The Digestive System PDF

8/27/24, 6:15 PM Platform | Study Fetch The Digestive System Overview of the Digestive System (00:00:13 - 00:00:35) The digestive system is responsible for the breakdown and absorption of food into the body Anything you ea...

8/27/24, 6:15 PM Platform | Study Fetch The Digestive System Overview of the Digestive System (00:00:13 - 00:00:35) The digestive system is responsible for the breakdown and absorption of food into the body Anything you eat, whether food or drink, goes through the digestive tract and gets absorbed into the bloodstream for use by the body's cells Lesson Overview (00:00:35 - 00:00:50) We will discuss the digestive pathway and the accessory organs that help with digestion and absorption Intracellular vs. Extracellular Digestion (00:00:50 - 00:01:07) Intracellular digestion: Some organisms like amoebas use pseudopods to pull in food and digest it within the cytoplasm of their cells Humans perform extracellular digestion: We eat food, which is then absorbed through an internal tract that runs through the body Nutrients are extracted, and waste (feces) is expelled Mechanical vs. Chemical Digestion (00:01:22 - 00:02:21) Mechanical Digestion: The physical breakdown of food, such as chewing and the churning action of the stomach Breaks food into smaller particles to aid chemical digestion Chemical Digestion: The chemical breakdown of food using enzymes Cleaves bonds in fats, proteins, and carbohydrates to make them smaller and easier to absorb The Digestive Pathway (00:02:21 - 00:02:54) The digestive system travels from the mouth, down the esophagus, through the stomach and intestines, and ends at the colon, rectum, and anus Mouth and Salivary Glands https://www.studyfetch.com/platform/studyset/66cd116dd279f5220d947c66/material/66ce2d05f069b6d6b89c5eba/document?go=note 1/9 8/27/24, 6:15 PM Platform | Study Fetch (00:02:54 - 00:03:19) The mouth is the first site of mechanical and chemical digestion Teeth mechanically break down food Saliva contains the enzyme amylase, which begins breaking down carbohydrates Salivary glands (parotid, submandibular, and sublingual) secrete saliva Pharynx and Esophagus (00:03:40 - 00:05:28) The pharynx separates the trachea from the esophagus The epiglottis blocks the trachea when swallowing to prevent choking The esophagus uses peristalsis (rhythmic muscle contractions) to move food from the pharynx to the stomach The upper esophagus is under voluntary, skeletal muscle control, while the lower esophagus is under involuntary, smooth muscle control Stomach (00:05:41 - 00:06:56) The stomach is the site of both mechanical and chemical digestion The cardiac sphincter connects the esophagus to the stomach and prevents reflux of stomach contents back into the esophagus The churning and acidic environment of the stomach further break down food Mechanical and Chemical Digestion in the Stomach (00:06:56 - 00:07:10) The churning and swirling motion of the stomach helps to break down food into smaller pieces, which speeds up the chemical digestion process. (00:07:10 - 00:07:20) The gastric pits in the stomach are the sites where acids and gastric enzymes are produced. (00:07:20 - 00:07:31) The G cells in the stomach release gastric, which stimulates the parietal and chief cells. (00:07:31 - 00:07:42) The parietal cells secrete hydrochloric acid (HCl), and the chief cells secrete pepsinogen, a zymogen (inactive enzyme). (00:07:42 - 00:07:55) The HCl secreted by the parietal cells activates the pepsinogen secreted by the chief cells, converting it into the active enzyme pepsin. (00:07:55 - 00:08:13) Pepsinogen is secreted as a zymogen to prevent it from activating and digesting the stomach lining before it reaches the food. https://www.studyfetch.com/platform/studyset/66cd116dd279f5220d947c66/material/66ce2d05f069b6d6b89c5eba/document?go=note 2/9 8/27/24, 6:15 PM Platform | Study Fetch (00:08:13 - 00:08:26) The pepsinogen needs to be exposed to the acid in the stomach to be converted into the active pepsin, which then begins breaking down proteins. (00:08:26 - 00:08:41) Mucus cells in the stomach produce mucus to protect the stomach lining from the acid and digestive enzymes. (00:08:41 - 00:08:53) Pepsin is not specific and can digest the stomach lining as easily as it can digest the food, so the mucus layer is essential for protection. (00:08:53 - 00:09:08) The partially digested food mixture in the stomach is called chyme, and it exits the stomach through the pyloric sphincter. (00:09:08 - 00:09:20) The chyme, now mixed with stomach acid and digestive enzymes, enters the duodenum, the first part of the small intestine. (00:09:20 - 00:09:31) The pyloric sphincter is a smooth muscle sphincter that controls the exit of chyme from the stomach into the duodenum. (00:09:31 - 00:09:42) Both mechanical and chemical digestion occur in the mouth and stomach. In the mouth, chewing and salivary amylase begin the digestive process. (00:09:42 - 00:10:00) In the stomach, the churning motion, pepsin, gastric lipase, and hydrochloric acid further the mechanical and chemical digestion of the food. (00:10:00 - 00:10:13) The key gastric cells to remember are the G cells, parietal cells, chief cells, and mucus cells. Digestion and Absorption in the Small Intestine (00:10:13 - 00:10:24) 90% of digestion and nutrient absorption occurs in the small intestine, which includes the duodenum, jejunum, and ileum. (00:10:24 - 00:10:34) The small intestine is the main organ responsible for digestion and absorption, not the stomach. (00:10:34 - 00:10:49) The small intestine is where the majority of fats, sugars, and proteins are absorbed from the food. (00:10:49 - 00:10:59) The small intestine needs to protect itself from the acidic chyme entering from the stomach. (00:11:13 - 00:11:29) https://www.studyfetch.com/platform/studyset/66cd116dd279f5220d947c66/material/66ce2d05f069b6d6b89c5eba/document?go=note 3/9 8/27/24, 6:15 PM Platform | Study Fetch Goblet cells in the small intestine secrete mucus to protect the epithelium from the acid and digestive enzymes. (00:11:29 - 00:11:40) Neutralization of the acid also occurs in the small intestine, with the help of bicarbonate secreted by the pancreas. (00:11:40 - 00:11:59) Cholecystokinin (CCK) triggers the release of pancreatic enzymes and bile to aid in the digestion and absorption of fats and proteins. (00:11:59 - 00:12:21) CCK also inhibits gastric emptying to prevent more acidic chyme from entering the small intestine too quickly. (00:12:21 - 00:12:31) The pancreatic enzymes secreted include amylase, lipase, trypsin, and chymotrypsin. (00:12:31 - 00:12:45) Bile, secreted by the gallbladder, also aids in the digestion and absorption of fats. (00:12:45 - 00:12:57) The mnemonic "Amy's Lips, Tricky" can be used to remember the pancreatic secretions triggered by CCK. (00:12:57 - 00:13:08) Villi in the small intestine greatly increase the surface area for nutrient absorption. (00:13:08 - 00:13:20) Without the villi, the small intestine would not be able to absorb nearly as many nutrients and vitamins. (00:13:20 - 00:13:32) Microvilli on the surface of the enterocytes (absorptive cells) further increase the surface area for absorption. (00:13:32 - 00:13:51) Diseases like celiac disease can damage the villi, leading to malabsorption of nutrients. The Small Intestine (00:14:02 - 00:14:20) The crypts located between the villi are where the stem cells are found These stem cells produce new epithelial cells for the intestine The new epithelial cells grow and move up the villi over time (00:14:20 - 00:14:34) Glucose and amino acids are absorbed into capillaries They need to cross through the enterocytes to get into the blood (00:14:34 - 00:14:47) There are very tight junctions between the intestinal cells This forms a barrier to prevent bacteria and the microbiome from entering the bloodstream directly (00:14:47 - 00:15:02) The intestines are not a sterile place, containing a lot of bacteria and microbes https://www.studyfetch.com/platform/studyset/66cd116dd279f5220d947c66/material/66ce2d05f069b6d6b89c5eba/document?go=note 4/9 8/27/24, 6:15 PM Platform | Study Fetch There needs to be a filter barrier to prevent anything from the intestines from entering the bloodstream (00:15:02 - 00:15:13) The tight junctions and epithelial layer act as a filter barrier (00:15:13 - 00:15:25) Antibodies are also secreted to sit on the epithelial layer and help ward off pathogens (00:15:25 - 00:15:38) Glucose and amino acids are absorbed into the capillaries They are first taken in intracellularly by the enterocytes, then transported into the blood (00:15:38 - 00:15:53) Fatty acids and glycerol are absorbed into the lacteals They then pass through the capillaries into the intestinal blood flow (00:15:53 - 00:16:15) The blood flow from the intestines goes to the portal vein and then to the liver Mnemonic for small intestine parts: "DJI" - Duodenum, Jejunum, Ileum Digestion occurs in the duodenum, absorption occurs in the duodenum and ileum (00:16:15 - 00:16:28) Patients can develop "short gut syndrome" if too much of the intestine is surgically removed This can lead to malabsorption of nutrients (00:16:28 - 00:16:40) Knowing where absorption occurs is clinically important (00:16:40 - 00:16:51) Important hormones to know are CCK (cholecystokinin) and secretin (00:16:51 - 00:17:02) CCK triggers the release of pancreatic enzymes CCK also triggers many other processes to get digestion going (00:17:02 - 00:17:13) Important digestive enzymes are amylase, lipase, pepsin, trypsin, and chymotrypsin The Large Intestine (00:17:13 - 00:17:24) The large intestine is a site of: Water absorption Mineral absorption Vitamin B and K production and absorption (00:17:24 - 00:17:38) The appendix is a projection from the cecum It may harbor the gut microbiome and help repopulate it if needed (00:17:38 - 00:18:05) Appendicitis can occur if something gets trapped in the appendix, causing inflammation https://www.studyfetch.com/platform/studyset/66cd116dd279f5220d947c66/material/66ce2d05f069b6d6b89c5eba/document?go=note 5/9 8/27/24, 6:15 PM Platform | Study Fetch (00:18:05 - 00:18:18) Mnemonic for the roles of the large intestine: "SALT" - Salt absorption, water absorption, vitamin production/absorption (00:18:40 - 00:18:51) The large intestine is also a site of bile reabsorption (00:18:51 - 00:19:09) Bile that is secreted by the liver is reabsorbed in the large intestine This allows the body to recycle and reuse the bile, rather than having to constantly produce it de novo The Gallbladder (00:19:09 - 00:19:26) The gallbladder stores bile produced by the liver Bile is used to emulsify fats to aid in their absorption (00:19:26 - 00:19:37) Bile travels from the liver down the hepatic duct into the gallbladder It is stored in the gallbladder until released (00:19:37 - 00:19:48) Cholecystokinin signals the gallbladder to release the stored bile (00:19:48 - 00:19:59) The bile then travels down the common bile duct into the small intestine Bile acts like a detergent to break up fat droplets and emulsify them The Pancreas (00:20:11 - 00:20:22) The pancreas is an incredibly important organ with two main functions: Exocrine pancreas Endocrine pancreas (00:20:22 - 00:20:38) The exocrine pancreas will be the focus here, the endocrine pancreas will be covered later (00:20:38 - 00:20:57) The exocrine pancreas contains the islets of Langerhans, which have alpha and beta cells This is the endocrine portion of the pancreas (00:20:57 - 00:21:14) Outside the islets is the exocrine pancreatic tissue that secretes digestive enzymes The Pancreas and Digestive Enzymes https://www.studyfetch.com/platform/studyset/66cd116dd279f5220d947c66/material/66ce2d05f069b6d6b89c5eba/document?go=note 6/9 8/27/24, 6:15 PM Platform | Study Fetch (00:21:14 - 00:22:26) The pancreas secretes digestive enzymes that flow down the pancreatic duct and into the common bile duct. These enzymes include: Bicarbonate ions to neutralize hydrochloric acid Pancreatic amylase to break down carbohydrates Pancreatic lipase to break down fats, working with bile Trypsin and chymotrypsin to further break down proteins Enzyme Activation Cascade(00:22:26 - 00:24:35) The pancreas secretes these enzymes as inactive zymogens to prevent them from digesting the pancreas itself. Trypsinogen is activated to trypsin by an endopeptidase once it reaches the small intestine. Trypsin then activates more trypsin and also activates chymotrypsin, creating a cascade of enzyme activation. If this activation happens prematurely in the pancreas, it can lead to pancreatitis. The Pancreas and Diabetes(00:24:35 - 00:26:56) The pancreas also has an endocrine function, producing insulin from beta cells in the islets of Langerhans. In type 1 diabetes, the beta cells are autoimmunely destroyed, leading to a lack of insulin. In type 2 diabetes, the body becomes resistant to insulin, leading the pancreas to overproduce it until it eventually fails. Mnemonic for Islet Cells(00:26:56 - 00:27:21) Beta cells secrete insulin to make diabetics feel better. Alpha cells secrete glucagon when glucose is gone. The Liver (00:27:34 - 00:28:27) The liver has two lobes and is a very important organ. The falciform ligament is a useful landmark when examining the liver. The gallbladder is tucked under the edge of the liver, so the liver can be palpated to check for gallbladder issues. The Liver and Portal System (00:28:27 - 00:28:42) The liver is located under the ribs on the right side of the body The portal system is an important concept for understanding blood flow through the intestines and back to the liver (00:28:42 - 00:28:53) The portal system is very important for understanding blood flow from the intestines to the liver (00:28:53 - 00:29:04) Blood flows from the aorta into the intestines to pick up nutrients This blood does not directly recirculate, but instead travels to the portal vein (00:29:04 - 00:29:20) The blood from the intestines and stomach collects in the portal vein https://www.studyfetch.com/platform/studyset/66cd116dd279f5220d947c66/material/66ce2d05f069b6d6b89c5eba/document?go=note 7/9 8/27/24, 6:15 PM Platform | Study Fetch The portal vein then drains into the liver (00:29:20 - 00:29:33) The portal vein flows into capillary beds in the liver The blood then returns to the vena cava and back to the heart (00:29:33 - 00:29:52) The liver has an important role in cleaning and detoxifying substances from the blood This is why the blood flows through the liver before returning to the heart (00:29:52 - 00:30:04) The liver detoxifies any toxins or substances from the food you've eaten The liver also supports the digestive system (00:30:04 - 00:30:15) The liver plays an important role in the circulatory system It helps recycle red blood cells and hemoglobin (00:30:15 - 00:30:26) Hemoglobin is broken down into bilirubin, which the liver secretes in bile This allows the body to get rid of old hemoglobin (00:30:26 - 00:30:39) If the liver is not functioning properly, bilirubin can build up, causing jaundice (yellowing of the skin and eyes) (00:30:39 - 00:30:49) Severe jaundice can occur if the liver is not excreting bilirubin properly (00:30:49 - 00:31:03) The liver also plays an important role in glucose metabolism: Stores glucose as glycogen Can break down glycogen (glycogenolysis) to release glucose when needed (00:31:03 - 00:31:20) The liver can also perform gluconeogenesis: Creating new glucose from glycerol and amino acids (00:31:20 - 00:31:35) Mnemonics for liver glucose metabolism: Glycogenesis - creating glycogen Glycogenolysis - breaking down glycogen Gluconeogenesis - creating new glucose (00:31:35 - 00:31:47) The liver also converts ammonia (a toxic byproduct) into urea for excretion (00:31:47 - 00:32:16) Glycogenesis - creating glycogen Glycogenolysis - breaking down glycogen Gluconeogenesis - creating new glucose from other molecules https://www.studyfetch.com/platform/studyset/66cd116dd279f5220d947c66/material/66ce2d05f069b6d6b89c5eba/document?go=note 8/9 8/27/24, 6:15 PM Platform | Study Fetch (00:32:16 - 00:32:29) The liver converts ammonia into urea, which is less toxic and can be excreted (00:32:29 - 00:33:04) When amino acids are broken down, ammonia is produced The liver metabolizes this ammonia into urea, which is then excreted by the kidneys (00:33:04 - 00:33:17) The liver is also the site of production for many blood proteins, like albumin (00:33:17 - 00:33:40) If the liver fails, it stops producing albumin, leading to fluid buildup in tissues (edema) This is because albumin provides the osmotic force to pull fluid back into the bloodstream (00:33:40 - 00:34:09) Without albumin production from the liver, excess fluid builds up in tissues (00:34:09 - 00:34:24) The liver also produces many blood clotting factors Liver failure can lead to problems with blood clotting (coagulopathy) (00:34:24 - 00:34:38) One of the first signs of liver failure is decreased production of antithrombin III, a clotting factor (00:34:38 - 00:34:50) Mnemonic for accessory digestive organs: Gall bladder, Liver, Pancreas (GAIL'S PANTS) (00:34:50 - 00:35:07) The gallbladder stores and concentrates bile, aiding in fat digestion If the gallbladder is removed, the liver has to work harder to produce enough bile (00:35:07 - 00:35:17) The liver, gallbladder, and pancreas are all essential for proper digestive function (00:35:17 - 00:35:29) Mnemonic for liver functions: Blood (maintenance, storage, detoxification) Globin (hemoglobin recycling) Protect (detoxification, protein production) (00:35:29 - 00:35:43) Liver functions: Blood maintenance, storage, detoxification Glucose metabolism (glycogenesis, glycogenolysis, gluconeogenesis) Protein metabolism (production of blood proteins like albumin and clotting factors) (00:35:43 - 00:35:53) The liver is an essential and complex organ with many vital functions in the body. https://www.studyfetch.com/platform/studyset/66cd116dd279f5220d947c66/material/66ce2d05f069b6d6b89c5eba/document?go=note 9/9