Which body system breaks down food into smaller molecules and absorbs nutrients?

• A. Digestive system
• B. Immune system
• C. Respiratory system
• D. Excretory system

Answer: (A)

Breaking down food into smaller molecules and absorbing nutrients is the job of the digestive system. It uses both mechanical and chemical processes to convert food into usable building blocks. Chewing and stomach churning physically break things apart, while enzymes and acids in the mouth, stomach, and especially the small intestine chemically digest proteins, carbohydrates, and fats. The small intestine then takes up these nutrients through its lining, which is covered with folds and tiny projections called villi to maximize absorption into the bloodstream (and fats into the lymphatic system). The large intestine mainly absorbs water, but the key step of turning food into absorbable nutrients happens primarily in the digestive tract. Other systems handle different roles—defense against disease, breathing, and waste elimination—so they don't perform this nutrient-absorption function.