Overview
This lecture covers the structure and function of the human digestive system, highlighting its major organs, processes, and key accessory components.
Unique Mammal Digestion Facts
- Platypuses are mammals that lay eggs and lack a true stomach with gastric glands.
- The absence of a stomach in platypuses does not mean they lack a digestive system.
Major Functions of the Digestive System
- The digestive system has four main tasks: ingestion, digestion, absorption, and elimination.
- Ingestion involves taking in food.
- Digestion is the breakdown of biomolecule polymers into building blocks by physical and chemical means.
- Absorption is the uptake of nutrients into the body for cellular use.
- Elimination is the removal of undigested waste.
Digestive System Anatomy & Processes
Mouth and Esophagus
- Digestion starts in the mouth with saliva containing enzymes like amylase for chemical digestion.
- Teeth mechanically grind food; saliva lubricates and neutralizes acids.
- Food forms a bolus and is swallowed, passing the epiglottis to avoid the trachea.
- Peristalsis (muscle contractions) moves food down the esophagus.
Stomach
- The stomach stores up to 2 liters of food and is highly acidic for chemical digestion.
- Gastric juices with HCl and enzymes like pepsin digest proteins.
- Mechanical churning occurs, forming a semi-liquid called chyme.
- Sphincters separate the stomach from the esophagus and intestines.
- Mucus and rapid cell division protect the stomach lining from self-digestion.
Small Intestine
- The small intestine (duodenum, jejunum, ileum) completes digestion of carbs, lipids, proteins, and nucleic acids.
- Digestive enzymes and juices from the liver, gallbladder, and pancreas are released here.
- Most nutrient absorption happens here via villi and microvilli that increase surface area.
- Absorbed nutrients enter the bloodstream through capillaries in villi.
Large Intestine and Elimination
- The large intestine mainly reabsorbs water and houses beneficial bacteria.
- Feces contain undigested materials and bacteria, stored in the rectum until expelled.
Accessory Organs and Hormones
- The liver produces bile for lipid breakdown and is involved in nutrient metabolism.
- The gallbladder stores bile.
- The pancreas provides digestive enzymes and neutralizes stomach acid.
Disorders and Additional Topics
- Common digestive disorders include celiac disease (small intestine), diverticulitis (large intestine), and heartburn (stomach acid in the esophagus).
- Hormones also play important roles in digestion.
Key Terms & Definitions
- Peristalsis — wave-like muscle contractions that move food through the digestive tract.
- Epiglottis — flap that prevents food from entering the windpipe during swallowing.
- Chyme — semi-liquid mixture of partially digested food from the stomach.
- Villi/Microvilli — small projections in the small intestine that increase surface area for nutrient absorption.
- Sphincter — ring-like muscle controlling passage between digestive organs.
Action Items / Next Steps
- Review provided science article links (if available) for more on platypus digestion evolution.
- Explore more about digestive hormones and disorders in video or class materials.