Overview
This lecture provides an overview of the human digestive system, its major organs and functions, and contrasts it with unique features in some mammals like the platypus.
Platypus and the Stomach
- Platypuses are mammals that lay eggs and lack a true stomach with gastric glands.
- Unlike many mammals, platypuses still have a digestive system despite lacking a typical stomach.
Major Functions of the Human Digestive System
- The digestive system accomplishes ingestion (eating), digestion (breaking down food), absorption (taking in nutrients), and elimination (removing waste).
Digestive Tract Overview
- Food enters through the mouth, where mechanical (teeth) and chemical (saliva enzymes) digestion begins.
- Saliva contains enzymes like amylase (breaks down carbs) and buffers that protect tooth health.
- The epiglottis prevents food from entering the windpipe during swallowing.
- Peristalsis moves food down the esophagus to the stomach.
Stomach and Small Intestine
- The stomach stores food, uses gastric juice (with HCl and pepsin) for chemical digestion, and churns food mechanically.
- Protective mucus and fast cell turnover prevent the stomach from digesting itself.
- Chyme leaves the stomach and enters the small intestine (duodenum, jejunum, ileum), where most chemical digestion and absorption occur.
- Enzymes and juices from the pancreas, liver, and gallbladder aid digestion in the small intestine.
Nutrient Absorption
- Most nutrients (e.g., sugars, amino acids, vitamins, lipids) are absorbed in the small intestine via villi and microvilli, which increase surface area.
- Villi are rich in capillaries to facilitate nutrient transport to the bloodstream.
Large Intestine and Elimination
- The large intestine (colon) reabsorbs water and houses beneficial bacteria, some of which produce vitamins.
- Undigested material forms feces, which are stored in the rectum and expelled via the anus.
Accessory Organs and Regulation
- The liver produces bile for lipid digestion and is involved in metabolism.
- The gallbladder stores bile.
- The pancreas releases digestive enzymes and neutralizes stomach acid.
- Various hormones regulate digestive processes.
Digestive System Disorders
- Celiac disease (small intestine), diverticulitis (large intestine), and heartburn (esophagus/stomach junction) are common digestive disorders.
Key Terms & Definitions
- Ingestion — taking in food.
- Digestion — breaking down food into smaller components.
- Absorption — uptake of nutrients into the bloodstream.
- Elimination — removal of undigested waste.
- Peristalsis — wave-like muscle contractions moving food.
- Chyme — semifluid mass of partly digested food from the stomach.
- Villi/Microvilli — small finger-like projections in the small intestine that increase surface area for absorption.
- Accessory Organs — organs like liver, pancreas, gallbladder aiding digestion.
Action Items / Next Steps
- Review linked articles and videos for more on the platypus and digestive system disorders.
- Memorize the four main digestive functions and key digestive tract structures.