Chapter_2

Learning Objectives

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

  • Describe actions and origins of digestive secretions.

  • Understand the anatomical details of the GI tract.

  • Explain features and activities of intestinal cells that facilitate nutrient absorption.

  • Detail the process of nutrient delivery from the GI tract to body cells via the vascular system, including types of lipoproteins.

  • Discuss how hormones, nerves, and bacteria influence GI tract health and activities.

  • Explain foodborne illnesses in humans and methods for ensuring food safety.

Anatomy of the Digestive Tract

  • Selective Functionality: The digestive tract selectively absorbs materials of nutritive value while excreting non-nutritive materials.

  • Gastrointestinal Tract: A flexible, muscular tube extending from mouth to anus.

  • Nutrient absorption occurs through the digestive tract cells.

Gastrointestinal Tract Organs

  • Mouth: Chews and mixes food with saliva.

  • Pharynx: Directs food to the esophagus.

  • Esophagus: Passes food to the stomach.

  • Stomach: Churns food and mixes it with gastric juices.

  • Small Intestine: Secretes enzymes and absorbs nutrients into blood and lymph.

  • Large Intestine (Colon): Absorbs water, minerals, and passes waste.

  • Rectum: Stores waste.

  • Anus: Opens to allow elimination.

Digestive Process Overview

  • Mouth: Digestion begins with chewing and saliva; bolus formation.

  • Esophagus: Controlled via sphincter muscles, transporting bolus to the stomach.

  • Stomach: Releases gastric juices to form chyme; gastric lipase digests fats.

  • Small Intestine: Further digestion with pancreatic juices and bile. Nutrient absorption primarily occurs here.

  • Large Intestine: Absorption of water; fiber passes through undigested, aiding digestion.

Digestive Secretions and Functions

  • Salivary Glands: Secrete saliva, containing amylase for starch digestion.

  • Stomach Gastric Juices: Composed mainly of water, enzymes, and hydrochloric acid; protects and breaks down proteins.

  • Pancreas and Liver: Add enzymes and bile to assist in fat digestion and neutralize chyme.

Involuntary Muscles and Glands

  • Gastrointestinal Motility: Includes peristalsis (muscular contractions) and segmentation (mixing contents).

  • These contractions help liquefy and mix contents, aiding in digestion.

Absorptive System: The Small Intestine

  • Villi and Microvilli: Increase surface area for nutrient absorption.

  • Specific cells in GI tract segments absorb nutrients differently; nutrient travel is adjusted for efficiency.

Transport of Nutrients

  • Vascular System: Blood transports nutrients from the digestive system to the body, with key pathways through the hepatic portal vein to the liver.

  • Lymphatic System: Conveys fat-soluble nutrients to the bloodstream via the heart.

Lipoproteins

  • VLDL: Transports triglycerides; created by the liver.

  • LDL: Carries cholesterol and triglycerides.

  • HDL: Transports excess cholesterol back to the liver.

Health Implications of Lipoproteins

  • High LDL and low HDL are associated with increased risk of heart disease.

  • Lifestyle choices like diet and exercise can influence lipoprotein levels.

Food Safety

  • Foodborne illness is a leading concern, particularly for vulnerable populations.

  • Proper food handling and safety practices are essential to prevent illness:

    • Recognize symptoms and seek medical help if necessary.

    • Understand differences between foodborne infection and intoxication.

Safe Food Handling

  • Employ practices such as proper cooking temperatures, clean preparation areas, and quick refrigeration of leftovers.

Conclusion

  • Understanding digestion, absorption, and safety can enhance nutritional health and prevent foodborne illnesses.