Module 3: Digestion and Absorption

Overview of Gastrointestinal (GI) tract

Digestion occurs in the gastrointestinal (GI) tract

Consists of:

• series of organs

• outer layers of muscles

• inner mucosal layer of glands and absorptive cells

Components of GI Tract

Primary Organs

• Mouth

• Esophagus

• Stomach

• Small Intestine

• Large Intestine

• Rectum

Accessory Organs

• Salivary glands

• Liver

• Pancreas

• Gallbladder

Functions of GI tract

Digestion

• Move food through the GI tract

• Break down food into smaller compounds

• Primary site is in small intestine

Absorption

• Cross lining of intestine to enter body - moves nutrients from GI tract to circulatory system

The period of time it takes food to travel the length of the digestion tract is transit time

Digestion involves mechanical and chemical processes

• Mechanical

  • chewing, grinding, mixing and peristalsis

    • peristalsis- wavelike muscular movements

  • movement of food through GI tract

• Chemical

  • secretions and enzymes

    • breakdown of large compounds into smaller units for absorption

  • Undigested and unabsorbed substances eliminated by defacation

Segment of digestive processes

• Sphincters

  • circular muscles that separate organs of GI tract

    • “one-way doors”

      • prevent food from flowing backwards

• 3 main sphincters

  • Lower esophageal sphincter (LES)

  • Pyloric sphincter

  • Ileocecal valave

• Complications associated with improper function

  • Heartburn and vomiting

Why must digestion occur before absorption?

• Complex macronutrients must be broken down to smaller units to be absorbed

  • Carbohydrates

  • Proteins

  • Fats

• Some food components do not require digestion in order to be absorbed

  • Water

  • Minerals

  • Vitamins

  • Small molecules (ex. glucose)

The nose

• Flavor perception is 80% smell

• Five primary tastes:

  • Sweet

  • Salty

  • Sour

  • Bitter

  • Umami

• Strong connection between brain, digestive system, and smell

Mouth and esophagus

• Digestive process begins in mouth

  • Mechanical Digestion:

    • Chewing

    • Mixing

    • Swallowing

  • Chemical Digestion

    • Saliva is added to help lubricate food

    • Enzymes are released

      • Salivary amylase

    • Bolus formed (moves through esophagus)

The Stomach (A j-shaped organ)

• Mechanical Digestion

  • Mixes food with gastric secretions

  • Liquifies food into chyme

  • 1-4 Liter capacity

• Chemical Digestion

  • Entrance of food stimulates production of gastric juices

    • Hydrochloric acid

    • Mucus

    • Pepsinogen and hormone gastrin

Functions and Importance

Anatomy of the Stomach

Food leaves the stomach and enters the small intestine

• Pyloric sphincter

  • separates stomach from small intestine

• 3 mL of chyme squirted at intervals into small intestine

  • facilitates neutralization of stomach acid

The small intestine is the primary site of digestion and absorption

• 3 sections

  • Duodenum

  • Jejunum

    • where most digestion and absorption occurs

  • Ileum

• Nutrient absorption assisted by

  • Villi

  • Microvilli

    • extend from the villi into the lumen (interior) of the small intestine

    • secrete digestive enzymes

The villi of the small intestine

Accessory organs facilitate digestion in small intestine

• Pancreas

  • Secretes alkaline bicarbonate

    • neutralizes acidic chyme from stomach

  • Liver

    • Produces bile

  • Gallbladder

    • Stores and releases bile

      • assists in digestion of fats

Water soluble and fat soluble nutrients are absorbed differently

• Water-soluble nutrients absorbed through the intestinal wall

  • Capillaries

    • carry nutrients to the liver

  • Fat and fat-soluble substances are absorbed into

    • Lacteals

    • Then travel through lymph system

Almost all nutrients absorbed in small intestine

• Duodenum and upper part of jejunum

  • Most minerals, except for the electrolytes

• Jejunum and upper ileum

  • carbs., amino acids, and water-soluble vitamins

• Terminal segment of ileum

  • Lipids and fat-soluble vitamins

  • Vitamin B12

• Digestive contents at terminus of ileum

  • some water, plant fibers, indigestible items, and bacteria

Undigested food components move to large intestine

• Ileocecal valve

  • Separates ileum from cecum of large intestine

  • Prevents reflux of waste into small intestine

• Large intestine

  • 5’ long

  • ~12-24 hour transit time

• Functions: propulsion, absorption of water and sodium, preparation of waste for defecation

Microbiota maintain health and function of large intestine

• Large intestine contains ~100 trillion bacteria

  • Produce vitamin K and some B vitamins

  • Ferment (break down) plant fibers to short chain fatty acids

  • Metabolize any remaining nutrients

Digestion and Absorption of Carbohydrates

Digestion and Absorption of Fat

Digestion and Absorption of Protein

Digestion and absorption of other food components

• Vitamins:

  • No digestion needed

  • Fat soluble vitamin absorption facilitated by dietary fat

    • Transported in chylomicron

  • Water soluble vitamins absorbed into the blood

    • Vitamin B12:

      • Required intrinsic factor for absorption

      • Absorbed in distal ileum

• Alcohol:

  • Some alcohol is absorbed in stomach

  • Most is absorbed in the small intestine

  • Travels to the liver for detoxification

Digestion and absorption of other food components - 2

• Minerals:

  • Absorbed all along the small intestine

  • Sodium is best absorbed with glucose and amino acids

  • Some nutrients are absorbed based on the body’s need

    • Iron and calcium

• Water:

  • Most is absorbed in small intestine

    • Small amount in large intestine

  • Absorbed by following sodium and other nutrients across intestinal wall

Nutrient digestion and absorption in the GI tract

Conditions of the GI Tract

• Erosion that occurs in lining of stomach or upper part of small intestine

  • ulcer or peptic ulcer

  • Primary cause:

    • Heliobacter pylori or H. pylori

      • oral-oral and fecal-oral exposure

  • Other causes of ulcers:

    • regular use of aspirin

    • excessive use of alcohol

    • smoking

H. pylori crossing mucus layer of stomach

Table 6.3 Diseases and Syndromes of the Digestive Tract

Table 6.3 Diseases and Syndromes of the Digestive Tract - 2

Bristol stool chart

Food Trends: Maintaining a Healthy Digestive Tract