3.7

Lecture Overview

  • Course: Lecture 3.7: Upper GI Tract and Stomach

  • Instructor: Tim Maley, PhD

  • Contact Information: maleytim@arizona.edu

  • Office Hours:

    • Monday: 10:30-11:30 am

    • Friday: 11:30-12:30 pm

Learning Objectives (Saladin, Ch. 25)

  1. Saliva Composition and Functions

    • Understand what saliva is made of and its various roles in digestion.

  2. Saliva Secretion Regulation

    • Learn mechanisms for controlling when and how saliva is secreted.

  3. Swallowing Process

    • Identify structures involved and the stages of swallowing.

  4. Smooth Muscle in GI Tract

    • Discuss the structure and role of smooth muscle in digestion.

  5. Stomach Regions and Functions

    • Outline major areas of the stomach and their specific functions.

  6. Stomach Cell Types and Secretions

    • Learn different cell types in the stomach and what substances they secrete.

  7. HCl Secretion Process

    • Explain how hydrochloric acid is secreted, including relevant transporters.

  8. Response to Food Ingestion

    • Discuss how stomach secretory cells and muscles react to food intake.

Digestion in the Mouth

  • Mechanical Digestion:

    • Mastication (chewing) forms a bolus of food.

  • Chemical Digestion:

    • Salivary amylase starts starch digestion.

    • Lingual lipase initiates triglyceride breakdown.

    • Digestion is pH dependent.

Salivary Glands and Ducts

  • Key Structures:

    • Parotid Gland and its duct, Submandibular Gland, Sublingual Gland.

    • Salivary Composition:

      • Enzymes (amylase, lipase), mucus, water, bicarbonate, lysozyme (antibacterial).

Regulation of Saliva Secretion

  • Mechanisms:

    • Involvement of pressure and chemoreceptors in the mouth.

    • Various inputs lead to salivary secretion controlled by the autonomic nervous system.

    • Distinctions between parasympathetic and sympathetic responses.

Swallowing Process (Deglutition)

  • Stages:

    1. Voluntary Stage: Oral phase where bolus is pushed to pharynx.

    2. Involuntary Stage: Pharyngeal and esophageal phases follow.

  • Esophageal Stage:

    • Upper esophageal sphincter relaxes, and peristalsis begins to move bolus down.

    • Time taken for solids (4-8s) and liquids (1s) to reach the stomach.

Anatomy of the Stomach

  • Regions:

    • Cardia, Body, Fundus, Antrum, Pylorus.

  • Muscular Layers:

    • Contains longitudinal, circular, and oblique layers.

Mechanical Digestion in the Stomach

  • Processes:

    • Gentle mixing of the bolus with gastric juices forms chyme.

    • Waves of contraction mix and push chyme toward the pyloric sphincter.

Stomach Cell Types

  • Cell Types:

    • Surface mucous cells: secrete mucus

    • Mucous neck cells: secrete mucus

    • Parietal cells: secrete hydrochloric acid and intrinsic factor

    • Chief cells: secrete pepsinogen and gastric lipase

    • G cells: secrete gastrin hormone.

The Gastric Phase – Chemical Digestion

  • HCl Function:

    • Roughly 1-3L HCl produced daily

    • Activates pepsin for protein breakdown, denatures proteins to facilitate digestion, kills pathogens, and inactivates salivary amylase.

  • Intrinsic Factor Production:

    • Key for absorption of Vitamin B12, essential for RBC production.

Chemical Digestion in the Stomach

  • Protein Digestion:

    • HCl denatures proteins and converts pepsinogen to pepsin.

  • Fat Digestion:

    • Continues with gastric lipase (effective at low pH).

  • Microbial Control:

    • HCl serves to kill microbes present in food.

Absorption of Substances in the Stomach

  • Minimal Absorption:

    • Stomach absorbs water, electrolytes, certain drugs (like aspirin), and alcohol.


  • Course: Lecture 3.7: Upper GI Tract and Stomach

  • Instructor: Tim Maley, PhD

  • Contact Information: maleytim@arizona.edu

  • Office Hours:

    • Monday: 10:30-11:30 am

    • Friday: 11:30-12:30 pm

Learning Objectives (Saladin, Ch. 25)

  1. Saliva Composition and Functions

    • Understand the complex composition of saliva, which includes water, electrolytes, enzymes (like amylase and lipase), mucins, and antibacterial agents such as lysozyme.

    • Discuss its critical roles including digestion, oral hygiene, and facilitating swallowing.

  2. Saliva Secretion Regulation

    • Learn about the neural and hormonal control mechanisms governing saliva secretion.

    • Distinctions between parasympathetic responses (stimulating secretion) and sympathetic responses (inhibiting secretion) during stress.

  3. Swallowing Process

    • Identify key anatomical structures involved in swallowing, including the tongue, pharynx, and esophagus.

    • Discuss the three phases of swallowing: the voluntary phase, pharyngeal phase, and esophageal phase, including reflexes involved.

  4. Smooth Muscle in GI Tract

    • Discuss the unique structure of smooth muscle in the GI tract, including its arrangement in layers and its role in peristalsis and segmentation during digestion.

    • Explain the significance of myenteric and submucosal plexuses in regulating gut motility.

  5. Stomach Regions and Functions

    • Outline the major areas of the stomach: the cardia, body, fundus, antrum, and pylorus, describing their anatomical features and functional contributions to digestion.

  6. Stomach Cell Types and Secretions

    • Learn about the various cell types in the gastric mucosa: surface mucous cells, mucous neck cells, parietal cells, chief cells, and G cells.

    • Discuss the specific substances each cell type secretes, such as hydrochloric acid from parietal cells and pepsinogen from chief cells.

  7. HCl Secretion Process

    • Explain the cellular mechanisms of hydrochloric acid secretion, focusing on the role of proton pumps and the importance of histamine, acetylcholine, and gastrin in the regulatory pathway.

  8. Response to Food Ingestion

    • Discuss the physiological responses of stomach secretory cells and muscles to food intake, including the role of gastric distension and chemical stimuli in signaling gastric secretion and motility.

Digestion in the Mouth

  • Mechanical Digestion:

    • The process of mastication (chewing) involves coordinated movements of the jaw, tongue, and teeth, forming a bolus that is easily swallowable.

  • Chemical Digestion:

    • Salivary amylase initiates the digestion of starches into simpler sugars, while lingual lipase begins the breakdown of triglycerides, particularly in infants.

    • The efficacy of digestion is influenced by the pH of the saliva and the food's properties.

Salivary Glands and Ducts

  • Key Structures:

    • Major salivary glands include the parotid gland and its duct, submandibular gland, and sublingual gland, which collectively produce a range of saliva components.

    • Salivary Composition:

      • Components include enzymes (such as amylase and lipase), mucus for lubrication, water for solubility, bicarbonate to neutralize acids, and lysozyme for its antibacterial properties.

Regulation of Saliva Secretion

  • Mechanisms:

    • Chewing, tasting, and the anticipation of food stimulate salivary secretion via pressure and chemoreceptors in the mouth.

    • Saliva secretion is controlled by the autonomic nervous system, particularly through reflexes that are predominantly parasympathetic in nature, enhancing digestion.

    • Discuss the contrasting functions of the sympathetic nervous system, which can reduce saliva flow under stressful conditions.

Swallowing Process (Deglutition)

  • Stages:

    1. Voluntary Stage: The oral phase where the tongue pushes the bolus back towards the pharynx.

    2. Involuntary Stages: Pharyngeal phase involves the reflex action of swallowing, closing of the airway, and esophageal phase is marked by peristalsis movement.

  • Esophageal Stage:

    • The upper esophageal sphincter relaxes to allow the bolus to enter the esophagus, and peristalsis transports the bolus downwards to the stomach, generally taking 4-8 seconds for solids and about 1 second for liquids to reach the stomach.

Anatomy of the Stomach

  • Regions:

    • The stomach comprises five main regions: the cardia (where the esophagus meets the stomach), body (the main part), fundus (the upper rounded portion), antrum (the lower part), and pylorus (which leads into the small intestine).

  • Muscular Layers:

    • The stomach features three layers of muscle: longitudinal, circular, and oblique, facilitating the churning action necessary for digestion.

Mechanical Digestion in the Stomach

  • Processes:

    • The stomach performs gentle mixing of the bolus with gastric juices, forming a semi-liquid substance called chyme.

    • Waves of contraction mix chyme effectively and push it toward the pyloric sphincter for entry into the small intestine.

Stomach Cell Types

  • Cell Types:

    • Different types of gastric secretory cells are crucial:

      • Surface mucous cells: secrete a protective mucus layer

      • Mucous neck cells: provide additional mucus for lubrication

      • Parietal cells: produce hydrochloric acid and intrinsic factor for B12 absorption

      • Chief cells: secrete pepsinogen (activated to pepsin) and gastric lipase for fat digestion

      • G cells: release gastrin hormone, stimulating gastric motility and secretions.

The Gastric Phase – Chemical Digestion

  • HCl Function:

    • The stomach produces approximately 1-3 liters of hydrochloric acid daily, essential for activating pepsinogen to pepsin, denaturing proteins to enhance enzymatic breakdown, killing pathogens, and inactivating salivary amylase.

  • Intrinsic Factor Production:

    • Intrinsic factor is vital for the absorption of vitamin B12 from the intestines, a necessary component for the production of red blood cells.

Chemical Digestion in the Stomach

  • Protein Digestion:

    • Hydrochloric acid denatures proteins and converts pepsinogen into pepsin, leading to effective protein digestion.

  • Fat Digestion:

    • The breakdown of fats continues in the stomach via gastric lipase, which is active at low pH levels, assisting in emulsification.

  • Microbial Control:

    • Hydrochloric acid contributes to microbial control by killing harmful microorganisms that may be present in ingested food.

Absorption of Substances in the Stomach

  • Minimal Absorption:

    • While the primary function of the stomach is digestion, it can absorb certain substances such as water, electrolytes, and specific drugs like aspirin and alcohol in limited quantities due to the permeability of its epithelium.