Biology 221 Anatomy & Physiology II TOPIC 8 Digestive System Chapter 18 pp. 651-677 E. Lathrop-Davis / E. Gorski / S. Kabrhel Digestive System Functions The digestive system: • provide nutrients in usable form; and • removes unusable wastes from the body. Digestive System Overview There are two main groups of organs: • The alimentary canal (also known as the gastrointestinal or GI tract) is the tube through which food passes. – The GI tract is responsible for digestion and absorption of food. – Organs of the GI tract include the mouth, pharynx, esophagus, stomach, small intestine, and large intestine. Fig. 24.1, p. 888 Digestive System Overview • Accessory organs are organs, glands and structures which aid digestion but not part of GI tract itself. – Accessory organs include the teeth, tongue, salivary glands, pancreas, liver, and gall bladder. Fig. 24.1, p. 888 Processes of Digestion There are 6 main processes involved in digestion: • Ingestion is the entrance of food and drink into the mouth. • Mechanical digestion is the physical breakdown into smaller pieces. • Propulsion is movement of materials through the gut. • Chemical digestion is the breakage of bonds in molecules to form smaller compounds. • Absorption is the uptake of nutrients from lumen (if it’s not absorbed, it’s not usable). • Defecation is the removal of indigestible material (like cellulose) Fig. 24.2, p. 889 Peritoneum • The peritoneum is a serous membrane. • The parietal peritoneum lines the abdominal cavity. – Organs posterior to the parietal peritoneum are retroperitoneal. • The visceral peritoneum (also called the serosa) covers surfaces of most abdominal organs. – Mesenteries are a double layer of peritoneum extending from the body wall to certain digestive organs – Intraperitoneal organs are ones found in the mesentaries. See also Fig. 24.5, p. 891 Fig,. 24.30, p. 929 Peritoneum • The peritoneal cavity is the fluid-filled “space” between the visceral and parietal peritoneum. • Peritonitis is any inflammation of the peritoneum. It makes movement within the gut more difficult and painful. See also Fig. 24.5, p. 891 Fig,. 24.30, p. 929 Splanchnic Circulation Arteries serving the digestive organs are branches of the abdominal aorta. • The celiac trunk is the first branch of the abdominal aorta. It is very short; gives rise to: – hepatic artery, which serves the liver, gall bladder, stomach, and duodenum; – left gastric artery serving the stomach and inferior esophagus; and the – splenic artery serving the spleen, stomach, and pancreas. Fig. 20.22, p. 761 Fig. 20.22, p. 759 Splanchnic Circulation • The superior mesenteric artery supplies the small intestine and most of large intestine, and the pancreas. • The inferior mesenteric artery serves the large intestine. Fig. 20.22, p. 761 Fig. 20.22, p. 759 Hepatic Circulation: Hepatic portal system • The hepatic portal system consistes of veins draining digestive organs and carrying nutrient-rich blood to the liver. – The gastric vein drains the stomach. – The superior mesenteric vein drains the small intestine. – The splenic vein drains spleen. ° The inferior mesenteric vein drains the large intestine and empties into the splenic vein. • Venous blood from the hepatic portal system mixes with arterial blood (from the hepatic artery) in liver. Fig. 20.27, p. 771 Hepatic Circulation: Hepatic Veins • drain venous blood from liver into inferior vena cava Fig. 20.27, p. 771 Mucosa • The mucosa is the mucous membrane lining the gut. • The mucosa consists of: – an epithelial lining; – a lamina propria of areolar connective tissue; and – the muscularis mucosae of smooth muscle. http://www.usc.edu/hsc/dental/ghisto/gi/d_1.html Mucosa: Epithelium • Type of tissue comprising the epithelium varies depending on location. – Stratified squamous epithelium is found in mouth, esophagus and anal canal where abrasion is likely. – Simple columnar epithelium is found in stomach and intestines where secretion and absorption are important. • The epithelium secretes mucus, digestive enzymes, and hormones. • It provides an intact barrier to protect against entry of bacteria. http://www.usc.edu/hsc/dental/ghisto/gi/d_15.html http://www.usc.edu/hsc/dental/ghisto/gi/c_2.html Mucosa: Lamina Propria • The lamina propria is a ayer of areolar connective tissue. • Blood capillaries nourish epithelium, absorb and transport digested nutrients. • Lymphatic capillaries provide drainage for interstitial fluid and transport fats to the venous circulation. http://www.usc.edu/hsc/dental/ghisto/gi/d_60.html Mucosa: Muscularis Mucosae • The muscularis mucosae consists of smooth muscle. • It is used for local movement and to hold mucosa in folds (small intestine) http://education.vetmed.vt.edu/Curriculum/VM8054/Labs/Lab19/EXAMPLES/ Exileum.htm Submucosa • The submucosa is dense connective tissue superficial to the mucosa. • It is highly vascularized with many lymphatic vessels. • Lymph nodules help protect against diseases. – MALT, or mucosa-associated lymphatic tissue, is common especially in the small intestine (as Peyer’s Patches) and large intestines. http://education.vetmed.vt.edu/Curriculum/VM8054/Labs/Lab19/EXAMPLES/ Exileum.htm Muscularis Externa (Muscularis) • The mucularis consists of two layers in most organs (3 in stomach). – The circular layer runs around the tube. – The longitudinal layer runs the length of the tube. • Peristalsis moves material through gut by alternating contraction of circular and longitudinal muscle. • Segmentation is a series of ring-like contractions that help mix material with digestive enzymes in small intestine Fig. 24.3, p. 890 Serosa • The visceral peritoneum, or serosa, is composed of simple squamous epithelium (mesothelium) with areolar CT. • The adventitia is a dense connective tissue covering without epithelium. It is found around the esophagus. • Retroperitoneal organs have both a serosa (of parietal peritoneum) and adventitia (on the side abutting body wall). Enteric Nervous System Intrinsic nerve plexuses consist of enteric neurons. – Enteric neurons are able to act independently of central nervous system. – They communicate with each other to control GI activity. • There are two main enteric plexuses. – The submucosal nerve plexus regulates glands in submucosa and smooth muscle of muscularis mucosae. – The myenteric nerve plexus regulates activity of muscularis externa (with aide of submucosal nerve plexus). Central Nervous System Control • Enteric nerve plexuses linked to CNS by visceral afferent (sensory) fibers • The digestive system receives motor input from the sympathetic and parasympathetic divisions of the autonomic nervous system. – Parasympathetic outflow generally increases activity. – Sympathetic outflow generally decreases activity. Mouth (Oral Cavity) • The oral oriface is the anterior opening. • The mouth is continuous posteriorly with the oropharynx. • The lips and cheeks keep food in oral cavity. • There ar three layers of tissue in the mouth wall: – the mucosa (stratified squamous epithelium); – the submucosa; and – the muscularis externa, which consists of skeletal muscle for voluntary control. Mouth: Palate • The palate separates the oral and nasal cavities and is why you can chew and breath at the same time. • The hard palate consists of the palatine process of maxilla and the palatine bones. • The soft palate is composed of muscle only. – It prevents food from entering nasopharynx during swallowing. – The uvula is the part that hangs down in middle. Fig. 24.7, p. 895 Mouth: Arches • The palatoglossal arch anchors the soft palate to the tongue. • The palatopharyngeal arch anchors the soft palate to the wall of the oropharynx. • The fauces refers to the area between these arches. The palatine tonsils are located in the fauces. Fig. 24.7, p. 895 Mouth: Tongue • The lingual tonsil sits at base of tongue and protects against invasion by bacteria (see Topic 5 - Lymphatic System). • Taste buds contain receptors for taste. They are scattered over the tongue in papillae. Fig. 24.8, p. 896 Mouth: Tongue • The tongue forms a bolus, or ball, of food, making it easier to swallow. – The tongue also keeps the food between the teeth. • Muscles are served by the Hypoglossal (XII) nerve (see A&P I Unit VI). – Intrinsic muscles within tongue (not attached to bone) allow the tongue to change shape for swallowing and speech. – Extrinsic muscles attach to bone or the soft palate and alter tongue position (protrusion, retraction, side-to-side). Fig. 24.7, p. 895 Salivary Glands • The salivary glands produce saliva. • There are two groups of salivary glands: – The intrinsic glands (or buccal glands) lie within the oral cavity. – The 3 pairs of extrinsic glands (see A&P I Unit VI for innervation) lie outside the cavity. They are: ° the parotid glands (connected to oral cavity by parotid duct; mumps is a viral infection of the parotid glands); ° the sublingual glands; and ° the submandibular glands. Fig. 24.9, p. 897 Saliva • Mucus cells in the salivary glands are less common than serous cells and produce mucus to lubricate the food. • Serous cells produce a watery saliva that is composed mostly (97-99.5%) of water. – This saliva is slightly acidic (pH ~ 6.8). – It contains several types of solutes including: ° electrolytes (ions such as Na+, K+, Cl-, PO4=, HCO3-), ° metabolic wastes (urea, uric acid), and ° proteins. Salivary Proteins Salivary proteins include: • mucin, the glycoprotein portion of mucus that lubricates oral cavity; • lysozyme, which has antibacterial properties; • IgA antibodies that prevent antigens from attaching to the mucus membrane; • defensins secreted by neutrophils that act as local antibiotic and chemotatic agents when the mucosa is damaged; and • salivary amylase, which begins starch hydrolysis. Control of Salivation • The sympathetic division of the ANS either: – stimulates production of mucin-rich saliva, or – inhibits salivation altogether at high levels. • The parasympathetic division of ANS stimulates activity. – Chemoreceptors (excited most by acidic substances) and baroreceptors (excited by mechanical stimuli) send messages to the salivatory nuclei in pons and medulla. This results in increased parasympathetic motor output. – Parasympathetic motor output results in increased salivation. Control of Salivation • The salivary nuclei are also under psychological control, wherein they respond to visual and olfactory stimuli, even thoughts of food, by increasing parasympathetic output. • Salivary nuclei are also stimulated by irritation to the lower GI tract. • Parasympathetic nerves that stimulate salivation are the: – the facial, which goes to the submandibular and sublingual salivary glands; and – the glossopharyngeal, which goes to the parotid salivary glands. Teeth • Teeth lie in the alveoli of the mandible and maxilla (see A&P I axial skeleton lab). • Primary dentition refers to the 20 deciduous teeth (also called milk or baby teeth). – The roots are absorbed as the permanent teeth grow in causing the baby teeth to fall out. Fig. 24.10, p. 899 Teeth Permanent dentition refers to the 32 adult teeth. Each jaw contains: • 4 Incisors (central and lateral) • 2 Canines (eyeteeth) • 4 Bicuspids = premolars • Molars – 2 first molars – 2 second molars – 2 third molars, which are also called wisdom teeth, and may become impacted as they grow in. When this happens they are surgically removed. Fig. 24.10, p. 899 Tooth Structure • The crown is the part visible above the gum line. It is covered by enamel (the hardest substance in body) with dentin under it. • The neck is the area between the crown and the root. • The root has no enamel; its dentin is under the cementum. – Cementum is calcified connective tissue covering the dentin of root. It attaches the root to the periodontal ligament which anchors tooth into the alveolus. – The pulp cavity houses blood vessels and nerves that enter or leave via the apical foramen in the root canal (narrowed end of the cavity). Fig. 24.11, p. 900 Pharynx • Only oropharynx and laryngopharynx are involved in digestion (nasopharynx is only respiratory). • The pharynx is lined with nonkeratinized stratified squamous epithelium to protect against abrasion. • Mucus-producing glands in submucosa produce mucus that lubricates food. • Skeletal muscle in the wall responds to somatic reflexes to move food quickly past laryngopharynx. • There is no serosa or adventitia around the pharynx. Esophagus • The esophagus runs from the laryngopharynx through the mediastinum to the stomach. • All 4 tissue layers are present in its wall. – The mucosa consists of stratified squamous epithelium. – The submucosa consists of dense irregular connective tissue with mucus-secreting esophageal glands. http://www.usc.edu/hsc/dental/ghisto/gi/c_2.html Esophagus • The muscularis changes muscle type as it goes down. – The top 1/3 is skeletal muscle – The middle 1/3 is a mix of skeletal and smooth muscle. – The bottom 1/3 is smooth muscle. • The adventitia is the dense connective tissue covering of the esophagus. http://www.usc.edu/hsc/dental/ghisto/gi/d_8.html http://www.usc.edu/hsc/dental/ghisto/gi/d_3.html Structures Associated with the Esophagus • The upper esophageal sphincter controls movement of material from pharynx into esophagus. • The esophageal hiatus is the opening in the diaphragm that allows the esophagus to pass from the thoracic cavity into the abdominal cavity. • The gastroesophageal (cardiac) sphincter is a thickening of smooth muscle at the inferior end of the esophagus. – It is aided by diaphragm in closing the bottom of the esophagus. – This helps prevent reflux of acidic gastric juice. Esophageal Disorders • Heartburn results from a failure of the lower esophageal sphincter to close completely allowing acidic gastric juice into esophagus. • Hiatus hernia refers to the protrusion of the superior portion of stomach above diaphragm through the esophageal hiatus. • An esophageal ulcer is an erosion of the esophageal wall due to chronic reflux of stomach acid. An ulcer represents a chemical burn to the esophageal wall. Digestive Processes in Mouth, Pharynx and Esophagus • Ingestion is taking food into the mouth. • Mechanical digestion in the mouth consists of mastication by teeth (with aid of tongue) and formation of a bolus. • Chemical digestion of carbohydrates starts with salivary amylase produced by the salivary glands. – Amylase breaks starch and glycogen into smaller fragments (including maltose [disaccharide] if left long enough, which is why a cracker starts to taste sweet if left in the mouth). – Activity of salivary amylase continues until it reaches acid environment of the stomach. Digestive Processes in Mouth, Pharynx and Esophagus • Absorption in the mouth, pharynx and esophagus is almost nil with the exception of some drugs, such as nitroglycerine. • Movement of food in these areas includes deglutition (swallowing) and peristalsis through the esophagus. – Deglutition moves food from oral cavity to stomach. ° The process is voluntary in the oral cavity (buccal phase) and reflexive in the pharynx. - Think About It: What is the importance of having deglutition be reflexive in the pharynx? – Peristalsis occurs and is involuntary where smooth muscle is found. Fig. 24.13, p.904 Stomach: Gross Anatomy • The cardiac region (cardia) is the first area into which the food goes. • The fundus is a temporary storage area at the upper left of the stomach. • The body is the main portion of the stomach. – The greater curvature runs along the inferior surface of the stomach; – the lesser curvature, along the upper surface. • The pyloric region is the distal portion leading into the small intestine. – The pyloric sphincter controls movement of chyme into small intestine. Fig. 24.14, p. 905 Stomach: Gross Anatomy • Arterial blood is supplied by the gastric arteries and the splenic artery. (See 20.22, p. 758) • Venous blood is drained by the gastric veins, which empty into the splenic vein or directly into the hepatic portal vein. http://medlib.med.utah.edu/WebPath/GIHTML/GI194.html http://medlib.med.utah.edu/WebPath/GIHTML/GI194.html Fig. 24.14, p. 905 Stomach Histology • The mucosa consists of simple columnar epithelium. – The muscularis mucosae throws mucosa into folds called rugae. • The submucosa consists of connective tissue. • The muscularis has 3 layers that allow it to create mixing waves in addition to peristalsis. The three layers are from outside to inside: – the longitudinal layer; – the circular layer; and – the oblique layer . • The serosa covers the stomach. http://www.gutfeelings.com/STOMACH.HTML Fig. 24.14, p. 905 Microscopic Anatomy • The surface of the epithelium is composed mainly of goblet cells that secrete mucus. • Gastric pits are downward extensions of the epithelium. – Tight junctions between epithelial cells prevent acidic gastric juice from reaching underlying layers. – Gastric pits contain gastric glands which secrete gastric juice. Four types of cells are present. Fig. 24.15, p. 906 http://www.usc.edu/hsc/dental/ghisto/gi/d_15.html Gastric Pit Cells • Mucous neck cells secrete bicarbonate-rich mucus • Parietal (oxyntic) cells secrete: – HCl ( which is buffered by bicarbonate rich mucus to protect the mucosa); and – intrinsic factor (which is essential to absorption of Vit. B12 by small intestine). • Chief (zymogenic) cells secrete: – pepsinogen (inactive form of the protease pepsin for protein hydrolysis); and – minor amounts of lipases (which hydrolyze lipids). Gastric Pit Cells • Enteroendocrine cells release hormones and hormonelike products into the lamina propria where they are picked up by blood and carried to other digestive organs. – Gastrin is generally stimulatory. – Histamine stimulates H+ secretion. – Somatostatin is generally inhibitory. Digestive Processes in Stomach • Mechanical digestion in the stomach is the result of mixing waves, which help break food into smaller particles. • Chemical digestion in the stomach produces chyme with a pH of about 2. – Acid (HCl) secreted by parietal cells breaks some bonds and activates pepsinogen into pepsin. – Pepsin hydrolyses proteins and is produced as pepsinogen by chief cells. ° Think About It: What would happen if pepsin were activated in the gastric pit? – Rennin is a protease secreted in children that acts on milk proteins. Digestive Processes in Stomach • Movement in the stomach includes: – mixing waves that mix food with acid and enzymes; and – peristalsis that moves material through stomach and into small intestine. • Absorption is limited to lipid soluble substances such as: – alcohol, – aspirin, and – some other drugs. Regulation of Gastric Secretion Gastric secretion is controlled by the autonomic nervous system and hormones. • Hormonal control is accomplished by locally secreted hormones. – Gastrin from the stomach stimulates secretion. – Somatostatin, gastric inhibitory protein (GIP), and cholecystokinin from the small intestine inhibit secretion. Somatostatin is also produced by the stomach itself. See Fig. 24.16, p. 910 Regulation of Gastric Secretion • Neural control is accomplished by the ANS and local enteric nerve reflexes. – Autonomic control starts in the CNS. ° The parasympathetic division works through the Vagus (X) nerve and stimulates activity. ° The sympathetic division works through thoracic spinal nerves and inhibits activity. – Local enteric nerve reflexes are based on distension of the stomach or duodenum. ° Distension of the stomach stimulates gastric activity ° Distension of the duodenum inhibits gastric activity. See Fig. 24.16, p. 910 Stimulation of Gastric Secretion • The cephalic phase involves subconscious activity resulting from the thought or sight of food and from stimulation of areas of the hypothalamus by the smell or taste of food. – Impulses sent via the Vagus nerve stimulate secretion of gastric juice. • The gastric phase starts as food enters the stomach. Distension and certain chemicals (especially caffeine and peptides) stimulate receptors resulting in: – release of gastrin from the stomach; – local reflexes; and – long reflexes involving the medulla oblongata and Vagus nerves. Stimulation of Gastric Secretion • The intestinal phase starts as food enters the duodenum (proximal portion of the small intestine). – Initially, partially digested food entering the duodenum causes the intestine to release enteric gastrin, which stimulates gastric activity. – As the duodenum is stretched by additional acidic chyme from the stomach and as the pH decreases, the enterogastric reflex starts and inhibits gastric activity. Fig. 24.16, p. 910 Inhibition of Gastric Secretion • During the cephalic (cerebral) phase, loss of apetite decreases parasympathetic impulses going to the stomach. • During the gastric phase, excess acidity in the stomach inhibits gastrin secretion. Emotional upset triggers the fight-or-flight response, thus sending sympathetic impulses instead of parasympathetic impulses. • During the intestinal phase, further distension of the duodenum and accumulation of fatty, acidic, or hypertonic chyme results in local reflexes that inhibit gastric activity. Fig. 24.16, p. 910 Gastric Disorders • Gastritis is an inflammation of underlying layers of the stomach wall. • Gastric ulcers are eroded areas of the stomach wall where it has been chemically burned. – Helicobacter infections associated with ~90% of all ulcers, although it is uncertain as to whether it is causitive agent or just opportunistic. – Non-infectious ulcers are associated with persistent inflammation and with chronic ingestion of nonsteroidal anti-inflammatory drugs like aspirin, which are absorbed through the gastric mucosa. Gastric Disorders (con’t) • Emesis, also called vomiting, is usually caused by ° extreme stretching of stomach or small intestine, or ° by the presence of irritants in the stomach (e.g., bacterial toxins, excessive alcohol, spicy foods, certain drugs). – The emetic center in the medulla initiates impulses: ° that lead to contraction of abdominal muscles (which increases the intra-abdominal pressure); ° relax the cardiac sphincter; and ° raise the soft palate to closes off the nasal passages. – Excessive vomiting results in dehydration and metabolic alkalosis (increased blood pH). Small Intestine: Gross Structure • The small intestine has a diameter of about 2.5 cm. It is approximately ~ 2-4 m (8-13’) in length in a living person. (In a cadaver, it is 6-7 m [2021’] long because the muscle is not contracted.) • The small intestine designed for secretion (especially proximal end) and absorption – It is the site of most chemical digestion. – It is also the site of most absorption Fig. 24.21, p. 916 Small Intestine: Gross Structure • The pH of chyme entering the small intestine is about 2, it is buffered to between pH 7 and pH 8 by alkaline secretions from the duodenal (Bruner’s) glands. • There are three areas of small intestine: – the duodenum is the first 25 cm; – the jejunum is the middle portion; and – the ileum is the distal portion that empties into the large intestine. Fig. 24.21, p. 916 Small Intestine: Duodenum • The duodenum receives acidic chyme from the stomach. • The hepatopancreatic ampulla is formed from the union of the common bile duct and the pancreatic duct. – The ampulla opens into the duodenum via the major duodenal papilla. – The hepatopancreatic sphincter (sphincter of Oddi) controls entry of fluid from ampulla. • Duodenal (Brunner’s) glands located in the submucosa secrete an alkaline mucus that helps neutralize the acidity of the chyme. http://www.usc.edu/hsc/dental/ghisto/gi/d_36.html Fig. 24.20, p. 915 Small Intestine: Jejunum & Ileum • The jejunum extends from duodenum to ileum. Digestion and absorption continue. • The ileum extends from the jejunum to the large intestine. Digestion and absorption continue. – The ileocecal valve controls movement of chyme from the ileum of the small intestine into the caecum (cecum) of the large intestine. http://www.usc.edu/hsc/dental/ghisto/gi/d_43.html http://www.usc.edu/hsc/dental/ghisto/gi/d_53.html Small Intestine: Innervation • Parasympathetic impulses supplied by Vagus nerve stimulates activity in the small intestine. • Sympathetic impulses supplied by thoracic splanchnic nerves inhibit activity in the small intestine. • Enteric nerves act locally to control movement. Fig. 14.4, p. 517 Fig. 14.5, p. 519 Small Intestine: Blood Supply • Arterial blood is supplied by – the common hepatic artery, which serves duodenum; and – the superior mesenteric artery, which serves most of small intestine. • Venous drainage is provided by the superior mesenteric vein, which drains the entire small intestine. The superior mesenteric vein is part of the hepatic portal system. Fig. 20.22, p. 761 Fig. 20.27, p. 771 Small Intestine: Overview of Special Anatomical Features • The plicae circularis are circular folds of the mucosa. • Villi are fingerlike projections of intestinal wall that increase overall surface area. • Microvilli are projections of individual cell membranes, thus increasing surface area of the cells to increase absorption. http://remf.dartmouth.edu/images/humanMicrovilliTEM/source/1.html http://www.usc.edu/hsc/dental/ghisto/gi/c_43.html See Fig. 24.21, p. 916 Small Intestine: Plicae Circularis • The plicae circularis are deep, permanent circular folds of the mucosa and submucosa. • Their presence forces chyme to spiral through lumen of the small intestine. This: – mixes chyme with intestinal juice and – slows movement of the chyme through the small intestine. Think About It: Why is this helpful? http://www.shu.edu/ha/anirefs/8751.htm http://www.udel.edu/Biology/Wags/histopage/colorpage/csi/csiipcv.gif Fig. 24.21, p. 916 Small Intestine: Villi • Villi are small finger-like projections of the mucosa (over 1 mm tall). See also Fig. 24.22, p. 917; • Each villus contains: Fig. 24.21, p. 916 – a blood capillary bed; – a lacteal to drain the tissue; and – smooth muscle, which allows the villus to shorten and lengthen. This ° increases contact between the villus and chyme in the lumen, and ° “milks” the lacteal to move the lymph. http://www.udel.edu/Biology/Wags/histopage/colorpage/csi/csivv.GIF http://www.udel.edu/Biology/Wags/histopage/colorpage/csi/csiivgc.GIF Small Intestine: Microvilli • Microvilli are extensions of cell membrane and are sometimes called the “brush border”. • Functions of the microvilli include: – secretion of brush border enzymes; and – increasing the surface area for absorption by individual cells. Fig. 24.21, p. 916 See also Fig. 24.22, p. 917 http://remf.dartmouth.edu/images/humanMicrovilliTEM/source/1.html Small Intestine: Mucosa • The mucosa is renewed every 3-6 days.See Fig. 24.21, p. 916 • It consists of – simple columnar epithelium with ° numerous goblet cells that secrete mucus; and ° absorptive cells that absorb nutrients. - Absorptive cells are bound by tight junctions, which means most substances go through the cell to be http://www.udel.edu/Biology/Wags/histopage/ absorbed. colorpage/csi/csidmbg.GIF - Microvilli increase the surface area for absorption. – a lamina propria in which blood vessels and lacteals are found; and – intestinal crypts (next slide). http://www.usc.edu/hsc/dental/ghisto/gi/c_38.html Small Intestine: Mucosa – Intestinal crypts (crypts of Lieberkuhn) extend down between the villi. ° Most cells of the crypt secrete “intestinal juice”, a watery fluid rich in enzymes. ° Paneth cells secrete lysozyme, which is an antibacterial compound. See Fig. 24.21, p. 916 http://www.udel.edu/Biology/Wags/histopage/colorpage/csi/csidmbg.GIF http://www.usc.edu/hsc/dental/ghisto/gi/c_38.html Small Intestine: Submucosa • Peyer’s patches, which are part of the MALT (see Topic 5) are clusters of lymphatic tissue for protection against disease. • Duodenal (Brunner’s) glands are found only in the duodenum. – They secrete an alkaline mucus rich in bicarbonate (HCO3-), which raises the pH of chyme from less than pH 3 (highly acidic) to between pH 7 and pH 8 (slightly alkaline). – Recall from general biology, that pH affects protein configuration and therefore affects enzyme activity. http://medicine.ucsd.edu/pathology/~som213/HistologyImageBank/chapter_4/slide_61_ peyers/pages/a.188.8.131.52.htm http://www.udel.edu/Biology/Wags/histopage/colorpage/csi/csidmbg.GIF Small Intestine: Muscularis & Serosa • The muscularis consists of the two layers of smooth muscle (circular and longitudinal) and creates two kinds of movement: – peristalsis moves chyme through intestine; and – segmentation mixes chyme with intestinal juice. ° Chyme moves between segments a few cm at a time. ° Intrinsic control occurs via intrinsic pacemaker cells in longitudinal muscle. – intensity altered by nervous system and hormones. ° parasympathetic impulses increase strength of contraction; sympathetic impulses decrease it. Small Intestine: Muscularis & Serosa • The serosa (visceral peritoneum) is the outer layer of the small intestine wall. – Mesenteries are formed from visceral peritoneum and attach the small intestine to posterior body wall. – Intraperitoneal organs are those that are completely surrounded and supported by mesenteries. The jejunum and ileum are intraperitoneal. Small Intestine: Digestive Processes • Mechanical digestion occurs via bile salts secreted by the liver – Bile salts are stored in and released from gall bladder in response to cholecystokinin. – Bile salts emulsify fat globules (make them into smaller droplets) to increase the surface area lipases have available to work on. • Chemical digestion hydrolyses macromolecules. All macromolecule types are broken down by enzymes in the small intestine. See Fig. 24.33, p. 933 Small Intestine: Lipid Digestion • Pancreatic lipase from the pancreas hydrolyses triglycerides in the small intestine. • Most common lipids are neutral fats (triglycerides). – glycerol + 1 fatty acid = monoglyceride – glycerol + 2 fatty acids = diglyceride – glycerol + 3 fatty acids = triglyceride • Triglycerides cleaved into glycerol and 3 fatty acids or monoglycerides and 2 fatty acids. Fig. 2.14, p. 48 Small Intestine: Protein Digestion • Pancreatic and intestinal proteases break proteins into amino acids • Pancreatic proteases are trypsin, chymotrypsin and carboxypolypeptidase. – These are secreted as inactive precoursers (trypsinogen, chymotrypsinogen, and procarboxypolypeptidase, respectively) to protect the intestinal mucosa (and the pancreas) from being digested – These cut large proteins into small peptides. See Fig. 2.17, p. 52 Small Intestine: Protein Digestion • Intestinal proteases include aminopeptidase, carboxypeptidase, and dipeptidase. – These cleave small peptides into individual amino acids that can be absorbed. See Fig. 2.17, p. 52 Small Intestine: Carbohydrate Digestion • Starches are long chains of glucose. Fig. 2.13, p. 46 – Starches are cleaved into short chains (oligosaccharides) and maltose (a disaccharide) by pancreatic amylase secreted by pancreas. • Disaccharides hydrolyzed by intestinal (brush border) enzymes specific to the disaccharide: – maltase cleaves maltose; – lactase cleaves lactose; and – sucrase cleaves sucrose. • Lactose intolerance results from a deficiency of lactase. Lactose is digested by bacteria in the large intestine instead, causing gas production. Small Intestine: Nucleic Acid Digestion • Pancreatic nucleases (pancreatic ribonuclease and pancreatic deoxyribonuclease) cleave nucleic acids into individual nucleotides. • Nucleosidases and phosphatases cleave the nucleotides into their components: sugars (ribose or deoxyribose), phosphates, and nitrogen bases. Fig. 2.22, p. 58 Small Intestine: Absorption of Carbohydrates • Absorption moves nutrients from lumen into cells, thence into interstitial fluid to blood or lymph • Carbohydrates are absorbed as monosaccharides by: – cotransport with Na+ (based on setting up Na+ gradient using active transport; glucose and galactose); or – facilitated transport (fructose). Small Intestine: Absorption of Proteins and Nucleic Acids • Proteins are absorbed as amino acids by cotransport with Na+ (based on setting up Na+ gradient using active transport). – Proteins rarely taken up intact (absorbed peptides may cause food allergies that disappear as the mucosa matures). • Nucleic acids are actively transported into epithelial cells as their components: sugar (ribose or deoxyribose), phosphate, and nitrogen bases Small Intestine: Absorption of Lipids • Lipids combine with bile salts to form micelles, which deliver the lipids to the microvilli. – Lipids are absorbed passively through lipid bilayer of the microvilli as glycerol and fatty acids or monoglycerides. – Once in the cell, they recombine into triglycerides, which combine with proteins to form chylomicrons. These are then released into the interstitial fluid. ° Most chylomicrons enter lymph through lacteals (lymphatic capillaries) in the villi and are transported to the subclavian veins where they join the blood circulation. Small Intestine: Absorption of Vitamins • Fat-soluble vitamins (DAKE) are incorporated into micelles and absorbed in same manner as fats (passively through lipid bilayer). • Water-soluble vitamins (C, B complex) are mostly absorbed by diffusion. – The exception is B12, which must bind to intrinsic factor produced in stomach to be actively absorbed in the ileum (recognition of B12-intrinsic factor complex by receptors in plasma membrane of cells triggers active receptor-mediated endocytosis). Small Intestine: Absorption of Electrolytes • Most are actively absorbed throughout small intestine. • Absorption based on how much is in food • The Na+/K+ pump plays role in moving Na+ into the epithelial cells and K+ out (into the lumen). • K+ passively absorbed based on gradient created by absortpion of water from the lumen. Small Intestine: Absorption of Electrolytes • Iron (Fe) and calcium (Ca) only absorbed in duodenum depending on the needs of the body. – Iron actively transported into cells where it becomes bound to ferritin. In the blood iron is carried bound to transferrin. (See topic 1) – Calcium absorption regulated by vitamin D which serves as cofactor in Ca transport. Vitamin D synthesis begins in the skin. Parathyroid hormone (PTH) stimulates the final formation of Vit. D by the kidneys when blood calcium is low. Small Intestine: Movement • Peristalsis moves chyme through the intestine. • Segmentation mixes chyme with intestinal juices. Fig. 24.3, p. 890 Hormonal Control of Small Intestine Activity Gastrin secreted by stomach is stimulatory. It stimulates: – contraction of intestinal smooth muscle; and – relaxation of ileocecal valve. Vasoactive intestinal peptide (VIP) from the duodenum acts on the duodenum to stimulate secretion of bicarbonate-rich intestinal juice to neutralize the chyme. Somatostatin from stomach and duodenum inhibits blood flow and absorption from the small intestine. Nervous System Control of Small Intestine Activity • Sympathetic impulses decrease activity. • The gastroileal reflex, initiated by increased activity in stomach, is a long reflex involving brain and parasympathetic innervation. – Parasympathetic impulses increase activity in the small intestine. Accessory Glands: Liver Gross Anatomy • The liver is the largest gland in body and weighs approximately 1.4 kg (a little over 3 lbs). • It is mainly located in the upper right hypochondriac region and extends into the epigastric region. • There are 4 primary lobes: right, left, caudate, quadrate. • The liver is covered by serosa except for uppermost region just under diaphragm Fig. 24.1, p. 888 See Fig. 24.23, p. 919 http://telpath2.med.utah.edu/WebPath/LIVEHTML/LIVER002.html Liver: Hepatic Ducts Hepatic ducts carry bile from the liver to the small intestine. • The right hepatic duct serves the right lobe. • The left hepatic duct serves the other 3 lobes. • The common hepatic duct is formed from the union of the right and left hepatic ducts. – The common hepatic duct joins the cystic duct of gall bladder to form common bile duct, which Fig. 24.20, p. 915main pancreatic duct to form hepatojoins with pancreatic ampulla. Liver: Ligaments • The falciform ligament is a piece of mesentery that separates right and left lobes. – It suspends liver from diaphragm and anterior abdominal wall. • The round ligament (ligamentum teres) is a remnant of the umbilical vein in the inferior portion of the falciform ligament. • The ligamentum venosum is a remnant of the ductus venosus and runs through the liver. http://storm.aecom.yu.edu/virtualDissector/New_online_dissector/Abdomen/Abdom en5-6/photos/STEP3/PAGES/ligamentum%20venosum_jpg.htm http://www.shu.edu/ha/imgs/00000/8000/000/8052.jpg See Fig. 24.23, p. 919 Liver: Blood supply • The hepatic artery supplies oxygen-rich arterial blood. • The hepatic portal vein carries nutrient-rich venous blood from the stomach, intestines, pancreas, and spleen (see lab for vessels) to the liver. • The hepatic vein drains venous blood from the liver into the inferior vena cava. Fig. 20.27, p. 771 Liver: Microscopic Anatomy • The liver is designed to filter and process nutrientrich blood. • It is composed of lobules which are hexagonal structure with a portal triad at each corner. The portal triad is composed of: – a branch of hepatic artery; – a branch of hepatic portal vein; and – a bile duct. http://www.usc.edu/hsc/dental/ghisto/gi/d_88.html Fig. 24.24, p. 921 Liver: Microscopic Anatomy (con’t) • Sinusoids are specialized capillaries in which venous and arterial blood mix. – Hepatocytes (liver cells) just outside the walls of the sinusoid perform the functions of the liver. – The walls of the sinusoids are incomplete and Kupffer cells are space irregularly along the wall. These fixed macrophages remove debris, bacteria, and worn out RBCs. Fig. 24.24, p. 921 Liver: Microscopic Anatomy (con’t) • A central vein drains each lobule. – These veins join to form hepatic veins • Bile canaliculi are channels between the hepatocytes. – They carry bile and join to form bile ducts. – Bile flow is counter to blood flow. http://www-edlib.med.utah.edu/WebPath/LIVEHTML/LIVER003.html Fig. 24.24, p. 921 Liver Functions Liver functions include: • Processing blood-borne nutrients, including: – storing glucose (as glycogen); – storing fat-soluble vitamins; and – storing iron (Fe); • Detoxifying poisons (toxins); • Production of plasma proteins (see Topic 1); • Cleansiong the blood of debris, including bacteria and worn out RBCs; and • Production of bile and bile salts. Liver Functions: Bile • Bile consists of bile salts, bile pigments, cholesterol, neutral fats, phospholipids, electrolytes in water. • Bile salts aid digestion of fat because they: – emulsify (break up) fat globules into droplets; and – form micelles (which ferry fats to mucosal wall). • Bile salts are conserved by the enterohepatic circulation in which some is reabsorbed by the ileum and returned to liver via hepatic portal system. • The main bile pigment is bilirubin, which is: – formed from breakdown of hemoglobin, and – metabolized by bacteria in large intestine (it becomes a brown pigment). Control of Bile Production Bile production is stimulated by • bile salts returning via hepatic portal blood; and by • secretin (a hormone secreted by the small intestine in response to fats in chyme). Fig. 24.25, p. 923 Liver Disorders/Disease • Hepatitis is an inflammation of the liver, often caused by viral infection (HVA, HVB, HVC, HVD). – Hepatitis can be transmitted enterically (HVA) or through blood (HVB, HVC, HVD). – Blood-borne viruses are linked to chronic hepatitis and cirrhosis. • Cirrhosis is a chronic disease characterized by growth and replacement of hepatic tissue by scar tissue. • Jaundice is a yellowing of the skin due to a build up of bilirubin from liver disease or excessive destruction of RBCs (e.g., neonatal jaundice). Accessory Glands: Gall Bladder • The gall bladder lies in a depression on ventral surface of liver. • It is a thin-walled, muscular sac that holds about 50 ml of bile. • It is designed to store and concentrate bile. • It releases bile via the cystic duct. • Histologically, it has 4 tissue layers: – The mucosa has cells with microvilli to increase reabsorption of water. – The submucosa consists of dense CT. – The muscularis of smooth muscle contracts to expel bile – The serosa is present over the ventral portion only. http://www.usc.edu/hsc/dental/ghisto/gi/d_91.html Control of Bile Release • Bile produced by the liver backs up into the gall bladder when the hepatopancreatic sphincter is closed. • The gall bladder releases bile into the cystic duct when stimulated by cholecystokinin (secreted by duodenum) and/or parasympathetic impulses. • Release is inhibited by somatostatin produced by the stomach and duodenum. Fig. 24.25, p. 923 Disorders of the Gall Bladder • Gallstones (biliary calculi) result from crystallization of cholesterol due to excess of cholesterol or too little bile salts. These hard structures can become lodged in the duct system. • Obstructive jaundice is yellowish coloration of skin caused by build up of bile pigments resulting from blockage of bile ducts. Accessory Glands: Pancreas • The pancreas is mostly retroperitoneal; the head is encircled by duodenum, the tail abuts the spleen. • Acinar cells (acini) secrete pancreatic juice rich in enzymes. – The enzymes are stored in inactive form in zymogen granules within the cells until released. – Pancreatic juice is excreted through the main and accessory pancreatic ducts. • The islets of Langerhans are the endocrine cells of the pancreas. – Beta cells secrete insulin. – Alpha cells secrete glucagon. – Delta cells secrete somatostatin. http://www.usc.edu/hsc/dental/ghisto/gi/d_95.html See Fig. 24.20, p. 915 Composition of Pancreatic Juice • Pancreatic juice is watery and rich in bicarbonate (HCO3-). – Bicarbonate makes it alkaline and neutralizes the acidity of chyme entering the duodenum from the stomach. • Digestive enzymes (see Small Intestine: Digestion): – Proteases are produced and released as zymogens (inactive precursors). ° Trypsin is released as trypsinogen and activated by enterokinase enzyme in brush border cells. ° Carboxypeptidase & chymotrypsin are activated from precursors by trypsin. Pancreatic Enzymes (con’t) Digestive enzymes produced by the pancreas include: • pancreatic amylase, which hydrolyzes starch and glycogen (animal “starch”) into short carbohydrate chains and maltose; • pancreatic lipases, which hydrolyze neutral fats into fatty acids and glycerol (or mono- and diglycerides); • pancreatic nucleases, which hydrolyze nucleic acids into nucleotides; and • pancreatic nucleosidases, which hydrolyze nucleotides into ribose, nitrogen bases and phosphate. Control of Pancreatic Secretion • Secretin stimulates acini to produce juice rich in bicarbonate. – Secretin is released from the small intestine in response to acidic chyme entering the duodenum. • Cholecystokinin (CCK) is released from duodenum in response to fatty or protein-rich chyme. – CCK stimulates the acini to secrete juice rich in enzymes. • The Vagus nerve stimulates secretion during the cephalic and gastric phases of digestion. Fig. 24.28, p. 925 Pancreas’ Endocrine Role: Insulin • Insulin is secreted when blood glucose increases. • Insulin lowers blood sugar by: – stimulating uptake by body cells (except liver, kidney and brain); – stimulating glycogen formation in liver and skeletal muscle; – stimulating carbohydrate metabolism in most cells; and – inhibiting gluconeogenesis (conversion of fats and protein to glucose) in liver. • In the absence of insulin, fat is released and metabolized resulting in the formation of keto acids, which lower the pH of the blood if uncompensated. Pancreas’ Endocrine Role: Glucagon • Glucagon is secreted in response to low blood glucose. • Glucagon increases blood sugar by: – promoting breakdown of glycogen (glycogenolysis) by the liver; – stimulating synthesis of glucose from lactic acid and noncarbohydrate sources (gluconeogenesis) by liver; – stimulating release of glucose into blood by the liver; and – inhibiting uptake and use of carbohydrates by skeletal muscle. Skeletal muscle can metabolize fats and amino acids as well. Disorders of the Pancreas Pancreatitis is an inflammation of the pancreas. • It is sometimes caused by excessive fat in blood. • Pancreatitis results in release and activation of enzymes within pancreas (pancreas digests itself). • Most severe cases are due to alcohol abuse. Large Intestine • The large intestine is located primarily in abdominal cavity, distal end is in pelvic cavity. • The large intestine is larger in diameter, but shorter (~1.5 m) than the small intestine. • Modifications seen in the large intestine include: – reduction of longitudinal layer of muscularis to form strips called the teniae coli; – pocket-like sacs called haustra formed by motor tone of the teniae coli; and – epiploic appendages, small, adipose-filled pouches of visceral peritoneum. Fig. 24.29, p. 928 Large Intestine: Subdivisions • The cecum (caecum) is the first, small pouch-like area of the large intestine; it receives chyme from the ileum. – The vermiform appendix contains lymphatic tissue for protection against disease. • The “colon” forms the bulk of the large intestine and is composed of: – the ascending colon, that rises along the right side; – the transverse colon, that goes across just under the umbilicus; Fig. 24.29, p. 928 – the descending colon, that goes down along the left side; and – the sigmoid colon, that enters the pelvic cavity. Large Intestine: Subdivisions • The rectum is the pouch just before the anal canal. It stores feces until they are released. • The anal canal is the final portion of the large intestine and serves as the passageway for removal of feces. Fig. 24.29, p. 928 Large Intestine: Anal Canal • The mucosa of the anal canal is arranged as long folds called anal columns. – The mucosa is composed of stratified squamous epithelium. – Anal sinuses secrete mucus when compressed by feces. The mucus helps lubricate and makes defecation easier. • Sphincters control defecation. – The internal anal sphincter is composed Fig. of 24.29, p. 928 smooth muscle and is involuntary. – The external anal sphincter is composed of skeletal muscle and is under voluntary control. Large Intestine: Histology • The mucosa is thicker than in small intestine and contains crypts with numerous goblet cells. – Most of the mucosa is simple columnar epithelium. – Stratified squamous epithelium is present in the anal canal where it protects against abrasion. • The submucosa is thinner than in small intestine and has less lymphatic tissue. • The longitudinal layer of the muscularis is modified as strips of muscle called the teniae coli, causes the colon to form bag-like structures called haustra. • The serosa covers all of the colon except the region in the pelvic cavity (sigmoid colon and rectum). See Fig. 24.31, p. 930 http://www.usc.edu/hsc/dental/ghisto/gi/d_60.html Intestinal Flora • Resident bacteria present in the large intestine are dominated by Escherichia coli (E. coli). • These bacteria ferment some indigestible carbohydrates resulting in mixture of irritating acids and gases. • They also synthesize several B vitamins and vit. K. Digestion in Large Intestine • There is no additional breakdown of molecules except by bacteria. • Reabsorption of water and electrolytes by the large intestine are very important to water and electrolyte balance. – Severe diarrhea can lead to severe dehydration. • Vitamins produced by bacteria are absorbed. Movements in Large Intestine Movement in the large intestine forms feces. • Haustral churning is a slow process in which distention of a hastrum stimulates contraction which moves food into next haustrum. – This process mixes food residue and aids water reabsorption. • Mass peristalsis refers to long, slow movements along the length of the large intestine, wherein feces are forced toward the rectum. – Mass peristlsis is stimulated by gastrocolic reflexes based on stretching of stomach. Defecation To release feces, the two sphincters must relax. • Entrance of feces into the anal canal results in parasympathetic reflex relaxation of smooth muscle (internal) sphincter. • The external sphincter (skeletal muscle) is controlled by voluntary nerve impulses. – Damage to the descending tracts of the spinal cord or cerebral cortex result in loss of ability to control defecation. Fig. 24.32, p. 931 Large Intestine: Disorders • Appendicitis is an inflammation of the appendix, usually caused by bacterial infection. Rupture of the • Diarrhea results when the colon is irritated and watery stools are produced. – Diarrhea is caused by irritants, bacterial or viral disease. – Loss of water and electrolytes can lead to dehydration and electrolyte imbalances. Large Intestine: Disorders • Constipation refers to the condition in which hard stools are produced due to increased time for water reabsorption. – Diets low in fiber may lead to constipation. – Constipation can also lead to electrolyte and pH imbalances. • Hemorrhoids are due to inflammation of the superficial anal veins. They can either be external or internal and are often painful. • Colitis is any inflammation of the colon. Large Intestine: Disorders • Diverticulosis is the formation of small pouches (called diverticula) in the mucosa of the large intestine. – This is common in the elderly, especially in people with poor diets low in fiber. Fiber holds water and keeps the feces more pliable. • Diverticulitis is an inflammation of the diverticula. – Inflammed diverticula can rupture leading to peritonitis. • Crohn’s disease is a chronic inflammation of the GI tract, usually involving the ileum or large intestine. Changes in the wall occur.