3. Microbiota Functions: The large intestine is home to normal microbiota that make up about 30% of fecal dry weight. They synthesize vitamin K, produce acids, and help break down some cellulose into glucose (though this glucose isn't absorbed). They also generate gases, or flatus, which varies based on diet and bacterial population. 4. Electrolyte and Water Absorption: The movement of sodium (Na) and chloride (Cl) into epithelial cells, driven by various transport mechanisms, facilitates water absorption through osmosis.  LARGE INTESTINE MOVEMENT IN THE LARGE INTESTINE: 1. Segmental and Peristaltic Movements: The large intestine has less frequent segmental mixing compared to the small intestine. Peristaltic waves mainly move chyme along the colon toward the anus. 2. Mass Movements: These occur most often after meals, typically starting about 15 minutes post-breakfast and lasting 10-30 minutes. They are driven by: - Gastrocolic Reflex: Triggered by stomach distension. - Duodenocolic Reflex: Triggered by duodenum distension. These reflexes stimulate peristalsis and are influenced by parasympathetic nerves and hormones like cholecystokinin and gastrin.  LARGE INTESTINE MOVEMENT IN THE LARGE INTESTINE: 3. Defecation: The process involves: - Coordination: Contractions move feces toward the anus while relaxing the internal and external anal sphincters. - Parasympathetic Reflexes: Manage most of the defecation reflex. - Internal Anal Sphincter: Maintains resting pressure through tonic contractions. - External Anal Sphincter: Under conscious control, allows for voluntary defecation or retention. Increased abdominal pressure triggers its contraction to prevent untimely expulsion.  LARGE INTESTINE MOVEMENT IN THE LARGE INTESTINE: 4. Conscious Control: The brain controls the external anal sphincter, allowing voluntary defecation or retention. The defecation reflex is brief and usually reinitiated by mass movements, with local and parasympathetic reflexes contributing to the process.  VE ORGANS DIGESTI RECTUM AND ANUS The rectum connects the large intestine to the anus. It acts as a reservoir where stool accumulates before being ready for elimination. The anus marks the exit point for food waste. Muscles, nerves, and mucous membranes work together to facilitate healthy bowel movements that you can control.  DIGESTION AND ABSORPTION  AND ABSORPTION DIGESTION Digestion – breakdown of food to molecules small enough to be absorbed into the blood. a. Mechanical Digestion – breaks large food particles apart into smaller ones. b. Chemical Digestion – breaking covalent chemical bonds in organic molecules in digestive enzymes: 1. Carbohydrates -> monosaccharides 2. Lipids -> fatty acids and monoglycerides 3. Proteins -> amino acids Vitamins, minerals, and water are not broken down. Absorption – which molecules are moved out of the digestive tract into the blood for distribution throughout the body.  AND ABSORPTION DIGESTION 1. Introduction to Absorption Absorption through mucosa: Some small molecules (e.g., alcohol, aspirin) diffuse through the stomach epithelium. Most molecules, however, need transport mechanisms to cross the intestinal wall. These transport mechanisms include facilitated diffusion, active transport, and secondary transport (symport and antiport). Membranes: The intestinal epithelial cells have two distinct sides. Apical membrane - lumen (cavity on digestive tract where food and liquids pass through) faces the digestrive tract, and the basolateral membrane (for transferring nutrients from epithelial cells into the bloodstream) faces the blood vessels. Each membrane has specific transport proteins that ensure one-way movement of molecules from the digestive tract into the blood.  AND ABSORPTION DIGESTION 1. Introduction to Absorption Routes of transport: Water - soluble molecules like glucose and amino acids enter the hepatic portal system and travel to the liver. Lipid metabolism products are coated with proteins and enter lymphatic capillaries called lacteals, eventually reaching the thoracic duct, which empties into the left subclavian vein.  AND ABSORPTION DIGESTION 2. Carbohydrates Digestion Types of carbohydrates: The ingested carbohydrates include: polysaccharides (starches), disaccharides (e.g., sucrose, lactose), and monosaccharides (e.g., glucose, fructose).  AND ABSORPTION DIGESTION 2. Carbohydrates Digestion Digestion process: 1.In the oral cavity: Begins with salivary amylase breaking down starches. This process continues until food reaches the stomach, where the acidity deactivates amylase. 2.In the small intestine: Pancreatic amylase resumes digestion, breaking polysaccharides into disaccharides and monosaccharides. Disaccharidases, bound to the microvilli, break disaccharides into glucose, galactose, and fructose.  AND ABSORPTION DIGESTION 2.5 Carbohydrates Absorption 1. Absorption of monosaccharides: Glucose and galactose are absorbed into intestinal cells by Na⁺ symport. 2. driven by the Na⁺ gradient maintained by the Na⁺-K⁺ pump. 3. Fructose is absorbed by facilitated diffusion. 4. Once absorbed, monosaccharides enter the blood capillaries of the intestinal villi and travel to the liver through the hepatic portal system, where non-glucose monosaccharides are converted to glucose.  ND ABSORPTION DIGESTION A 3. LIPIDS Types of lipids: Includes triglycerides, phospholipids, cholesterol, steroids, and fat-soluble vitamins. Triglycerides, made of three fatty acids and glycerol, are the most common. Lipase enzymes: Enzyme that breaks down triglycerides into free fatty acids There are three lipases: pancreatic lipase (primary enzyme), lingual lipase, and gastric lipase. Lingual and gastric lipases are important in infants, while pancreatic lipase is dominant in adults. Emulsification: Bile salts emulsify large lipid droplets into smaller ones, increasing surface area for enzyme action. This step is crucial for lipid digestion, as lipases are water-soluble and can only act on the surface of lipid droplets.  ND ABSORPTION DIGESTION A 3.5 Lipids Absorption 1. ·Bile salts form micelles around small lipid droplets, aiding in their diffusion through the intestinal epithelial cell membranes. 2. Attachment of micelle to plasma membrane 3. ·Inside the cells, lipids are reassembled into triglycerides by free fatty acids and packaged into chylomicrons (90% triglyceride, 5% cholesterol, 4% phospholipid, and 1% protein). 4. ·Chylomicrons leave the epithelial cells via exocytosis and enter the lacteals of the lymphatic system. They are transported to adipose tissue or the liver, where lipids are stored, used as energy, or converted into other molecules.  ND ABSORPTION DIGESTION A 4. Lipoprotein Transport Lipids are transported in the blood as lipoproteins, which are combinations of lipids and proteins. They are classified by density: Chylomicrons: Very low density, 99% lipid, 1% protein. Very-low-density lipoproteins (VLDL): 92% lipid, 8% protein. Low-density lipoproteins (LDL): 75% lipid, 25% protein. LDL is often called "bad cholesterol" because excess LDL leads to cholesterol deposition in arteries. High-density lipoproteins (HDL): 55% lipid, 45% protein. HDL is "good cholesterol" because it helps remove cholesterol from tissues and transports it to the liver for excretion.  ND ABSORPTION DIGESTION A 4. Lipoprotein Transport 1. LDL, which carries cholesterol and other lipids, is transported through the blood to various tissues. Cells have specialized receptors for LDL located in pits on their surface. 2. When LDL binds to these receptors, the pits on the cell surface, which are a small part (2%) of the cell surface area, form endocytotic vesicles. The cell then engulfs the LDL through receptor-mediated endocytosis. 3. The endocytotic vesicle, containing LDL, merges with a lysosome inside the cell. Inside the lysosome, LDL is broken down, and its components are made available for the cell to use.  ND ABSORPTION DIGESTION A 5. Proteins Digestion Begins in the stomach: Pepsin breaks proteins into smaller polypeptide chains. Continues in the small intestine: Pancreatic enzymes break down polypeptides into smaller peptides. Final breakdown: Peptidases in the microvilli of the small intestine convert peptides into amino acids. Absorption of amino acids: Amino acids are transported into epithelial cells by specific carrier proteins Dipeptides and tripeptides are absorbed by H⁺ symport, then further broken down inside the cells. Amino acids enter the hepatic portal system and travel to the liver, where they are either modified or released into the bloodstream.  AND ABSORPTION DIGESTION 5.5 Protein Absorption 1. Acidic and most neutral amino acids enter epithelial cells using a symport mechanism with Na+ (sodium) ions, similar to glucose transport. Basic amino acids enter via facilitated diffusion, which doesn't require Na+. 2. Dipeptides and tripeptides enter epithelial cells through a symport mechanism with H+ (hydrogen) ions. More amino acids enter as dipeptides or tripeptides than as single amino acids. Dipeptidases and tripeptidases break down dipeptides and tripeptides into individual amino acids. 3. Individual amino acids then leave the epithelial cell. 4. Enter the hepatic portal system, which carries them to the liver. In the liver, amino acids can be modified or released into the bloodstream for distribution throughout the body.  AND ABSORPTION DIGESTION 6. Water and Ions Absorption Water absorption: About 9 liters of water enter the digestive tract daily, and about 92% is absorbed in the small intestine, with another 6-7% absorbed in the large intestine. Water moves through osmosis depending on the osmotic gradient between the chyme and the surrounding fluids As nutrients are absorbed in the small intestine, the concentration of solutes (osmotic pressure) in the chyme decreases. This causes water to move from the small intestine into the surrounding extracellular fluid, following the osmotic gradient. Once in the extracellular fluid, the water can enter the bloodstream. Because of this shift in osmotic pressure, nearly all the water that enters the small intestine whether from food, drink, or digestive secretions is reabsorbed back into the body. This process helps maintain fluid balance during digestion.  AND ABSORPTION DIGESTION 6. Water and Ions Absorption Ions absorption: Sodium (Na⁺), potassium (K⁺), calcium (Ca²⁺), magnesium (Mg²⁺), and phosphate (PO₄³⁻) are absorbed by active transport. Chloride (Cl⁻) ions move passively following Na⁺, but active transport occurs in the ileum (last part of small intestine). 6.5 Water and Ion Balance in the Digestive System The balance of water, ions, and nutrients is important for proper digestive function. Sodium and glucose absorption help regulate water reabsorption by osmosis, aiding in hydration and ion replacement.  ANATOMY CLASS THANK YOU FOR LISTENING!  ANATOMY CLASS PRESENTED BY: Malaquilla, Gave-Riel H. Dela Cruz, John Angelo C. Borre, Tiffany Blas, Jairus Shin. R  ANATOMY CLASS REFERENCES: VanPutte, C. L., Regan, J. L., Russo, A. F., & Seeley, R. R. (2023). Seeley’s anatomy & physiology (Thirteenth edition. International student edition). McGraw Hill.