Ch 2

DIGESTION & ABSORPTION

LEARNING OUTCOMES

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

• 2.1 Describe the path that food takes during digestion and the muscular actions of digestion.

• 2.2 Describe the actions and origins of the digestive secretions.

• 2.3 Describe the anatomical details of the GI tract and the features and activities of intestinal cells that facilitate nutrient absorption.

• 2.4 Describe the process of nutrient delivery from the GI tract to body cells by the vascular system and the different types of lipoproteins.

• 2.5 Describe how hormones, nerves, and bacteria influence the health and activities of the GI tract.

• 2.6 Explain the causes and effects of foodborne illnesses in humans and the methods of ensuring food safety.

OVERVIEW OF DIGESTION & ABSORPTION

• Humans are omnivorous, meaning they can digest and absorb nutrients from a variety of sources:

  • Plants

  • Animals

  • Fungi

  • Bacteria

• Digestion: Process by which food components are broken down into basic units.

  • Renders all foods into basic units of:

  • Carbohydrates

  • Fats

  • Proteins

• Absorption: Process by which substances are taken up from the digestive tract into circulation.

THE PROCESS OF DIGESTION

SELECTIVITY OF THE DIGESTIVE TRACT

• The GI tract does not absorb everything consumed; it is designed to:

  • Break down materials of nutritive value (like carbohydrates, proteins, and fats) into smaller components.

  • Absorb these nutrients through the intestinal wall into the bloodstream or lymph for use by the body.

• The digestive system simultaneously:

  • Rejects or ignores most nonnutritive materials (e.g., fiber, toxins, indigestible compounds).

  • Eliminates nonnutritive materials from the body as feces, along with other waste products.

ENERGY-YIELDING NUTRIENTS

• Carbohydrates, fats, and proteins must be broken down into their basic building blocks before the body can absorb them:

  • Carbohydrates → Simple sugars (e.g., glucose)

  • Proteins → Amino acids

  • Fats → Fatty acids and glycerol

Rate of Digestion Depends on Meal Composition

• High in simple sugars (e.g., white bread, juice) → Fast digestion

• High in fat (e.g., fried food, cheese) → Slower digestion, as fats delay stomach emptying

• Mixed meals (with fats, proteins, and fiber) take longer to digest than meals made of simple carbohydrates alone.

OTHER NUTRIENTS

• Vitamins, minerals, and water:

  • Generally do not require breakdown and are absorbed early in the small intestine.

• Undigested Residues:

  • Dietary fiber and other indigestible substances are not broken down or absorbed but move through the colon and help form the semisolid mass (feces).

  • Some fibers are fermented by gut bacteria, producing beneficial compounds.

EXCRETION

• The final waste (feces) contains:

  • Indigestible substances

  • Bacteria

  • Dead cells

  • Very little nutritive value; most useful components have been absorbed by this point.

TWO TYPES OF DIGESTION

1. Mechanical Digestion

• Definition: Physical breakdown of food into smaller pieces.

• Purpose: Increases surface area for enzymes to act more effectively during chemical digestion.

• Occurs in:

  • Mouth: Chewing (mastication)

  • Stomach: Churning and mixing by muscular contractions

  • Small intestine: Segmentation movements mix contents with digestive juices

2. Chemical Digestion

• Definition: Breakdown of food molecules into their chemical building blocks by digestive enzymes and acids.

• Purpose: Converts complex molecules into absorbable units.

• Involves:

  • Saliva: Amylase breaks down starch

  • Stomach: Acid and enzymes break down proteins

  • Pancreas & intestines: Enzymes break down carbohydrates, proteins, and fats

ANATOMY OF THE DIGESTIVE TRACT

• Major organs:

  • Mouth

  • Esophagus

  • Stomach

  • Small intestine

  • Large intestine

• Accessory organs:

  • Teeth

  • Tongue

  • Salivary glands

  • Liver

  • Gall bladder

  • Pancreas

• Definition of Digestive System: All the organs and glands associated with the ingestion and digestion of food.

THE GASTROINTESTINAL TRACT

• Description: The digestive tract is a flexible, muscular tube that runs from the mouth to the anus, referred to as the gastrointestinal (GI) tract.

• Nutrients enter the body through the cells of the digestive tract.

• Principal organs: stomach and intestines.

• Note: Only when a nutrient or other substance passes through the cells of the digestive tract wall does it actually enter the body.

DIGESTIVE JUICES

• Five different organs secrete digestive juices:

  • Salivary glands

  • Stomach

  • Small intestine

  • Liver (via gallbladder)

  • Pancreas

SECRETION DETAILS

1. Salivary Glands

   • Digestive Juice: Saliva

   • Key Components: Water, mucus, amylase (enzyme)

   • Main Function: Begins carbohydrate digestion in the mouth

2. Stomach

   • Digestive Juice: Gastric Juice

   • Key Components: Hydrochloric acid (HCl), pepsin (enzyme), mucus

   • Main Function: Starts breakdown of proteins and some fats

3. Pancreas

   • Digestive Juice: Pancreatic Juice

   • Key Components: Bicarbonate, amylase, lipase, proteases

   • Main Function: Neutralizes stomach acid; digests carbohydrates, fats, proteins

4. Liver (via gallbladder)

   • Digestive Juice: Bile

   • Key Components: Bile salts, cholesterol

   • Main Function: Emulsifies fats to aid in fat digestion (not an enzyme)

5. Small Intestine

   • Digestive Juice: Intestinal Juice

   • Key Components: Bile salts, enzymes (e.g., peptidases, disaccharidases), mucus

   • Main Function: Completes digestion of proteins and carbohydrates

FOOD TRAVEL PATH

• Food travels through the digestive tract in this order:

  • Mouth

  • Esophagus

  • Lower Esophageal Sphincter

  • Stomach

  • Pyloric Sphincter

  • Duodenum

  • Jejunum

  • Ileum

  • Ileocecal Valve

  • Large Intestine (Colon)

  • Rectum

  • Anus

THE MOUTH

• Beginning of Digestion:

  1. Teeth and Chewing (Mechanical Digestion)

  • Function: Break food into smaller pieces.

  • Purpose: Increases the surface area for enzymes to act on food.

  1. Salivary Glands

  • Type: Exocrine glands

  • Function: Secrete saliva into the mouth through ducts.

  1. Saliva

  • Description: A watery fluid that moistens food to assist with swallowing.

  • Begins chemical digestion of carbohydrates.

  • Contains:

    • Water

    • Salts

    • Enzymes

      • Principal Enzyme: Salivary amylase - begins breaking down amylose (a type of starch) into smaller sugar molecules.

  1. Bolus

  • Definition: The soft, moist mass of food formed in the mouth after chewing and mixing with saliva, which is swallowed and passed down the esophagus.

THE TONGUE

• Five Basic Tastes:

  • Sweet: Signals energy-rich nutrients (e.g., sugar)

  • Sour: Detects acidity (e.g., citrus fruits)

  • Salty: Detects essential electrolytes (e.g., sodium)

  • Bitter: Often a warning signal for toxins

  • Umami: A savory taste from glutamate (e.g., meats, broth, soy sauce)

• Regions of the Tongue:

  • Most Sensitive To:

  • Tip of tongue - Sweet

  • Sides of tongue - Sour and salty

  • Back of tongue - Bitter

• Note: Modern science shows all parts of the tongue can detect all tastes, but these are traditional zones.

SMELL ENHANCES TASTE

• Description: Olfactory receptors in the nose detect airborne molecules from food.

• Observation: Much of what we perceive as "flavor" actually comes from smell.

  • This is why food tastes bland when you have a stuffy nose.

THE ESOPHAGUS

• Function of the Esophagus:

  • Transfers a mass of swallowed food into the stomach.

  • Entrance to the esophagus is near the larynx and the opening of the trachea.

EPIGLOTTIS FUNCTIONING

1. Swallowing: Stimulates peristalsis.

   • Definition: Peristalsis is a series of involuntary, wave-like muscle contractions that move food through the digestive tract.

     • In the esophagus, peristalsis:

     • Begins after swallowing.

     • Pushes the bolus down the esophagus toward the stomach.

     • Continues throughout the entire GI tract, moving contents forward.

2. When you swallow, the epiglottis closes over the trachea to prevent choking.

3. After swallowing, the epiglottis returns to its usual position, allowing you to talk and breathe again.

THE STOMACH

LOWER GASTROESOPHAGEAL SPHINCTER (LES)

• Description: A muscular ring at the junction of the esophagus and stomach.

• Function:

  • Opens to let food enter the stomach.

  • Closes tightly to prevent stomach acid and food from flowing back (reflux).

STOMACH FUNCTIONS

• Churning & Mixing:

  • Mechanical digestion: The stomach churns food using muscular contractions.

  • Chemical digestion: Food is mixed with gastric juice, starting the breakdown of:

  • Proteins

  • Some fats

• Gastric Glands and Gastric Juice:

  • Gastric glands are exocrine glands located in the stomach lining.

  • Secretion Includes:

  • Water

  • Hydrochloric acid (HCl) - lowers pH, denatures proteins, kills bacteria

  • Enzymes like pepsin - begins protein digestion

• Chyme:

  • Definition: A semi-liquid mixture of partially digested food and gastric juices.

  • Leaves the stomach and enters the small intestine in small amounts.

• Absorption in the Stomach: Very few nutrients are absorbed here, with exceptions including:

  • Some alcohol

  • Certain drugs (e.g. aspirin)

PYLORIC SPHINCTER

• Location: Between the stomach and small intestine.

• Function: Prevents intestinal fluid from refluxing into the stomach.

  • Takes typically 2-4 hours for food to pass.

  • Chyme is released bit by bit into the small intestine.

REGULATION OF DIGESTION & ABSORPTION

SYSTEMS INVOLVED

• Nervous System:

  • Function: Coordinates movement and secretions throughout the digestive tract.

• Hormonal (Endocrine) System:

  • Function: Regulates digestive secretions and the rate of digestion through hormones released into the bloodstream.

NERVOUS SYSTEM COMPONENTS

• Enteric Nervous System (ENS):

  • A network of nerves within the GI tract that controls:

  • Peristalsis (muscle contractions)

  • Secretion of digestive juices

• Autonomic Nervous System (ANS):

  • **Parasympathetic (