Digestion in Small Intestine — Core Principles

Core Principles

The small intestine is the primary site for complete digestion and absorption of nutrients. It's divided into the duodenum, jejunum, and ileum. The duodenum receives acidic chyme from the stomach, which is then neutralized by bicarbonate from the pancreas.

Here, bile from the liver/gallbladder emulsifies fats, and pancreatic juice (containing amylase, lipases, and proteases like trypsin and chymotrypsin) begins the major breakdown of carbohydrates, fats, and proteins.

As food moves into the jejunum and ileum, brush border enzymes (e.g., maltase, sucrase, lactase, peptidases) complete the digestion, breaking down disaccharides into monosaccharides and small peptides into amino acids.

Fats are broken into monoglycerides and fatty acids. The small intestine's inner lining is highly folded with villi and microvilli, vastly increasing the surface area for efficient absorption. Monosaccharides and amino acids enter the bloodstream, while fats are re-packaged into chylomicrons and enter the lymphatic system.

Hormones like secretin and CCK regulate these processes.

Important Differences

vs Digestion in Stomach

Aspect	This Topic	Digestion in Stomach
Primary Function	Initial protein digestion, food storage, mechanical churning.	Complete digestion of all macronutrients, primary site of nutrient absorption.
pH Environment	Highly acidic (pH 1.5-3.5) due to HCl.	Alkaline (pH 7-8) due to pancreatic bicarbonate.
Key Enzymes	Pepsin (for proteins), Gastric lipase (minor fat digestion).	Pancreatic amylase, lipases, proteases (trypsin, chymotrypsin), brush border enzymes (maltase, sucrase, lactase, peptidases).
Macronutrient Digestion	Mainly protein digestion begins; minor fat digestion; no carbohydrate digestion.	Complete digestion of carbohydrates, proteins, and fats.
Accessory Organ Secretions	None directly, only gastric secretions.	Receives bile from liver/gallbladder and pancreatic juice from pancreas.
Absorption	Limited absorption (water, alcohol, some drugs).	Extensive absorption of digested nutrients, water, electrolytes, vitamins.
Structural Adaptations	Rugae (folds) for expansion.	Plicae circulares, villi, microvilli for vast surface area increase.

Digestion in the stomach primarily focuses on initiating protein breakdown in a highly acidic environment and mechanically churning food. In contrast, the small intestine is the central hub for the complete chemical digestion of all macronutrients (carbohydrates, proteins, fats) and the subsequent absorption of nearly all nutrients. It operates in an alkaline environment, utilizing a broad spectrum of enzymes from the pancreas and its own brush border, along with bile for fat emulsification. Its intricate structure with villi and microvilli provides an enormous surface area, making it exceptionally efficient for absorption, a function largely absent in the stomach.