By the end of this lesson, you should be able to:

  1. Identify major parts of the digestive system
  2. Explain how food is digested at each step of digestion
  3. Identify key enzymes in digestion
  4. Describe the role of hormones and the nervous system in digestion
  5. Identify common disorders associated with the digestive system 

Lesson Summary

  1. The digestive system is responsible for  breaking down our food and making it available for absorption
  2. The digestive system consists of the mouth, esophagus, stomach, small intestine and large intestine. Accessory organs such as the liver, gallbladder and pancreas help in digestion
  3. Food is broken down in the mouth by chewing and mixing it with saliva to form a bolus. Saliva contains digestive enzymes called amylase that partially digests carbohydrates
  4. The swallowing reflex cause a valve called the epiglottis to cover the trachea and prevent food from getting into our breathing passage while we swallow.
  5. Once we swallow, food is passed quickly in a matter of seconds to our stomach by a process called peristalsis which is a contraction process that pushes food forward along the alimentary canal (digestive tract)
  6. At the end of the esophagus and just before the stomach is a a valve called the gastroesophageal sphincter which opens up to receive food in the stomach. When this valve does not work right, food in the stomach gets backed up in the esophagus giving us heartburn.
  7. Once food is in our stomach a hormone called gastrin is secreted from the stomach into the blood to cause the release of gastric juices. This juice contains water, mucus, hydrochloric acid (HCl), pepsinogen, gastric lipase, and intrinsic factor.
    • Mucus protects the lining of the stomach from acid damage
    • HCl denatures protein to make it easier to digest
    • Pepsinogen – an enzyme that converts to another form called pepsin when exposed to acid. Pepsin digests protein to allow for easier digestion later on
    • Gastric lipase – Begins digestion of lipids (fats and oils)
    • Intrinsic factor – Facilitates absorption of vitamin B12  
  8. The stomach has strong muscles that mixes and churns food into a semi-liquid mass called chyme.
  9. At the end of the stomach there is another muscular valve called the pyloric sphincter that prevents food from leaving the stomach too fast and also prevent food from going back into the stomach once it has exited
  10. The small intestine is where digestion is completed and most nutrients is absorbed. It consists of three parts that are progressively longer
    • duodenum
    • Jejunum
    • Illeum
  11. The surface of the small intestine contains tiny projections called villi which also has tiny projections on them called microvilli. These projects increase the surface area of the small intestine and increases the rate of digestion
  12. Apart from the presence of villi and microvilli, the small intestine is heavily folded into circular folds that helps to increase surface area and increase digestion rate
  13. Each villus has capillaries and lymph vessels that takes nutrients from the small intestine to the blood.
  14. The capillaries collects water-soluble nutrients such as water soluble vitamins, monosaccharides glucose) and amino acids, and passes it directly into the bloodstream via the hepatic portal vein where the nutrients are taken to the liver and then to the heart for distribution
  15. Digested fats are unable to be deposited into the blood directly since fats do not mix with water well. Therefore they are pre-packaged into water-soluble circular vessels known as chylomicrons. These vessels are too big to enter the capillaries, however, they can enter the lymphatic vessels. The lymphatic vessels bypass the liver and take the chylomicrons to the upper part of the chest near to the heart where they are deposited directly into the blood.   
  16. Nutrients are absorbed by one of three methods
    • Simple diffusion: Movement of nutrients from high to low concentration
    • Facilitated diffusion: Movement of nutrients from high to low concentration with the help of a carrier protein
    • Active transport: Movement of nutrients from low to high concentration with the help of a carrier protein and energy
    • Endocytosis: Nutrient engulfs cell membrane and migrates into the cell taking the nutrients inside the cell.  
  17. At the end of the small intestine, just where the large intestine begins, there is a a muscular valve known as the ileocecal sphincter. This valve prevents backflow of fecal matter into the small intestine.
  18. The large intestine consists of the cecum, ascending colon, descending colon  transverse colon (transverse means “across), the sigmoid colon (sigmoid means S-shaped), rectum, and anus
  19. The function of the large intestine includes reabsorption of water and nutrients, fermentation of undigested carbohydrates and fiber, production of biotin and vitamin K, and expulsion of unwanted waste
  20. The rectum stores waste until enough accumulates to trigger a defecation response.
  21. Feces passes through the anus which is controlled by an internal and external sphincter. The external sphincter can be voluntarily controlled so that you can delay defecating until the appropriate time.
  22. The liver, gallbladder and pancreas are accessory organs in digestion
    • The liver is a “factory” that uses “raw materials” from digestion to make glycogen (storage form of glucose), proteins, cholesterol, and bile salts
    • The gallbladder is a holding sack for bile salts made in the liver. Bile salts are emulsifies that prevent fats from clumping together, as this would make them more difficult to digest
    • The pancreas is a gland that produces enzymes that are deposited in the small intestine to complete digestion of food. These enzymes include lipase, proteases and amylases
  23. The nervous system and endocrine system, affects digestion e.g.
    • When we are are stressed the nervous system slows down saliva secretion, reduce peristalsis and divert blood flow from the digestive organs to the muscles
    • When our stomach is empty the hormone ghrelin (I call it the greedy hormone to remember its function) is released in the blood causing the brain (part of the nervous system) to respond with feelings of hunger that may be accompanied by dizziness, headache and weakness
    • Insulin causes glucose to be absorbed into cells in the body when concentration of glucose is high in the blood
    • Glucagon causes glucose to be released from cells into the blood when glucose levels are low in the blood
  24. Common Digestive Disorders
    • Tooth decay
    • Periodontal disease: Inflammation of the gums caused by infection or plaque
    • Dysphagia: Difficulty swallowing due to tumor, trauma or other obstruction in the throat
    • Gastroesophageal reflux disease (GERD): Condition where fluid from the stomach flows into the esophagus causing heartburn. Certain foods such as fatty foods, spicy foods and alcohol can contribute to heartburn, Lifestyle practices such as smoking, overeating, wearing tight clothing reclining soon after eating can also contribute to heartburn.  
    • Gastroenteritis: Inflammation of the stomach caused by a virus or bacteria
    • Peptic ulcer: Erosion in the stomach or intestine wall caused by the bacteria Helicobacter pylori
    • Gallstone: Hard rocklike substance that forms in the gallbladder to to presence of high cholesterol in the bile  
    • Flatulence: Release of intestinal gas from the rectum. Exacerbated by eating too fast, drinking carbonated beverages, and eating foods with indigestible carbohydrates
    • Constipation: Slow movement of waste through. Commonly due to inadequate fiber and water intake the colon
    • Diarrhea: Loose stools. It can cause dehydration and loss of electrolytes (minerals). It’s often caused by virus or bacteria infection
    • Hemorrhoids: Swollen veins in the rectum and anus leading to bloody stool. Can be caused by constipation and straining to defecate  
    • Irritable bowel syndrome (IBS): Abnormal contractions in the colon that are painful. These contractions may constipation when they flow down the flow of matter in the colon or diarrhea when they speed up flow. Other symptoms may be loss of appetite, weight loss and fatigue.
    • Celiac disease: An autoimmune disease whereby eating gluten causes the immune system to attach the villi in the small intestine, damaging them and reducing nutrient absorption. The only cure is a strict non-gluten diet. Symptoms may include bloating, cramping, diarrhea, flatulence, joint pain, and skin rash.
    • Crohn’s disease: painful swelling of the intestine, occurring mainly in the illeum. Bloody stool, weight loss and anemia (lack of healthy red blood cells) are some of the common symptoms
    • Colon cancer: Cancer (abnormal tissue growth) in the colon. A common cause of cancer deaths along with lung, liver, stomach and breast cancer. It may be treated by radiation, chemotherapy or surgery to remove part or entire colon.  

Key Definitions

    1. Amylase: Enzymes responsible for breaking down carbohydrates. Present in the saliva and also secreted by the pancreas
    2. Epiglottis: Valve that covers the trachea and prevent food from getting into the respiratory tract when we swallow
    3. Bolus: The semi-solid mass produced after chewing food
    4. Esophagus: Part of the digestive tract between the mouth and the stomach
    5. Peristalsis: Involuntary contraction and relaxation in the digestive tract that results in food being push forward
    6. Gastroesophageal sphincter: Valve at the end of the esophagus that controls the flow of food into the stomach
    7. Hydrochloric acid: Acid in the stomach that is responsible for denaturing proteins and activation the protein enzyme pepsinogen
    8. Pepsinogen: A precursor enzymes that converts to pepsin when exposed to hydrochloric acid in the stomach. Pepsin is responsible for digesting protein in the stomach  
    9. Gastric lipase: An enzyme that breaks down lipids
    10. Chyme: The semi-liquid food slurry produced after mixing and partial digestion in the stomach
    11. Pyloric sphincter: Valve at the end of the stomach that controls the flow of food out of the stomach
    12. Duodenum: First part of the small intestine
    13. Jejunum: Second part of the small intestine
    14. Ileum: Third part of the small intestine
    15. Villi: tiny projections in the small intestine that increases surface area and nutrient absorption
    16. Bile: An emulsifier made in the liver and stored in the gallbladder. It is used to to aid in the digestion of lipids
    17. Gallbladder: Organ that stores bile from the liver
    18. Pancreas: Organ that produces several enzymes and secrete them into the small intestine during digestion. It also produces insulin and glucagon which regulates blood glucose
    19. Insulin: Hormone that controls blood glucose level. When glucose level is high in the blood, insulin is secreted which causes increased uptake of glucose by tissue cells   
    20. Glucagon: Hormone that controls blood glucose level. When glucose level is low in the blood, glucagon is secreted which causes the liver to convert stored glucose from glycogen
    21. Ghrelin: Hormone that causes us to feel hungry when our stomach is empty
    22. Colon: part of the large intestine consisting of the ascending colon, transverse colon, descending colon and the sigmoid colon
    23. Ileocecal sphincter: valve controlling the rate at which undigested matter enters the large intestine
    24. Rectum: Where waste is stored in the large intestine
    25. Anus: Where waste exit the large intestine

Reference: Blake S. B. (2017). Nutrition and You, 4th Edition. Hoboken, NJ: Pearson. 

Courtney Simons
Courtney Simons is a food science professor. He holds a BS degree in food science and a Ph.D. in cereal science from North Dakota State University.
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