The small intestine is the primary site for the completion of food breakdown and the subsequent absorption of nutrients. This extensive, coiled organ resides between the stomach and the large intestine. Although named “small,” it is remarkably long, measuring approximately 20 to 25 feet in adults, but its narrow diameter gives it the designation. It converts ingested food into usable fuel and building blocks for the body.
The Three Anatomical Divisions
The small intestine is organized into three distinct segments, each with specialized functions that facilitate digestion. The first segment is the duodenum, a short, C-shaped tube that receives partially digested food, called chyme, from the stomach. Contents from the pancreas and gallbladder enter here, mixing with the acidic chyme to initiate chemical digestion.
Following the duodenum is the jejunum, which constitutes about two-fifths of the small intestine’s total length. The jejunum is the major location for chemical digestion, breaking down macromolecules into absorbable components. It is also the principal region where the vast majority of nutrient absorption occurs.
The final and longest section is the ileum, which connects to the large intestine at the ileocecal valve. The ileum is specifically responsible for reclaiming unabsorbed bile salts and absorbing the final remaining nutrients. Notably, it is the only site for the absorption of vitamin B12, accomplished via a specialized transport system.
Specialized Features for Absorption
The small intestine is structurally engineered to maximize its surface area, which significantly enhances nutrient absorption efficiency. The inner wall is lined with permanent, circular folds, known as plicae circulares, which slow the movement of chyme and increase exposure to the absorptive surface. Projecting from these folds are millions of tiny, finger-like structures called villi.
Each villus is covered in epithelial cells, and each cell possesses microscopic projections called microvilli. This “brush border” created by the microvilli further amplifies the surface area. This vast area ensures that nutrients are efficiently captured and transported.
Beneath the epithelial layer of each villus is a network of capillaries and a single specialized lymphatic vessel called a lacteal. Water-soluble nutrients, such as simple sugars and amino acids, are absorbed directly into the capillary network, which drains into the hepatic portal vein toward the liver. Conversely, most absorbed dietary fats enter the lacteal, which transports them into the lymphatic system, bypassing the immediate circulatory system.
Primary Roles in Chemical Digestion and Nutrient Absorption
Chemical digestion in the small intestine relies on enzymes from the pancreas and those embedded in the intestinal brush border. Pancreatic amylase continues the breakdown of complex carbohydrates, while brush border enzymes, such as lactase and sucrase, complete the process by splitting disaccharides into absorbable monosaccharides. Proteins, partially broken down in the stomach, are further cleaved by pancreatic proteases like trypsin and chymotrypsin into smaller peptides.
Brush border peptidases perform the final digestion of peptides into single amino acids. Fats are water-insoluble, requiring emulsification by bile salts to form tiny droplets called micelles. Pancreatic lipase then hydrolyzes the triglycerides within the micelles into monoglycerides and free fatty acids.
Nutrient absorption employs various transport mechanisms to move these products across the intestinal lining. Amino acids and most monosaccharides, including glucose, are actively transported across the cell membrane, often coupled with the movement of sodium ions. These water-soluble molecules then exit the cell via facilitated diffusion into the bloodstream.
Monoglycerides and fatty acids diffuse into the epithelial cells where they are reassembled into triglycerides. These new triglycerides are packaged with proteins and phospholipids into large transport vesicles called chylomicrons. Because of their size, chylomicrons are expelled from the cell and enter the lacteals, ultimately joining the systemic circulation through the lymphatic system.
Immune Defense and Hormonal Regulation
Beyond core digestive functions, the small intestine plays a role in both immune defense and the coordination of digestion. The organ contains numerous patches of specialized lymphoid tissue, particularly concentrated in the ileum, known as Peyer’s patches. These patches function as surveillance centers, monitoring intestinal contents for harmful microorganisms and foreign antigens.
Peyer’s patches contain immune cells that sample the environment and initiate a protective immune response if a pathogen is detected, helping to prevent infection from entering the bloodstream. This localized immune system is a sophisticated part of the body’s defense against the massive microbial load present in the gut.
The small intestine also regulates digestion through the release of hormones into the bloodstream. When acidic chyme enters the duodenum, specialized cells release the hormone secretin. Secretin stimulates the pancreas to release a bicarbonate-rich fluid that neutralizes stomach acid, establishing the optimal pH for digestive enzymes.
Another hormone, cholecystokinin (CCK), is released in response to fats and proteins in the chyme. CCK signals the pancreas to secrete digestive enzymes and simultaneously causes the gallbladder to contract, releasing bile into the duodenum.