Survey
* Your assessment is very important for improving the workof artificial intelligence, which forms the content of this project
* Your assessment is very important for improving the workof artificial intelligence, which forms the content of this project
Anatomy and Physiology Biology 2402 Chapter-24 The Digestive System OVERVIEW Organization • The Digestive system is organized into two groups of organs: – The Gastrointestinal Tract or alimentary canal: a long tube extending from the mouth to the anus. • Includes mouth, pharynx, esophagus, stomach, small intestine, large intestine, and anus – The accessory structures: glands, organs, and other structures that produce and release secretions into the gastrointestinal tract, and contribute to the mechanical and chemical digestion of food. • Includes teeth, tongue, salivary glands, liver, gall bladder, and pancreas. An Introduction to the Digestive System • The Digestive System – Acquires nutrients from environment – Anabolism • Uses raw materials to synthesize essential compounds – Catabolism • Decomposes substances to provide energy cells need to function • Breakdown of carbohydrates, proteins, lipids and nucleic acids. The Digestive Processes Digestion includes six basic processes. • Ingestion is taking food into the mouth (eating). • Secretion is the release, by cells within the walls of the GI tract and accessory organs, of water, acid, buffers, and enzymes into the lumen of the tract. • Mixing and propulsion result from the alternating contraction and relaxation of the smooth muscles within the walls of the GI tract. • Digestion proper is the breakdown of large food molecules into simple molecules. It includes: – Mechanical digestion: consists of movements of the GI tract that mix, soften, and liquefy the food. – Chemical digestion: a series of enzymatic reactions that break down polysaccharides, lipids, proteins, and nucleic acids into smaller molecules. • Absorption is the passage of nutrients generated by digestion from the lumen of the GI tract into blood or lymph. • Defecation is the release of solid waste (generated by digestion) from the rectum. LAYERS OF THE GI TRACT The basic layers of the GI tract from the inside to the outside are : 1. Mucosa 2. Submucosa 3. Muscularis 4. Serosa LAYERS OF THE GI TRACT Mucosa • The mucosa lines the lumen of the GI tract; it consists of an epithelium, lamina propria, and muscularis mucosa. –The epithelium consists of various cells: stratified cells for protection, simple columnar cells for secretion and absorption, mucus secreting cells for protection and lubrication, and enteroendocrine cells that release hormones involved in the regulation of digestion. –The lamina propria anchors the mucosa and provides a framework for blood vessels, lymph vessels, and nerves. Many mucosa-associated lymph tissues (MALT) provide on-site defense against potential pathogens present in food. – The muscularis mucosa provides elasticity to the mucosa to enhance digestion and absorption. Submucosa • The submucosa is made of areolar connective tissue housing a large number of glands, nerves, blood vessels, and lymphatic tissue. Muscularis The muscularis is made of at least two layers of smooth muscle fibers (involuntary): – the innermost layer is the circular layer – the outermost layer is the longitudinal layer Together the two layers contract rhythmically during a process called peristalsis to mix, crush, and propel the food through the GI tract. The muscularis is highly vascular and houses the myenteric plexus or plexus of Auerbach. This vast network of nerves, neurons, and ganglia controls both parasympathetic & sympathetic innervation of circular and longitudinal muscle layers Movement Serosa • The serosa is made of two layers: – An inner connective tissue layer which is made of areolar connective tissue: it covers the other layers, and lead blood vessels and nerves into them. – An outer epithelial layer which attaches the GI tract to the peritoneum folds. Structure of the Peritoneum • The peritoneum is the largest serous membrane of the body. • The parietal peritoneum lines the wall of the abdominal cavity. • The visceral peritoneum covers some of the organs and constitutes their serosa. • The space between the parietal and visceral layers of the peritoneum is called the peritoneal cavity and it contains serous fluid. PERITONEUM Regions of the Peritoneum • Parietal peritoneum • Visceral peritoneum • Mesentery • Mesocolon • Falciform ligament • Lesser omentum • Greater omentum Functions of the Peritoneum • • Organs located on the posterior abdominal wall behind the peritoneum are said to be retroperitoneal. In the adult, the aorta, inferior vena cava, kidneys, adrenal glands, the pancreas, duodenum, ascending and descending colons are retroperitoneal. Organs covered by the folds of the peritoneum are said to be intraperitoneal: they include stomach, small intestine (jejunum and ileum), transverse colon, liver and gallbladder. • Mesenteries – Are double sheets of peritoneal membrane – Suspend portions of digestive tract within peritoneal cavity by sheets of serous membrane that connect parietal peritoneum with visceral peritoneum. • The Lesser Omentum – Stabilizes position of stomach • The Greater omentum • Extends inferiorly between the body wall and the anterior surface of small intestine • Hangs like an apron From lateral and inferior borders of stomach Figure 24-2b Mesenteries Lesser omentum Transverse colon Ascending colon Greater omentum (cut) Transverse mesocolon Mesocolon of ascending and descending colons fused to posterior portion of the parietal peritoneum Mesentery proper (mesenterial sheet) Descending colon Small intestine Sigmoid colon A diagrammatic view of the organization of mesenteries in an adult. MOUTH (Oral Cavity) Parts of the Mouth Lips and Cheeks • Vestibule • Teeth • Hard & soft palate • Uvula • Tongue • Pharyngeal Arches • Fauces Structure of the Oral Cavity • The mouth or oral cavity is formed by the cheeks, hard and soft palate, lips , and tongue. • The vestibule of the mouth is the space between the cheeks and lips (external boundary), and the gums and teeth (internal boundary). • The oral cavity proper is the space that extends from the gums and teeth (outermost boundary) to the fauces (innermost boundary). • The fauces are the opening that leads to the pharynx and larynx. Oral Cavity (continued) • Lips and cheeks keep the food within the mouth. • The tongue is a muscular organ that occupies most of the floor of the mouth. It moves food around the mouth during mastication, mixes it with saliva, and shapes it into the bolus which is swallowed later. Lingual papillae (taste buds) detect food flavor. • The roof of the mouth is called the palate, which is subdivided into hard and soft palates. • The uvula is an extension of the soft palate: it contributes to sound modulation, and prevents the entry of food in the nasal cavity during swallowing. Salivary Glands The Three Major Pairs of Salivary Glands: • parotid • Submandibular (submaxillary) • sublingual Salivary Glands (continued) • The salivary glands are located around the oral cavity: - The parotid glands are located below the ears and over the masseters. - The submandibular glands are located under the lower edge of mandible. - The sublingual glands are located underneath the tongue in the floor of the mouth • These glands are exocrine glands: they produce saliva which is delivered via ducts that empty into the oral cavity. Saliva • Saliva is a watery secretion from the salivary glands. • Water makes up 99.5% of saliva, and the remaining 0.5% is a mixture of mucus, solutes such as salts, various organic substances, and enzymes. • Functions of Saliva – Lubricating the mouth – Moistening and lubricating materials in the mouth – Dissolving chemicals that stimulate taste buds and provide sensory information – Initiating digestion of complex carbohydrates by the enzyme salivary amylase. Salivation • Salivation is the secretion of saliva. • Salivation is a nearly constant phenomenon that helps cleanse the mouth and keep it moist. • Salivation can be triggered by a number of stimuli including the smell, taste, sight, or even the thought of food. • It can be interrupted by emotions such as fear (dry mouth). • Salivation is under sympathetic and parasympathetic control. Structure of a Tooth • Crown • Neck • Roots • Pulp cavity Composition of Teeth • The crown is the visible portion of the enamel above the gums (gingivae). • The enamel is the hardest part of the tooth. It is made of calcium phosphate like bone, but unlike bone it does not contain collagen. • The crown of a tooth is made of a thick enamel surface overlaying dentin, the main tissue of the tooth. • Dentin is a calcified connective tissue. It is protected by cementum in the root of the tooth. • Cementum is a thin bone like tissue linked to the alveoli of the maxilla and the mandible by periodontal ligaments. (This immovable joint is called a gomphosis). Dental Pulp • Dental Pulp is located at the center of a tooth within the pulp cavity and the root canal(s). • Pulp is a soft tissue made of various connective tissue cells such as odontoblasts, and fibroblasts. It produces dentin and other tissues of the tooth. • Blood vessels and nerves enter and exit a tooth via the root canals, supplying blood and innervation. • Gingivae (gums) cover the maxilla and mandible. Dentition • Dentition is the kind, the number, and the arrangement of teeth in an individual. • Deciduous or primary dentition (baby teeth) is present in infants and children between 6 months, when it starts appearing, to 6 years of age, when it starts being replaced. • Permanent or secondary dentition starts to appear in 6 years old children and is completed between age 17 and 21. • In the adult there are 4 types of teeth: incisors, canines, premolars and molars. Deciduous and Permanent Dentition Mastication • Mastication or chewing is the mechanical breakdown of food in the mouth. Teeth tear, crush, and grind the food into a soft paste called bolus. • The tongue moves food around the oral cavity, mixing crushed food with saliva: this allows chemical digestion to begin, softening of the food, and neutralization of acids in food. • At the end of mastication, the bolus is swallowed. Pharynx (The Throat) • The pharynx is a funnel shaped tube that links the nasal cavity, the oral cavity, and the larynx. • It is subdivided into the nasopharynx, the oropharynx, and the laryngopharynx (hypopharynx). • The nasopharynx is not involved in the digestive system, and the uvula is designed to prevent food from entering the nasopharynx during swallowing. • The oropharynx and laryngopharynx are involved in swallowing. Esophagus • The esophagus is a long tube that connects the pharynx to the stomach. • Layers lining the esophagus – Mucosa: made of Stratified Squamous Non-keratinized epithelium. – Submucosa: contains mucus secreting glands that lubricate the lining of the esophagus. – Muscularis: made of longitudinal and circular smooth muscle layers that allow propulsion of food. – Adventitia: Outermost connective tissue layer. • The pharyngeal (upper) opening of the esophagus is guarded by the upper esophageal sphincter, while the gastric (lower) opening is guarded by the lower esophageal sphincter. Esophagus Deglutition • Deglutition or swallowing is the passage of food from the mouth to the esophagus through the pharynx. • Is divided into three phases • Buccal phase • Pharyngeal phase 1. 2. • Esophageal phase During the buccal phase of deglutition the tongue pushes the soft and humid bolus towards the back of the oral cavity, into the oropharynx. The pharyngeal stage starts when the presence of food in the oropharynx triggers the deglutition reflex: the larynx is lifted, the epiglottis bends and covers the air passages; the uvula blocks access to the nasopharynx, and pharyngeal muscles push the bolus towards the esophagus. Deglutition (continued) 3. The esophageal stage starts when the lifted pharynx causes the upper esophageal sphincter to relax. – The bolus enters the esophagus causing the esophageal muscle layers to contract rhythmically: this is peristalsis. – Deglutition ends when the upper esophageal sphincter contracts, closing the esophagus, and the larynx and epiglottis move back to their initial positions. – One can breath again while food moves towards the stomach. – As the peristaltic wave reaches the lower (cardiac) esophageal sphincter, it relaxes and opens the stomach. Phases of Deglutition Deglutition Problems • Problems may occur during deglutition ‒ When one swallows too fast, the epiglottis may fail to cover the glottis in time to prevent food from entering the air passages. In this case breathing is impaired. Coughing may remove the obstruction, but more serious measures may be attempted to unblock the air passages (backslaps, abdominal thrusts, chest thrusts). ‒ When one swallows a large, moderately masticated amount of food, propulsion is slow and painful. ‒ When the lower esophageal sphincter fails to close properly, stomach acids may enter the lower esophagus causing a burning sensation best known as a heartburn. The Stomach • Major Functions of the Stomach 1. Storage of ingested food 2. Mechanical breakdown of ingested food 3. Disruption of chemical bonds in food material by acid and enzymes 4. Production of intrinsic factor, a glycoprotein required for absorption of vitamin B12 in small intestine Stomach • The stomach is shaped like an expanded J • Short lesser curvature forms medial surface • Long greater curvature forms lateral surface • The entrance to the stomach is guarded by the cardiac sphincter, and the exit is guarded by the pyloric sphincter. Structure of the Stomach Regions of the Stomach •Cardia •Fundus •Body •Pylorus Anatomy of the Stomach • The stomach is a very muscular organ covered by a tough serosa (connective tissue layer) attached to the visceral peritoneum. • Unlike other organs of the GI tract the muscularis of the stomach is made of three thick smooth muscle layers: longitudinal layer (outermost), circular layer (middle), and oblique layer (innermost). – These 3 muscle layers allow the stomach to contract forcefully to mix gastric juice with food, breakdown the bolus mechanically and liquefy it. They also allow the stomach to eject this liquefied food (chyme) into the duodenum of the small intestine. • Submucosa Gastric Mucosa • The rough gastric mucosa (stomach lining) is made of simple columnar epithelial cells. • Ridges of the gastric mucosa called rugae give it a rough appearance. They increase the mechanical properties of the stomach and allow it to contract and return to its original shape. • Infoldings of the mucosa are called gastric pits shallow depressions that open onto the gastric surface. Gastric Layers Gastric Glands • There are 4 main types of gastric gland cells in gastric pits: – Chief cells (exocrine) secrete the inactive enzymes pepsinogen and gastric lipase (zymogens). – Parietal cells (exocrine) secrete hydrochloric acid (HCl), and a glycoprotein called intrinsic factor – Mucous neck cells (exocrine) secrete mucus - protects the mucosa from the very acidic gastric juice – G cells (endocrine) secrete gastrin. • Combined secretion of chief cells (Pepsin) + Parietal Cells (HCl) + Mucous cells (mucous) is called gastric juice. Gastric Gland Cells Digestion in the Stomach • In the stomach there are two types of digestion: – Mechanical digestion occurs when peristaltic contractions mix food and gastric juice: the muscularis contracts forcefully to mix the contents of the stomach. This action liquefies the food into acid chyme. – Chemical digestion occurs when pepsin breaks down proteins into peptides, and lipases (lingual and gastric) break down certain fats. – Meanwhile intrinsic factor binds vitamin B12 which will be absorbed later in the ileum. • The products of this digestion are not absorbed through the gastric mucosa. However some drugs such as aspirin and alcohol can pass through the lining of the stomach. Phases of Gastric Secretion • Gastric secretion is divided into 3 phases: cephalic, gastric, and intestinal. – Cephalic phase: taste, smell, thought, or presence of food in the mouth trigger gastric juice secretion by the gastric glands – Gastric phase: Presence of food in the stomach stretches the gastric mucosa and cause continued secretion of gastric juice in the stomach. – Intestinal phase: Gastric emptying sends chyme into the duodenum. A very acidic chyme (pH below 2), or presence of digested fats in chyme trigger a series of events that result in decreased secretion of gastric juice. • All these events involve the medulla oblongata, vagus nerves, and hormones. Phases of Gastric Secretion Pancreas • The pancreas is a leaf-shaped organ with three main parts: the head, the body, and the tail. • The head of the pancreas is attached to the lesser curvature of the C-shaped duodenum via the pancreatic duct (duct of Wirsung) and the accessory pancreatic duct (duct of Santorini). • It has both endocrine and exocrine functions: its endocrine cells secrete hormones (insulin, glucagon, somatostatin), and its exocrine cells secrete pancreatic juice which contains digestive enzymes and bicarbonate ions. Pancreas (continued) Pancreatic Islet (Islet of Langerhans) • Pancreatic acini: secrete pancreatic juice • Pancreatic islet cells: secrete hormones Pancreatic Juice • During digestion, the pancreas secrets pancreatic juice which is a mixture of digestive enzymes and bicarbonate ions. • The digestive enzymes are secreted by basophilic acinar cells. – Pancreatic amylase for starch digestion. – Trypsinogen, chymotrypsinogen, procarboxypeptidase, and proelastase for protein digestion. – Pancreatic lipase for lipid digestion. – Deoxyribonuclease and ribonuclease for nucleic acid digestion. • Bicarbonate ions are produced by centroacinar cells. Pancreatic Juice (continued) • Pancreatic enzymes are secreted as zymogens (inactive form of the enzyme). • Most of these zymogen are activated by trypsin. • Trypsin itself is activated by enterokinase which is secreted by the absorptive cells of the small intestine. • Pancreatic juice is slightly alkaline, due to the presence of bicarbonate ions. It neutralizes acid chyme in the duodenum. Secretion of Pancreatic Juice • Pancreatic juice is collected by the pancreatic duct through a network of tubes called the pancreatic ductal system. • The duct of Wirsung joins the common bile duct to form the hepatopancreatic ampulla. There, bile and pancreatic juice mix. They are released into the duodenum through the major duodenal papilla. • Pancreatic juice can be released alone into the duodenum via the accessory pancreatic duct. • Secretion and release of pancreatic juice are regulated by 2 duodenal hormones: secretin and CCK (cholecystokinin). Hormonal Regulation of the Pancreas Hormonal Regulation of the Pancreas • Presence of fatty acids and amino acids in the duodenum triggers the release of secretin and CCK by the duodenal glands (enteroendocrine glands). • Secretin and CCK have different effects on the pancreas: – Secretin stimulates secretion of bicarbonate ions. – CCK stimulates secretion of digestive enzymes. • Duodenal glands release also Gastric Inhibitory Polypeptide (GIP). It inhibits stomach acid secretion, and is now believed to also induce the secretion of insulin in presence of glucose. Liver and Gallbladder Liver and Gallbladder • The liver is the largest organ in the abdominopelvic cavity. • It is located in the upper abdominal cavity, right underneath the diaphragm. • The gallbladder is attached to the posterior surface of the right hepatic lobe. • Right and left lobes of the liver are unequally separated by the falciform ligament: the right lobe is significantly larger than the left lobe. • The liver is an important, multitasking organ in the body. However its main contribution to digestion is the secretion of bile. Liver Lobules Liver Lobules (cont’d) • Liver cells are called hepatocytes. • They are arranged into regular structures called liver lobules: the hepatocytes are lined up along sinusoids that converge towards a central vein. • Other cells called Kupffer cells are also associated with liver lobules. They are macrophages specialized in red blood cell recycling. The Liver • The Functions of the Liver 1. Metabolic regulation: The liver regulates: – Composition of circulating blood – Nutrient metabolism and storage – Waste product removal – Drug inactivation 2. Hematological regulation: largest blood reservoir of the body. – Phagocytosis and antigen presentation – Synthesis of plasma proteins – Removal of circulating hormones – Removal of antibodies – Removal or storage of toxins 3. Bile production Production of Bile • Hepatocytes produce bile which is collected by bile canaliculi. These small vessels converge to form bile ducts. • The main bile ducts of the liver are the right and the left hepatic duct. • These two ducts join to form the common hepatic duct. • The common hepatic duct joins the cystic duct from the gallbladder to form the common bile duct. • This duct delivers bile to the hepatopancreatic ampulla where it mixes with pancreatic juice. Bile • Bile is a yellowish-green fluid produced by the liver and stored in the gallbladder. • It contains two main types of substances: – Bile salts which are involved in digestion, Bile salts emulsify fats, i.e. they break up fat globules into fat droplets that lipases can break down. – Bile pigments and cholesterol which are excretory products of red blood cell recycling. Bile pigments contribute to the color of feces. • Gallstones (choleliths) are formed when cholesterol concretizes. They can obstruct the cystic duct, causing pain. They can be removed with drugs, sound waves, and surgery. Duodenal Hormones and Bile Secretion Duodenal hormones regulate bile secretion. When chyme with fatty acids and amino acids enters the duodenum, the duodenal glands secrete CCK and secretin: – secretin stimulates the production of bile by hepatocytes. – CCK stimulates the gallbladder to contract and release bile. • Gastric Inhibitory Peptide (GIP) – Is secreted when fats and carbohydrates enter small intestine Gallbladder • The gallbladder (or cholecyst) is a muscular, pear shaped pouch that stores and releases bile. • The gallbladder has three smooth muscle layers that contract to eject bile into the cystic duct. • The cystic duct leads to the common bile duct. The Small Intestine • The Small Intestine – Plays key role in digestion and absorption of nutrients – 90% of nutrient absorption occurs in the small intestine – Divided into three sections: Duodenum, Jejunum, and Ileum The Small Intestine • The Duodenum – The segment of small intestine closest to stomach – 25 cm (10 in.) long – “Mixing bowl” that receives chyme from stomach and digestive secretions from pancreas and liver – Functions of the duodenum: • To receive chyme from stomach • To neutralize acids before they can damage the absorptive surfaces of the small intestine The Small Intestine • The Jejunum – Is the middle segment of small intestine – 2.5 meters (8.2 ft) long – Is the location of most: • Chemical digestion • Nutrient absorption – Has few plicae circulares • The Ileum – The final segment of small intestine – 3.5 meters (11.48 ft) long – Ends at the ileocecal valve • A sphincter that controls flow of material from the ileum into the cecum of the large intestine Small Intestine Layers of the Small Intestine Intestinal Mucosa – Circular folds or plicae circularis are folds of the mucosa and submucosa that increase the surface of absorption – Villi are finger-like projections of the mucosa that are lined with mucous secreting goblet cells, and simple columnar absorptive cells called enterocytes. They also increase the absorption surface area. – The mucosal layer of each villus contains blood capillaries and a central lymphatic capillary called a lacteal. – Intestinal juice is secreted by epithelial cells of the crypts of Lieberkhun (intestinal glands) located at the base of each villus. It contains important enzymes called brush border enzymes that finish digestion. – Microvilli: extensions of the plasma membrane of the luminal surface of enterocytes. These bristle-like structures increase the surface of absorption further, and release brush border enzymes for digestion. Brush Border Enzymes • Brush border enzymes include: – Aminopeptidase and dipeptidase - split dipeptides into amino acids. – Sucrase, lactase, maltase - break disaccharides into monosaccharides. – Enterokinase - activates trypsinogen to produce trypsin. Trypsin then activates the precursors of chymotrypsin and carboxypeptidase. – carbohydrases: dextrinase and glucoamylase. – Nucleosidase and Phosphatase: remove phosphate group and separate pentose sugar from nitogenous base. Absorption of Nutrients in the Small Intestine. • Monosaccharides, amino acids, dipeptides and tripeptides, and short chain fatty acids are transported through mucosa cells to the blood • Long fatty acids, cholesterol, and monoglycerides are transported through the mucosa cells to the lymph. • Vitamins and minerals are absorbed along with other nutrients: lipid soluble vitamins are absorbed with lipids, while water soluble vitamins are absorbed by diffusion. • Absorption mechanisms vary depending on the nature of the transported material. – Transport mechanisms include: simple diffusion, facilitated diffusion, primary and secondary active transport, exocytosis, and endocytosis. Absorption Mechanisms Facts About Absorption • It lasts approximately 4 to 6 hours. • The majority of nutrients are absorbed through the intestinal mucosa. • Most of the water that enters the GI tract, including drinking water and water secreted by accessory glands is absorbed in the small intestine. • Peristalsis and segmentation propel food through the small intestine. • These movements are coordinated by the enteric nervous system, and hormones that respond to the presence of chyme in the small intestine. Large Intestine • The large intestine or colon is shorter in length than the small intestine, but larger in diameter. • The colon is made of haustrae, pouches which increase the surface area of the colon for absorption of water and electrolytes. • The first part of the colon is a blind pouch called the cecum which is connected to the ileum of the small intestine. • The ileocecal sphincter (valve) separates the cecum from the ileum. • The vermiform (wormlike) appendix is attached to the cecum: it may have a function in colic defense, but appendectomy (removal of the appendix) does not appear to have serious effect on the colon. Regions of the Colon • • • • • • • • cecum ascending colon transverse colon descending colon sigmoid colon rectum. anal canal anus Histology of the Colon Functions of the Colon • • • Absorption of the remaining water in chyme Production and storage of feces. Long process (12 hours): – – – – – Peristaltic waves from the ileum relax the ileocecal valve. Liquid chyme enters the colon. The ileocecal valve prevents backflow of chyme Colic muscle movements push chyme along: 3 types of colic movements: • • • minor peristaltic waves: slow and sluggish movement haustral churning: segmentation contractions which serve to mix the contents to enhance absorption. mass peristalsis: large movements which occur at intervals, usually associated with meals. Triggered by presence of food in the stomach. Figure 24-28 Digestive Secretion and Absorption of Water Digestive Secretions Dietary Input Food and drink 2000 mL Saliva 1500 mL Gastric secretions 1500 mL 5000 mL Water Reabsorption Liver (bile) 1000 mL Pancreas (pancreatic juice) 1000 mL 9000 mL Small intestine reabsorbs 7800 mL Intestinal secretions 2000 mL 1200 mL Colon reabsorbs 1250 mL 150 mL lost in feces 1400 mL Colonic mucous secretions 200 mL Feces formation and Defecation Reflex • Chyme turns from a liquid into a slush and then into firm feces. • Vitamins such as vitamin K and certain B vitamins are produced with the help of bacteria, and absorbed along with water and electrolytes. • Feces are stored in the rectum. • Defecation occurs when: – Pressure is exerted on the rectum, and on the internal anal sphincter (made of smooth muscle = involuntary). – The internal anal sphincter relaxes causing the need to defecate. – Feces are released when the external anal sphincter relaxes (skeletal muscle, voluntary) , and the rectal muscles contract. Feces formation and Defecation Reflex