Digestive System: The Stomach and Peristalsis

Practice questions covering the structure and function of the stomach, components of gastric juice, and the movement of food through the digestive tract.

What is peristalsis?

Peristalsis is the process of rhythmic, involuntary muscle contractions that move food or fluid along the gastrointestinal tract, such as from the esophagus to the stomach.

What is the capacity of the J-shaped stomach and how does it expand?

The stomach can store about $1.5\,L$ of food and has numerous ridges that allow it to expand.

What are the two main functions of the stomach mentioned in the text?

The stomach is the site of food storage and initial protein digestion.

How many muscle layers does the stomach have and what is their purpose?

The stomach contains three layers of muscle running in different directions to churn the food.

What is the function of the lower esophageal sphincter ($LES$)?

The $LES$ regulates the movement of food into the stomach and prevents food and acid from being regurgitated up into the esophagus.

What is the function of the pyloric sphincter?

The pyloric sphincter regulates the movement of food and stomach acids into the small intestine.

What are the three main activities that occur in the stomach?

1. Secretion of gastric fluids/juice; 2. Contractions to mix food with gastric fluids; 3. Physical (churning) and chemical (protein digestion) digestion.

What is the composition of gastric juice?

Gastric juice includes mucus, hydrochloric acid ($HCl$), pepsinogens, and other substances.

How much gastric fluid is typically produced following a large meal?

Approximately $500\,mL$ of fluid.

What are the two primary roles of hydrochloric acid ($HCl$) in the stomach?

$HCl$ kills many harmful substances (pathogens) ingested with food and converts pepsinogen into its active form, pepsin.

What is the role of the enzyme pepsin?

Pepsin is a protein-digesting enzyme that breaks long amino acid chains in proteins into shorter chains called polypeptides.

What is the normal pH range inside the stomach?

The pH normally ranges between $2.0$ and $3.0$, but it may approach $1.0$.

How does the stomach protect itself from the low pH and pepsin?

A layer of alkaline mucus protects the stomach lining from being digested.

What causes the pain known as heartburn?

Heartburn is caused when a weak lower esophageal sphincter ($LES$) allows stomach acid to enter the esophagus, which lacks a protective mucous layer.

What is chyme?

Chyme is the name for the mixture of partially digested food and gastric juice.

What is the typical timeframe for gastric emptying after a meal?

Gastric emptying occurs within two to six hours after a meal.

Why does the chemical digestion of starches and fats decrease in the stomach?

The low pH of the stomach denatures the amylase and lipase that were secreted in the mouth.