Digestive System, Part II & Metabolism: Physical and Chemical Mechanisms, Hormonal Regulation, and Enzymatic Pathways

Digestive System, Part II & Metabolism

Lesson 1 – Physical Mechanisms of Digestion

The digestive system utilizes both physical and chemical processes to break down food into absorbable nutrients. Physical mechanisms involve the movement and mixing of food through various organs.

• Mastication (chewing): Digestive Activity: Digests (D) Organs: Tongue, Teeth Definition: The mechanical breakdown of food into smaller pieces to increase surface area for enzymes.

• Deglutition (swallowing): Digestive Activity: Propels (P) Organs: Mouth, Pharynx, Esophagus Definition: The process of moving food from the mouth to the stomach via the esophagus.

• Peristalsis: Digestive Activity: Contract → move (P) Organs: Esophagus, Stomach, SI Definition: Rhythmic contractions of smooth muscle that propel food through the digestive tract.

• Gastric Churning: Digestive Activity: Contract → mix (D, P) Organs: Stomach (D); Propels to duodenum (P) Definition: Mixing of stomach contents to form chyme and periodic propulsion through the pyloric sphincter.

• Segmentation: Digestive Activity: Contract → mix (D) Organs: Small Intestine (SI) Definition: Localized contractions that mix chyme and increase contact with digestive enzymes.

• Colon Churning: Digestive Activity: Contract → move (P) Organs: Large Intestine (LI) Definition: Movement of contents through the colon for water absorption and feces formation.

Additional info: The pyloric sphincter regulates the passage of chyme from the stomach to the duodenum, typically allowing only small amounts to pass at a time.

Lesson 2 – Hormonal Regulation of Digestion

Digestion is regulated by several hormones that coordinate the activity of digestive organs in response to the presence of food and its breakdown products.

Stimulus	Production Site	Hormone	Target Site	Action
Stomach distension	Stomach (G-cells)	Gastrin	Cardiac sphincter, Pyloric sphincter, Ileocecal valve, Stomach (mucosa)	Closes cardiac sphincter, opens pyloric sphincter and ileocecal valve, increases gastric juice secretion and stomach motility
Partially digested proteins/fats in SI	Duodenal mucosa	CCK (Cholecystokinin)	Acini cells (pancreas), Gallbladder, Sphincter of Oddi	Stimulates digestive enzyme secretion, gallbladder contraction (bile release), opens sphincter of Oddi
Acidic chyme in SI	Duodenal mucosa	Secretin	Hepatocytes, Pancreatic duct, Stomach	Stimulates bile secretion, bicarbonate secretion, decreases stomach secretion and motility

Insulin and Glucagon also play roles in metabolism and blood glucose regulation.

Lesson 3 – Digestive Chemicals /Aids

Chemical digestion involves enzymes and other chemicals that break down food into absorbable molecules.

Digestive Chemical	Production Site	Actions
Bile	Hepatocytes	Emulsifies fats, works with lipase
Hydrochloric acid (HCl)	Stomach (parietal cells)	Denatures proteins, kills bacteria, activates pepsinogen
Bicarbonate ion (HCO3-)	Pancreatic duct, Gall bladder (some)	Neutralizes acid chyme
Mucus	Mucosa (stomach & SI)	Neutralizes HCl
Salivary amylase	Salivary glands	Begins starch digestion

Lesson 4 – Physio-EX Simulation: Enzyme Assays

Laboratory simulations help illustrate the effects of pH and temperature on enzyme activity, particularly amylase.

• Effect of pH: Amylase activity is highest at neutral pH (7.0). Acidic (pH 2) and basic (pH 9) conditions decrease enzyme activity.

• Effect of boiling: Boiling denatures enzymes, preventing starch digestion.

• Salivary amylase in the stomach: Inactive due to acidic pH; cannot digest starch.

• Experimental results: If both starch and maltose are present, breakdown is incomplete; some enzyme remains active.

Key Equations:

• Enzyme activity rate: $\text{Rate} = k[\text{Enzyme}][\text{Substrate}]$

• Starch breakdown: $\text{Starch} \xrightarrow{\text{Amylase}} \text{Maltose}$

Digestive Enzymes and Their Pathways

Enzymatic pathways are specific to each macromolecule and involve sequential breakdown into absorbable units.

Macromolecule	Intermediate Product	End Product
Carbohydrates	Disaccharides	Monosaccharides
Lipids	Glycerol + 3 fatty acids	Fatty acids, glycerol
Proteins	Dipeptides	Amino acids
Nucleic acids	Nucleotides	Sugars, bases, phosphate

Digestive Enzymes Table:

Enzyme	Produced by	Action Site	Substrate	End Product
Amylase	Salivary glands, Pancreas	Mouth, Duodenum/SI	Starch (amylose)	Maltose
Maltase, Sucrase, Lactase	Duodenal mucosa	Duodenum/SI	Maltose, Sucrose, Lactose	Glucose + Glucose, Glucose + Fructose, Glucose + Galactose
Pepsin	Stomach	Stomach	Proteins	Polypeptides
Trypsin, Chymotrypsin	Pancreas	Duodenum/SI	Polypeptides	Dipeptides
Carboxypeptidase	Pancreas, Duodenal mucosa	Duodenum/SI	Polypeptides	Amino acids
Lipase	Pancreas	Duodenum/SI	Triglycerides	3 Fatty acids + 1 Glycerol
Deoxyribonuclease (DNAse), Ribonuclease (RNAse)	Pancreas	Duodenum/SI	DNA, RNA	Nucleotides

Metabolic Regulation: Insulin and Glucagon

Insulin and glucagon are key hormones regulating metabolism, especially blood glucose levels.

• Insulin: Released during absorptive state; stimulates glycogenesis, glycolysis, lipogenesis, protein anabolism; inhibits glycogenolysis, gluconeogenesis, lipolysis, protein catabolism.

• Glucagon: Released during postabsorptive state; stimulates glycogenolysis, gluconeogenesis, lipolysis, protein catabolism; inhibits glycogenesis, glycolysis, lipogenesis, protein anabolism.

• Clinical relevance: Insulin deficiency leads to diabetes mellitus (DM).

Additional info: Glycogenesis is the formation of glycogen from glucose; glycolysis is the breakdown of glucose to pyruvate; gluconeogenesis is the formation of glucose from non-carbohydrate sources.