NOTA STRUKTUR & FUNGSI GI TREK.pdf

# Structure of the digestive tract The structure of the digestive tract is composed of distinct layers and is supported by accessory organs, all of which work in coordination for efficient digestion, absorption, and elimination [2](#page=2). ### 1.1 The alimentary tract layers The alimentary tract is generally formed by four distinct layers, with the exception of the mouth and pharynx. These layers, from outer to inner, are the adventitia/peritoneum, the muscular layer, the submucosa, and the mucosa [2](#page=2). #### 1.1.1 Adventitia / peritoneum (outermost layer) The outermost layer varies depending on its location. In the thoracic cavity, it consists of loose fibrous tissue, termed adventitia. In the abdominal cavity, this layer is a serous membrane known as the peritoneum [3](#page=3). ##### 1.1.1.1 Peritoneum The peritoneum is the largest membrane in the body. It is a serous membrane that protects and covers the abdominal organs. It comprises two layers [3](#page=3): * **Parietal layer:** Lines the inner surface of the abdominal wall [3](#page=3). * **Visceral layer:** Covers the abdominal organs [3](#page=3). A small amount of clear fluid is present between these layers, which reduces friction as the abdominal organs move [3](#page=3). > **Tip:** The fluid in the peritoneal cavity is crucial for smooth organ movement within the abdomen. Organs that are almost entirely covered by the peritoneum include the stomach, liver, gallbladder, and intestines. Organs partially covered include the spleen, pancreas, duodenum, the end of the intestines, and pelvic organs [4](#page=4). Folds of the peritoneum have specific names and functions: * **Mesentery:** A peritoneal fold attached to the posterior abdominal wall. It serves to support and anchor organs, and blood vessels and nerves pass through it to reach the organs [4](#page=4). * **Greater omentum:** A peritoneal fold that hangs from the anterior abdominal wall. It stores fat and can help to isolate infections [4](#page=4) [5](#page=5). * **Lesser omentum:** A peritoneal fold extending from the lower edge of the liver to the lesser curvature of the stomach [4](#page=4). The peritoneum has several important functions: * Maintains the position of blood vessels and nerves, supporting and stabilizing abdominal and pelvic organs [4](#page=4). * Produces serous fluid to prevent friction during organ movement [5](#page=5). * Stores fat within the omentum and mesentery [5](#page=5). * Contains lymphatic vessels and lymph nodes, contributing to immune defense [5](#page=5). * Can absorb water and electrolytes, a principle utilized in peritoneal dialysis [5](#page=5). * Helps control localized infections by isolating inflamed areas [5](#page=5). #### 1.1.2 Muscular layer This layer consists of two sub-layers: * **Outer layer:** Composed of longitudinal muscle fibers [5](#page=5). * **Inner layer:** Composed of circular muscle fibers [5](#page=5). Blood vessels, lymphatic vessels, sympathetic nerves, and parasympathetic nerves are located between these muscle layers, forming the mesentery, and supply the muscle and blood vessels [5](#page=5). The muscular layer's functions are vital for digestion: * **Peristalsis:** Muscle contractions propel food through the tract [5](#page=5). * **Mixing:** Muscle contractions help mix food with digestive juices [5](#page=5). * **Regulation of movement:** Sphincter muscles, rich in circular fibers, slow down food movement, allowing adequate time for digestion and absorption. They also prevent the backflow of food [5](#page=5). ##### 1.1.2.1 Peristalsis When food is swallowed, smooth muscles in the digestive tract wall contract, pushing food downwards in an wave-like motion. When the circular muscles contract, the longitudinal muscles relax, propelling the bolus of food forward [6](#page=6). #### 1.1.3 Submucosa layer This layer is composed of loose connective tissue and some elastic fibers. It houses blood vessels, nerves, lymphatic vessels, and lymphatic tissue. It contains arteries, arterioles, veins, and capillaries. The nerve supply here is the submucosal plexus (also known as Meissner's plexus), which includes sympathetic nerves that reduce muscle contraction and parasympathetic nerves that initiate contraction and stimulate juice production [6](#page=6). #### 1.1.4 Mucosa layer The innermost layer of the digestive tract, the mucosa, consists of three sub-layers: * **Mucous membrane:** Formed by columnar epithelium, it has three primary functions: protection, secretion, and absorption [7](#page=7). * **Lamina propria:** A layer of loose connective tissue containing blood vessels that supply nutrients to the inner layers and lymphatic tissue [7](#page=7). * **Muscularis mucosa:** A thin layer of smooth muscle [7](#page=7). In areas exposed to mechanical injury, the epithelium may be squamous with glands and secretions. In areas where absorption and secretion of digestive juices occur, the columnar epithelium includes goblet cells that secrete mucus. Beneath this surface are specialized cells or glands that excrete digestive juices containing enzymes to break down food into the tract lumen [7](#page=7). ### 1.2 Blood supply The digestive tract receives its blood supply primarily from branches of the abdominal aorta. Key arteries include [7](#page=7): * Celiac artery (supplying the esophagus, stomach, and upper duodenum) * Superior mesenteric artery (supplying the small intestine, cecum, ascending colon, and transverse colon) [8](#page=8). * Inferior mesenteric artery (supplying the descending colon and rectum) [8](#page=8). Venous drainage follows a similar pattern, with blood from the thoracic region draining into the superior vena cava, and blood from the abdominal region, particularly the gut, draining into the portal vein before reaching the liver. Blood from the rectum drains into the internal iliac vein [8](#page=8). ### 1.3 Nerve supply The alimentary tract is innervated by both divisions of the autonomic nervous system [8](#page=8). * **Parasympathetic nervous system:** Primarily through the vagus nerve, its stimulation causes muscle contraction along the tract and stimulates the secretion of digestive juices [8](#page=8). * **Sympathetic nervous system:** Fibers originate from the thoracic and lumbar spinal cord, forming plexuses in the thorax, abdomen, and pelvis. Its action is to inhibit muscle contraction and reduce juice secretion [8](#page=8). Two major plexuses are found within the wall of the alimentary tract, derived from both sympathetic and parasympathetic innervation: * **Myenteric plexus (Auerbach's plexus):** Located between the two muscle layers [8](#page=8). * **Submucosal plexus (Meissner's plexus):** Located in the submucosa, associated with secretory glands [8](#page=8). ### 1.4 Accessory digestive organs These organs play a crucial role in the digestive process but are not part of the continuous alimentary tract. They include [2](#page=2): * Three pairs of salivary glands [2](#page=2). * The pancreas [2](#page=2). * The liver and gallbladder [2](#page=2). These organs are anatomically and physiologically linked to the digestive tract, ensuring the smooth and complete process of digestion, absorption, and elimination [2](#page=2). * * * # Organs of the digestive system This section details the organs that constitute the alimentary canal and the accessory organs aiding digestion, outlining their locations and initial functions [1](#page=1). ### 2.1 The alimentary canal organs The alimentary canal is a long tube that acts as the pathway for food, starting from the mouth and ending at the anus. It comprises several organs responsible for preparing food for absorption and utilization by the body's cells [1](#page=1). #### 2.1.1 Mouth (Oral cavity) The mouth, or oral cavity, is surrounded by muscles and bones and is crucial for receiving food [9](#page=9). * **Boundaries:** * Anterior: Lips [9](#page=9). * Posterior: Connects to the oropharynx [9](#page=9). * Lateral: Cheeks [9](#page=9). * Superior: Hard and soft palate [9](#page=9). * Inferior: Tongue and floor of the mouth tissues [9](#page=9). * **Lining:** The oral cavity is lined with a mucous membrane composed of stratified squamous epithelium and contains small mucosal glands [9](#page=9). * **Divisions:** * **Oral cavity proper:** The central space containing the tongue [9](#page=9). * **Vestibule:** The space between the teeth and the inner surface of the cheeks [9](#page=9). ##### 2.1.1.1 Structures forming the mouth * **Lips:** Located at the anterior part of the mouth [10](#page=10). * **Cheeks:** Located at the sides of the mouth [10](#page=10). * **Tongue:** Forms the floor of the mouth [10](#page=10). * **Structure:** A voluntary muscular organ attached posteriorly to the hyoid bone and inferiorly by a mucosal fold called the frenulum. Its superior surface has stratified squamous epithelium with papillae (taste buds) [10](#page=10). * **Types of papillae:** * **Vallate papillae:** Located at the base of the tongue in a V-shape [10](#page=10). * **Fungiform papillae:** Mostly found at the tip and sides of the tongue [10](#page=10). * **Filiform papillae:** The most abundant, located on the anterior two-thirds of the tongue's surface [10](#page=10). * **Blood supply:** Lingual branches of the external carotid artery; venous drainage via lingual veins into the internal jugular vein [11](#page=11). * **Nerve supply:** Hypoglossal nerve (involuntary muscles), lingual branch of the mandibular nerve (sensation, temperature, touch), facial and glossopharyngeal nerves (special sensations) [11](#page=11). * **Functions:** Chewing and swallowing, speech, touch, and taste (sweet at the tip, salty at the tip and sides, bitter near the root, sour on the sides) [11](#page=11). * **Palate:** Forms the roof of the mouth [10](#page=10). * **Hard palate:** Anterior part, composed of maxillary and palatine bones [10](#page=10). * **Soft palate:** Posterior part, separates the nasopharynx from the oral cavity. It ends in a cone-shaped projection called the uvula. The soft palate has four folds: two posterior forming the palatopharyngeal arches and two anterior forming the palatoglossal arches. The palatine tonsils are lymphatic tissues located between the palatopharyngeal arches [10](#page=10). ##### 2.1.1.2 Teeth Teeth are hard, bone-like structures embedded in the alveolar sockets of the mandible and maxilla [11](#page=11). * **Types:** Individuals have deciduous (temporary) teeth and permanent teeth [11](#page=11). * **Deciduous teeth:** 20 teeth that begin to emerge around 6 months and are complete by 24 months [11](#page=11). * **Permanent teeth:** Begin replacing deciduous teeth around age 6, with 32 teeth fully erupted by age 24 [11](#page=11). * **Structure:** * **Crown:** The part protruding from the gums [12](#page=12). * **Neck:** The region between the crown and the root [12](#page=12). * **Root:** The part embedded in the gums [12](#page=12). * **Dentin:** The primary hard material of the tooth [12](#page=12). * **Enamel:** A thin, very hard layer covering the dentin in the crown [12](#page=12). * **Cementum:** Covers the tooth root, anchoring it to the socket [12](#page=12). * **Pulp cavity:** Contains blood vessels, lymphatic vessels, and nerves, with a small opening at the root apex called the foramen [12](#page=12). * **Types and Functions:** * **Incisors:** 8 teeth with flat, sharp edges and one root; used for biting and cutting food [13](#page=13). * **Canines:** 4 teeth with pointed, sharp surfaces and one root; used for tearing food [13](#page=13). * **Premolars:** 8 teeth with broad, flat surfaces and two roots; used for grinding and crushing food [13](#page=13). * **Molars:** 12 teeth with broad surfaces; upper molars have three roots, lower molars have two roots; used for grinding and crushing food [13](#page=13). * **Blood and Nerve Supply:** Arteries are branches of the maxillary artery; veins drain into the jugular veins. Upper teeth are supplied by the maxillary nerve, lower teeth by the mandibular nerve [13](#page=13). #### 2.1.2 Pharynx Also known as the throat, the pharynx is a funnel-shaped tube extending from the internal nares to the larynx [14](#page=14). * **Parts:** * **Nasopharynx:** Upper part, behind the nose; contains adenoids and openings of auditory tubes [14](#page=14). * **Oropharynx:** Middle part, behind the mouth; contains palatine and lingual tonsils [14](#page=14). * **Laryngeal pharynx:** Lower part, connects to the esophagus and larynx; serves as a passageway for both food and air [14](#page=14). * **Structure:** Composed of skeletal muscle and lined with mucous membrane [14](#page=14). * **Function:** Involved in breathing and swallowing; muscle contractions in the oropharynx and laryngopharynx help move food into the esophagus [14](#page=14). #### 2.1.3 Esophagus The esophagus is an alimentary canal that connects to the stomach [15](#page=15). * **Characteristics:** Approximately 25 cm long with a diameter of 2-3 cm, lined with stratified squamous epithelium [15](#page=15). * **Location:** Situated anterior to the vertebral column and posterior to the trachea and heart [15](#page=15). * **Connections:** Connects to the pharynx, passes through the diaphragm, and slightly curves upward before joining the stomach to prevent regurgitation [15](#page=15). * **Sphincters:** * **Upper esophageal sphincter (Cricopharyngeal sphincter):** Prevents air from entering the esophagus [15](#page=15). * **Lower esophageal sphincter (Cardiac sphincter):** Prevents the reflux of gastric acid into the esophagus [15](#page=15). * **Microscopic Structure (Four basic layers of the alimentary canal wall):** * **Adventitia (outer layer):** Fibrous and elastic tissue [15](#page=15). * **Muscular layer:** Contains longitudinal and circular muscle fibers [15](#page=15). * **Submucosa:** Areolar tissue, blood vessels, lymphatic vessels, and nerves [15](#page=15). * **Mucosa (inner layer):** Stratified squamous epithelium with glands producing mucus [16](#page=16). * **Blood Supply:** Thoracic region by esophageal arteries from the aorta; abdominal region by inferior phrenic arteries and inferior left gastric artery (branches of the celiac artery) [16](#page=16). * **Venous Drainage:** Thoracic region to the azygos and hemiazygos veins; abdominal region to the inferior left gastric vein [16](#page=16). ##### 2.1.3.1 Functions of the mouth, pharynx, and esophagus * **Food bolus formation:** Food is chewed by teeth, manipulated by the tongue and cheek muscles, mixed with saliva, and formed into a soft bolus ready for swallowing [16](#page=16). * **Swallowing (Deglutition):** Occurs in three stages after bolus formation [16](#page=16): 1. **Voluntary stage:** Mouth closes, tongue and cheek muscles push the bolus into the pharynx [16](#page=16). 2. **Pharyngeal stage:** A reflex initiated by the oropharynx walls, coordinated by the brainstem; pharyngeal muscles contract to move food down, while other passages are closed (soft palate seals nasopharynx, tongue seals mouth, epiglottis covers larynx) [17](#page=17). 3. **Esophageal stage:** The presence of the bolus stimulates peristalsis, propelling it through the esophagus to the stomach [17](#page=17). * **Peristalsis:** Rhythmic contractions of smooth muscles in the digestive tract wall that propel food downward in a wave-like motion. Circular muscle contraction causes longitudinal muscle relaxation and vice versa, pushing the bolus forward [17](#page=17). #### 2.1.4 Stomach (Gaster) The stomach is a J-shaped, sac-like organ located in the epigastric region, between the 5th and 9th ribs [17](#page=17). * **Location:** Connects directly to the esophagus and hangs freely in the left hypochondriac region of the abdominal cavity [17](#page=17). * **Relations:** * Anterior: Left lobe of the liver and anterior abdominal wall [18](#page=18). * Posterior: Pancreas, spleen, abdominal aorta, inferior vena cava, left kidney, and left adrenal gland [18](#page=18). * Inferior: Transverse colon [18](#page=18). * Left: Diaphragm and spleen [18](#page=18). * Right: Liver and abdomen [18](#page=18). * **Structure:** Divided into three main parts [18](#page=18): * **Fundus:** Located to the left of the cardiac sphincter, preventing food backflow [18](#page=18). * **Body:** The large, main part of the stomach [18](#page=18). * **Pyloric part (Pyloric antrum):** The lower, curved portion leading to the pyloric sphincter and then the duodenum; the pyloric sphincter prevents undigested food from entering the duodenum [18](#page=18). * **Curvatures:** * **Greater curvature:** The convex anterior surface [18](#page=18). * **Lesser curvature:** The concave posterior surface [18](#page=18). * **Wall Layers:** Consists of four layers [19](#page=19): * **Outer layer (Serosa):** Peritoneal membrane, known as the omentum in the stomach, covering the entire organ [19](#page=19). * **Muscular layer:** Composed of three layers of smooth muscle: longitudinal (outer), circular (middle), and oblique (inner). This arrangement allows for churning and peristaltic movements, aiding in digestion and breakdown of food into a liquid state (chyme) before it passes to the duodenum [19](#page=19). * **Submucosa:** Loose connective tissue with elastic fibers, containing blood vessels, lymphatic vessels, and nerves [19](#page=19). * **Mucosa:** The inner lining, formed by columnar epithelium that produces mucus. When the stomach is empty, it folds into rugae, which allow for expansion when food is present. The mucosa contains glands that produce gastric juice and enzymes [20](#page=20). * **Gastric Glands and Secretions:** * **Surface mucous cells & Mucous neck cells:** Produce mucus to lubricate and protect the stomach lining from mechanical injury and acid [20](#page=20). * **Chief cells (Zymogen cells/Peptic cells):** Located in the body and fundus; produce pepsinogen (for protein digestion) and renin (for milk digestion in infants) [20](#page=20). * **Parietal cells (Oxyntic cells):** Located in the pyloric region; produce hydrochloric acid (HCl) and intrinsic factor (essential for Vitamin B12 absorption, which is vital for red blood cell formation). They also produce gastrin hormone to stimulate gastric juice production [20](#page=20). * **Endocrine G cells:** Produce the hormone gastrin [20](#page=20). * **Blood Supply:** Receives oxygenated blood from the gastric artery and branches of the splenic artery. Gastric veins drain deoxygenated blood rich in nutrients into the portal vein [21](#page=21). * **Nerve Supply:** Sympathetic stimulation from the celiac plexus slows peristalsis and reduces gastric juice production. Vagus nerve stimulation (parasympathetic) speeds up peristalsis and increases gastric juice production [21](#page=21) [22](#page=22). * **Gastric Juice Composition and Function:** Approximately 2 liters are produced daily [22](#page=22). * **Mucus:** Protects the stomach lining from acid and mechanical damage, lubricates food [22](#page=22). * **Hydrochloric acid (HCl):** Kills microorganisms, activates pepsinogen, acidifies food (inhibiting salivary amylase), and provides an optimal environment for pepsin [22](#page=22). * **Intrinsic factor:** Aids in Vitamin B12 absorption [23](#page=23). * **Pepsinogen:** Activated by HCl to pepsin, initiating protein digestion into polypeptides [23](#page=23). * **Gastrin:** Hormone that stimulates gastric gland secretion [23](#page=23). * **Functions of the Stomach:** 1. Temporary food storage, allowing pepsin to convert proteins to polypeptides [23](#page=23). 2. Chemical digestion: Pepsin breaks down proteins into polypeptides [23](#page=23). 3. Mechanical breakdown: Three muscle layers churn food, and gastric juice dilutes it into chyme [23](#page=23). 4. Limited absorption of water, alcohol, and lipid-soluble medications [23](#page=23). 5. Protection from microbes due to HCl [23](#page=23). 6. Provides iron in a soluble form for absorption [23](#page=23). 7. Produces intrinsic factor for Vitamin B12 absorption in the terminal ileum [23](#page=23). 8. Controls the passage of stomach contents to the duodenum; the pyloric antrum empties chyme in small amounts as it becomes sufficiently acidified and diluted [23](#page=23). 9. Secretes gastrin, which increases HCl production [23](#page=23). 10. Gastric juice dilutes food into chyme [23](#page=23). #### 2.1.5 Small intestine The small intestine plays a critical role in digestion and absorption, receiving stomach contents about 2-4 hours after a meal [24](#page=24). * **Characteristics:** A tube extending from the pyloric sphincter to the ileocecal valve, approximately 6.5 meters (21 feet) long [24](#page=24). * **Divisions:** * **Duodenum:** The first and shortest part, about 25-30 cm long, C-shaped and encircling the head of the pancreas. It contains the ampulla of Vater, regulated by the sphincter of Oddi, where the common bile duct and main pancreatic duct open [24](#page=24) [25](#page=25). * **Jejunum:** The middle section, approximately 1 meter long [25](#page=25). * **Ileum:** The final section, about 2 meters long, connecting to the cecum of the large intestine via the ileocecal sphincter/valve [25](#page=25). * **Structure of the Wall:** Consists of the four basic layers of the alimentary canal [26](#page=26): * **Outer layer (Serosa):** Peritoneal membrane (mesentery), which supports the jejunum and ileum and allows passage of nerves [26](#page=26). * **Muscular layer:** Inner circular and outer longitudinal smooth muscle fibers [26](#page=26). * **Submucosa:** Loose connective tissue with elastic fibers, containing blood and lymphatic ducts, and nerves [26](#page=26). * **Mucosa:** The inner lining, composed of simple columnar epithelium. It has folds (circular folds or plica circulares) that are permanent and increase the surface area for absorption. Protruding from the mucosal surface are millions of finger-like projections called villi (0.5-1 mm long). Each villus contains blood capillaries and a central lymphatic vessel called a lacteal. Nutrients are absorbed through the villi walls into the blood capillaries (glucose and amino acids) and lymphatic ducts (fatty acids and glycerol). Glands between the villi produce intestinal juice containing enzymes that complete carbohydrate, protein, and fat digestion. The mucosa also contains lymphatic nodules; solitary lymphatic follicles and aggregated lymphatic follicles (Peyer's patches) are found, especially in the lower ileum [27](#page=27). * **Blood Supply and Nerve Supply:** Oxygenated blood is supplied by the superior mesenteric artery; deoxygenated blood rich in nutrients is drained by the mesenteric veins into the portal vein. The small intestine is supplied by sympathetic and parasympathetic nerves [29](#page=29) [30](#page=30). * **Intestinal Juice:** Produced by intestinal glands, it is watery, clear yellowish, and alkaline (pH 7.6) [31](#page=31). * **Secretions and Functions:** * **Goblet cells:** Produce mucus to protect the intestinal lining [31](#page=31). * **Simple columnar epithelium (Absorptive cells):** Contain brush border enzymes (maltase, sucrase, lactase, peptidases) that complete carbohydrate digestion (disaccharides to monosaccharides) and protein digestion (peptides to amino acids) [31](#page=31). * **S cells:** Secrete secretin (hormone) which stimulates the pancreas to release bicarbonate-rich juice [31](#page=31). * **CCK cells:** Secrete cholecystokinin (hormone) which inhibits gastric emptying, stimulates pancreatic enzyme-rich juice secretion, stimulates bile ejection from the gallbladder, and induces satiety [31](#page=31). * **K cells:** Secrete glucose-dependent insulinotropic peptide (GIP), which induces insulin secretion [32](#page=32). * **Simple columnar epithelium (lining cells):** Secrete enterokinase, which activates trypsinogen and chymotrypsinogen [32](#page=32). * **Functions of the Small Intestine:** 1. Produces intestinal fluid containing brush border enzymes (enterokinase, peptidase, maltase, sucrase, lactase) and mucus [32](#page=32). 2. Completes the digestion of carbohydrates, proteins, and fats within enterocytes [32](#page=32). 3. Produces hormones like secretin and cholecystokinin to aid digestion [32](#page=32). 4. Provides immune protection via lymphatic nodules and Peyer's patches [32](#page=32). 5. Absorption of digested nutrients through the villi into blood capillaries (amino acids, glucose) and lacteals (fatty acids, glycerol) [32](#page=32). #### 2.1.6 Large intestine The large intestine extends from the cecum to the anus, serving as the final passageway for waste material. It is approximately 1.5 meters long [33](#page=33). * **Divisions:** 1. **Cecum:** The initial part, about 5.8 cm long, located in the right iliac fossa. The ileocecal valve prevents backflow into the cecum. The appendix, containing lymphoid tissue, protrudes from the cecum [33](#page=33). 2. **Ascending colon:** Runs upward from the cecum in the right lumbar region, turns left at the hepatic flexure to become the transverse colon [34](#page=34). 3. **Transverse colon:** Crosses horizontally from right to left across the abdomen, from the hepatic flexure to the splenic flexure, where it turns downward as the descending colon [34](#page=34). 4. **Descending colon:** Extends downward in the left abdominal region, turning medially in the left iliac fossa to become the sigmoid colon [34](#page=34). 5. **Sigmoid colon:** Located in the pelvis, continuing downwards to become the rectum [34](#page=34). 6. **Rectum:** About 13 cm long, located in front of the sacrum, with the uterus anteriorly in females and the bladder anteriorly in males; it opens into the anal canal [34](#page=34). 7. **Anus:** A canal about 3.8 cm long, with two sphincters controlling it: the internal anal sphincter (smooth muscle) and the external anal sphincter (voluntary skeletal muscle). The upper part of the anal canal is lined with mucosa, while the lower part is skin [34](#page=34). * **Structure of the Colon:** Consists of four layers [34](#page=34): * **Outer layer (Serosa):** Peritoneal membrane covering the cecum, transverse colon, and sigmoid colon [35](#page=35). * **Muscular layer:** Two layers of involuntary smooth muscle: longitudinal (outer) and circular (inner). The longitudinal muscle forms three bands called teniae coli, which are shorter than the colon, causing it to pucker. The circular muscle layer is thinner and covers the entire colon. The anal sphincters are thickenings of the circular muscle layer [35](#page=35). * **Submucosa:** Loose connective tissue with elastic fibers, containing blood and lymphatic vessels and nerves. Lymphoid tissue is more abundant here due to the appendix [35](#page=35). * **Mucosa:** Lined with columnar epithelium containing mucus-producing glands. Unlike the small intestine, the colonic mucosa lacks villi and folds, except for the anus which has stratified squamous epithelium [36](#page=36). * **Blood Supply and Nerve Supply:** Oxygenated blood is supplied by the superior mesenteric artery (cecum, ascending, transverse colon) and inferior mesenteric artery (lower colon, rectum). Deoxygenated blood is drained by mesenteric veins into the portal vein. The colon receives sympathetic and parasympathetic innervation, but the external anal sphincter is under voluntary control [36](#page=36) [37](#page=37). * **Functions of the Colon:** 1. **Absorption:** Approximately 1000-1500 ml of chyme enters the cecum daily; this volume is significantly reduced by the absorption of sodium and water, especially in the cecum and ascending colon. The colonic mucosa produces alkaline mucus, not digestive juices. Most unabsorbed water, salts, and some glucose are absorbed. The colon also absorbs some medications, forming the basis for rectal suppositories. Less than 250 ml of water is excreted with feces [38](#page=38). 2. **Microbial activity:** Undigested food material is broken down by commensal microorganisms. Intestinal bacteria synthesize vitamins K, riboflavin, nicotinic acid, biotin, and folic acid [38](#page=38). 3. **Mucus secretion:** Lubricates the colon, facilitating the expulsion of feces [38](#page=38). 4. **Flatus production:** Intestinal gas is produced as a byproduct of bacterial fermentation [38](#page=38). 5. **Elimination:** Undigested materials are eliminated from the rectum as feces [38](#page=38). * **Defecation:** When feces reach the rectum, water is absorbed, concentrating the feces. Distension of the rectal wall stimulates stretch receptors, leading to the urge to defecate [38](#page=38) [39](#page=39). * **Feces:** Typically semi-solid, soft, and brown due to bile pigments (bilirubin and biliverdin). The amount produced daily (around 100 grams) varies with diet. Feces are about 70% water and contain undigested material (cellulose, undigested fats, protein remnants), dead epithelial cells, and microorganisms [39](#page=39). * * * # Functions of the digestive system The digestive system is responsible for the mechanical and chemical breakdown of food, the absorption of nutrients, and the elimination of waste products. ### 3.1 Overview of digestive processes The digestive system performs a series of complex functions to process ingested food. These functions include breaking down food into smaller molecules, absorbing these molecules into the bloodstream, and eliminating indigestible waste [16](#page=16) [32](#page=32) [38](#page=38). ### 3.2 Mechanical breakdown of food Mechanical breakdown involves the physical manipulation of food to increase its surface area for enzymatic action [23](#page=23). #### 3.2.1 Mastication and bolus formation * Food is chewed by teeth and manipulated by the tongue and cheek muscles [16](#page=16). * Food mixes with saliva to form a soft mass or bolus, which is ready for swallowing [16](#page=16). * The duration of food in the mouth depends on the amount and type of food, requiring thorough chewing before swallowing [16](#page=16). #### 3.2.2 Peristalsis and segmentation * The muscular layers of the digestive tract contract to propel food forward through a process called peristalsis [5](#page=5). * Muscle contractions also help mix food with digestive juices, aiding in its breakdown [23](#page=23) [5](#page=5). * Sphincter muscles, composed of many circular fibers, slow down the movement of food, allowing adequate time for digestion and absorption. These sphincters also prevent the backflow of food [5](#page=5). ### 3.3 Chemical breakdown of food Chemical breakdown involves the use of enzymes and acids to break down complex food molecules into simpler substances [23](#page=23) [32](#page=32). #### 3.3.1 Gastric digestion The stomach plays a crucial role in chemical digestion [23](#page=23). * **Gastric juice:** This secretion, comprising contributions from surface mucous cells, mucous neck cells, parietal cells, and chief cells, contains hydrochloric acid (HCl) and pepsinogen [23](#page=23). * **Hydrochloric acid (HCl):** HCl serves multiple functions, including: * Activating pepsinogen into its active enzyme form, pepsin [23](#page=23). * Providing an acidic environment optimal for pepsin activity [23](#page=23). * Killing microorganisms ingested with food, thus protecting against infection [23](#page=23). * Denaturing proteins, making them more accessible to pepsin [23](#page=23). * Dissolving iron, facilitating its absorption [23](#page=23). * **Pepsin:** This enzyme initiates the digestion of proteins, breaking them down into polypeptides [23](#page=23). * **Intrinsic factor:** Produced by parietal cells, it is essential for the absorption of vitamin B12 in the terminal ileum [23](#page=23). * **Gastrin:** This hormone, secreted by G cells, stimulates the gastric glands to produce more gastric juice, including HCl [23](#page=23). #### 3.3.2 Intestinal digestion The small intestine is the primary site for completing chemical digestion and absorbing nutrients [32](#page=32). * **Intestinal fluid:** The small intestine produces intestinal fluid containing various enzymes, known as brush border enzymes: * Enterokinase: Activates trypsinogen and chymotrypsinogen [32](#page=32). * Peptidase: Breaks down peptides into amino acids. * Maltase: Breaks down maltose into glucose. * Sucrase: Breaks down sucrose into glucose and fructose. * Lactase: Breaks down lactose into glucose and galactose. * Mucus: Lubricates the intestinal lining. * **Hormonal regulation:** Hormones such as secretin and cholecystokinin are produced in the small intestine and aid in the digestive process [32](#page=32). ### 3.4 Absorption of nutrients Absorption is the process by which digested nutrients pass from the digestive tract into the bloodstream or lymphatic system [32](#page=32). #### 3.4.1 Absorption in the stomach * The stomach absorbs a limited amount of water, alcohol, and lipid-soluble medications [23](#page=23). #### 3.4.2 Absorption in the small intestine * The inner lining of the small intestine, characterized by simple columnar epithelium, is covered in villi and microvilli, greatly increasing the surface area for absorption [32](#page=32). * Digested nutrients are absorbed through the villi into: * **Blood capillaries:** Amino acids and glucose are absorbed into the blood capillaries [32](#page=32). * **Lacteals:** Fatty acids and glycerol are absorbed into the lacteals, which are lymphatic vessels within the villi [32](#page=32). #### 3.4.3 Absorption in the large intestine * The large intestine primarily absorbs water and electrolytes, reducing the volume of indigestible material [38](#page=38). * About 1000-1500 ml of chyme enters the cecum daily from the small intestine [38](#page=38). * Significant water absorption occurs in the cecum and ascending colon [38](#page=38). * The colonic mucosa absorbs some drugs, which is the basis for rectal suppositories [38](#page=38). * The colon also absorbs salt and glucose. Less than 250 ml of water is ultimately eliminated with the feces [38](#page=38). ### 3.5 Elimination of waste Elimination, or defecation, is the process of expelling undigested material from the body [38](#page=38). * **Role of the colon:** The colon processes the remaining indigestible material from the small intestine [38](#page=38). * **Bacterial activity:** Commensal microorganisms in the colon break down undigested food materials. These bacteria are beneficial, synthesizing essential vitamins such as vitamin K, riboflavin, nicotinic acid, biotin, and folic acid [38](#page=38). * **Mucus production:** The colon's mucous membrane produces alkaline mucus that lubricates the colon, facilitating the passage of feces through the rectum and anus [38](#page=38). * **Flatus production:** The colon produces intestinal gas (flatus), mainly as a byproduct of bacterial fermentation [38](#page=38). * **Defecation process:** When undigested material (feces) reaches the rectum, it accumulates. Water is absorbed from the feces into the rectal wall, making the feces more solid over time. Peristalsis throughout the digestive tract, stimulated by nerve impulses, propels the contents towards elimination. The ileocecal valve contracts and allows contents from the ileum to enter the cecum, initiating this final stage of movement [38](#page=38). ### 3.6 Protective and storage functions The peritoneum, a serous membrane lining the abdominal cavity, also plays a role in the digestive system's functions [5](#page=5). * **Lubrication:** It produces serous fluid to reduce friction between abdominal organs during movement [5](#page=5). * **Fat storage:** The omentum and mesentery, parts of the peritoneum, store fat [5](#page=5). * **Immunity:** The peritoneum contains lymphatic vessels and nodes that help resist microbial invasion and can isolate localized inflammation, preventing the spread of infection [5](#page=5). * **Absorption:** The peritoneum can absorb water and electrolytes, a principle utilized in peritoneal dialysis [5](#page=5). > **Tip:** The muscular layers of the digestive tract, consisting of longitudinal and circular muscle fibers, are crucial for both propelling food (peristalsis) and mixing it with digestive juices. The presence of a third oblique muscle layer in the stomach further enhances its churning ability [23](#page=23) [5](#page=5). > **Example:** The stomach's ability to break down proteins with pepsin is a key chemical digestion step, converting large protein molecules into smaller polypeptides that can be further processed in the small intestine [23](#page=23). * * * # Blood and nerve supply to the digestive tract The digestive tract receives a rich blood supply primarily from the abdominal aorta and its major branches, while its innervation is controlled by the autonomic nervous system, comprising both parasympathetic and sympathetic fibers [7](#page=7) [8](#page=8). ### 4.1 Arterial supply The arterial supply to the digestive tract originates from the abdominal aorta, which gives rise to several key arteries [7](#page=7): * **Celiac Trunk:** Branches further to supply various organs. * **Left gastric artery:** Supplies the stomach [7](#page=7). * **Splenic artery:** Supplies the spleen and parts of the stomach and pancreas [8](#page=8). * **Hepatic artery:** Supplies the liver [8](#page=8). * **Superior mesenteric artery:** This major artery supplies oxygenated blood to the small intestine, cecum, ascending colon, and transverse colon [29](#page=29) [36](#page=36) [8](#page=8). * **Inferior mesenteric artery:** This artery supplies oxygenated blood to the descending colon and rectum [36](#page=36) [8](#page=8). * **Esophageal arteries:** Supply the esophagus [7](#page=7). #### 4.1.1 Specific organ arterial supply * **Stomach (Gaster):** Receives oxygenated blood from the gastric arteries and branches of the splenic artery [21](#page=21). * **Esophagus:** Supplied by esophageal arteries branching from the abdominal aorta [15](#page=15) [7](#page=7). * **Teeth:** Arteries are branches of the maxillary artery for the upper teeth and are related to the lingual artery for the tongue [11](#page=11) [13](#page=13). * **Colon:** The cecum, ascending colon, and transverse colon are supplied by the superior mesenteric artery, while the descending colon and rectum are supplied by the inferior mesenteric artery [36](#page=36). ### 4.2 Venous drainage The venous drainage of the digestive tract largely mirrors the arterial supply, with a significant portion converging into the portal venous system before reaching the liver [8](#page=8). * **Thoracic region:** Superior vena cava [8](#page=8). * **Abdominal region:** The portal vein collects nutrient-rich, deoxygenated blood from the stomach, intestines, and spleen, transporting it to the liver for processing [21](#page=21) [29](#page=29) [36](#page=36) [8](#page=8). * **Rectum:** Internal iliac veins drain the rectum [8](#page=8). #### 4.2.1 Specific organ venous drainage * **Stomach (Gaster):** Gastric veins drain deoxygenated blood, rich in nutrients and specific substances, into the portal vein [21](#page=21). * **Teeth:** Venous drainage leads to the jugular veins for the tongue and is related to the maxillary artery's venous counterparts for teeth [11](#page=11) [13](#page=13). * **Colon:** Deoxygenated blood is collected by the mesenteric veins and drains into the portal vein [36](#page=36). ### 4.3 Nervous supply The digestive tract is innervated by the autonomic nervous system, which comprises two main divisions: parasympathetic and sympathetic. Within the walls of the alimentary tract, two major nerve plexuses are found [8](#page=8). * **Parasympathetic nervous system:** * Primarily originates from the vagus nerve (cranial nerve) [8](#page=8). * Stimulation of the parasympathetic system generally promotes muscle contractions along the digestive tract and increases the secretion of digestive juices [8](#page=8). * **Sympathetic nervous system:** * Fibers originate from the spinal cord in the thoracic and lumbar regions, forming plexuses in the thorax, abdomen, and pelvis [8](#page=8). * Sympathetic stimulation typically inhibits muscle contractions and reduces the secretion of digestive juices [8](#page=8). #### 4.3.1 Intrinsic nerve plexuses Located within the walls of the digestive tract, these plexuses are influenced by both sympathetic and parasympathetic inputs [8](#page=8): * **Myenteric plexus (Auerbach's plexus):** Situated between the longitudinal and circular muscle layers of the intestinal wall. It primarily controls gut motility [8](#page=8). * **Submucosal plexus (Meissner's plexus):** Located within the submucosa, near the secretory glands. It influences secretion and local blood flow [8](#page=8). #### 4.3.2 Specific organ innervation * **Stomach (Gaster):** Receives sympathetic nerve stimulation from the celiac plexus, which slows peristalsis and reduces gastric juice production [21](#page=21). * **Colon:** Innervated by both sympathetic and parasympathetic nerves [30](#page=30) [37](#page=37). * **Anus:** The external anal sphincter is controlled by voluntary skeletal muscles, while the internal sphincter is innervated by the autonomic nervous system [36](#page=36) [37](#page=37). * **Tongue:** Supplied by the hypoglossal nerve for involuntary muscles, branches of the mandibular nerve for sensation (temperature and touch), and the facial and glossopharyngeal nerves for special sensations [11](#page=11). * **Teeth:** Upper teeth are innervated by branches of the maxillary nerve, while lower teeth receive innervation from the mandibular nerve [13](#page=13). > **Tip:** Understanding the dual innervation of the digestive tract by the autonomic nervous system is crucial. The parasympathetic system is generally stimulatory for digestion, while the sympathetic system is inhibitory. The intrinsic plexuses (myenteric and submucosal) allow for localized control of digestive functions. > **Example:** When you eat a meal, the parasympathetic nervous system (via the vagus nerve) is activated, increasing stomach contractions and the release of digestive enzymes. Conversely, during a stressful situation ("fight or flight"), the sympathetic nervous system dominates, slowing down digestion to divert resources to essential survival functions. * * * ## Common mistakes to avoid * Review all topics thoroughly before exams * Pay attention to formulas and key definitions * Practice with examples provided in each section * Don't memorize without understanding the underlying concepts

| Term | Definition | |------|------------| | Alimentary Canal | A long, continuous tube in the body that serves as the pathway for food, starting from the mouth and ending at the anus, responsible for digestion and absorption. | | Peritoneum | The largest membrane in the body, composed of serous fluid, which protects organs in the abdominal cavity by lining and enveloping them with two layers: parietal (lining the abdominal wall) and visceral (covering the abdominal organs). | | Mesentery | A fold of peritoneum located in the posterior abdominal wall, which supports and suspends abdominal organs, containing blood vessels and nerves that branch to these organs. | | Peristalsis | A wave-like series of muscle contractions in the walls of the digestive tract that propels food forward, involving the coordinated relaxation of longitudinal muscles and contraction of circular muscles. | | Submucosa | A layer of the digestive tract wall composed of loose connective tissue, elastic fibers, blood vessels, nerves, and lymphatic tissue, found beneath the mucosa. | | Mucosa | The innermost layer of the digestive tract wall, consisting of an epithelial lining, lamina propria (connective tissue), and muscularis mucosa (smooth muscle), responsible for protection, secretion, and absorption. | | Adventitia | The outermost layer of connective tissue in some parts of the digestive tract, particularly in the esophagus, which is fibrous and helps anchor the organ. | | Myenteric Plexus (Auerbach's Plexus) | A network of nerves located between the longitudinal and circular muscle layers of the digestive tract wall, controlling muscular contractions. | | Submucosal Plexus (Meissner's Plexus) | A network of nerves located in the submucosa layer of the digestive tract wall, regulating glandular secretions and blood flow. | | Pepsinogen | An inactive precursor enzyme produced by chief cells in the stomach, which is converted to pepsin by hydrochloric acid and begins protein digestion. | | Hydrochloric Acid (HCl) | A strong acid produced by parietal cells in the stomach that kills microorganisms, activates pepsinogen, and creates an acidic environment for optimal pepsin activity. | | Chyme | A semi-fluid mass of partially digested food that is mixed with digestive juices and passed from the stomach into the duodenum. | | Villi | Finger-like projections lining the inner surface of the small intestine, which significantly increase the surface area for nutrient absorption into the bloodstream and lymphatic system. | | Lacteal | A lymphatic capillary located within each villus of the small intestine, responsible for absorbing digested fats (fatty acids and glycerol). | | Enterokinase | An enzyme produced by the small intestine that activates trypsinogen (from the pancreas) into trypsin, initiating the digestion of proteins into smaller peptides. | | Bile | A digestive fluid produced by the liver and stored in the gallbladder, which emulsifies fats, aiding in their digestion and absorption. | | Feces | The waste product of digestion, consisting of undigested material, bacteria, and dead epithelial cells, which is eliminated from the body through the anus. | | Defecation | The process of eliminating feces from the body, involving the coordinated contraction of abdominal muscles and relaxation of the anal sphincters. |