Physiology is the study of how living organisms and their various components function. The small intestine plays a central role in the human digestive system, processing ingested food and transforming it into usable forms for the body.
Structural Foundations for Function
The small intestine is a long, coiled tube, typically measuring 3 to 5 meters in length, extending from the stomach to the large intestine. It is divided into three segments: the duodenum, jejunum, and ileum. The duodenum receives partially digested food from the stomach. The jejunum is a primary site for nutrient absorption, while the ileum, the final section, absorbs remaining nutrients, including specific vitamins and bile salts.
The internal lining of the small intestine is highly specialized for functional efficiency. It features circular folds (plicae circulares), macroscopic folds of the mucosal and submucosal layers. Projecting from these folds are millions of tiny, finger-like villi, each about 0.5 to 1 mm long. Each villus is covered by cells with even smaller, microscopic projections called microvilli, forming the brush border.
These successive levels of folding—circular folds, villi, and microvilli—collectively amplify the surface area dramatically. This extensive surface area, estimated to be 60 to 120 times greater than a smooth tube, is fundamental for efficient digestion and absorption. The increased contact allows for more thorough interaction between digestive enzymes, nutrients, and absorptive cells, ensuring the body extracts maximum sustenance from food.
Breaking Down Food
Chemical digestion in the small intestine involves a coordinated effort of various enzymes, transforming complex food molecules into simpler units. Pancreatic enzymes, delivered from the pancreas into the duodenum, play a key role in this process. Pancreatic amylase breaks down carbohydrates into smaller sugars, while pancreatic lipases break down fats into fatty acids and monoglycerides. Proteases like trypsin and chymotrypsin dismantle proteins into smaller peptides.
The small intestine’s own brush border enzymes further refine the digestive process. Disaccharidases, such as lactase, sucrase, and maltase, break down disaccharides into their constituent monosaccharides, like glucose, fructose, and galactose. Peptidases, including aminopeptidase and dipeptidase, complete protein digestion by cleaving small peptides into individual amino acids. These enzymes are embedded within the microvilli, ensuring digestion occurs directly at the absorptive surface.
Bile, produced by the liver and stored in the gallbladder, is released into the duodenum and is important for fat digestion. Bile salts act as emulsifiers, breaking large fat globules into smaller droplets. This emulsification increases the surface area of fats, allowing pancreatic lipase to access and break them down more effectively. Without bile, fat digestion and absorption would be impaired.
Absorbing Life’s Essentials
Following the extensive breakdown of food, the small intestine orchestrates the absorption of these digested nutrients. Monosaccharides (the simplest carbohydrates) and amino acids (protein building blocks) are primarily absorbed across the intestinal lining into the capillaries within the villi. From there, they enter the bloodstream and are transported directly to the liver for further processing.
Fats, broken down into fatty acids and monoglycerides, follow a different absorption pathway. These lipid components are absorbed into the epithelial cells, reassembled into triglycerides, and then packaged into chylomicrons. Chylomicrons are too large to enter the capillaries directly, so they are absorbed into specialized lymphatic vessels within the villi called lacteals. The lymphatic system then transports these fats to the bloodstream, bypassing the liver initially.
In addition to macronutrients, the small intestine absorbs water, vitamins, and minerals. Water absorption occurs largely by osmosis, following the movement of absorbed solutes. Vitamins and minerals, depending on their type, are absorbed through various mechanisms, including active transport and facilitated diffusion. For instance, iron is primarily absorbed in the duodenum, while vitamin B12 and bile salts are absorbed in the terminal ileum.
Orchestrating Movement and Control
The small intestine exhibits distinct patterns of motility for mixing food with digestive juices and propelling it forward. Segmentation contractions are localized, rhythmic constrictions of the circular muscles that divide the chyme into segments. This back-and-forth movement thoroughly mixes the chyme with enzymes and ensures maximum exposure of nutrients to the absorptive surface. Segmentation is the most frequent motor pattern.
Peristalsis, in contrast, involves wave-like contractions of the intestinal muscles that propel the chyme progressively along the digestive tract. While segmentation primarily mixes, peristalsis ensures unidirectional movement towards the large intestine. These motor activities are regulated by the enteric nervous system, an intrinsic network of nerves within the intestinal wall, allowing local control.
Hormonal signals further coordinate the small intestine’s functions with other digestive organs. When acidic chyme enters the duodenum, small intestine cells release hormones like secretin and cholecystokinin (CCK). Secretin stimulates the pancreas to release bicarbonate, neutralizing the stomach acid and creating an optimal environment for enzyme activity. CCK, released in response to fats and proteins, triggers the pancreas to secrete digestive enzymes and stimulates the gallbladder to contract, releasing bile into the duodenum. These hormonal actions synchronize digestive processes for efficient nutrient extraction.