D1.2 - The Digestive System (Updated 2024) PDF

D1.2 - The Digestive System I can… Identify the principal structures of the digestive system (mouth, esophagus, stomach, sphincters, small and large intestines, liver, pancreas, gallbladder) Describe the processing of matter through the digestive system into the circulatory system...

D1.2 - The Digestive System I can… Identify the principal structures of the digestive system (mouth, esophagus, stomach, sphincters, small and large intestines, liver, pancreas, gallbladder) Describe the processing of matter through the digestive system into the circulatory system The Digestive System There are two main divisions of the digestive system: 1. Accessory Organs: Those which aid in digestion by secreting enzymes, but do not actually handle food (such as salivary glands, liver, gallbladder, and pancreas). 2. Digestive Tract: Those which actually move food through the body (mouth, esophagus, stomach, small & large intestine, rectum, anus) The Digestive System The Movement of Food 1. Food enters the digestive tract through the mouth. Here, teeth begin to break down food physically and enzymes secreted by salivary glands begin to break down food chemically. 2. Partially-digested food is then pushed down the esophagus into the stomach using a process called peristalsis. 3. In the stomach, muscle contractions continue physical digestion, and enzymes & stomach acids continue chemical digestion. Some water & salts are absorbed into the bloodstream. 4. Food then enters the small intestine. Enzymes from the pancreas & liver are secreted into the small intestine to finish chemically digesting food. The digested nutrients are then absorbed into the bloodstream through specialized structures called villi and microvilli. 5. After it is processed by the small intestine, food is then passed into the large intestine. Here, any remaining water & salt is absorbed. Bacteria in the large intestine also help release vitamins from food. 6. Next, waste enters the rectum where it is held until it is excreted. What remains of the food we have eaten exits the body through the anus. The Mouth Digestion begins in the mouth Salivary glands in the mouth secrete saliva, which contains an enzyme known as salivary amylase which breaks down starch into disaccharides (chemical digestion) Physical digestion also occurs in the mouth, where teeth chew up food into smaller pieces. As you chew, your tongue pushes the bolus (ball of food) to the back of your mouth for swallowing The bolus enters the esophagus, moving towards the stomach through wavelike contractions of the esophageal tissue known as peristalsis The Mouth Food is chewed and formed into a bolus by the tongue Physical digestion breaks down food into smaller pieces ○ This increases surface area for chemical digestion The Esophagus Once swallowed, food travels from the mouth to the stomach via the esophagus Bolus of food stretches the walls of the esophagus and activates muscles that create rhythmic waves of contractions (peristalsis) Works even if you are upside-down! The Stomach Before the bolus enters the stomach from the esophagus, it must pass through the esophageal sphincter, which usually remains tightly shut to prevent stomach acid from splashing up into the esophagus (a.k.a. heartburn). Food undergoes both physical and chemical digestion in the stomach as well ○ Chemical by the stomach acid ○ Physical by the churning process that takes place as its muscular layers contract The Stomach Peristalsis pushes food against bottom of stomach, churning and breaking it into smaller pieces that mix with gastric juices to produce chyme (thick liquid) Gastric juice is responsible for chemical digestion in stomach ○ Made of: water, mucus, salts, hydrochloric acid, and enzymes ○ Has a pH of 1-3 ○ Helps to soften and break down proteins in chyme and kill bacteria Protein Digestion Pepsin, a protein-digesting enzyme, starts breaking down protein present in your food A thick mucus prevents the acid from digesting the stomach lining (weak spots due to stress can lead to ulcers) Protein Digestion Stomach doesn’t digest the proteins that make up its own cells Three methods of protection: 1. Doesn’t secrete gastric juice until food is present 2. Secretes mucus, which prevents gastric juice from harming lining of stomach 3. Produces a protein digesting enzyme called pepsin Pepsin becomes alive when hydrochloric acid is present Absorption in Stomach Absorbs small amounts of salts, water, anti-inflammatory medicines (ex. aspirin and alcohol) Pyloric sphincter controls exit of stomach contents into small intestine Ingestion Summary Saliva contains amylase enzymes that initiate breakdown, dissolving food particles, activating the taste buds, and lubricating the food Teeth bite, grind, tear, and crush food into smaller particles After food is swallowed, movement through the esophagus is regulated by peristalsis, contractions of muscles Sphincter muscles regulate movement of food in and out of stomach Digestive fluids in the stomach include HCl, pepsinogens, and mucus The Small Intestine Although some physical digestion occurs in the small intestine, its main function is to complete the digestion of macromolecules (carbs, proteins & lipids) and to absorb the resulting nutrients The Small Intestine Has a small diameter compared to the large intestine 4x the length of large intestine Surface is covered in fingerlike projections, called villi Each villi has thousands of microscopic extensions, called microvilli Divided into three regions: ○ Duodenum ○ Jejunum ○ Ileum Digestion and Absorption Most chemical digestion in small intestine occurs in duodenum Enzymatic digestion of macromolecules is performed by: ○ Carbohydrases: Digest carbohydrates ○ Lipases: Digest larger fats ○ Proteases: Digest larger polypeptides ○ Nucleases: Digest nucleic acids The Small Intestine - Duodenum The first 30 cm of the small intestine U-shaped Shortest and widest of the three regions Important site for chemical digestion of chyme The Small Intestine - Jejunum Contains more folds and secretory glands than the duodenum Continues to break down food The Small Intestine - Ileum Contains fewer and smaller villi Absorbs nutrients and pushes remaining undigested material into large intestine The Large Intestine The large intestine is only 1.5 meters long, but it has a greater diameter than the small intestine Is the final part of the digestive tract Shorter than the small intestine, but has a larger diameter Digestion does not occur here The large intestine’s main function is to concentrate and eliminate waste material The Large Intestine Colon: The largest part of the large intestine, needs to store waste long enough to reabsorb water from it Cellulose (carbs from plants!) is still undigested in the large intestine, however it provides bulk to our waste! Wastes build up in the large intestine Receptors in intestinal walls provide info to the central nervous system, which results in a bowel movement! If you don’t eat enough cellulose (veggies), then you probably don’t poop enough and toxins stay in your body longer than they should Colon cancer directly correlates to poor diet The Mystery of the Appendix It was previously believed that the appendix was a vestigial organ (has no purpose) However, it has recently been hypothesized that the appendix serves as a storage site for the “good” bacteria which aid in digestion As food is passed from the small intestine into the large intestine, the appendix is thought to add bacteria which aid in digestion as needed The appendix can become inflamed or infected if it absorbs “bad” bacteria or toxins ○ Severe cases require surgical removal; failing to do so could result in death Symptoms of appendicitis include pain & tenderness in the lower right abdomen, nausea, vomiting, and fever Accessory Organs Pancreas Liver Gallbladder Accessory Organs - Pancreas Delivers 1 L of pancreatic fluid to the duodenum each day Contains enzymes including: ○ Trypsin and chymotrypsin: Proteases that digest proteins into amino acids ○ Pancreatic amylase: Carbohydrases that digest starch into disaccharides in small intestine ○ Lipase: Digests lipids into fatty acids Digests proteins into smaller polypeptides, polysaccharides into simple sugars, and fats into fatty acids Contains bicarbonate which helps to neutralize hydrochloric acid in stomach ○ Gives chyme in duodenum a slightly alkaline pH of 8 Accessory Organs - Liver Produces bile which is made up of bile salts and bile pigments ○ Bile salts emulsify the fat (causes it to form tiny globules) ○ Tiny globules of fat have a large SA:V ratio and the lipase enzymes work on them more efficiently Accessory Organs - Gallbladder Bile is sent to gallbladder for storage The gall bladder then “injects” bile salts as needed into the small intestine. Fat containing chyme in the duodenum stimulates gallbladder to contract and inject the bile into the duodenum High cholesterol from excess fatty foods can cause gallstones to form; can be painful, and depending on the size, may need to be surgically removed.