Bio 224 - Human Digestive system, March 17, 2025
Introduction to Accessory Digestive Organs
Accessory organs play a significant role in the digestive process.
The digestive process initiates in the oral cavity when food (bolus) enters.
Oral Cavity
Mechanical and Chemical Digestion
Mechanical Breakdown:
Involves chewing food to reduce particle size.
Increases surface area for more efficient enzymatic action.
Salivary Glands:
Types of Salivary Glands:
Parotid gland (located at the back of the upper jaw).
Sublingual gland (under the tongue).
Submandibular gland (at the back of the lower jaw).
Saliva Components:
Contains amylase for starch digestion.
Contains lipases for fat digestion (acting on triglycerides).
Salivary mucus moisten food, acting as a lubricant to aid swallowing.
Antimicrobial proteins help kill germs in food.
Pharynx and Esophagus
Transition from Oral Cavity to Stomach
The moist bolus moves through the pharynx, a shared pathway for food and air.
Swallowing Reflex:
An autonomic process that occurs involuntarily.
Epiglottis closes over the trachea to prevent food from entering the airway.
Pharyngeal sphincter (upper esophageal sphincter) relaxes to allow bolus entry into the esophagus.
Peristalsis:
Rhythmic contractions of smooth muscles propel the bolus down the esophagus.
Involves coordinating circular muscles (narrowing) and longitudinal muscles (lengthening) to push bolus in one direction.
Stomach
Mechanical and Chemical Digestion
Gastric Secretions:
Contains hydrochloric acid (pH 1.5 to 2) that denatures proteins and kills bacteria.
Secretes pepsinogen (precursor for pepsin) to digest proteins.
Gastric lipase starts fat digestion.
Intrinsic factor aids in Vitamin B12 absorption.
Chyme:
The semisolid food mixture formed in the stomach after mixing with gastric juices.
Protective Mechanism:
Mucus protects the stomach lining from acidic damage.
Gastric Pit Cells
Types of Cells in Gastric Pits:
Mucous Cells: Secrete mucus for protection.
Parietal Cells: Produce gastric acid.
Chief Cells: Secrete pepsinogen and gastric lipase.
Heartburn
Heartburn results from the backflow of stomach acid into the esophagus due to improper functioning of the lower esophageal sphincter.
Triggered by overeating or consumption of spicy/fatty foods.
Small Intestine
Chemical Digestion and Absorption
Connects to the stomach where chyme enters.
pH Shift:
Small intestine has an alkaline environment (due to bicarbonate secretion) necessary for enzyme function.
Segments of Small Intestine:
Duodenum (first 25 cm), Jejunum, Ileum.
Villi and Microvilli:
Increase surface area for nutrient absorption with finger-like projections (villi) and smaller projections (microvilli).
Nutrients (except lipids) are absorbed into blood capillaries; lipids are absorbed into lymphatic vessels.
Enterocytes
Fastest Turnover Cells:
Enterocytes in the intestinal epithelium regenerate every 3-5 days due to exposure to harsh digestive conditions.
Ensures efficient absorption of nutrients as older cells are regularly replaced.
Introduction to Accessory Digestive Organs
Accessory organs play a significant role in the digestive process.
The digestive process initiates in the oral cavity when food (bolus) enters.
Oral Cavity
Mechanical and Chemical Digestion
Mechanical Breakdown:
Involves chewing food to reduce particle size.
Increases surface area for more efficient enzymatic action.
Salivary Glands:
Types of Salivary Glands:
Parotid gland (located at the back of the upper jaw).
Sublingual gland (under the tongue).
Submandibular gland (at the back of the lower jaw).
Saliva Components:
Contains amylase for starch digestion.
Contains lipases for fat digestion (acting on triglycerides).
Salivary mucus moisten food, acting as a lubricant to aid swallowing.
Antimicrobial proteins help kill germs in food.
Pharynx and Esophagus
Transition from Oral Cavity to Stomach
The moist bolus moves through the pharynx, a shared pathway for food and air.
Swallowing Reflex:
An autonomic process that occurs involuntarily.
Epiglottis closes over the trachea to prevent food from entering the airway.
Pharyngeal sphincter (upper esophageal sphincter) relaxes to allow bolus entry into the esophagus.
Peristalsis:
Rhythmic contractions of smooth muscles propel the bolus down the esophagus.
Involves coordinating circular muscles (narrowing) and longitudinal muscles (lengthening) to push bolus in one direction.
Stomach
Mechanical and Chemical Digestion
Gastric Secretions:
Contains hydrochloric acid (pH 1.5 to 2) that denatures proteins and kills bacteria.
Secretes pepsinogen (precursor for pepsin) to digest proteins.
Gastric lipase starts fat digestion.
Intrinsic factor aids in Vitamin B12 absorption.
Chyme:
The semisolid food mixture formed in the stomach after mixing with gastric juices.
Protective Mechanism:
Mucus protects the stomach lining from acidic damage.
Gastric Pit Cells
Types of Cells in Gastric Pits:
Mucous Cells: Secrete mucus for protection.
Parietal Cells: Produce gastric acid.
Chief Cells: Secrete pepsinogen and gastric lipase.
Heartburn
Heartburn results from the backflow of stomach acid into the esophagus due to improper functioning of the lower esophageal sphincter.
Triggered by overeating or consumption of spicy/fatty foods.
Small Intestine
Chemical Digestion and Absorption
Connects to the stomach where chyme enters.
pH Shift:
Small intestine has an alkaline environment (due to bicarbonate secretion) necessary for enzyme function.
Segments of Small Intestine:
Duodenum (first 25 cm), Jejunum, Ileum.
Villi and Microvilli:
Increase surface area for nutrient absorption with finger-like projections (villi) and smaller projections (microvilli).
Nutrients (except lipids) are absorbed into blood capillaries; lipids are absorbed into lymphatic vessels.
Enterocytes
Fastest Turnover Cells:
Enterocytes in the intestinal epithelium regenerate every 3-5 days due to exposure to harsh digestive conditions.
Ensures efficient absorption of nutrients as older cells are regularly replaced.
Introduction to Accessory Digestive Organs
Accessory organs play a significant role in the digestive process.
The digestive process initiates in the oral cavity when food (bolus) enters.
Oral Cavity
Mechanical and Chemical Digestion
Mechanical Breakdown:
Involves chewing food to reduce particle size.
Increases surface area for more efficient enzymatic action.
Salivary Glands:
Types of Salivary Glands:
Parotid gland (located at the back of the upper jaw).
Sublingual gland (under the tongue).
Submandibular gland (at the back of the lower jaw).
Saliva Components:
Contains amylase for starch digestion.
Contains lipases for fat digestion (acting on triglycerides).
Salivary mucus moisten food, acting as a lubricant to aid swallowing.
Antimicrobial proteins help kill germs in food.
Pharynx and Esophagus
Transition from Oral Cavity to Stomach
The moist bolus moves through the pharynx, a shared pathway for food and air.
Swallowing Reflex:
An autonomic process that occurs involuntarily.
Epiglottis closes over the trachea to prevent food from entering the airway.
Pharyngeal sphincter (upper esophageal sphincter) relaxes to allow bolus entry into the esophagus.
Peristalsis:
Rhythmic contractions of smooth muscles propel the bolus down the esophagus.
Involves coordinating circular muscles (narrowing) and longitudinal muscles (lengthening) to push bolus in one direction.
Stomach
Mechanical and Chemical Digestion
Gastric Secretions:
Contains hydrochloric acid (pH 1.5 to 2) that denatures proteins and kills bacteria.
Secretes pepsinogen (precursor for pepsin) to digest proteins.
Gastric lipase starts fat digestion.
Intrinsic factor aids in Vitamin B12 absorption.
Chyme:
The semisolid food mixture formed in the stomach after mixing with gastric juices.
Protective Mechanism:
Mucus protects the stomach lining from acidic damage.
Gastric Pit Cells
Types of Cells in Gastric Pits:
Mucous Cells: Secrete mucus for protection.
Parietal Cells: Produce gastric acid.
Chief Cells: Secrete pepsinogen and gastric lipase.
Heartburn
Heartburn results from the backflow of stomach acid into the esophagus due to improper functioning of the lower esophageal sphincter.
Triggered by overeating or consumption of spicy/fatty foods.
Small Intestine
Chemical Digestion and Absorption
Connects to the stomach where chyme enters.
pH Shift:
Small intestine has an alkaline environment (due to bicarbonate secretion) necessary for enzyme function.
Segments of Small Intestine:
Duodenum (first 25 cm), Jejunum, Ileum.
Villi and Microvilli:
Increase surface area for nutrient absorption with finger-like projections (villi) and smaller projections (microvilli).
Nutrients (except lipids) are absorbed into blood capillaries; lipids are absorbed into lymphatic vessels.
Enterocytes
Fastest Turnover Cells:
Enterocytes in the intestinal epithelium regenerate every 3-5 days due to exposure to harsh digestive conditions.
Ensures efficient absorption of nutrients as older cells are regularly replaced.