What does mechanical digestion in the stomach involve?

• A. Secretion of digestive enzymes to break down proteins
• B. Absorption of nutrients into the bloodstream
• C. Production of bile to emulsify fats
• D. Churning and mixing of food by muscular contractions

Answer: (D)

Verified step by step guidance

1. Understand the concept of mechanical digestion: Mechanical digestion refers to the physical breakdown of food into smaller pieces without altering its chemical structure. This process prepares food for chemical digestion.
2. Identify the role of the stomach in mechanical digestion: The stomach uses muscular contractions, specifically peristalsis and churning, to mix and break down food into smaller fragments. This action helps expose food particles to digestive enzymes for chemical digestion.
3. Clarify the difference between mechanical and chemical digestion: Mechanical digestion involves physical processes like churning and mixing, while chemical digestion involves enzymatic reactions that break down macromolecules into their building blocks (e.g., proteins into amino acids).
4. Focus on the muscular contractions in the stomach: The stomach's walls contain layers of smooth muscle that contract rhythmically to churn food and mix it with gastric juices, forming a semi-liquid substance called chyme.
5. Eliminate incorrect options: Secretion of digestive enzymes, absorption of nutrients, and production of bile are chemical or accessory processes, not mechanical digestion. The correct answer is the churning and mixing of food by muscular contractions.