Digestive System PDF

Digestive System Marie A. Román Martínez, PhD Department of Biology Office hours: by appointment Email: [email protected] Copyright-This presentation is intended for educational purpose only. No part of this presentation may be reproduced or transmitted in any form without written permission. Obje...

Digestive System Marie A. Román Martínez, PhD Department of Biology Office hours: by appointment Email: [email protected] Copyright-This presentation is intended for educational purpose only. No part of this presentation may be reproduced or transmitted in any form without written permission. Objectives 1. Identify the structures composing the alimentary canal (digestive tract). 10. Describe the location and functions of the liver. 11. Explain how bile release is stimulated. 2. Identify the accessory organs of the digestive 12. Describe digestion in the small intestine. system. 13. Describe the structure and functions of the 3. Compare mechanical and chemical digestion. large intestine. 4. Describe the role of digestive enzymes. 14. Describe the major disorders of the digestive 5. Describe digestion in the mouth. system. 6. Describe the basic structure of a tooth. 7. Describe the location and function of the pharynx, esophagus, stomach. 8. Explain the control of gastric secretions. 9. Describe the control and functions of pancreatic secretions. 2 Liver Liver is the largest gland in the body at 1.4 kg (3 lb). Located mostly in the right upper quadrant just below the diaphragm. Dark, reddish brown color. Falciform ligament Joins liver to diaphragm and anterior abdominal wall. Separates the two main lobes. Main lobes are the right lobe and left lobe. 3 Liver Blood Supply Hepatic artery proper brings oxygenated blood to the hepatocytes (liver cells). Hepatic portal vein brings deoxygenated, nutrient rich blood from the digestive tract. Hepatocytes remove, modify, or add substances to the blood before it leaves the liver via the hepatic veins. 4 Histology of liver lobules in pig liver Liver Microscopic structure Hepatic lobules are the structural and functional units. Hexagonal cylinder. https://histology.siu.edu/erg/GI152b.htm Has a central vein with sheets of hepatocytes radiating outward. Hepatic triad Located at corners where several lobules meet. Consists of: Branch of hepatic artery proper Branch of hepatic portal vein Interlobular bile ductule 5 Liver Hepatic sinusoids Blood-filled spaces between the hepatocyte sheets. Carry blood from hepatic artery proper and hepatic portal vein to central vein. Site of material exchange between blood and hepatocytes. Have macrophages to remove debris and bacteria. Central veins merge to form hepatic veins. 6 Ducts of the Liver Bile is collected into the right and left hepatic ducts. Common hepatic duct forms with union of right and left hepatic ducts. Common hepatic duct and cystic duct from gallbladder form the bile duct. The bile duct carries bile to the duodenum. Gallbladder: Stores bile between meals. Bile duct 7 Liver Functions Production and secretion of bile →a substance that aids in the digestion and absorption of lipids. It also produces and secretes heparin, a blood anticoagulant and plasma proteins. Role in blood glucose homeostasis and carbohydrate, lipid and protein metabolism. Stores triglycerides, glycogen, iron, and vitamins A, D, E, K and B12. Detoxifies blood by modifying drugs and harmful chemicals to form less toxic compounds. Removes worn out blood cells and bacteria by phagocytosis. 8 Bile Yellowish, green liquid continuously produced by the liver. Consists of water, bile salts, bile pigments, cholesterol, minerals. Bilirubin (yellow-colored) is a waste product of hemoglobin breakdown that are excreted through the bile. Jaundice Bile salts emulsify lipids in chyme. Increases surface area of fats exposed to water and lipases. Aid in absorption of fatty acids, cholesterol, and lipid-soluble vitamins by the small intestine. 9 Bile Release of Bile: Bile usually enters the duodenum only when chyme is present. When intestine is empty, the sphincter at base of bile duct constricts. Forces bile into gallbladder for storage. In response to lipid-rich chyme, cholecystokinin is released. Causes the gallbladder to contract. Relaxes sphincter at base of bile duct. Bile is injected into the small intestine. 10 Small Intestine Small intestine is 2.5 cm wide, 12-15 ft in length. It begins at the pyloric sphincter and ends at the large intestine. Site of most digestion and absorption of nutrients. Three segments: Duodenum—First and shortest section. Jejunum –Middle section. Ileum– Last and longest section. Ileal orifice Suspended by mesentery →double folds of the peritoneum that provides support but allow movement. 11 Small Intestine Mucosa has numerous intestinal villi. Provide a very large surface area. Villus anatomy: Covered in simple columnar epithelium. Have a centrally located lacteal and a blood capillary network. Tiny pits at base of villi open into intestinal glands, which secrete intestinal juice. Microvilli increase surface area. Folds in epithelial cell plasma membranes. “Brush border” 12 Intestinal Juice Slightly alkaline with abundant water and mucus. Forms appropriate environment for actions of bile salts and pancreatic digestive enzymes. Regulation of intestinal secretion: Mechanical stimulation due to presence of chyme. Activates secretion of intestinal juice and enzymes. Neural reflex due to intestinal wall stretch caused by the chyme. Parasympathetic action potentials increase the rate of intestinal secretions. 13 Digestion and Absorption in the Small Intestine General events: Vigorous segmentation mixes chyme with bile, pancreatic juice, and intestinal juice. Emulsification of fats occurs. Digestion of carbohydrates, proteins, and lipids occur due to pancreatic and intestinal enzymes. Brush-border enzymes complete the process of digestion. Digestion of disaccharides to monosaccharides: Maltase Sucrase Lactase Peptidases 14 15 Absorption of Carbohydrates and Fats Absorption of carbohydrates: Simple sugars (glucose, fructose, galactose) are absorbed across the epithelium and into the capillaries of the intestinal villi. Absorption of fats: Short-chain fatty acids are absorbed by simple diffusion. All other lipids require alternate absorption. Bile salts interact and form micelles. Micelles absorb large fatty acids, monoglycerides, cholesterol, phospholipids, steroids, and lipid-soluble vitamins. 16 Absorption of Carbohydrates and Fats Absorption of fats: Micelles transport lipids to intestinal brush border. Lipids enter epithelial cells by simple diffusion. Triglycerides are reformed. Lipids and lipid-soluble vitamins are packaged into chylomicrons. Chylomicrons are released from the epithelial cells by exocytosis. Chylomicrons move into lacteals and are carried by lymphatic vessels to the left subclavian vein, where lymph from the intestine enters the blood. 17 Absorption of Proteins and Other Materials Absorption of proteins: Amino acids are actively absorbed into villi capillaries. Other absorbed materials: Water, minerals, and water-soluble vitamins enter villi capillaries. Blood leaving intestines flows to the liver via the hepatic portal vein: Absorbed materials are processed before blood enters general circulation. 18 Glucagon-like peptide-1 Produced by the cells of the small intestine. Main action→ stimulate insulin secretion and to inhibit glucagon secretion. It has also been shown to slow gastric emptying. GLP-1 receptors are expressed in many regions of the brain. GLP-1 also has been shown to reduce food intake. Semaglutide (Ozempic, Wegovy, Rybelsus) belongs to a class of medications known as glucagon-like peptide 1 receptor agonists. 19 Glucagon-like peptide-1 Image obtained from Ussher and Drucker et al. 2023 https://www.nature.com/articles/s41569-023-00849-3 20 Large Intestine Chyme asses from the small intestine into large intestine through the ileal orifice. 6.5 cm wide, 1.5 m long. Consist of four segments: Cecum: Pouch below ileal orifice Appendix no digestive function it contributes to the immune defense of the body. Colon: Ascending colon Transverse colon Descending colon Sigmoid colon Rectum Anal canal 21 Large Intestine Anus is kept closed except during defecation by the: Internal anal sphincter: Involuntary controlled External anal sphincter: Voluntarily controlled 22 Large Intestine Taeniae coli Three longitudinal bands of muscle running length of colon. Contraction forms haustra. Large intestine is supported by mesentery. Mucosa possesses no villi. Epithelium contains numerous goblet cells. 23 Large Intestine Chyme entering the large intestine contains: Water Minerals Bacteria Undigested and unabsorbed materials No digestive enzymes are produced by the large intestine. Intestinal bacteria decompose the undigested food residues. Yield B vitamins and vitamin K. Produce flatus. Mucosa produces large amounts of mucus Lubrication and protection against abrasion. 24 Large Intestine Main function of large intestine is absorption of water, some minerals, and vitamins as contents move slow through the colon. End product is feces. Contains large amounts of bacteria, mucus, water, and non-digested food molecules. Movements: Segmentation and peristalsis are sluggish. Undergo vigorous peristalsis 2 to 4 times per day. Usually after a meal. Moves descending and sigmoid colon contents toward rectum. 25 Large Intestine Movements: Defecation reflex: Rectum fills with feces and its wall is stretched. Parasympathetic action potentials are sent to the rectum. Stimulate muscular contractions that increase rectal pressure. Internal anal sphincter is relaxed. If external anal sphincter is voluntarily relaxed, defecation occurs. If external anal sphincter stays voluntarily contracted, defecation is postponed. 26 Disorders of the Digestive System Digestive system disorders are grouped as: Inflammatory disorders Noninflammatory disorders 27 Inflammatory Disorders Appendicitis: Acute inflammation of appendix. Symptoms include referred pain in umbilical region, nausea. Treatment is surgical removal of the appendix. Diverticulitis: Development of small, saclike outpockets on colon. Develops from diet lacking sufficient fiber. Inflammation causes pain, bloating, or diarrhea. 28 Inflammatory Disorders Periodontal disease: Inflammation, bleeding gingivae, in addition to degeneration of gingivae, cement, periodontal ligaments, and dental alveoli. Causes loss of teeth. Caused by poor dental hygiene. Peritonitis: Acute inflammation of peritoneum. Results from bacteria entering peritoneal cavity due to contamination. Example: accidents, surgery, ruptures intestine or appendix. 29 Noninflammatory Disorders Constipation: Defecation is difficult due to hard, dry feces. Feces stay in colon too long: Too much water is absorbed from feces. Diarrhea: Production of watery feces. Due to abnormally rapid peristalsis in colon. Decreases time available to absorb water. Caused by inflammation and chronic stress. 30 Noninflammatory Disorders Eating disorders: Caused by obsessive concern about weight control. Most common in young adult females. Two major types: Anorexia nervosa Self-imposed starvation. Causes malnutrition and physiological changes. Patients see themselves as overweight. Death due to complications of prolonged starvation. Bulimia Frequent overeating and purging by selfinduced vomiting. Associated with fears of being overweight, depression and stress. Causes electrolyte imbalances, erosion of tooth enamel and constipation. 31 Obesity Obesity results from consuming food containing more kilocalories than is necessary to meet the body’s energy needs. Causes of obesity include heredity, eating habits, inadequate exercise, and poor diet. Excess kilocalories are stored as fats in adipose tissue. The only way to get rid of stored fat is to consume fewer kilocalories than needed so the stored fat will be mobilized and broken down. Exercise increases the body’s need for energy, which helps burn fat more rapidly. 32 Glucagon-like peptide-1 Produced by the cells of the small intestine. Main action→ stimulate insulin secretion and to inhibit glucagon secretion. It has also been shown to slow gastric emptying. GLP-1 receptors are expressed in many regions of the brain. GLP-1 also has been shown to reduce food intake. Semaglutide (Ozempic, Wegovy, Rybelsus) belongs to a class of medications known as glucagon-like peptide 1 receptor agonists. 33 Glucagon-like peptide-1 Image obtained from Ussher and Drucker et al. 2023 https://www.nature.com/articles/s41569-023-00849-3 34 https://www.viddler.com/embed/6360b8e5 35