Digestion and Absorption Flashcards

Digestion and Absorption Part 1

Learning Objectives

• Overview of how food moves through the digestive system.

• Basic anatomy of the digestive system and accessory organs.

• Muscles of the GI tract.

• Introduction to some GI tract secretions (more in part 2).

GI Tract Overview

• The GI tract is the "tube" that food travels through. Starting at the mouth and ending at the rectum/anus.

• The purpose is to break down food for absorption.

• Major parts:

  • Mouth: Mechanical digestion (chewing) to break food into smaller pieces. Saliva for lubrication, enzymes, and lysozyme to kill microbes.

  • Esophagus: A tube connecting the mouth and stomach. Primarily for transport.

  • Stomach

  • Small Intestine

  • Large Intestine

  • Rectum and Anus

Small Intestine

• Three parts:

  • Duodenum

  • Jejunum

  • Ileum

• Most digestion and absorption occur here.

Colon (Large Intestine)

• Parts:

  • Ascending colon

  • Transverse colon

  • Descending colon

  • Sigmoid colon (S-shaped)

  • Rectum and anus

Accessory Organs

• Liver: Produces bile.

• Gallbladder: Stores bile.

• Pancreas: Adds secretions into the GI tract to aid in digestion.

GI Tract as a Tube

• The stomach is the largest part in terms of volume to serve as a reservoir since humans tend to eat meals at discrete times rather than grazing throughout the day.

• Food is slowly released into the small intestine for digestion and absorption.

• Typically, hunger returns about 3-4 hours after eating.

Muscle and Tissue Layers of the GI Tract

• Lumen: The inside opening of the tube.

• Mucosa: The inner lining of the GI tract.

• Submucosa: Layer beneath the mucosa.

• Muscle layers: Surround the tube for muscular contractions.

• Blood Vessels: Extensive blood supply to the GI tract.

Muscular Contractions

• Segmentation: Contractions that squish food into smaller pieces (mechanical digestion) increasing surface area.

• Peristalsis: Wave-like contractions that move food in one direction towards the anus.

• Unidirectional movement is important for efficient digestion and prevents bacterial overgrowth.

Valves (Sphincters)

• Muscular areas that contract to control the flow of contents.

• Lower Esophageal Sphincter: Between the esophagus and stomach; prevents acidic stomach contents from going back up into the esophagus (gastric reflux/heartburn).

• Pyloric Sphincter (Pylorus): Between the stomach and small intestine; controls the flow of contents from the stomach into the small intestine.

• Hepato-pancreatic Sphincter: Where bile and pancreatic juice leave the ducts into the small intestine. (Less detail).

• Ileocecal Valve: Between the ileum (end of the small intestine) and the large intestine.

Importance of Sphincters

• Lower Esophageal Sphincter: Prevents heartburn and damage to the esophagus from stomach acid.

• Pyloric Sphincter: Prevents dumping syndrome, ensures controlled release of stomach contents into the small intestine. Allows time for the stomach to mix, churn, and kill microbes with hydrochloric acid.

• Ileocecal Valve: Prevents microbes from the colon from entering the small intestine, which could cause small intestinal bacterial overgrowth (SIBO). Also helps keep food moving in one direction.

• Anal Sphincters: control when you go to the bathroom.

Moving Through the GI Tract

• The Mouth

• Epiglottis: Opens and closes depending on breathing or swallowing.

  • Open when breathing, allowing air into the trachea.

  • Closes when swallowing, directing food into the esophagus.

  • If food enters the trachea, it can lead to infection and pneumonia.

Dysphagia

• Difficulty swallowing potentially caused by stroke where it's harder for those folks to coordinate the muscles movement in their throat to be able to swallow.

X-Ray Swallowing Study

• A liquid with radio-opaque dye is swallowed while being X-rayed.

• Aspiration: When liquid enters the trachea instead of the esophagus, which is dangerous.

Tube Feeding Considerations

• A tube going into the GI tract can interfere with the epiglottis.

• Elevate the head of the bed to reduce the risk of tube feeding contents going into the trachea, especially when the patient is not fully awake.

Stomach

• Muscles contract to mix and churn food.

• Hydrochloric acid (low pH) kills microbes and aids in protein digestion.

• Pylorus rhythmically opens to allow small amounts of stomach contents into the duodenum.

Small Intestine details

• Duodenum, jejunum, and ileum.

• Ileocecal valve prevents contents from moving back from the colon into the small intestine.

• Most digestion and absorption occur in the small intestine.

Bile, Enzymes, and Bicarbonate

• Bile from the gallbladder, enzymes, and bicarbonate from the pancreas enter the small intestine through ducts.

• Bicarbonate neutralizes the acidic contents from the stomach to protect the small intestine.

Small Intestine Lining Specialization

• Villi: Finger-like projections that increase surface area.

• Each villus contains a blood supply (red and blue) and a lacteal (lymph system, green).

• Enterocytes: Cells of the small intestine.

• Microvilli (Brush Border): Tiny projections on top of enterocytes, further increasing surface area.

  • Total surface area of the small intestine, when stretched out, is about the size of a tennis court.

  • Brush border secretes enzymes for final digestion of macronutrients.

Colon (Large Intestine) function

• Ascending, transverse, descending, sigmoid, rectum, and anus.

• The colon does not have as much digestion/absorption as the small intestine.

• Contains a lot of microbes that are important for human health.

• Microbes:

  • Feed on fiber.

  • Produce vitamin K and biotin.

  • Ferment fiber.

  • Colonocytes (cells of the colon) can benefit from the leftover materials after fermentation.

Accessory Organs details

• Liver: Produces bile, an emulsifier for fat.

• Gallbladder: Stores and concentrates bile, releasing it into the duodenum when needed.

• Pancreas: Releases digestive enzymes for macronutrients and bicarbonate to neutralize stomach acid.

GI Tract blood supply

• Vast blood supply is crucial.

• Infarcted bowel: Obstruction of blood flow can cause tissue damage or death. Life-threatening.

• The blood supply provides oxygen and nutrients.

• Enterocytes absorb nutrients, passing the rest into the bloodstream for the body to use.

• Nutrients stay outside the body until broken down and absorbed.

Nerve Innervation

• Nerves are essential for muscle function in the GI tract.

• Gastroparesis: An example of nerve damage affecting GI function.

  • Can be caused by diabetes, which damages nerves.

  • Leads to delayed gastric emptying.

  • Results in:

    • Decreased appetite.

    • Weight loss.

    • Malnutrition.

    • Abdominal pain.

Gastroparesis Mechanism

• Normally, the stomach mixes and turns food, then slowly releases it into the duodenum.

• Nerves stimulate stomach muscles and affect the pylorus.

• In gastroparesis, nerve damage impairs stomach muscle function and pyloric opening.

The GI tract includes the mouth, esophagus, stomach, small intestine (duodenum, jejunum, ileum), large intestine (colon), rectum, and anus. Accessory organs include the liver, gallbladder, and pancreas.

• Mouth: Mechanical digestion (chewing) breaks food into smaller pieces. Saliva lubricates and contains enzymes, like amylase which starts the breakdown of carbohydrates, and lysozyme to kill microbes.

• Esophagus: Primarily for transport of food to the stomach. No significant digestion or absorption occurs.

• Stomach: Muscles contract to mix and churn food. Hydrochloric acid (low pH) kills microbes and aids in protein digestion by activating pepsin. The pylorus rhythmically opens to allow small amounts of stomach contents into the duodenum.

• Small Intestine: Most digestion and absorption occur here.

  • Duodenum: Bile from the gallbladder, enzymes, and bicarbonate from the pancreas enter to aid digestion. Bile emulsifies fats. Pancreatic enzymes break down carbohydrates, proteins, and fats. Bicarbonate neutralizes the acidic contents from the stomach to protect the small intestine.

  • Jejunum: Continued digestion and absorption of nutrients.

  • Ileum: Absorption of nutrients (e.g., B12) and bile salts. The ileocecal valve prevents contents from moving back from the colon into the small intestine.

• Colon (Large Intestine): The colon does not have as much digestion/absorption as the small intestine. Contains a lot of microbes that are important for human health. Microbes feed on fiber, produce vitamin K and biotin, and ferment fiber. Colonocytes (cells of the colon) can benefit from the leftover materials after fermentation.

• Accessory Organs Impact:

  • Liver: Produces bile, an emulsifier for fat.

  • Gallbladder: Stores and concentrates bile, releasing it into the duodenum when needed.

  • Pancreas: Releases digestive enzymes for macronutrients and bicarbonate to neutralize stomach acid.

Nutrient Absorption Location:

• Carbohydrates: Primarily absorbed in the small intestine (duodenum and jejunum) as glucose, fructose, and galactose.

• Proteins: Digested into amino acids and absorbed in the small intestine (duodenum and jejunum).

• Fats: Emulsified by bile and digested by pancreatic lipase into fatty acids and monoglycerides, absorbed in the small intestine.

• Vitamins and Minerals: Absorbed in various parts of the small and large intestines, depending on the specific nutrient (e.g., B12 in the ileum).

• Water: Absorbed throughout the small and large intestines.