Which organ handles the majority of digestion and nutrient absorption?

• A. Stomach
• B. Mouth
• C. Small intestine
• D. Large intestine

Answer: (C)

The small intestine is structured for maximizing digestion and nutrient uptake. After chyme leaves the stomach, it mixes with bile and pancreatic enzymes in the small intestine, where most chemical digestion occurs. The duodenum starts this process, and the jejunum does the bulk of nutrient absorption, with the ileum absorbing bile acids and vitamin B12. The lining features enormous surface area from folds, villi, and microvilli, plus many transporters and enzymes on the brush border, so nutrients like glucose, amino acids, fatty acids, vitamins, and minerals move into the bloodstream efficiently. In addition, brush-border enzymes finish the final steps of digestion right at the enterocytes.

By contrast, the stomach mainly digests proteins and forms a semi-liquid chyme, with limited absorption. The large intestine absorbs water and electrolytes and hosts bacterial activity that makes some vitamins, but it is not where most nutrient absorption happens.