Digestion in Stomach — Core Principles

Core Principles

Digestion in the stomach is a vital step where food undergoes both mechanical and chemical breakdown. Mechanically, the stomach's muscular walls churn and mix food with gastric juices, forming a semi-liquid called chyme.

Chemically, specialized gastric glands secrete gastric juice, a potent mixture containing hydrochloric acid (HCl), pepsinogen, gastric lipase, mucus, and intrinsic factor. HCl creates an acidic environment (pH 1.

5-3.5), which denatures proteins, kills pathogens, and activates pepsinogen into pepsin. Pepsin is the primary enzyme for protein digestion, breaking them into smaller polypeptides. Gastric lipase initiates minor fat digestion.

Mucus protects the stomach lining from the corrosive acid and enzymes, while intrinsic factor is crucial for Vitamin B12 absorption. This entire process is tightly regulated by neural and hormonal signals, preparing the chyme for its controlled release into the small intestine.

Important Differences

vs Digestion in Mouth

Aspect	This Topic	Digestion in Mouth
Primary Function	Mechanical breakdown (chewing), initiation of carbohydrate and minor fat digestion.	Mechanical churning, initiation of protein digestion, antimicrobial action.
pH Environment	Neutral to slightly alkaline (pH $6.7-7.0$).	Highly acidic (pH $1.5-3.5$).
Key Enzymes	Salivary amylase (carbohydrates), Lingual lipase (minor fat).	Pepsin (proteins), Gastric lipase (minor fat).
Major Nutrient Digested	Carbohydrates (starch).	Proteins.
Protective Mechanism	Saliva moistens and lubricates food, antibodies provide minor defense.	Thick mucus layer and bicarbonate protect against acid and enzymes.
Product	Bolus (soft, moist mass of food).	Chyme (acidic, semi-liquid paste).

Digestion in the mouth primarily involves mechanical chewing and the initial enzymatic breakdown of carbohydrates by salivary amylase in a neutral pH environment, forming a bolus. In contrast, digestion in the stomach is characterized by vigorous mechanical churning and the commencement of protein digestion by pepsin in a highly acidic environment, transforming the bolus into chyme. The stomach also provides a crucial antimicrobial barrier and secretes intrinsic factor for Vitamin B12 absorption, functions not present in the mouth. These differences highlight the specialized roles of each organ in the digestive continuum.