a and o lecture 16

Enzymes and Their Roles

• Lingual Lipase: An enzyme secreted in saliva that begins the digestion of lipids (fats).

• Amylase and Protease: Enzymes that help in breaking down carbohydrates (amylase) and proteins (protease). The suffix "-ase" indicates that these are enzymes.

Swallowing Process

• Muscle Coordination: Over 22 muscle groups work together in a coordinated fashion to move food from the oral cavity to the stomach.

• Buccal Phase: The first phase of swallowing where the food is voluntarily pushed back into the pharynx by the tongue.

• Involuntary Phase: After the food reaches the pharynx, involuntary muscles take over, which push the food down through the esophagus.

Pharynx and Esophagus

• Tongue Role: Presses against the palate to push food (bolus) into the pharynx.

• Stretch Receptors: Located in the pharynx; respond to the presence of food, stimulating muscle contractions.

• Peristalsis: The circular and longitudinal muscles in the esophagus contract rhythmically to propel food down to the stomach.

• Sphincter Muscles: Gastroesophageal sphincter prevents the reflux of stomach contents back into the esophagus (which is harmful due to acidic content).

Muscle Types in Digestive Tract

• Striated Muscle: Found in the upper esophagus and under voluntary control.

• Smooth Muscle: Predominant in the lower esophagus and elsewhere in the gastrointestinal tract; under involuntary control.

Gastroesophageal Sphincter

• Function: Prevents acidic stomach contents from moving back into the esophagus.

• Importance: Protects esophageal tissues from damage.

Mucosal Function

• Mucus Production: Mucus serves as a protective barrier against pathogens and acidic content in the gastrointestinal system.

• Histological Features: Pharynx and esophagus lined with stratified squamous epithelium to withstand mechanical stress.

Muscularis Externa

• Structure: Comprised of layers of circular and longitudinal smooth muscles helping in peristalsis.

Peristalsis

• Mechanism: Alternating contractions of circular and longitudinal muscles push food through the gastrointestinal tract.

• Role: Essential for moving food effectively and mixing its contents, especially in the small intestine, to facilitate digestion and absorption.

Stomach Anatomy

• Structure: Can expand significantly, holding up to 4 liters of food.

• Specializations: Has three muscle layers (longitudinal, circular, and oblique) allowing for effective churning and mixing of food with gastric juices (chyme).

Gastric Environment

• Acidic Conditions: Stomach contents have a pH of around 1, enabling the breakdown of food and killing pathogens.

• Gastric Juice Components: Includes pepsinogen (activated to pepsin) and hydrochloric acid.

• Mucosal Barrier: Simple columnar epithelium secretes a thick layer of mucus to protect the stomach's lining from acidic damage.

Secretions and Control Mechanisms

• Cell Types:

  • Chief Cells: Secrete pepsinogen.

  • Parietal Cells: Secrete hydrochloric acid and intrinsic factor (important for vitamin B12 absorption).

• Regulation of Secretions:

  • Cephalic Phase: Preparation for digestion upon seeing, smelling, or tasting food.

  • Gastric Phase: Food entering stomach increases secretion of gastric juices and motility through hormone gastrin.

  • Intestinal Phase: Chyme released into small intestine progressively, neutralizing acidity before further digestion.

Absorption and Digestion in the Stomach

• Nutrient Absorption: Primarily occurs in the small intestine; stomach’s role is mainly mechanical and enzymatic breakdown.

• Digestive Agents: Enzymes in the stomach facilitate the initial breakdown of proteins and fats, while complex carbohydrates are mainly digested in later sections of the gastrointestinal tract.