Overview
This lecture explains the journey of food through the digestive system, focusing on the mechanical and chemical processes of digestion up to the stomach.
Functions of the Digestive System
- Digestion is the breakdown of food into molecules.
- Absorption is the uptake of nutrients into the bloodstream.
- Waste products that are not absorbed are eliminated as feces.
Mechanical Digestion in the Mouth
- The tongue, a muscular hydrostat, moves food for effective chewing.
- Front teeth (incisors, canines) tear food with concentrated force.
- Back teeth (molars) grind food using greater leverage.
- Chewing increases the surface area of food, aiding enzyme action.
Chemical Digestion in the Mouth
- Salivary enzymes begin chemical digestion (amylase for starch, lipase for fat).
- Lysozymes in saliva help kill bacteria.
Swallowing and Passage to the Stomach
- The uvula closes nasal passages; the epiglottis closes the trachea during swallowing.
- These closures prevent food from entering the nasal cavity or airway.
- The esophagus moves food to the stomach by peristalsis (waves of muscle contraction).
- Esophageal and other sphincters prevent reflux and separate GI tract regions.
The Stomach: Mechanical & Chemical Digestion
- The stomach has three muscle layers that churn and grind food.
- Hydrochloric acid creates a low pH, killing bacteria and aiding digestion.
- Chief cells secrete pepsinogen (inactive protease), activated to pepsin in acid to digest proteins.
- Parietal cells secrete hydrochloric acid.
- Mucous neck cells secrete mucus that protects the stomach lining from acid and enzymes.
Protection of the Stomach Lining
- Thick, buffered mucus protects epithelial cells from stomach acid.
- Acid and enzymes travel through mucus channels, preventing direct contact with the stomach wall.
Key Terms & Definitions
- Digestion — breaking food into molecules.
- Absorption — transport of nutrients into the bloodstream.
- Tongue (muscular hydrostat) — muscle organ that manipulates food.
- Peristalsis — wave-like muscle movements that move food through the GI tract.
- Sphincter — muscular ring sealing GI tract sections.
- Hydrochloric acid — highly acidic stomach secretion aiding digestion.
- Parietal cells — stomach cells producing hydrochloric acid.
- Chief cells — stomach cells secreting pepsinogen.
- Pepsinogen/Pepsin — enzyme precursor activated by acid to digest proteins.
- Mucous neck cells — cells producing protective mucus in the stomach.
Action Items / Next Steps
- Prepare for upcoming lecture on nutrient absorption in the small intestine.