Which digestive organ is the major site of nutrient absorption?

Question

BlackTom AI · Accepted Answer

The small intestine is the major site of nutrient absorption because its structure and physiology are specialized to take up the end products of digestion.

1) Relevant knowledge points and concepts
- Digestive tract roles: ingestion, propulsion, digestion, absorption, elimination.
- Absorption requires large surface area, thin epithelium, rich blood/lymphatic supply.
- The small intestine (duodenum, jejunum, ileum) has folds, villi, and microvilli that dramatically increase surface area and possess transport mechanisms for sugars, amino acids, fatty acids, vitamins, minerals, and water.
- Other organs: esophagus (transport), stomach (mechanical/chemical digestion; limited absorption), large intestine (water, electrolytes, some vitamins).

2) Step-by-step reasoning
- Food is mechanically and chemically broken down in stomach and duodenum into absorbable units: monosaccharides, amino acids, fatty acids/monoglycerides.
- Most chemical digestion of carbohydrates, proteins, and fats is completed in the small intestine by pancreatic enzymes and brush-border enzymes.
- The jejunum and ileum epithelial cells actively and passively transport these nutrients into capillaries (for sugars/amino acids) or into lacteals as chylomicrons (for fats).

3) Intermediate steps/details
- Surface area amplification: circular folds (plicae circulares), villi (~0.5–1 mm projections), and microvilli (brush border) increase absorptive area to dozens or hundreds of square meters, maximizing contact with chyme.
- Capillary networks in villi rapidly carry absorbed monosaccharides and amino acids to hepatic portal circulation; lacteals transport lipids to lymphatics.

4) Why the correct answer is correct
- Because the small intestine combines completion of digestion with maximal absorptive surface and transport systems, it is uniquely suited to absorb the bulk of nutrients.

5) Why other options are incorrect
- Esophagus: primarily a conduit using peristalsis; no nutrient absorption.
- Stomach: mainly mechanical churn and protein digestion; only minimal absorption (e.g., some alcohol, aspirin), not primary nutrient uptake.
- Large intestine: absorbs water, electrolytes, and certain vitamins (K, some B vitamins) produced by flora, but not the main site for macronutrient absorption.

6) Conclusion
- Therefore, the correct answer is Small intestine, which matches the provided answer.

Which digestive organ is the major site of nutrient absorption?单项选择题

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Question at position 39 How do nutrients from food actually enter the body’s tissues?By mechanical digestion in the stomachBy filtration through the kidneysThrough absorption in the digestive tractThrough enzymatic breakdown in the esophagus

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